a

FAAH808330

AVI ATI ON MAI NTENANCETECHNI CI AN HANDBOOK

AVI ATI ON MAI NTENANCE TECHNI CI AN HANDBOOK

FAAH808330

GENERAL

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Preface The Aviation Maintenance Technician Handbook—General was developed as one of a series of three handbooks for persons preparing for mechanic certification with airframe or powerplant ratings, or both. It is intended that this handbook will provide basic information on principles, fundamentals, and technical procedures in the subject matter areas common to both the airframe and powerplant ratings. Emphasis in this volume is on theory and methods of application. The handbook is designed to aid students enrolled in a formal course of instruction preparing for FAA certifica‑ tion as a maintenance technician, as well as for current technicians who wish to improve their knowledge. This volume contains information on mathematics, aircraft drawings, weight and balance, aircraft materials, processes and tools, physics, electricity, inspection, ground operations, and FAA regulations governing the certification and work of maintenance technicians. New to this volume is a section addressing how successful aviation maintenance technicians incorporate knowledge and awareness of ethics, professionalism, and human factors in the field. Because there are so many different types of airframes and powerplants in use today, it is reasonable to expect that differences exist in the components and systems of each. To avoid undue repetition, the practice of using representative systems and units is implemented throughout the handbook. Subject matter treatment is from a generalized point of view, and should be supplemented by reference to manufacturers’ manuals or other textbooks if more detail is desired. This handbook is not intended to replace, substitute for, or supersede official regula‑ tions or manufacturer instructions. The companion handbooks to Aviation Maintenance Technician Handbook—General are the Airframe and Powerplant Mechanics Powerplant Handbook, AC 65-12A, and the Airframe and Powerplant Mechanics Airframe Handbook, AC 65-15A. This handbook may be purchased from the Superintendent of Documents, U.S. Government Printing Office (GPO), Washington, DC 20402-9325, or from the GPO website: http://bookstore.gpo.gov This handbook is also available for download, in pdf format, from the Regulatory Support Division’s (AFS-600) website: http://www.faa.gov/about/office_org/headquarters_offices/avs/offices/afs/afs600 This handbook is published by the U.S. Department of Transportation, Federal Aviation Administration, Airmen Testing Standards Branch, AFS-630, P.O. Box 25082, Oklahoma City, OK 73125. Comments regarding this publication should be sent, in email form, to the following address: [email protected]

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Aviation Maintenance Technician Handbook— General was produced by the Federal Aviation Administration (FAA) with the assistance of Aviation Supplies and Academics, Inc., and the following authors: Ann Riley, Bob Aardema, Pete Vosbury, Mary Ann Eiff, H.G. Frautschy, Ron Serkenburg, Don Shaffer, Tom Wild, and Terry Michmerhuizen. Grateful acknowledgment is extended to the contributors of photography for this hand‑ book. Contained throughout the book are photographs by H.G. Frautschy, Sam Chui, Jeremy Irish, Pete Vosbury, Matt Wallman, Fred Gay, Jim Groom, Emmanuel Perez, John Martin, Ad Vercruijsse, Marc Michel, Bill Blanchard, Wim Houquet, Jean-Luc Altherr, Andrew Doughty, Erwin van Dijck, Kathleen and Walter Hudson, Paul Lintott, Michael Davis, and Dave Baldwin.

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Contents Preface..................................................................iii Chapter 1. Mathematics..................................................... 1-1 Mathematics in Aviation Maintenance........................1-1 Whole Numbers..........................................................1-1 Addition of Whole Numbers..................................1-1 Subtraction of Whole Numbers.............................1-1 Multiplication of Whole Numbers.........................1-1 Division of Whole Numbers................................. 1-2 Fractions..................................................................... 1-2 Finding the Least Common Denominator............ 1-2 Addition of Fractions............................................ 1-2 Subtraction of Fractions........................................ 1-2 Multiplication of Fractions....................................1-3 Division of Fractions.............................................1-3 Reducing Fractions................................................1-3 Mixed Numbers......................................................... 1-4 Addition of Mixed Numbers................................. 1-4 Subtraction of Mixed Numbers............................ 1-4 The Decimal Number System.................................... 1-4 The Origin and Definition . .................................. 1-4 Addition of Decimal Numbers.............................. 1-5 Subtraction of Decimal Numbers......................... 1-5 Multiplication of Decimal Numbers..................... 1-5 Division of Decimal Numbers.............................. 1-6 Rounding Off Decimal Numbers.......................... 1-6 Converting Decimal Numbers to Fractions.......... 1-6 Converting Fractions to Decimals.........................1-7 Decimal Equivalent Chart......................................1-7 Ratio............................................................................1-7 Aviation Applications........................................... 1-9 Proportion.................................................................. 1-9 Extremes and Means............................................. 1-9 Solving Proportions............................................. 1-10 Percentage................................................................. 1-10 Expressing a Decimal Number as a Percentage............................................................ 1-10 Expressing a Percentage as a Decimal Number................................................................ 1-10 Expressing a Fraction as a Percentage................. 1-10 Finding a Percentage of a Given Number .......... 1-10 Finding What Percentage One Number Is of Another............................................................ 1-10

Finding a Number When a Percentage of It Is Known.......................................................... 1-11 Positive and Negative Numbers (Signed Numbers)................................................................... 1-11 Addition of Positive and Negative Numbers........ 1-11 Subtraction of Positive and Negative Numbers............................................................... 1-11 Multiplication of Positive and Negative Numbers............................................................... 1-11 Division of Positive and Negative Numbers.......1-12 Powers.......................................................................1-12 Special Powers . ..................................................1-12 Negative Powers..................................................1-12 Law of Exponents................................................1-12 Powers of Ten......................................................1-12 Roots....................................................................1-12 Square Roots........................................................1-13 Cube Roots...........................................................1-13 Fractional Powers................................................1-13 Functions of Numbers Chart.....................................1-13 Scientific Notation....................................................1-13 Converting Numbers from Standard Notation to Scientific Notation........................................... 1-16 Converting Numbers from Scientific Notation to Standard Notation............................. 1-16 Addition, Subtraction, Multiplication, and Division of Scientific Numbers........................... 1-16 Algebra...................................................................... 1-16 Equations............................................................. 1-16 Algebraic Rules................................................... 1-16 Solving for a Variable.......................................... 1-17 Use of Parentheses............................................... 1-17 Order of Operation............................................... 1-17 Order of Operation for Algebraic Equations....... 1-18 Computing Area of Two-Dimensional Solids........... 1-18 Rectangle............................................................. 1-18 Square.................................................................. 1-18 Triangle................................................................1-19 Parallelogram.......................................................1-19 Trapezoid.............................................................1-19 Circle................................................................... 1-20 Ellipse................................................................. 1-20 Wing Area........................................................... 1-20 Units of Area........................................................1-21



Computing Volume of Three-Dimensional Solids....1-21 Rectangular Solid.................................................1-21 Cube.................................................................... 1-22 Cylinder.............................................................. 1-22 Sphere................................................................. 1-23 Cone.................................................................... 1-23 Units of Volume.................................................. 1-23 Computing Surface Area ofThree-Dimensional Solids....................................................................... 1-23 Rectangular Solid................................................ 1-23 Cube.................................................................... 1-24 Cylinder.............................................................. 1-24 Sphere................................................................. 1-24 Cone ................................................................... 1-24 Trigonometric Functions.......................................... 1-24 Right Triangle, Sides and Angles........................ 1-24 Sine, Cosine, and Tangent................................... 1-24 Pythagorean Theorem......................................... 1-25 Measurement Systems............................................. 1-25 Conventional (U.S. or English) System.............. 1-25 Metric System..................................................... 1-25 Measurement Systems and Conversions............ 1-26 The Binary Number System..................................... 1-26 Place Values........................................................ 1-26 Converting Binary Numbers to Decimal Numbers............................................... 1-26 Converting Decimal Numbers to Binary Numbers . ............................................... 1-27

Chapter 2. Aircraft Drawings............................................. 2-1 Computer Graphics.................................................... 2-1 Purpose and Function of Aircraft Drawings.............. 2-1 Care and Use of Drawings......................................... 2-2 Types of Drawings..................................................... 2-2 Detail Drawing...................................................... 2-2 Assembly Drawing............................................... 2-2 Installation Drawing............................................. 2-3 Sectional View Drawings...................................... 2-3 Full Section...................................................... 2-3 Half Section..................................................... 2-3 Revolved Section............................................. 2-3 Removed Section............................................. 2-4 Title Blocks................................................................ 2-4 Drawing or Print Numbers.................................... 2-5 Reference and Dash Numbers.............................. 2-5 Universal Numbering System.................................... 2-5 Bill of Material........................................................... 2-6 Other Drawing Data................................................... 2-6

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Revision Block...................................................... 2-6 Notes..................................................................... 2-6 Zone Numbers...................................................... 2-6 Station Numbers and Location Identification on Aircraft............................................................. 2-7 Allowances and Tolerances................................... 2-7 Finish Marks......................................................... 2-7 Scale...................................................................... 2-7 Application........................................................... 2-7 Methods of Illustration............................................... 2-7 Applied Geometry................................................ 2-7 Orthographic Projection Drawings....................... 2-7 Detail View........................................................... 2-9 Pictorial Drawings...............................................2-11 Perspective Drawings.....................................2-11 Isometric Drawings.........................................2-11 Oblique Drawings...........................................2-11 Exploded View Drawings...............................2-11 Diagrams..............................................................2-11 Installation Diagrams......................................2-11 Schematic Diagrams...................................... 2-13 Block Diagrams............................................. 2-13 Wiring Diagrams........................................... 2-15 Flowcharts........................................................... 2-15 Troubleshooting Flowchart............................ 2-15 Logic Flowchart............................................. 2-15 Lines and Their Meanings........................................2-16 Centerlines...........................................................2-16 Dimension Lines..................................................2-16 Extension Lines...................................................2-17 Sectioning Lines..................................................2-17 Phantom Lines.....................................................2-17 Break Lines..........................................................2-17 Leader Lines........................................................2-17 Hidden Lines........................................................2-17 Outline or Visible Lines.......................................2-17 Stitch Lines..........................................................2-17 Cutting Plane and Viewing Plane Lines..............2-17 Drawing Symbols.....................................................2-17 Material Symbols.................................................2-18 Shape Symbols.....................................................2-18 Electrical Symbols...............................................2-18 Reading and Interpreting Drawings..........................2-18 Drawing Sketches.................................................... 2-20 Sketching Techniques......................................... 2-21 Basic Shapes....................................................... 2-21 Repair Sketches.................................................. 2-21 Care of Drafting Instruments................................... 2-21 Graphs and Charts.................................................... 2-21

Reading and Interpreting Graphs and Charts...... 2-21 Nomograms......................................................... 2-21 Microfilm and Microfiche........................................ 2-22 Digital Images.......................................................... 2-22

Chapter 3. Physics............................................................... 3-1 Matter......................................................................... 3-1 Characteristics of Matter....................................... 3-1 Mass and Weight.............................................. 3-1 Attraction......................................................... 3-1 Porosity............................................................ 3-2 Impenetrability................................................. 3-2 Density . .......................................................... 3-2 Specific Gravity .............................................. 3-2 Energy........................................................................ 3-3 Potential Energy.................................................... 3-3 Kinetic Energy...................................................... 3-4 Force, Work, Power, and Torque................................ 3-4 Force..................................................................... 3-4 Work...................................................................... 3-4 Friction and Work................................................. 3-5 Static Friction.................................................. 3-6 Sliding Friction................................................ 3-6 Rolling Friction................................................ 3-6 Power.................................................................... 3-7 Torque................................................................... 3-7 Simple Machines........................................................ 3-8 Mechanical Advantage of Machines..................... 3-8 The Lever.............................................................. 3-9 First Class Lever.............................................. 3-9 Second Class Lever......................................... 3-9 Third Class Lever.......................................... 3-10 The Pulley........................................................... 3-10 Single Fixed Pulley........................................ 3-10 Single Movable Pulley................................... 3-10 Block and Tackle............................................3-11 The Gear..............................................................3-11 Inclined Plane..................................................... 3-13 Stress........................................................................ 3-14 Tension................................................................ 3-14 Compression....................................................... 3-14 Torsion................................................................ 3-14 Bending............................................................... 3-14 Shear................................................................... 3-15 Strain................................................................... 3-16 Motion...................................................................... 3-16 Uniform Motion.................................................. 3-16

Speed and Velocity.............................................. 3-16 Acceleration.........................................................3-17 Newton’s Law of Motion.....................................3-17 First Law.........................................................3-17 Second Law................................................... 3-18 Third Law...................................................... 3-18 Circular Motion.................................................. 3-18 Heat.......................................................................... 3-19 Heat Energy Units............................................... 3-19 Heat Energy and Thermal Efficiency.................. 3-20 Heat Transfer...................................................... 3-20 Conduction..................................................... 3-20 Convection..................................................... 3-21 Radiation........................................................ 3-22 Specific Heat....................................................... 3-22 Temperature........................................................ 3-23 Thermal Expansion/Contraction......................... 3-24 Pressure.................................................................... 3-24 Gauge Pressure................................................... 3-25 Absolute Pressure............................................... 3-25 Differential Pressure........................................... 3-25 Gas Laws.................................................................. 3-26 Boyle’s Law........................................................ 3-26 Charles’ Law....................................................... 3-26 General Gas Law................................................ 3-27 Dalton’s Law....................................................... 3-27 Fluid Mechanics....................................................... 3-27 Buoyancy............................................................ 3-28 Fluid Pressure..................................................... 3-29 Pascal’s Law....................................................... 3-30 Bernoulli’s Principle........................................... 3-32 Sound....................................................................... 3-32 Wave Motion....................................................... 3-33 Speed of Sound................................................... 3-33 Mach Number..................................................... 3-34 Frequency of Sound............................................ 3-34 Loudness............................................................. 3-34 Measurement of Sound Intensity........................ 3-34 Doppler Effect..................................................... 3-34 Resonance........................................................... 3-35 The Atmosphere....................................................... 3-35 Composition of the Atmosphere......................... 3-35 Atmospheric Pressure......................................... 3-36 Atmospheric Density.......................................... 3-37 Water Content of the Atmosphere....................... 3-37 Absolute Humidity........................................ 3-38 Relative Humidity.......................................... 3-38 Dew Point...................................................... 3-38

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Vapor Pressure............................................... 3-38 Standard Atmosphere.......................................... 3-38 Aircraft Theory of Flight......................................... 3-39 Four Forces of Flight.......................................... 3-39 Bernoulli’s Principle and Subsonic Flow........... 3-39 Lift and Newton’s Third Law............................. 3-40 Airfoils................................................................ 3-41 Camber .......................................................... 3-41 Chord Line .................................................... 3-41 Relative Wind ................................................ 3-41 Angle of Attack ............................................. 3-41 Boundary Layer Airflow .................................... 3-41 Boundary Layer Control................................ 3-42 Wingtip Vortices................................................. 3-42 Axes of an Aircraft.............................................. 3-42 Aircraft Stability ................................................ 3-43 Static Stability................................................ 3-43 Dynamic Stability.......................................... 3-43 Longitudinal Stability.................................... 3-44 Lateral Stability............................................. 3-44 Directional Stability....................................... 3-45 Dutch Roll...................................................... 3-45 Flight Control Surfaces....................................... 3-45 Flight Controls and the Lateral Axis.............. 3-45 Flight Controls and the Longitudinal Axis.... 3-46 Flight Controls and the Vertical Axis............. 3-46 Tabs................................................................ 3-47 Supplemental Lift-Modifying Devices.......... 3-48 High-Speed Aerodynamics................................. 3-49 Compressibility Effects................................. 3-49 The Speed of Sound....................................... 3-50 Subsonic, Transonic, and Supersonic Flight.............................................................. 3-50 Shock Waves.................................................. 3-50 High-Speed Airfoils....................................... 3-51 Aerodynamic Heating.................................... 3-52 Helicopter Aerodynamics................................... 3-53 Helicopter Structures and Airfoils................. 3-53 Helicopter Axes of Flight.............................. 3-56 Helicopters in Flight...................................... 3-58 Weight-Shift Control, Flexible Wing Aircraft Aerodynamics..................................................... 3-60 Powered Parachute Aerodynamics..................... 3-62

Chapter 4. Aircraft Weight & Balance.............................. 4-1 Need and Requirements for Aircraft Weighing.......... 4-1 Weight and Balance Terminology.............................. 4-2 Datum................................................................... 4-2 viii

Arm....................................................................... 4-2 Moment................................................................. 4-3 Center of Gravity.................................................. 4-3 Maximum Weight................................................. 4-3 Empty Weight....................................................... 4-4 Empty Weight Center of Gravity.......................... 4-4 Useful Load.......................................................... 4-4 Minimum Fuel...................................................... 4-5 Tare Weight........................................................... 4-5 Procedures for Weighing an Aircraft......................... 4-5 General Concepts.................................................. 4-5 Weight and Balance Data...................................... 4-7 Weight and Balance Equipment.......................... 4-13 Scales............................................................. 4-13 Spirit Level ................................................... 4-14 Plumb Bob..................................................... 4-14 Hydrometer.................................................... 4-15 Preparing an Aircraft for Weighing..................... 4-15 Fuel System................................................... 4-15 Oil System..................................................... 4-16 Miscellaneous Fluids..................................... 4-16 Flight Controls............................................... 4-16 Other Considerations..................................... 4-16 Weighing Points............................................. 4-16 Center of Gravity Range..................................... 4-17 Empty Weight Center of Gravity Range........ 4-18 Operating Center of Gravity Range............... 4-18 Standard Weights Used for Aircraft Weight and Balance......................................................... 4-18 Example Weighing of an Airplane...................... 4-18 Loading an Aircraft for Flight.................................. 4-20 Example Loading of an Airplane........................ 4-20 Weight and Balance Extreme Conditions................ 4-21 Example Forward and Aft Extreme Condition Checks................................................ 4-21 Equipment Change and Aircraft Alteration.............. 4-22 Example Calculation After an Equipment Change.............................................. 4-22 The Use of Ballast.................................................... 4-23 Loading Graphs and CG Envelopes......................... 4-25 Helicopter Weight and Balance................................ 4-27 General Concepts................................................ 4-27 Helicopter Weighing........................................... 4-27 Weight and Balance —Weight-Shift Aircraft and Powered Parachutes.......................................... 4-29 Weight-Shift Aircraft.......................................... 4-29 Powered Parachutes............................................ 4-30 Weight and Balance for Large Airplanes................. 4-30 Built-In Electronic Weighing.............................. 4-30

Mean Aerodynamic Chord.................................. 4-31 Weight and Balance Records................................... 4-32

Chapter 5. Aircraft Materials, Processes, & . Hardware........................................................... 5-1 Aircraft Metals........................................................... 5-1 Properties of Metals.............................................. 5-1 Hardness.......................................................... 5-1 Strength............................................................ 5-1 Density............................................................. 5-1 Malleability...................................................... 5-1 Ductility........................................................... 5-1 Elasticity.......................................................... 5-1 Toughness........................................................ 5-2 Brittleness........................................................ 5-2 Fusibility.......................................................... 5-2 Conductivity.................................................... 5-2 Thermal Expansion.......................................... 5-2 Ferrous Aircraft Metals......................................... 5-2 Iron................................................................... 5-2 Steel and Steel Alloys...................................... 5-2 Types, Characteristics, and Uses of Alloyed Steels................................................................ 5-4 Electrochemical Test............................................. 5-5 Nonferrous Aircraft Metals................................... 5-5 Aluminum and Aluminum Alloys.................... 5-6 Wrought Aluminum......................................... 5-7 Effect of Alloying Element.............................. 5-8 Hardness Identification.................................... 5-8 Magnesium and Magnesium Alloys................ 5-9 Titanium and Titanium Alloys......................... 5-9 Copper and Copper Alloys.............................5-11 Nickel and Nickel Alloys............................... 5-12 Substitution of Aircraft Metals........................... 5-12 Metalworking Processes..................................... 5-12 Hot Working.................................................. 5-12 Internal Structure of Metals........................... 5-14 Heat-Treating Equipment.............................. 5-15 Soaking...........................................................5-17 Cooling...........................................................5-17 Quenching Media...........................................5-17 Quenching Equipment................................... 5-19 Heat Treatment of Ferrous Metals ..................... 5-19 Behavior of Steel During Heating and Cooling.......................................................... 5-19 Hardening...................................................... 5-20 Hardening Precautions................................... 5-20 Tempering...................................................... 5-21

Annealing....................................................... 5-21 Normalizing................................................... 5-21 Casehardening............................................... 5-21 Heat Treatment of Nonferrous Metals................ 5-22 Aluminum Alloys.......................................... 5-22 Alclad Aluminum.......................................... 5-23 Solution Heat Treatment................................ 5-23 Straightening After Solution Heat Treatment....................................................... 5-24 Precipitation Heat Treating............................ 5-24 Heat Treatment of Aluminum Alloy Rivets............................................................. 5-26 Heat Treatment of Magnesium Alloys........... 5-26 Heat Treatment of Titanium........................... 5-27 Hardness Testing............................................ 5-28 Forging................................................................ 5-30 Casting................................................................ 5-30 Extruding.............................................................5-31 Cold Working/Hardening.....................................5-31 Nonmetallic Aircraft Materials.................................5-31 Wood................................................................... 5-32 Plastics................................................................ 5-32 Transparent Plastics............................................ 5-32 Composite Materials........................................... 5-32 Advantages/Disadvantages of Composites.... 5-33 Composite Safety........................................... 5-33 Fiber Reinforced Materials............................ 5-33 Laminated Structures..................................... 5-34 Reinforced Plastic.......................................... 5-34 Rubber................................................................. 5-35 Natural Rubber.............................................. 5-35 Synthetic Rubber........................................... 5-35 Shock Absorber Cord............................................... 5-36 Seals......................................................................... 5-36 Packings.............................................................. 5-36 O-Ring Packings................................................. 5-36 Backup Rings...................................................... 5-37 V-Ring Packings................................................. 5-38 U-Ring Packings................................................. 5-38 Gaskets................................................................ 5-38 Wipers................................................................. 5-38 Sealing Compounds............................................ 5-38 One Part Sealants................................................ 5-38 Two Part Sealants................................................ 5-39 Aircraft Hardware.................................................... 5-39 Identification....................................................... 5-39 Threaded Fasteners............................................. 5-39 Classification of Threads............................... 5-40 Aircraft Bolts................................................. 5-41 ix

General Purpose Bolts................................... 5-41 Close Tolerance Bolts.................................... 5-41 Identification and Coding.............................. 5-41 Special-Purpose Bolts.................................... 5-43 Aircraft Nuts.................................................. 5-45 Aircraft Washers............................................ 5-50 Installation of Nuts, Washers, and Bolts........ 5-51 Aircraft Rivets.................................................... 5-54 Standards and Specifications......................... 5-56 Solid Shank Rivets......................................... 5-56 Blind Rivets................................................... 5-60 Rivet Identification........................................ 5-64 Special Shear and Bearing Load Fasteners.... 5-66 Screws............................................................ 5-70 Riveted and Rivetless Nutplates.................... 5-71 Hole Repair and Hole Repair Hardware........ 5-74 Safetying Methods . ...................................... 5-79

Chapter 6. Aircraft Cleaning & Corrosion Control......... 6-1 Corrosion Control...................................................... 6-1 Types of Corrosion................................................ 6-2 Direct Chemical Attack................................... 6-3 Electrochemical Attack.................................... 6-3 Forms of Corrosion.................................................... 6-4 Surface Corrosion................................................. 6-4 Dissimilar Metal Corrosion.................................. 6-4 Intergranular Corrosion......................................... 6-5 Stress Corrosion.................................................... 6-6 Fretting Corrosion................................................. 6-6 Factors Affecting Corrosion....................................... 6-7 Climate.................................................................. 6-7 Foreign Material................................................... 6-7 Preventive Maintenance............................................. 6-7 Inspection................................................................... 6-8 Corrosion Prone Areas............................................... 6-8 Exhaust Trail Areas............................................... 6-8 Battery Compartments and Battery Vent Openings............................................................... 6-8 Bilge Areas............................................................ 6-8 Wheel Well and Landing Gear.............................. 6-9 Water Entrapment Areas....................................... 6-9 Engine Frontal Areas and Cooling Air Vents........ 6-9 Wing Flap and Spoiler Recesses........................... 6-9 External Skin Areas.............................................. 6-9 Miscellaneous Trouble Areas.............................. 6-10 Corrosion Removal.................................................. 6-10 Surface Cleaning and Paint Removal................. 6-10 Corrosion of Ferrous Metals.................................... 6-11 Mechanical Removal of Iron Rust...................... 6-11



Chemical Removal of Rust................................. 6-13 Chemical Surface Treatment of Steel................. 6-13 Removal of Corrosion from Highly Stressed Steel Parts........................................................... 6-13 Corrosion of Aluminum and Aluminum Alloys....... 6-13 Treatment of Unpainted Aluminum Surfaces............................................................... 6-13 Treatment of Anodized Surfaces......................... 6-14 Treatment of Intergranular Corrosion in Heat‑Treated Aluminum Alloy Surfaces............. 6-14 Corrosion of Magnesium Alloys.............................. 6-15 Treatment of Wrought Magnesium Sheet and Forgings....................................................... 6-15 Treatment of Installed Magnesium Castings...... 6-16 Treatment of Titanium and Titanium Alloys............ 6-16 Protection of Dissimilar Metal Contacts.................. 6-16 Contacts Not Involving Magnesium................... 6-16 Contacts Involving Magnesium.......................... 6-16 Corrosion Limits...................................................... 6-16 Processes and Materials Used in Corrosion Control..................................................................... 6-17 Metal Finishing................................................... 6-17 Surface Preparation............................................. 6-17 Chemical Treatments............................................... 6-18 Anodizing........................................................... 6-18 Alodizing............................................................ 6-18 Chemical Surface Treatment and Inhibitors....... 6-18 Chromic Acid Inhibitor....................................... 6-19 Sodium Dichromate Solution............................. 6-19 Chemical Surface Treatments............................. 6-19 Protective Paint Finishes.......................................... 6-19 Aircraft Cleaning..................................................... 6-19 Exterior Cleaning................................................ 6-19 Interior Cleaning................................................. 6-20 Types of Cleaning Operations....................... 6-21 Nonflammable Aircraft Cabin Cleaning Agents and Solvents...................................... 6-21 Flammable and Combustible Agents............. 6-21 Container Controls......................................... 6-22 Fire Prevention Precautions........................... 6-22 Fire Protection Recommendations................. 6-22 Powerplant Cleaning................................................ 6-23 Solvent Cleaners...................................................... 6-23 Dry Cleaning Solvent......................................... 6-23 Aliphatic and Aromatic Naphtha........................ 6-23 Safety Solvent..................................................... 6-24 Methyl Ethyl Ketone (MEK).............................. 6-24 Kerosene............................................................. 6-24 Cleaning Compound for Oxygen Systems......... 6-24

Emulsion Cleaners................................................... 6-24 Water Emulsion Cleaner..................................... 6-24 Solvent Emulsion Cleaners................................. 6-24 Soaps and Detergent Cleaners................................. 6-24 Cleaning Compound, Aircraft Surfaces.............. 6-24 Nonionic Detergent Cleaners.............................. 6-25 Mechanical Cleaning Materials............................... 6-25 Mild Abrasive Materials..................................... 6-25 Abrasive Papers.................................................. 6-25 Chemical Cleaners................................................... 6-25 Phosphoric-Citric Acid....................................... 6-25 Baking Soda........................................................ 6-25

Chapter 7. Fluid Lines & Fittings....................................... 7-1 Rigid Fluid Lines....................................................... 7-1 Tubing Materials................................................... 7-1 Copper............................................................. 7-1 Aluminum Alloy Tubing.................................. 7-1 Steel................................................................. 7-1 Titanium 3AL-2.5V......................................... 7-1 Material Identification.......................................... 7-1 Sizes...................................................................... 7-2 Fabrication of Metal Tube Lines........................... 7-2 Tube Cutting.................................................... 7-2 Tube Bending................................................... 7-3 Tube Flaring..................................................... 7-5 Fittings..............................................................7-7 Beading.............................................................7-7 Fluid Line Identification....................................... 7-8 Fluid Line End Fittings......................................... 7-9 Universal bulkhead fittings.............................7-10 AN Flared Fittings..........................................7-10 MS Flareless Fittings...........................................7-10 Swaged Fittings..............................................7-10 Cryofit Fittings................................................7-13 Rigid Tubing Installation and Inspection.............7-13 Connection and Torque...................................7-13 Flareless Tube Installation.............................. 7-14 Rigid Tubing Inspection and Repair...............7-15 Flexible Hose Fluid Lines......................................... 7-17 Hose Materials and Construction......................... 7-17 Low, Medium, and High Pressure Hoses....... 7-17 Hose Identification.......................................... 7-17 Flexible Hose Inspection..................................... 7-18 Fabrication and Replacement of Flexible Hose..................................................................... 7-18 Flexible Hose Testing..................................... 7-18 Size Designations.................................................7-19

Hose Fittings........................................................7-19 Installation of Flexible Hose Assemblies............7-19 Hose Clamps....................................................... 7-22

Chapter 8 . Inspection Fundamentals.............................. 8-1 Basic Inspection Techniques/Practices...................... 8-1 Preparation................................................................. 8-1 Aircraft Logs.............................................................. 8-2 Checklists................................................................... 8-2 Publications................................................................ 8-3 Manufacturers’ Service Bulletins/Instructions...... 8-3 Maintenance Manual............................................ 8-3 Overhaul Manual.................................................. 8-4 Structural Repair Manual...................................... 8-4 Illustrated Parts Catalog........................................ 8-4 Code of Federal Regulations (CFRs).................... 8-4 Airworthiness Directives...................................... 8-4 Type Certificate Data Sheets................................. 8-4 Routine/Required Inspections.................................... 8-5 Preflight/Postflight Inspections............................. 8-5 Annual/100-Hour Inspections............................. 8-12 Progressive Inspections...................................... 8-12 Continuous Inspections....................................... 8-15 Altimeter and Transponder Inspections.............. 8-15 ATA iSpec 2200....................................................... 8-15 ATA Specification 100 ­Systems......................... 8-15 Special Inspections................................................... 8-16 Hard or Overweight Landing Inspection............ 8-16 Severe Turbulence Inspection/Over “G” ........... 8-16 Lightning Strike...................................................8-17 Fire Damage.........................................................8-17 Flood Damage......................................................8-17 Seaplanes.............................................................8-17 Aerial Application Aircraft..................................8-17 Special Flight Permits.............................................. 8-18 Nondestructive Inspection/Testing........................... 8-18 General Techniques............................................. 8-18 Visual Inspection................................................ 8-18 Borescope........................................................... 8-18 Liquid Penetrant Inspection................................ 8-19 Interpretation of Results................................ 8-19 False Indications............................................ 8-20 Eddy Current Inspection..................................... 8-20 Basic Principles............................................. 8-20 Ultrasonic Inspection.......................................... 8-21 Pulse Echo..................................................... 8-22 Through Transmission................................... 8-22 Resonance . ................................................... 8-22 xi

Acoustic Emission Inspection............................. 8-24 Magnetic Particle Inspection.............................. 8-24 Development of Indications........................... 8-25 Types of Discontinuities Disclosed............... 8-25 Preparation of Parts for Testing..................... 8-25 Effect of Flux Direction................................. 8-26 Effect of Flux Density................................... 8-27 Magnetizing Methods.................................... 8-27 Identification of Indications........................... 8-27 Magnaglo Inspection.......................................... 8-27 Magnetizing Equipment................................ 8-28 Indicating Mediums....................................... 8-30 Demagnetizing............................................... 8-30 Standard Demagnetizing Practice.................. 8-30 Radiographic.................................................. 8-30 Radiographic Inspection......................................8-31 Preparation and Exposure...............................8-31 Film Processing..............................................8-31 Radiographic Interpretation............................8-31 Radiation Hazards.......................................... 8-32 Inspection of Composites......................................... 8-32 Tap Testing.......................................................... 8-32 Electrical Conductivity....................................... 8-32 Inspection of Welds.................................................. 8-33

Chapter 9. Hand Tools & Measuring Devices................. 9-1 General Purpose Tools............................................... 9-1 Hammers and Mallets........................................... 9-1 Screwdrivers......................................................... 9-1 Pliers and Plier-Type Cutting Tools...................... 9-3 Punches................................................................. 9-3 Wrenches............................................................... 9-4 Special Wrenches.................................................. 9-5 Torque Wrench...................................................... 9-7 Strap Wrenches..................................................... 9-7 Impact Drivers...................................................... 9-8 Metal Cutting Tools................................................... 9-8 Hand Snips............................................................ 9-8 Hacksaws.............................................................. 9-8 Chisels................................................................... 9-9 Files..................................................................... 9-10 Files — Care and Use..................................... 9-10 Most Commonly Used Files [Figure 9-17].... 9-10 Care of Files................................................... 9-12 Drills................................................................... 9-12 Twist Drills.................................................... 9-12 Reamers.............................................................. 9-13

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Countersink..........................................................9-17 Taps and Dies............................................................9-17 Layout and Measuring Tools.................................... 9-18 Rules................................................................... 9-18 Combination Sets................................................ 9-18 Scriber................................................................. 9-21 Dividers and Pencil Compasses.......................... 9-21 Calipers............................................................... 9-21 Micrometer Calipers........................................... 9-21 Micrometer Parts................................................ 9-22 Reading a Micrometer........................................ 9-22 Vernier Scale....................................................... 9-23 Using a Micrometer............................................ 9-24 Slide Calipers...................................................... 9-25

Chapter 10. Basic Electricity.............................................. 10-1 Introduction to Electricity and Electronics.............. 10-1 General Composition of Matter............................... 10-1 Matter.................................................................. 10-1 Element............................................................... 10-1 Compound........................................................... 10-1 The Molecule...................................................... 10-1 The Atom............................................................ 10-2 Electrons, Protons, and Neutrons........................ 10-2 Electron Shells and Energy Levels..................... 10-2 Valence Electrons.......................................... 10-3 Ions................................................................ 10-3 Free Electrons................................................ 10-3 Electron Movement............................................ 10-3 Conductors..................................................... 10-3 Insulators....................................................... 10-3 Semiconductors............................................. 10-3 Metric Based Prefixes Used for Electrical Calculations ............................................................ 10-3 Static Electricity....................................................... 10-4 Attractive and Repulsive Forces......................... 10-4 Electrostatic Field............................................... 10-5 ESD Considerations............................................ 10-6 Magnetism................................................................ 10-7 Types of Magnets....................................................10-11 Electromagnetism.................................................. 10-12 Conventional Flow and Electron Flow...................10-16 Conventional Flow.............................................10-16 Electron Flow.....................................................10-16 Electromotive Force (Voltage)................................10-16 Current....................................................................10-17 Ohm’s Law (Resistance).........................................10-17 Resistance of a Conductor..................................... 10-19

Factors Affecting Resistance............................ 10-19 Resistance and Its Relation to Wire Sizing....... 10-20 Circular Conductors (Wires/Cables)............ 10-20 Rectangular Conductors (Bus Bars)............ 10-21 Power and Energy.................................................. 10-21 Power in an Electrical Circuit........................... 10-21 Power Formulas Used in the Study of Electricity.......................................................... 10-22 Power in a Series and Parallel Circuit.............. 10-23 Energy in an Electrical Circuit.......................... 10-23 Sources of Electricity........................................ 10-23 Pressure Source............................................ 10-23 Chemical Source.......................................... 10-23 Thermal Sources.......................................... 10-24 Light Sources............................................... 10-24 Schematic Representation of Electrical Components...................................................... 10-24 Conductors................................................... 10-24 Types of Resistors.................................................. 10-25 Fixed Resistor................................................... 10-25 Carbon Composition......................................... 10-25 Resistor Ratings................................................ 10-25 Wire-Wound...................................................... 10-27 Variable Resistors............................................. 10-27 Rheostat............................................................ 10-28 Potentiometer.................................................... 10-28 Linear Potentiometers....................................... 10-29 Tapered Potentiometers..................................... 10-29 Thermistors....................................................... 10-29 Photoconductive Cells...................................... 10-29 Circuit Protection Devices..................................... 10-30 Fuse................................................................... 10-30 Current Limiter................................................. 10-30 Circuit Breaker.................................................. 10-30 Arc Fault Circuit Breaker.............................10-31 Thermal Protectors.............................................10-31 Control Devices.................................................10-31 Switches........................................................10-31 Toggle Switch...............................................10-31 Pushbutton Switches . ................................. 10-32 Microswitches.............................................. 10-33 Rotary Selector Switches............................. 10-33 Lighted Pushbutton Switches...................... 10-33 DIP Switches............................................... 10-34 Relays.......................................................... 10-34 Series DC Circuits.................................................. 10-34 Introduction ..................................................... 10-34 Voltage Drops and Further Application of Ohm’s Law .................................................. 10-35

Voltage Sources in Series.................................. 10-36 Kirchhoff’s Voltage Law................................... 10-36 Voltage Dividers............................................... 10-37 Determining the Voltage Divider Formula ...... 10-39 Parallel DC Circuits............................................... 10-40 Overview........................................................... 10-40 Voltage Drops................................................... 10-40 Total Parallel Resistance................................... 10-40 Resistors in Parallel.......................................... 10-41 Two Resistors in Parallel.................................. 10-41 Current Source.................................................. 10-41 Kirchhoff’s Current Law.................................. 10-41 Current Dividers............................................... 10-42 Series-Parallel DC Circuits.................................... 10-42 Overview........................................................... 10-42 Determining the Total Resistance..................... 10-43 Alternating Current and Voltage............................ 10-44 AC and DC Compared...................................... 10-44 Generator Principles............................................... 10-44 Generators of Alternating Current.................... 10-46 Position 1..................................................... 10-47 Position 2..................................................... 10-47 Position 3..................................................... 10-48 Position 4..................................................... 10-48 Position 5..................................................... 10-48 Cycle and Frequency........................................ 10-48 Cycle Defined.............................................. 10-48 Frequency Defined....................................... 10-48 Period Defined............................................. 10-49 Wavelength Defined..................................... 10-49 Phase Relationships.......................................... 10-49 In Phase Condition....................................... 10-49 Out of Phase Condition................................ 10-49 Values of Alternating Current........................... 10-50 Instantaneous Value .................................... 10-50 Peak Value................................................... 10-50 Effective Value............................................. 10-50 Capacitance............................................................ 10-51 Capacitors in Direct Current............................. 10-51 The RC Time Constant..................................... 10-51 Units of Capacitance......................................... 10-52 Voltage Rating of a Capacitor........................... 10-52 Factors Affecting Capacitance.......................... 10-52 Types of Capacitors.......................................... 10-53 Fixed Capacitors.......................................... 10-53 Variable Capacitors...................................... 10-54 Capacitors in Series.......................................... 10-54 Capacitors in Parallel........................................ 10-55

xiii

Capacitors in Alternating Current..................... 10-55 Capacitive Reactance Xc.................................. 10-55 Capacitive Reactances in Series and in Parallel.......................................................... 10-56 Phase of Current and Voltage in Reactive Circuits.............................................................. 10-56 Inductance.............................................................. 10-57 Characteristics of Inductance............................ 10-57 The RL Time Constant...................................... 10-58 Physical Parameters.......................................... 10-58 Self-Inductance................................................. 10-58 Types of Inductors............................................ 10-59 Units of Inductance........................................... 10-59 Inductors in Series............................................ 10-59 Inductors in Parallel.......................................... 10-59 Inductive Reactance.......................................... 10-60 AC Circuits............................................................ 10-61 Ohm’s Law for AC Circuits.............................. 10-61 Series AC Circuits............................................. 10-61 Parallel AC Circuits.......................................... 10-63 Resonance......................................................... 10-65 Power in AC Circuits........................................ 10-66 True Power Defined..................................... 10-66 Apparent Power Defined............................. 10-66 Transformers..................................................... 10-66 Current Transformers........................................ 10-69 Transformer Losses........................................... 10-69 Power in Transformers...................................... 10-69 DC Measuring Instruments.................................... 10-69 D’Arsonval Meter Movement........................... 10-70 Current Sensitivity and Resistance................... 10-70 Damping........................................................... 10-71 Electrical Damping........................................... 10-71 Mechanical Damping........................................ 10-71 A Basic Multirange Ammeter........................... 10-71 Precautions................................................... 10-72 The Voltmeter......................................................... 10-72 Voltmeter Sensitivity......................................... 10-72 Multiple Range Voltmeters............................... 10-72 Voltmeter Circuit Connections.......................... 10-73 Influence of the Voltmeter in the Circuit........... 10-73 The Ohmmeter....................................................... 10-73 Zero Adjustment............................................... 10-73 Ohmmeter Scale.................................................10-74 The Multirange Ohmmeter................................10-74 Megger (Megohmmeter)....................................10-75 AC Measuring Instruments.................................... 10-76 Electrodynamometer Meter Movement............ 10-76 Moving Iron Vane Meter................................... 10-76 xiv

Inclined Coil Iron Vane Meter.......................... 10-78 Varmeters.......................................................... 10-78 Wattmeter.......................................................... 10-78 Frequency Measurement/Oscilloscope............. 10-79 Horizontal Deflection........................................ 10-79 Vertical Deflection............................................ 10-80 Tracing a Sine Wave......................................... 10-80 Control Features on an Oscilloscope................ 10-80 Flat Panel Color Displays for Oscilloscopes.... 10-82 Digital Multimeter............................................ 10-82 Basic Circuit Analysis and Troubleshooting.......... 10-82 Voltage Measurement....................................... 10-82 Current Measurement....................................... 10-83 Checking Resistance in a Circuit...................... 10-83 Continuity Checks............................................ 10-84 Capacitance Measurement................................ 10-84 Inductance Measurement.................................. 10-85 Troubleshooting the Open Faults in Series Circuit............................................................... 10-85 Tracing Opens with the Voltmeter.................... 10-85 Tracing Opens with the Ohmmeter . ................ 10-86 Troubleshooting the Shorting Faults in Series Circuit ................................................... 10-86 Tracing Shorts with the Ohmmeter................... 10-87 Tracing Shorts with the Voltmeter.................... 10-87 Troubleshooting the Open Faults in Parallel Circuit.................................................. 10-87 Tracing an Open with an Ammeter................... 10-88 Tracing an Open with an Ohmmeter................. 10-88 Troubleshooting the Shorting Faults in Parallel Circuit.................................................. 10-89 Troubleshooting the Shorting Faults in Series‑Parallel Circuit....................................... 10-89 Logic in Tracing an Open............................ 10-89 Tracing Opens with the Voltmeter .............. 10-90 Batteries................................................................. 10-90 Primary Cell...................................................... 10-90 Secondary Cell.................................................. 10-90 Battery Ratings................................................. 10-92 Life Cycle of a Battery..................................... 10-92 Lead-Acid Battery Testing Methods................. 10-92 Lead-Acid Battery Charging Methods.............. 10-93 Nickel-Cadmium Batteries..................................... 10-94 Chemistry and Construction............................. 10-94 Operation of Nickel-Cadmium Cells................ 10-94 General Maintenance and Safety Precautions................................................... 10-95 Sealed Lead Acid Batteries .............................. 10-95 Inverters................................................................. 10-96 Rotary Inverters................................................ 10-96

Permanent Magnet Rotary Inverter.................. 10-96 Inductor-Type Rotary Inverter.......................... 10-98 Static Inverters.................................................. 10-98 Semiconductors.....................................................10-101 Doping.............................................................10-101 PN Junctions and the Basic Diode.................. 10-102 Forward Biased Diode.................................... 10-103 Reverse Biased Diode..................................... 10-104 Rectifiers.............................................................. 10-104 Half-Wave Rectifier........................................ 10-105 Full-Wave Rectifier......................................... 10-106 Dry Disk.......................................................... 10-106 Types of Diodes.............................................. 10-107 Power Rectifier Diodes.............................. 10-107 Zener Diodes.............................................. 10-107 Special Purpose Diodes............................. 10-108 Light-Emitting Diode (LED)..................... 10-108 Liquid Crystal Displays (LCD)................. 10-108 Photodiode................................................. 10-108 Varactors.................................................... 10-108 Schottky Diodes......................................... 10-108 Diode Identification........................................ 10-109 Introduction to Transistors................................... 10-109 Classification................................................... 10-109 Transistor Theory............................................ 10-109 PNP Transistor Operation................................ 10-111 Identification of Transistors............................. 10-111 Field Effect Transistors.................................... 10-111 Metal-Oxide-Semiconductor FET (MOSFET).......................................................10-112 Common Transistor Configurations.................10-112 Common-Emitter Configuration......................10-112 Common-Collector Configuration................... 10-113 Common-Base Configuration.......................... 10-113 Vacuum Tubes....................................................... 10-113 Filtering................................................................. 10-114 Filtering Characteristics of Capacitors ........... 10-114 Filtering Characteristics of Inductors.............. 10-114 Common Filter Configurations........................ 10-115 Basic LC Filters............................................... 10-115 Low-Pass Filter........................................... 10-116 High-Pass Filter.......................................... 10-116 Band-Pass Filter.......................................... 10-117 Band-Stop Filter......................................... 10-117 Amplifier Circuits................................................. 10-118 Classification.................................................... 10-118 Class A........................................................ 10-118 Class AB..................................................... 10-118 Class B........................................................ 10-119

Class C........................................................ 10-119 Methods of Coupling...................................... 10-120 Direct Coupling......................................... 10-120 RC Coupling.............................................. 10-120 Impedance Coupling.................................. 10-120 Transformer Coupling............................... 10-120 Feedback..........................................................10-121 Operational Amplifiers.....................................10-121 Applications............................................... 10-122 Magnetic Amplifiers............................................. 10-122 Saturable-Core Reactor................................... 10-123 Logic Circuits....................................................... 10-124 Logic Polarity................................................. 10-124 Positive...................................................... 10-124 Negative..................................................... 10-124 Pulse Structure................................................ 10-124 Basic Logic Circuits....................................... 10-125 The Inverter Logic..................................... 10-125 The AND Gate........................................... 10-125 The OR Gate.............................................. 10-126 The NAND Gate........................................ 10-127 The NOR Gate........................................... 10-127 Exclusive OR Gate.................................... 10-128 Exclusive NOR Gate................................. 10-128 The Integrated Circuit..................................... 10-128 Microprocessors.............................................. 10-130 DC Generators..................................................... 10-130 Theory of Operation....................................... 10-130 Generation of a DC Voltage............................. 10-131 Position A................................................... 10-132 Position B.................................................. 10-132 Position C.................................................. 10-132 Position D...................................................10-133 The Neutral Plane.......................................10-133 Construction Features of DC Generators.........10-133 Field Frame......................................................10-133 Armature......................................................... 10-134 Gramme-Ring Armature..................................10-135 Drum-Type Armature.......................................10-135 Commutators................................................... 10-136 Armature Reaction.......................................... 10-137 Compensating Windings................................. 10-137 Interpoles........................................................ 10-138 Types of DC Generators................................. 10-138 Series Wound DC Generators.................... 10-138 Shunt Wound DC Generators.................... 10-139 Compound Wound DC Generators............ 10-139 Generator Ratings........................................... 10-140

xv

Generator Terminals......................................... 10-141 DC Generator Maintenance.................................. 10-141 Inspection......................................................... 10-141 Condition of Generator Brushes...................... 10-141 DC Motors............................................................10-142 Force between Parallel Conductors.................10-143 Developing Torque..........................................10-143 Basic DC Motor.............................................. 10-144 Position A....................................................10-145 Position B...................................................10-145 Position C...................................................10-145 Position D...................................................10-145 DC Motor Construction....................................... 10-146 Armature Assembly........................................ 10-146 Field Assembly................................................10-147 Brush Assembly...............................................10-147 End Frame........................................................10-147 Types of DC Motors..............................................10-147 Series DC Motor..............................................10-147 Shunt DC Motor............................................. 10-148 Compound DC Motor..................................... 10-148 Counter Electromotive Force (emf)......................10-149 Types of Duty...................................................10-149 Reversing Motor Direction..............................10-149 Motor Speed.................................................... 10-150 Energy Losses in DC Motors...........................10-151 Inspection and Maintenance of DC Motors.....10-152 AC Motors ...........................................................10-153 Types of AC Motors.........................................10-153 Three Phase Induction Motor..........................10-153 Rotating Magnetic Field..................................10-153 Construction of Induction Motor.................... 10-154 Induction Motor Slip....................................... 10-154 Single Phase Induction Motor.........................10-155 Shaded Pole Induction Motor..........................10-155 Split Phase Motor........................................... 10-156 Capacitor Start Motor..................................... 10-156 Direction of Rotation of Induction Motors..... 10-156 Synchronous Motor.........................................10-157 AC Series Motor..............................................10-159 Maintenance of AC Motors............................ 10-160 Alternators............................................................ 10-160 Basic Alternators and Classifications.............. 10-160 Method of Excitation...................................... 10-160 Number of Phases............................................ 10-161 Armature or Field Rotation.............................. 10-161 Single Phase Alternator.................................... 10-161 Two Phase Alternator.......................................10-162

xvi

Three Phase Alternator....................................10-162 Wye Connection (Three Phase).......................10-162 Delta Connection (Three Phase)......................10-162 Alternator Rectifier Unit..................................10-163 Brushless Alternator.........................................10-163 Alternator Rating.............................................10-165 Alternator Frequency.......................................10-165 Alternator Maintenance...................................10-165 Regulation of Generator Voltage.....................10-167 Voltage Regulation with a Vibrating-Type Regulator..........................................................10-167 Three Unit Regulators......................................10-167 Differential Relay Switch................................10-169 Overvoltage and Field Control Relays............10-170 Generator Control Units (GCU)............................10-170 Basic Functions of a Generator Control Unit.....................................................10-170 Voltage Regulation...........................................10-170 Overvoltage Protection....................................10-170 Parallel Generator Operations..........................10-170 Over-Excitation Protection..............................10-170 Differential Voltage . .......................................10-170 Reverse Current Sensing..................................10-170 Alternator Constant Speed Drive System............. 10-171 Hydraulic Transmission........................................10-172 Voltage Regulation of Alternators.........................10-178 Alternator Transistorized Regulators...............10-178

Chapter 11. Safety, Ground Operations, & . Servicing......................................................... 11-1 Shop Safety............................................................... 11-1 Electrical Safety................................................... 11-1 Physiological Safety....................................... 11-1 Fire Safety....................................................... 11-2 Safety Around Compressed Gases....................... 11-2 Safety Around Hazardous Materials.................... 11-2 Safety Around Machine Tools............................. 11-3 Flight Line Safety.................................................... 11-4 Hearing Protection.............................................. 11-4 Foreign Object Damage (FOD).......................... 11-4 Safety Around Airplanes...................................... 11-5 Safety Around Helicopters................................... 11-5 Fire Safety............................................................ 11-5 Fire Protection........................................................... 11-5 Requirements for Fire To Occur.......................... 11-5 Classification of Fires.......................................... 11-5 Types and Operation of Shop and Flight Line Fire Extinguishers............................................... 11-6

Inspection of Fire Extinguishers..........................11-8 Identifying Fire Extinguishers.............................11-8 Using Fire Extinguishers.....................................11-9 Tiedown Procedures..................................................11-9 Preparation of Aircraft.........................................11-9 Tiedown Procedures for Land Planes..................... 11-10 Securing Light Aircraft...................................... 11-10 Securing Heavy Aircraft.................................... 11-10 Tiedown Procedures for Seaplanes.................... 11-11 Tiedown Procedures for Ski Planes .................. 11-11 Tiedown Procedures for Helicopters................. 11-11 Procedures for Securing Weight-Shift Control Aircraft.................................................. 11-12 Procedures for Securing Powered Parachutes.......................................................... 11-12 Ground Movement of Aircraft................................ 11-13 Engine Starting and Operation........................... 11-13 Reciprocating Engines....................................... 11-13 Hand Cranking Engines..................................... 11-14 Extinguishing Engine Fires................................ 11-15 Turboprop Engines............................................. 11-15 Turboprop Starting Procedures.......................... 11-16 Turbofan Engines............................................... 11-17 Starting a Turbofan Engine................................ 11-17 Auxiliary Power Units (APUs).......................... 11-19 Unsatisfactory Turbine Engine Starts................ 11-19 Hot Start........................................................ 11-19 False or Hung Start....................................... 11-19 Engine Will Not Start................................... 11-19 Towing of Aircraft............................................. 11-19 Taxiing Aircraft.................................................. 11-21 Taxi Signals........................................................ 11-21 Servicing Aircraft.................................................... 11-22 Servicing Aircraft Air/Nitrogen, Oil, and Fluids................................................................. 11-22 Ground Support Equipment............................... 11-25 Electric Ground Power Units........................ 11-25 Hydraulic Ground Power Units.................... 11-26 Ground Support Air Units............................. 11-26 Ground Air Heating and Air Conditioning ................................................ 11-26 Oxygen Servicing Equipment............................ 11-26 Oxygen Hazards................................................. 11-27 Fuel Servicing of Aircraft....................................... 11-27 Types of Fuel and Identification........................ 11-27 Contamination Control...................................... 11-27 Fueling Hazards................................................. 11-28 Fueling Procedures............................................ 11-28 Defueling........................................................... 11-30

Chapter 12. Publications, Forms, & Records................... 12-1 Overview — Title 14 of the Code of Federal Regulations (14 CFR).............................................. 12-1 Maintenance Related Regulations...................... 12-5 14 CFR Part 1 — Definitions and Abbreviations................................................. 12-5 14 CFR Part 21 — Certification Procedures for Products and Parts.................................... 12-5 14 CFR Part 23 — Airworthiness Standards: Normal, Utility, Acrobatic, and Commuter Category Airplanes........................................ 12-5 14 CFR Part 25 — Airworthiness Standards: Transport Category Airplanes........................ 12-5 14 CFR Part 27 — Airworthiness Standards: Normal Category Rotorcraft.......................... 12-7 14 CFR Part 29 — Airworthiness Standards: Transport Category Rotorcraft . .................... 12-7 14 CFR Part 33 — Airworthiness Standards: Aircraft Engines............................................. 12-9 14 CFR Part 35 — Airworthiness Standards: Propellers....................................................... 12-9 14 CFR Part 39 — Airworthiness Directives....................................................... 12-9 14 CFR Part 43 — Maintenance, Preventive Maintenance, Rebuilding, and Alteration...... 12-9 14 CFR Part 45 — Identification and Registration Marking..................................... 12-9 14 CFR Part 47 — Aircraft Registration...... 12-10 14 CFR Part 65 — Certification: Airmen Other Than Flight Crewmembers................ 12-10 14 CFR Part 91 — General Operating and Flight Rules.................................................. 12-10 14 CFR Part 119 — Certification: Air Carriers and Commercial Operators............ 12-10 14 CFR Part 121 — Operating Requirements: Domestic, Flag, and Supplemental Operations.................................................... 12-11 14 CFR Part 125 — Certification and Operations: Airplanes Having a Seating Capacity of 20 or More Passengers or a Maximum Payload Capacity of 6,000 Pounds or More; and Rules Governing Persons on Board Such Aircraft ................. 12-12 14 CFR Part 135 — Operating Requirements: Commuter and On Demand Operations and Rules Governing Persons on Board Such Aircraft ....................................................... 12-12 14 CFR Part 145 — Repair Stations............. 12-13 14 CFR Part 147 — Aviation Maintenance Technician Schools...................................... 12-14 14 CFR Part 183 — Representatives of the Administrator............................................... 12-14 Detailed Explanation of Primary Regulations (Parts 43 and 91).................................................... 12-14 14 CFR Part 43 — Maintenance, Preventive Maintenance, Rebuilding, and Alteration......... 12-14 xvii

§43.1  Applicability..................................... 12-14 §43.2  Records of overhaul and rebuilding..................................................... 12-15 §43.3  Persons authorized to perform maintenance, preventive maintenance, rebuilding, and alterations........................... 12-15 §43.5  Approval for return to service after maintenance, preventive maintenance, rebuilding, or alteration............................... 12-15 §43.7  Persons authorized to approve aircraft, airframes, aircraft engines, propellers, appliances, or component parts for return to service after maintenance, preventive maintenance, rebuilding, or alteration...................................................... 12-16 §43.9  Content, form and disposition of maintenance, preventive maintenance, rebuilding, and alteration records (except inspections performed in accordance with parts 91 and 125, and §§135.411(a)(1) and 135.419 of this chapter)............................... 12-16 §43.10  Disposition of life-limited aircraft parts . .............................................. 12-17 §43.11  Content, form, and disposition of records for inspections conducted under parts 91 and 125, and §135.411(a)(1) and 135.419 of this chapter................................. 12-18 §43.12  Maintenance records: Falsification, reproduction, or alteration........................... 12-18 §43.13  Performance rules (general)............ 12-18 §43.15  Additional Performance Rules for Inspections............................................. 12-20 §43.16  Airworthiness limitations................ 12-20 §43.17  Maintenance, preventive maintenance, or alterations performed on U.S. aeronautical products by certain Canadian persons......................................... 12-20 Appendix A — Major Alterations, Major Repairs, and Preventive Maintenance......... 12-20 Appendix B — Recording of Major Repairs and Major Alterations.................................. 12-21 Appendix D — Scope and Detail of Items To Be Included in Annual and 100-Hour Inspections................................................... 12-21 Appendix E — Altimeter System Test and Inspection..................................................... 12-21 Appendix F — ATC Transponder Tests and Inspections................................................... 12-21 14 CFR Part 91 — General Operating and Flight Rules....................................................... 12-21 Subpart A — General.................................... 12-21 Subpart E — Maintenance, Preventive Maintenance, and Alterations...................... 12-22 Civil Air Regulations (CARs)................................ 12-25 CAR 3 — Airplane Airworthiness — Normal, Utility, Aerobatic, and Restricted Purpose Categories......................................................... 12-25 CAR 4a — Airplane Airworthiness................... 12-27 xviii

Suspected Unapproved Parts (SUPs)..................... 12-27 Other FAA Documents........................................... 12-27 Advisory Circulars (ACs)................................. 12-27 Airworthiness Directives (ADs)....................... 12-30 AD Content.................................................. 12-30 AD Number................................................. 12-30 Applicability and Compliance..................... 12-30 Alternative Method of Compliance............. 12-30 Aircraft Listings . ............................................. 12-30 Aircraft Specifications...................................... 12-30 Aviation Maintenance Alerts (AC 43-16)..........12-31 Supplemental Type Certificates (STC)..............12-31 Type Certificate Data Sheets (TCDS)................12-31 Non-FAA Documents............................................. 12-36 Air Transport Association (ATA) 100............... 12-36 Manufacturers’ Published Data......................... 12-36 Airworthiness Limitations................................ 12-37 Service Bulletins (SB)...................................... 12-37 Structural Repair Manual (SRM)...................... 12-38 Forms..................................................................... 12-38 Airworthiness Certificates................................ 12-38 Aircraft Registration......................................... 12-39 Radio Station License ...................................... 12-40 FAA Form 337 — Major Repair and Alteration.......................................................... 12-40 Records.................................................................. 12-43 Making Maintenance Record Entries............... 12-43 Temporary Records — 14 CFR §91.417(a)(1) and (b)(1).......................................................... 12-43 Permanent Records — 14 CFR §91.417(a)(2) and (b)(2).......................................................... 12-43 Electronic Records . ......................................... 12-43 Light Sport Aircraft (LSA) Maintenance............... 12-44 Aircraft Maintenance Manual........................... 12-45 Line Maintenance, Repairs, and Alterations......................................................... 12-45 Major Repairs and Alterations.......................... 12-46

Chapter 13. The Mechanic Certificate.............................. 13-1 Overview of the Maintenance Technician............... 13-1 The Mechanic Certificate — Maintenance Technician Privileges and Limitations................ 13-1 Mechanic Certification — General (by 14 CFR Section).................................................................... 13-1 65.3  Certification of Foreign Airmen Other Than Flight Crewmembers................................. 13-1 65.11  Application and Issue............................... 13-1 65.12  Offenses Involving Alcohol and Drugs................................................................... 13-1

65.13  Temporary Certificate.............................. 13-1 65.14  Security Disqualification......................... 13-2 65.15  Duration of Certificate............................. 13-2 65.16  Change of Name: Replacement of Lost or Destroyed Certificate...................................... 13-2 65.17  Test: General Procedure........................... 13-2 65.18  Written Tests: Cheating or Other Unauthorized Content......................................... 13-2 65.19  Retesting After Failure............................. 13-2 65.20  Applications, Certificates, Logbooks, Reports, and Records: Falsification, Reproduction, or Alteration................................ 13-2 65.21  Change of Address................................... 13-3 65.23  Refusal to Submit to a Drug or Alcohol Test........................................................ 13-3 Mechanic Certification — Specific (by 14 CFR Section).................................................................... 13-3 65.71  Eligibility Requirements: General........... 13-3 65.73  Ratings..................................................... 13-3 65.75  Knowledge Requirements........................ 13-3 65.77  Experience Requirements........................ 13-4 65.79  Skill Requirements................................... 13-4 65.80  Certificated Aviation Maintenance Technician School Students................................ 13-4 65.81  General Privileges and Limitations......... 13-4 65.83  Recent Experience Requirements............ 13-4

65.85  Airframe Rating: Additional Privileges............................................................ 13-4 65.87  Powerplant Rating: Additional Privileges............................................................ 13-4 65.89  Display of Certificate............................... 13-5 Inspection Authorization (by 14 CFR Section)........ 13-5 65.91  Inspection Authorization.......................... 13-5 65.92  Inspection Authorization: Duration......... 13-5 65.93  Inspection Authorization: Renewal.......... 13-5 65.95  Inspection Authorization: Privileges and Limitations................................................... 13-6 Ethics ...................................................................... 13-6 A Scenario........................................................... 13-6 Final Observation .......................................... 13-7 Human Factors......................................................... 13-9 FAA Involvement................................................ 13-9 Importance of Human Factors............................ 13-9 Definitions of Human Factors............................. 13-9 Brief History......................................................13-10 Current Approach...............................................13-11 Professionalism.......................................................13-14

Glossary.............................................................G-1 Index................................................................... I-1

xix

xx

Index 100-hour inspection......................................................13-4

A absolute pressure...........................................................3-25 acceleration...................................................................3-17 acceleration due to gravity..............................................3-1 AC circuits..................................................................10-61 AC generators.............................................................10-46 AC motor.................................................... 10-153, 10-160 acoustic emission inspection.........................................8-24 AC series motor........................................................10-159 action and reaction........................................................3-18 addition...........................................................................1-1 advisory circulars (ACs).............................................12-27 aerodynamic heating.....................................................3-52 aerodynamics................................................................3-39 aircraft cleaning............................................................6-19 aircraft logs.....................................................................8-2 aircraft maintenance manual.......................................12-45 aircraft nuts...................................................................5-45 aircraft registration......................................................12-39 Aircraft Specifications................................. 4-2, 4-7, 12-30 airfoils...........................................................................3-41 airframe and powerplant (A&P).................................12-13 Airloc fasteners.............................................................5-69 Air Transport Association (ATA) 100.........................12-36 Airworthiness Certificate.................................. 12-4, 12-38 Airworthiness Directives (ADs)................ 8-4, 12-9, 12-30 Alclad aluminum...........................................................5-23 Allen wrench...................................................................9-4 allowance........................................................................2-7 alodizing........................................................................6-18 alteration.....................................................................12-16 alterations....................................................................12-46 alternating current........................................... 10-44, 10-55 alternator............. 10-160, 10-161, 10-162, 10-165, 10-178 alternator rectifier unit...............................................10-163 altimeter inspections.....................................................8-15 aluminum....................................................... 5-6, 5-7, 6-13 aluminum alloy............................................ 5-6, 5-22, 6-13 ammeter........................................................... 10-70, 10-71 amp-hour.....................................................................10-92 ampere.........................................................................10-17 amplifier.................................................................... 10-118 AND gate..................................................................10-125 angle of attack...............................................................3-41 angle of incidence.........................................................3-41

annealing................................... 5-9, 5-13, 5-21, 5-26, 5-27 annual inspection................................................ 8-12, 13-5 anodizing............................................................. 6-14, 6-18 anti-servo tab.................................................................3-47 anti-torque rotor............................................................3-54 anti-torque systems.......................................................3-54 apparent power............................................................10-66 Archimedes’ principle...................................................3-28 area...................................................................... 1-18, 1-21 arm..................................................................................4-2 armature.............. 10-134, 10-135, 10-137, 10-146, 10-161 artificial aging...............................................................5-23 aspect ratio............................................................ 1-9, 3-41 assembly drawing............................................................2-2 atmosphere....................................................................3-35 atmospheric density......................................................3-37 atmospheric pressure........................................... 3-24, 3-36 atom...............................................................................10-2 attraction.........................................................................3-1 autorotation...................................................................3-59 Auxiliary Power Units (APUs)................................... 11-19 aviation fuel................................................................ 11-27 Aviation Maintenance Alerts.......................................12-31 axes of rotation..............................................................3-42

B balance tab....................................................................3-48 ballast............................................................................4-23 band-pass filter.......................................................... 10-117 band-stop filter.......................................................... 10-117 Barcol tester..................................................................5-29 barometer......................................................................3-35 battery............................................................. 10-90, 10-92 bending..........................................................................3-14 Bernoulli’s principle............................................ 3-32, 3-39 bill of material.................................................................2-6 binary number system...................................................1-26 blade flapping................................................................3-59 blind rivets....................................................................5-60 block and tackle............................................................ 3-11 block diagram................................................................2-13 bolts..................................................................... 5-41, 5-43 Boots self-locking.........................................................5-47 borescope......................................................................8-18 boundary layer..............................................................3-41 box-end wrenches...........................................................9-4 Boyle’s law....................................................................3-26 I-1

break line.......................................................................2-17 Brinell hardness tester...................................................5-28 British thermal unit.........................................................3-4 BTU..............................................................................3-19 buoyancy.......................................................................3-28 buoyant force................................................................3-28 buttock line......................................................................2-7

C calipers..........................................................................9-21 calorie............................................................................3-19 camber...........................................................................3-41 Camloc fasteners...........................................................5-69 capacitance........................................... 10-51, 10-52, 10-84 capacitive reactance....................................................10-56 capacitive reactance Xc...............................................10-55 capacitor............................................. 10-51, 10-52, 10-114 capacitors........................................................ 10-53, 10-55 capacitor start motor.................................................10-156 captive fasteners............................................................5-67 carbon composed resistor............................................10-25 carbon dioxide (CO2) extinguishers............................. 11-6 carbon steel.................................................... 5-2, 5-4, 5-20 carbon tetrachloride...................................................... 11-6 carburizing....................................................................5-21 casehardening...................................................... 5-21, 5-27 casting...........................................................................5-30 cathode-ray tube (CRT)...............................................10-79 centerline.......................................................................2-16 center of gravity (CG)........................................... 3-42, 4-3 center of gravity range..................................................4-17 CG envelope..................................................................4-25 Charles’ law..................................................................3-26 checklists.........................................................................8-2 cherrylock rivets............................................................5-62 chisels..............................................................................9-9 chord line......................................................................3-41 chrome-molybdenum steel..............................................5-4 chrome-nickel.................................................................5-4 chrome-vanadium...........................................................5-4 chromium........................................................................5-4 circle..............................................................................1-20 circuit breaker................................................. 10-30, 10-31 circuit protection devices............................................10-30 circular magnetization...................................................8-26 circular motion..............................................................3-18 classification of fires..................................................... 11-5 cleaning.........................................................................6-10 clevis bolts....................................................................5-43 close tolerance bolts......................................................5-41 Code of Federal Regulations (CFRs)..............................8-4 coefficient of linear expansion......................................3-24

I-2

cold working.................................................................5-31 combination sets............................................................9-18 combination wrench........................................................9-5 common denominators....................................................1-2 commutator................................................. 10-131, 10-136 composite material.............................................. 5-32, 5-33 compound......................................................................10-1 compound DC motor.................................................10-148 compound wound DC generator...............................10-139 compressibility effects..................................................3-49 compression..................................................................3-14 compression ratio............................................................1-9 compression wave.........................................................3-33 computer graphics...........................................................2-1 conduction.....................................................................3-20 conductors............ 10-3, 10-12, 10-19, 10-20, 10-21, 10-24 continuous inspections..................................................8-15 convection.....................................................................3-21 conventional flow........................................................10-16 conventional system......................................................1-25 copper............................................................................ 5-11 copper alloy................................................................... 5-11 corrosion................................................................. 6-1, 6-2 corrosion prone areas......................................................6-8 corrosion treatment.......................................................6-10 cosine............................................................................1-24 cotter pin............................................................. 5-79, 5-84 counter electromotive force (emf)............................10-149 countersink....................................................................9-17 coupling.....................................................................10-120 crowfoot wrench.............................................................9-6 Cryofit fittings...............................................................7-13 cube root........................................................................1-13 current.................................................. 10-17, 10-42, 10-83 current limiter..............................................................10-30 cutting plane line...........................................................2-17 cycle............................................................................10-48

D d’Arsonval meter........................................................10-70 Dalton’s law..................................................................3-27 dash number....................................................................2-5 datum....................................................................... 2-7, 4-2 DC circuit....................................................................10-21 DC generator.................................. 10-130, 10-138, 10-141 DC motor..........................10-142, 10-144, 10-147, 10-151 decimal....................................................1-4, 1-5, 1-6, 1-27 decimal number system...................................................1-4 defueling..................................................................... 11-30 denominator....................................................................1-2 density.............................................................................3-2 density altitude..............................................................3-37

depth micrometer..........................................................9-21 detail drawing..................................................................2-2 detailed inspection......................................................12-20 detail view.......................................................................2-9 Deutsch rivets................................................................5-74 diagonal pliers.................................................................9-3 dies................................................................................9-17 difference........................................................................1-1 differential pressure......................................................3-25 digital image..................................................................2-22 dimension line...............................................................2-16 diode.................... 10-102, 10-103, 10-104, 10-107, 10-109 direct chemical attack.....................................................6-3 direct current (DC).......................................... 10-17, 10-44 directional control.........................................................3-46 dissimilar metal corrosion...............................................6-4 dissymmetry of lift........................................................3-58 dividend...........................................................................1-2 dividers..........................................................................9-21 division............................................................................1-2 divisor.............................................................................1-2 doping.......................................................................10-101 Doppler effect...............................................................3-34 double flare......................................................................7-5 drag...............................................................................3-39 drills..............................................................................9-12 drive screws..................................................................5-71 dry disk rectifier........................................................10-106 dry powder extinguishers.............................................. 11-8 duckbill pliers..................................................................9-3 dynamic stability...........................................................3-43 Dzus fasteners...............................................................5-68

E eddy current..................................... 10-69, 10-152, 10-159 eddy current inspection.................................................8-20 effective value.............................................................10-50 elastic stop.....................................................................5-47 electrical symbol...........................................................2-18 electric ground power units......................................... 11-25 electricity.....................................................................10-23 electrochemical attack.....................................................6-3 electrodynamometer....................................................10-76 electromagnet.................................................. 10-12, 10-15 electromagnetism........................................................10-12 electromotive force (emf)...........................................10-16 electron flow................................................................10-16 electrons.............................................................. 10-2, 10-3 electrostatic discharge (ESD)........................................10-6 electrostatic field...........................................................10-5 empty weight...................................................................4-4 empty weight center of gravity.......................................4-4

empty weight center of gravity range...........................4-18 emulsion cleaners..........................................................6-24 energy....................................................... 3-3, 10-21, 10-23 engine fires.................................................................. 11-15 engine operation.......................................................... 11-13 equipment change.........................................................4-22 ethics.............................................................................13-6 exclusive NOR gate..................................................10-128 exclusive OR gate.....................................................10-128 exfoliation.....................................................................6-14 expansion wave.............................................................3-51 exploded view drawing................................................. 2-11 exponent........................................................................1-12 extension line................................................................2-17 extreme condition check...............................................4-21 extremes..........................................................................1-9 extruding.......................................................................5-31 eyebolt...........................................................................5-43

F FAA Form 337, Major Repair and Alteration.............12-15 ferromagnetic.............................................................. 10-11 ferrous metals.........................................................5-2, 6-11 field effect transistor................................................. 10-111 files................................................................................9-10 filtering...................................................................... 10-114 finish mark......................................................................2-7 fire extinguishers..................................................11-6, 11-8 fire protection.......................................................6-22, 11-5 fire safety....................................................................... 11-5 first class lever.................................................................3-9 fittings.............................................................................7-7 Cryofit........................................................................7-13 flared..........................................................................7-10 MS flareless...............................................................7-10 swaged.......................................................................7-10 universal....................................................................7-10 fixed capacitors...........................................................10-53 fixed resistor................................................................10-25 flaps...............................................................................3-48 flared fittings.................................................................7-10 flareless fittings...............................................................7-7 flareless tube....................................................................7-7 flare nut wrench...............................................................9-6 flat files..........................................................................9-10 flathead pin....................................................................5-79 flexible hose..................................................................7-18 flexible hose fluid lines.................................................7-17 fluid...............................................................................3-27 fluid line end fittings.......................................................7-9 fluid line identification....................................................7-8

I-3

fluid lines.........................................................................7-1 flexible hose...............................................................7-17 fluid pressure.................................................................3-29 force................................................................................3-4 Foreign Object Damage (FOD).................................... 11-4 forging...........................................................................5-30 four forces of flight.......................................................3-39 fractions................................................................... 1-2, 1-3 frequency.....................................................................10-48 frequency of sound........................................................3-34 fretting corrosion.............................................................6-6 friction..................................................................... 3-5, 3-6 fueling hazards............................................................ 11-28 fueling procedures....................................................... 11-28 fuel servicing............................................................... 11-27 fulcrum............................................................................4-3 full-wave rectifier......................................................10-106 full section.......................................................................2-3 fuse..............................................................................10-30

G gaskets...........................................................................5-38 gauge pressure...............................................................3-25 gear.......................................................................3-11, 3-12 general gas law..............................................................3-27 generator...............................10-44, 10-46, 10-133, 10-140 generator control units (GCU)..................................10-170 grades of fuel............................................................... 11-27 ground support air units.............................................. 11-26

H hacksaws.........................................................................9-8 half-round files..............................................................9-10 half-wave rectifier.....................................................10-105 half section......................................................................2-3 halogenated hydrocarbon extinguishers........................ 11-6 hammers..........................................................................9-1 hand cranking.............................................................. 11-14 hand files.......................................................................9-10 hand snips.......................................................................9-8 hand taps.......................................................................9-17 hardening.......................................................................5-20 hard landing inspection.................................................8-16 hardness testing.............................................................5-28 head of pressure............................................................3-29 hearing protection......................................................... 11-4 heat................................................................................3-19 heat-treating equipment................................................5-15 heat energy.......................................................... 3-19, 3-20 heat treatment............................................. 5-14, 5-19, 5-22 helicopter aerodynamics...............................................3-53

I-4

helicopter axes of flight.................................................3-56 helicopters....................................... 3-58, 4-27, 11-5, 11-11 helicopter weight and balance.......................................4-27 henry...........................................................................10-59 hermaphrodite caliper...................................................9-21 Hi-Tigue........................................................................5-67 hidden line.....................................................................2-17 high-pass filter........................................................... 10-116 hook spanner...................................................................9-6 hose identification.........................................................7-17 hot start........................................................................ 11-19 hot working...................................................................5-12 human factors................................................................13-9 hung start..................................................................... 11-19 hydraulic ground power units..................................... 11-26 hydraulic transmission..............................................10-172 hydrometer................................................. 3-2, 4-15, 10-92 hypersonic.....................................................................3-50 hypotenuse....................................................................1-24 hysteresis.......................................... 10-69, 10-152, 10-159

I illustrated parts catalog...................................................8-4 impact drivers..................................................................9-8 impenetrability................................................................3-2 improper fraction............................................................1-2 inclined coil iron vane meter.......................................10-78 inclined plane................................................................3-13 Inconel...........................................................................5-12 inductance....................................................... 10-57, 10-85 induction motor.........................................................10-154 inductive reactance......................................................10-60 inductor..........................................................10-59, 10-114 inductor-type inverter..................................................10-98 inertia............................................................................3-17 inside calipers................................................................9-21 inside micrometer..........................................................9-21 Inspection Authorization...............................................13-5 inspection of composites...............................................8-32 inspection of welds.......................................................8-33 inspections.......................................................................8-1 100-hour....................................................................13-4 altimeter.....................................................................8-15 annual.............................................................. 8-12, 13-5 eddy current...............................................................8-20 magnetic particle.......................................................8-24 transponder................................................................8-15 installation drawing.........................................................2-3 insulators.......................................................................10-3 integrated circuit.......................................................10-128 intergranular corrosion....................................................6-5 internal and external wrenching....................................5-49

interpole....................................................................10-138 inverter........................................................................10-96 ion.................................................................................10-3 isometric drawing.......................................................... 2-11

J jet fuel......................................................................... 11-27 Jo-bolt...........................................................................5-43 joules...........................................................................10-21

K K-Monel........................................................................5-12 kinematics.....................................................................3-16 kinetic energy..................................................................3-4 kinetic theory of gases..................................................3-26 Kirchhoff’s current law...............................................10-41 Kirchhoff’s voltage law..............................................10-36 knife file........................................................................ 9-11

L lag hinge........................................................................3-53 laminated structure........................................................5-34 lateral axis.....................................................................3-45 lateral control................................................................3-46 lateral stability...............................................................3-44 Law of Conservation.......................................................3-1 LC filters................................................................... 10-115 lead-acid battery................................... 10-90, 10-92, 10-93 lead-acid car................................................................10-90 lead-acid cell...............................................................10-92 leader line......................................................................2-17 lead float files................................................................ 9-11 leading edge slats..........................................................3-49 leading edge slots..........................................................3-48 least common denominator (LCD).................................1-2 left-hand rule...............................................................10-45 Lenz’s Law..................................................................10-58 lever.................................................................................3-9 lift........................................................................ 3-39, 3-40 light-emitting diode (LED).......................................10-108 lightning strike..............................................................8-17 light sport aircraft (LSA) maintenance.......................12-44 linear potentiometer....................................................10-29 liquid crystal display (LCD.......................................10-108 load cells.......................................................................4-13 loading graph................................................................4-25 lockbolts........................................................................5-43 lockwashers...................................................................5-50 lodestone............................................................10-8, 10-11 logic circuit................................................. 10-124, 10-125

logic flowchart..............................................................2-15 longitudinal axis............................................................3-46 longitudinal control.......................................................3-45 longitudinal magnetization............................................8-26 longitudinal stability.....................................................3-44 loudness........................................................................3-34 low-pass filter............................................................ 10-116

M machine...........................................................................3-8 machine screws.............................................................5-70 Mach number................................................................3-34 magnaglo inspection.....................................................8-27 magnesium.............................................................. 5-6, 5-9 magnesium alloys..................................5-7, 5-9, 5-26, 6-15 magnetic amplifier....................................................10-122 magnetic field................................................................10-7 magnetic flux.................................................................10-8 magnetic particle inspection.........................................8-24 magnetism.....................................................................10-7 magnetomotive force.................................................. 10-11 magnets....................................................................... 10-11 main rotor systems........................................................3-53 maintenance alteration...................................... 12-9, 12-14 maintenance records............................. 12-15, 12-18, 12-43 major alterations................................................ 12-20, 13-4 major repairs.......................................... 12-20, 12-46, 13-4 Malfunction or Defect Report.....................................12-31 mallets.............................................................................9-1 manganese..................................................................... 5-11 manufacturer’s maintenance manual.................. 8-3, 12-18 mass................................................................................3-1 Material Safety Data Sheet (MSDS)............................. 11-3 material symbol.............................................................2-18 matter.................................................................... 3-1, 10-1 maximum landing weight...............................................4-4 maximum ramp weight...................................................4-4 maximum takeoff weight................................................4-4 maximum weight............................................................4-3 maximum zero fuel weight.............................................4-4 maxwell....................................................................... 10-11 mean aerodynamic chord..............................................4-31 means..............................................................................1-9 measuring tools.............................................................9-18 mechanic.......................................................................13-1 mechanical advantage............................................. 3-8, 3-9 Mechanic Certificate.....................................................13-1 megohmmeter.............................................................10-75 metal-oxide-semiconductor FET (MOSFET)........... 10-112 metal cutting tools...........................................................9-8 metalworking process...................................................5-12 Methyl ethyl ketone (MEK)..........................................6-24

I-5

METO horsepower..........................................................4-5 metric............................................................................10-3 metric system................................................................1-25 micrometer.......................................................... 9-22, 9-24 micrometer calipers.......................................................9-21 microprocessor..........................................................10-130 mill files........................................................................9-10 minimum fuel..................................................................4-5 mixed numbers................................................................1-4 molecule........................................................................10-1 moment...........................................................................4-3 momentary pressure......................................................3-33 Monel............................................................................5-12 MOSFET................................................................... 10-112 motion...........................................................................3-16 moving iron vane meter..............................................10-76 MS flareless fittings......................................................7-10 multimeter...................................................................10-82 multiplication..................................................................1-1

N N-type semiconductor...............................................10-102 NAND gate...............................................................10-127 needlenose pliers.............................................................9-3 negative numbers.......................................................... 1-11 neutrons.........................................................................10-2 Newton’s first law.........................................................3-17 Newton’s second law....................................................3-18 Newton’s third law.............................................. 3-18, 3-40 nickel.............................................................................5-12 nickel-cadmium batteries............................................10-94 nickel-cadmium cells..................................................10-94 nickel alloy....................................................................5-12 nickel steel......................................................................5-4 nitriding.........................................................................5-22 nomogram.....................................................................2-21 non-self-locking nuts....................................................5-45 nondestructive inspection..............................................8-18 NOR gate..................................................................10-127 normalizing......................................................... 5-13, 5-21 normal shock wave.......................................................3-51 NOTAR.........................................................................3-55 nucleus..........................................................................10-2 numerator........................................................................1-2 nutplates........................................................................5-71

O oblique drawing............................................................ 2-11 oblique shock wave.......................................................3-51 octane rating................................................................ 11-27 Ohm’s law..................... 10-17, 10-18, 10-22, 10-35, 10-61

I-6

ohmmeter............................................. 10-70, 10-73, 10-74 ohms............................................................................10-18 one-view drawing............................................................2-9 open-end wrenches..........................................................9-4 operating center of gravity range..................................4-18 operational amplifier.................................................10-121 OR gate.....................................................................10-126 original equipment manufacturer (OEM)...................12-36 orthographic projection...................................................2-7 oscilloscope..................................................... 10-79, 10-80 outside calipers..............................................................9-21 outside micrometer........................................................9-21 overhaul manual..............................................................8-4 oxygen hazards............................................................ 11-27

P P-type semiconductor................................................10-102 packings........................................................................5-36 paint removal................................................................6-10 parallel AC circuits.....................................................10-63 parallel circuit................................................. 10-23, 10-40 parallel DC circuits.....................................................10-40 part number.....................................................................2-5 Pascal’s law...................................................................3-30 pencil compasses...........................................................9-21 penetrant inspection......................................................8-19 percentage............................................................1-10, 1-11 permanent ballast..........................................................4-23 permanent magnet inverter.........................................10-96 permeability..................................................................8-25 perspective drawing...................................................... 2-11 phantom line..................................................................2-17 phase...........................................................................10-49 photoconductive cells..................................................10-29 photodiode.................................................................10-108 pictorial drawing........................................................... 2-11 pin rivets........................................................................5-67 pitch...............................................................................3-34 plastics...........................................................................5-32 pliers................................................................................9-3 PN junction................................................. 10-102, 10-103 PNP transistor........................................................... 10-111 polyurethane enamel.....................................................6-19 porosity...........................................................................3-2 positive numbers........................................................... 1-11 potential difference.....................................................10-17 potential energy...............................................................3-3 potentiometer..............................................................10-28 power............................................................ 3-4, 3-7, 10-21 powered parachute.......................... 3-62, 4-29, 4-30, 11-12 powerplant cleaning......................................................6-23 power rectifier diode.................................................10-107

powers...........................................................................1-12 powers of ten.................................................................1-12 precipitation heat treating.............................................5-24 precipitation heat treatment...........................................5-27 preflight inspections........................................................8-5 pressure.........................................................................3-24 preventive maintenance............................. 6-7, 12-4, 12-20 preventive maintenance alteration................................12-9 primary cell.................................................................10-90 product............................................................................1-1 professionalism...........................................................13-14 progressive inspection....................................... 12-20, 13-5 progressive inspections.................................................8-12 properties of metals.........................................................5-1 proportion........................................................................1-9 protons..........................................................................10-2 pull-thru rivets...............................................................5-61 pulleys...........................................................................3-10 punches...........................................................................9-3 Pythagorean Theorem...................................................1-25

resonance.................................................. 3-35, 8-22, 10-65 retentivity.................................................................... 10-11 revision block..................................................................2-6 revolved section..............................................................2-3 rheostat........................................................................10-28 right-hand motor rule................................................10-144 rigid fluid lines................................................................7-1 rigid rotor system..........................................................3-53 rivets..............................................................................5-54 Rivnuts..........................................................................5-71 Rockwell hardness tester...............................................5-28 roll pin...........................................................................5-79 root................................................................................1-12 rotary inverter..............................................................10-96 roundnose pliers..............................................................9-3 round or rattail files.......................................................9-10 routine inspection........................................................12-20 rubber............................................................................5-35 rules...............................................................................9-18 rust.......................................................................6-11, 6-13

Q

S

quenching..................................................... 5-9, 5-17, 5-24 quotient...........................................................................1-2

SAE.................................................................................5-2 SAE numerical index......................................................5-3 saturable-core reactor................................................10-123 saturated........................................................................3-38 schematic diagram........................................................2-13 Schottky diode..........................................................10-108 scientific notation..........................................................1-13 screwdrivers....................................................................9-1 screws............................................................................5-70 scriber............................................................................9-21 sealants................................................................ 5-38, 5-39 sealing compound.........................................................5-38 secondary cell..............................................................10-90 second class lever............................................................3-9 sectional view drawing...................................................2-3 sectioning line...............................................................2-17 self-inductance............................................................10-58 self-locking nuts............................................................5-46 self-plugging rivets............................................. 5-60, 5-62 self-tapping screws........................................................5-71 semi-rigid rotor system.................................................3-53 semiconductors............................................... 10-3, 10-101 series-parallel DC circuit............................................10-42 series AC circuits........................................................10-61 series circuit......................................... 10-23, 10-34, 10-40 series DC circuit..........................................................10-34 series DC motor........................................................10-147 series wound DC generator.......................................10-138 service bulletins (SB).......................................... 8-3, 12-37 servicing aircraft......................................................... 11-22

R radiation........................................................................3-22 radical sign....................................................................1-13 radiographic inspection.................................................8-31 radiographic interpretation............................................8-31 ram pressure..................................................................3-30 rarefaction (expansion) wave........................................3-33 ratcheting box-end wrench..............................................9-5 ratio.................................................................................1-7 reamers..........................................................................9-13 reciprocating engines.................................................. 11-13 rectifiers.....................................................................10-104 reference number............................................................2-5 reheat treatment.............................................................5-24 reinforced plastic...........................................................5-34 relays...........................................................................10-34 remainder........................................................................1-2 removed section..............................................................2-4 repair...........................................................................12-16 repairmen............................................................ 13-1, 13-4 repair stations.................................................... 12-13, 13-4 residual fuel........................................................... 4-4, 4-15 resistance............ 10-17, 10-18, 10-19, 10-20, 10-43, 10-61 resistive circuit............................................................10-22 resistor color code.......................................................10-25 resistors.......................................................................10-25

I-7

servo tab........................................................................3-48 severe turbulence inspection.........................................8-16 shaded pole induction motor.....................................10-155 shape symbol.................................................................2-18 shear stress....................................................................3-15 sheet spring...................................................................5-48 shock absorber cord......................................................5-36 shock wave....................................................................3-50 shop safety.................................................................... 11-1 shunt DC motor.........................................................10-148 shunt resistor...............................................................10-70 shunt wound DC generator.......................................10-139 signed numbers............................................................. 1-11 silicon............................................................................5-12 sine................................................................................1-24 sine wave.....................................................................10-80 single fixed pulley.........................................................3-10 single flare.......................................................................7-5 single movable pulley...................................................3-10 single phase induction motor....................................10-155 sketches.........................................................................2-20 slide calipers..................................................................9-25 socket wrench..................................................................9-5 solenoid.......................................................................10-16 solid-state..................................................................10-101 solid-state inverter.......................................................10-99 solid shank rivets...........................................................5-56 solution heat treatment................................. 5-9, 5-23, 5-26 solvent cleaners.............................................................6-23 sound.............................................................................3-32 sound intensity..............................................................3-34 special flight permit.......................................................8-18 special inspections........................................................8-16 special purpose diode................................................10-108 specific gravity................................................................3-2 specific heat...................................................................3-22 specific weight................................................................3-2 speed.............................................................................3-16 speed of sound..................................................... 3-33, 3-50 sphere............................................................................1-23 spirit level............................................................ 4-14, 9-18 split phase motor.......................................................10-156 square files....................................................................9-10 square root.....................................................................1-13 squirrel cage motor...................................................10-153 stability..........................................................................3-43 stabilizing........................................................................5-9 stainless steel...................................................................5-4 stainless steel self-locking.............................................5-47 standard atmosphere......................................................3-38 standard weights............................................................4-18 static electricity.............................................................10-4 static inverter................................................... 10-98, 10-99

I-8

static pressure................................................................3-29 static stability................................................................3-43 station numbering system...............................................2-7 steel.................................................................................5-2 steel alloy........................................................................5-2 stitch line.......................................................................2-17 strain..............................................................................3-16 strap wrenches.................................................................9-7 stress..............................................................................3-14 stress corrosion................................................................6-6 stress relieving..............................................................5-27 structural repair manual (SRM).......................... 8-4, 12-38 structural screws............................................................5-70 subsonic speed..............................................................3-50 subtraction.......................................................................1-1 sum..................................................................................1-1 supersonic speed...........................................................3-50 supplemental type certificates (STC)..........................12-31 surface area...................................................................1-23 surface corrosion.............................................................6-4 suspected unapproved parts (SUPs)............................12-27 swaged fittings..............................................................7-10 switches................................................ 10-31, 10-32, 10-33 synchronous motor....................................................10-157

T tangent...........................................................................1-24 Taper-Lok......................................................................5-67 tapered potentiometer..................................................10-29 taper pin........................................................................5-79 taper tap.........................................................................9-17 taps................................................................................9-17 tap testing......................................................................8-32 tare weight.......................................................................4-5 taxiing aircraft............................................................. 11-21 taxi signals.................................................................. 11-21 Teflon™........................................................................7-18 temperature...................................................................3-23 tempering......................................................................5-21 tension...........................................................................3-14 thermal efficiency..........................................................3-20 thermal expansion.........................................................3-24 thermal hardening.........................................................5-27 thermal protectors.......................................................10-31 thermistors...................................................................10-29 thermocouples.............................................................10-24 thermoplastic material...................................................5-32 thermosetting plastic.....................................................5-32 third class lever.............................................................3-10 threaded fastener...........................................................5-39 thread micrometer.........................................................9-21 three phase induction motor......................................10-153

three square files...........................................................9-10 thrust................................................................... 3-18, 3-39 tiedown procedures............................................11-9, 11-11 titanium...............................................5-9, 5-10, 5-27, 6-16 titanium alloy............................................... 5-9, 5-10, 6-16 title block........................................................................2-4 tolerance..........................................................................2-7 torque......................................................... 3-4, 3-7, 10-143 torque wrench........................................................ 5-53, 9-7 torsion...........................................................................3-14 total pressure.................................................................3-30 towing aircraft............................................................. 11-19 transformer................................10-66, 10-67, 10-68, 10-69 transistor..........................................10-109, 10-111, 10-112 transonic speed..............................................................3-50 transponder inspections.................................................8-15 trapezoid........................................................................1-19 triangle..........................................................................1-19 triangular files...............................................................9-10 trigonometry..................................................................1-24 trim tabs........................................................................3-47 troubleshooting flowchart.............................................2-15 true power...................................................................10-66 tube beading....................................................................7-7 tube bending....................................................................7-3 tube cutting......................................................................7-2 tube flaring......................................................................7-5 tubing materials...............................................................7-1 turbofan engines.......................................................... 11-17 turboprop engines.............................................11-15, 11-16 turnbuckle.....................................................................5-78 turnlock fasteners..........................................................5-68 twist drills.....................................................................9-12 Type Certificate Data Sheet (TCDS).....4-2, 4-7, 8-4, 12-31

U ultrasonic inspection.....................................................8-21 uniform motion.............................................................3-16 universal fittings............................................................7-10 universal numbering system...........................................2-5 unusable fuel.................................................................4-18 useful load.......................................................................4-4

variable capacitors......................................................10-54 variable resistors.........................................................10-27 varmeter......................................................................10-78 velocity..........................................................................3-16 venturi................................................................. 3-32, 3-40 vernier scale..................................................................9-23 vertical axis...................................................................3-46 viewing plane line.........................................................2-17 visible line.....................................................................2-17 Vixen files..................................................................... 9-11 voltage............................................................. 10-44, 10-82 voltage divider............................................................10-37 voltage regulation......................................................10-178 voltmeter......................................................... 10-70, 10-72 volume................................................................. 1-21, 1-23 vortex generators...........................................................3-42

W warding file................................................................... 9-11 washers..........................................................................5-50 waterline..........................................................................2-7 water vapor....................................................................3-37 watt..............................................................................10-21 wattmeter.....................................................................10-78 wavelength....................................................................3-33 weighing points.............................................................4-16 weight.................................................................... 3-1, 3-39 weight-shift-control aircraft..................... 3-60, 4-29, 11-12 weight and balance records...........................................4-32 weight and balance report.............................. 4-1, 4-7, 4-32 whole numbers................................................................1-1 wingtip vortices.............................................................3-42 wire-wound resistors...................................................10-27 wiring diagram..............................................................2-15 wood file....................................................................... 9-11 work........................................................................ 3-4, 3-5 wrenches.........................................................................9-4

Z zener diode.................................................. 10-107, 10-179 zone number....................................................................2-6

V vacuum tube.............................................................. 10-113 valence electrons...........................................................10-3 varactor.....................................................................10-108

I-9

Mathematics in Aviation Maintenance

Many examples of using mathematical principles by the aviation mechanic are available. Tolerances in turbine engine components are critical, making it necessary to measure within a ten-thousandth of an inch. Because of these close tolerances, it is important that the aviation mechanic be able to make accurate measurements and mathematical calculations. An aviation mechanic working on aircraft fuel systems will also use mathematical principles to calculate volumes and capacities of fuel tanks. The use of fractions and surface area calculations are required to perform sheet metal repair on aircraft structures.

Whole Numbers Whole numbers are the numbers 0, 1, 2, 3, 4, 5, and so on. Addition of Whole Numbers Addition is the process in which the value of one number is added to the value of another. The result is called the sum. When working with whole numbers, it is important to understand the principle of the place value. The place value in a whole number is the value of the position of the digit within the number. For example, in the number 512, the 5 is in the hundreds column, the 1 is in the tens column, and the 2 is in the ones column. The place values of three whole numbers are shown in Figure 1-1.

Tens

Ones

Hundreds

Mathematics is woven into many areas of everyday life. Performing mathematical calculations with success requires an understanding of the correct methods and procedures, and practice and review of these principles. Mathematics may be thought of as a set of tools. The aviation mechanic will need these tools to successfully complete the maintenance, repair, installation, or certification of aircraft equipment.

Thousands

Ten Thousands

Place Value

3

5

2

6

9

7

4

9

35 shown as 269 shown as 12,749 shown as

1

2

Figure 1-1. Example of place values of whole numbers.

When adding several whole numbers, such as 4,314, 122, 93,132, and 10, align them into columns according to place value and then add.

4,314 122 93,132 + 10 97,578 This is the sum of the four whole numbers. Subtraction of Whole Numbers Subtraction is the process in which the value of one number is taken from the value of another. The answer is called the difference. When subtracting two whole numbers, such as 3,461 from 97,564, align them into columns according to place value and then subtract. 97,564 –3,461 94,103 This is the difference of the two whole numbers. Multiplication of Whole Numbers Multiplication is the process of repeated addition. For example, 4 × 3 is the same as 4 + 4 + 4. The result is called the product. Example: How many hydraulic system filters are in the supply room if there are 35 cartons and each carton contains 18 filters?

1-1



18 × 35 90 54 630 Therefore, there are 630 filters in the supply room.

Division of Whole Numbers Division is the process of finding how many times one number (called the divisor) is contained in another number (called the dividend). The result is the quotient, and any amount left over is called the remainder. quotient divisor dividend Example: 218 landing gear bolts need to be divided between 7 aircraft. How many bolts will each aircraft receive? 31 7 218 − 21 8 −7 1 The solution is 31 bolts per aircraft with a remainder of 1 bolt left over.

Fractions A fraction is a number written in the form N⁄D where N is called the numerator and D is called the denominator. The fraction bar between the numerator and denominator shows that division is taking place. 17 2 5 Some examples of fractions are: 18 3 8 The denominator of a fraction cannot be a zero. For example, the fraction 2⁄ 0 is not allowed. An improper fraction is a fraction in which the numerator is equal to or larger than the denominator. For example, 4⁄4 or 15⁄ are examples of improper fractions. 8 Finding the Least Common Denominator To add or subtract fractions, they must have a common denominator. In math, the least common denominator (LCD) is commonly used. One way to find the LCD is to list the multiples of each denominator and then choose the smallest one that they have in common. Example: Add 1⁄5 + 1⁄10 by finding the least common denominator.

1-2

Multiples of 5 are: 5, 10, 15, 20, 25, and on. Multiples of 10 are: 10, 20, 30, 40, and on. Notice that 10, 20, and 30 are in both lists, but 10 is the smallest or least common denominator (LCD). The advantage of finding the LCD is that the final answer is more likely to be in lowest terms. A common denominator can also be found for any group of fractions by multiplying all of the denominators together. This number will not always be the LCD, but it can still be used to add or subtract fractions. Example: Add 2⁄3 + 3⁄5 + 4⁄7 by finding a common denominator. A common denominator can be found by multiplying the denominators 3 × 5 × 7 to get 105. 2 3

3 5

4 7

70 105

63 105

60 105

193 105

1

88 105

Addition of Fractions In order to add fractions, the denominators must be the same number. This is referred to as having “common denominators.” Example: Add 1⁄7 to 3⁄7 1 7

1+3 7

3 7

4 7

If the fractions do not have the same denominator, then one or all of the denominators must be changed so that every fraction has a common denominator. Example: Find the total thickness of a panel made from 3⁄ -inch thick aluminum, which has a paint coating that 32 is 1⁄64-inch thick. To add these fractions, determine a common denominator. The least common denominator for this example is 1, so only the first fraction must be changed since the denominator of the second fraction is already 64. 3 32

1 64

3×2 32 × 2

1 64

6 64

1 64

6+1 64

7 64

Therefore, 7⁄64 is the total thickness. Subtraction of Fractions In order to subtract fractions, they must have a common denominator. Example: Subtract 2⁄17 from 10⁄17 10 − 2 17 17

10 − 2 17

8 17

If the fractions do not have the same denominator, then one or all of the denominators must be changed so that every fraction has a common denominator. Example: The tolerance for rigging the aileron droop of an airplane is 7⁄8 inch ± 1⁄5 inch. What is the minimum droop to which the aileron can be rigged? To subtract these fractions, first change both to common denominators. The common denominator in this example is 40. Change both fractions to 1⁄40, as shown, then subtract. 7 1 − 8 5

7×5 1×8 8×5 − 5×8

35 8 40 − 40

35 − 8 40

27 40

3 –5 8

7 5 –– 16

Figure 1-2. Center hole of the plate.

Therefore, 27⁄40 is the minimum droop. Multiplication of Fractions Multiplication of fractions does not require a common denominator. To multiply fractions, first multiply the numerators. Then, multiply the denominators. Example: 3 7 1 × × 5 8 2

3×7×1 5×8×2

21 80

The use of cancellation when multiplying fractions is a helpful technique which divides out or cancels all common factors that exist between the numerators and denominators. When all common factors are cancelled before the multiplication, the final product will be in lowest terms. Example: 14 × 3 15 7

2 1 14 × 3 15 7 5 1

2×1 5×1

2 5

Division of Fractions Division of fractions does not require a common denominator. To divide fractions, first change the division symbol to multiplication. Next, invert the second fraction. Then, multiply the fractions. Example: Divide 7⁄8 by 4⁄3 7 8

4 3

7 3 × 8 4

7×3 8×4

21 32

Example: In Figure 1-2, the center of the hole is in the center of the plate. Find the distance that the center of the hole is from the edges of the plate. To find the answer, the length and width of the plate should each be divided in half. First, change the mixed numbers to improper fractions:

5 7⁄16 inches = 87⁄16 inches 3 5⁄8 inches = 29⁄8 inches Then, divide each improper fraction by 2 to find the center of the plate. 87 16

2 1

87 1 × 16 2

87 inches 32

29 8

2 1

29 1 × 8 2

29 inches 16

Finally, convert each improper fraction to a mixed number: 87 32

87 32

2

23 inches 32

29 16

29 16

1

13 inches 16

Therefore, the distance to the center of the hole from each of the plate edges is 2 23⁄32 inches and 1 13⁄16 inches. Reducing Fractions A fraction needs to be reduced when it is not in “lowest terms.” Lowest terms means that the numerator and denominator do not have any factors in common. That is, they cannot be divided by the same number (or factor). To reduce a fraction, determine what the common factor(s) are and divide these out of the numerator and denominator. For example when both the numerator and denominator are even numbers, they can both be divided by 2. Example: The total travel of a jackscrew is 13⁄16 inch. If the travel in one direction from the neutral position is 7⁄ inch, what is the travel in the opposite direction? 16 13 7 − 16 16

13 − 7 16

6 16

1-3

The fraction 6⁄16 is not in lowest terms because the numerator (6) and the denominator (16) have a common factor of 2. To reduce 6⁄16, divide the numerator and the denominator by 2. The final reduced fraction is 3⁄8 as shown below. 6 6÷2 3 16 ÷ 2 16 8 Therefore, the travel in the opposite direction is 3⁄8 inch.

Subtraction of Mixed Numbers To subtract mixed numbers, find a common denominator for the fractions. Subtract the fractions from each other (it may be necessary to borrow from the larger whole number when subtracting the fractions). Subtract the whole numbers from each other. The final step is to combine the final whole number with the final fraction.

Therefore, the grip length of the bolt is 113⁄16 inches. (Note: The value for the overall length of the bolt was given in the example, but it was not needed to solve the problem. This type of information is sometimes referred to as a “distracter” because it distracts from the information needed to solve the problem.)

The Decimal Number System The Origin and Definition The number system that we use every day is called the decimal system. The prefix in the word decimal is a Latin root for the word “ten.” The decimal system probably had its origin in the fact that we have ten fingers (or digits). The decimal system has ten digits: 0, 1, 2, 3, 4, 5, 6, 7, 8, and 9. It is a base 10 system and has been in use for over 5,000 years. A decimal is a number with a decimal point. For example, 0.515, .10, and 462.625 are all decimal numbers. Like whole numbers, decimal numbers also have place value. The place values are based on powers of 10, as shown in Figure 1-4.

Example: What is the length of the grip of the bolt shown in Figure 1-3? The overall length of the bolt is 3 1⁄2 inches, the shank length is 3 1⁄8 inches, and the threaded portion is 1 5⁄16 inches long. To find the grip, subtract the length of the threaded portion from the length of the shank.

Place Value

3 1⁄8 inches – 1 5⁄16 inches = grip length To subtract, start with the fractions. Borrowing will be necessary because 5⁄16 is larger than 1⁄8 (or 2⁄16). From the whole number 3, borrow 1, which is actually 16⁄16. After borrowing, the first mixed number will now be 1-4

1,623,051

1

6

2

3

0

5

1

0.0531 32.4

3

Ten Thousandths

13 12

13 16

Thousandths

6

1

Hundredths

4 12

18 5 −1 16 16

Tenths

9 12

2

Ones

6

2 − 5 1 16 16

Tens

1 3

3

Hundreds

3 1 3 2 (4 + 2) + 4 3 4 1 feet of cargo room. 7 12

4

1 5 −1 8 16

Thousands

Example: The cargo area behind the rear seat of a small airplane can handle solids that are 4 3⁄4 feet long. If the rear seats are removed, then 2 1⁄3 feet is added to the cargo area. What is the total length of the cargo area when the rear seats are removed?

3

Ten Thousands

Addition of Mixed Numbers To add mixed numbers, add the whole numbers together. Then add the fractions together by finding a common denominator. The final step is to add the sum of the whole numbers to the sum of the fractions for the final result.

2 18⁄16 . (Because, 31⁄8 = 3 2⁄16 = 2 + 1 + 2⁄16 = 2 + 16⁄16 + 2⁄16 = 218⁄16.)

Hundred Thousands

A mixed number is a combination of a whole number and a fraction.

Millions

Mixed Numbers

Figure 1-3. Bolt dimensions.

0

0

5

3

1

2

4

Figure 1-4. Place values.

Addition of Decimal Numbers To add decimal numbers, they must first be arranged so that the decimal points are aligned vertically and according to place value. That is, adding tenths with tenths, ones with ones, hundreds with hundreds, and so forth.

2.34 Ohms

37.5 Ohms

Example: Find the total resistance for the circuit diagram shown in Figure 1-5. The total resistance of a series circuit is equal to the sum of the individual resistances. To find the total resistance, RT, the individual resistances are added together.

.09 Ohms

RT = 2.34 + 37.5 + .09

Figure 1-5. Circuit diagram.

Arrange the resistance values in a vertical column so that the decimal points are aligned and then add.

2.34 37.5 + .09 39.93

Therefore, the total resistance, RT = 39.93 ohms. Subtraction of Decimal Numbers To subtract decimal numbers, they must first be arranged so that the decimal points are aligned vertically and according to place value. That is, subtracting tenths from tenths, ones from ones, hundreds from hundreds, and so forth. Example: A series circuit containing two resistors has a total resistance (RT) of 37.272 ohms. One of the resistors (R1) has a value of 14.88 ohms. What is the value of the other resistor (R2)? R2 = RT – R1 = 37.272 – 14.88 Arrange the decimal numbers in a vertical column so that the decimal points are aligned and then subtract. 37.272 − 14.88 22.392 Therefore, the second resistor, R2 = 22.392 ohms. Multiplication of Decimal Numbers To multiply decimal numbers, vertical alignment of the decimal point is not required. Instead, align the numbers to the right in the same way as whole numbers are multiplied (with no regard to the decimal points or

M

place values) and then multiply. The last step is to place the decimal point in the correct place in the answer. To do this, “count” the number of decimal places in each of the numbers, add the total, and then “give” that number of decimal places to the result. Example: To multiply 0.2 × 6.03, arrange the numbers vertically and align them to the right. Multiply the numbers, ignoring the decimal points for now. 6.03 × 0.2 1206 (ignore the decimal points, for now) After multiplying the numbers, count the total number of decimal places in both numbers. For this example, 6.03 has 2 decimal places and 0.2 has 1 decimal place. Together there are a total of 3 decimal places. The decimal point for the answer will be placed 3 decimal places from the right. Therefore, the answer is 1.206. 6.03 × 0.2 1.206

2 decimal places 1 decimal place 3 decimal places

Example: Using the formula Watts = Amperes × Voltage, what is the wattage of an electric drill that uses 9.45 amperes from a 120 volt source? Align the numbers to the right and multiply. After multiplying the numbers, count the total number of decimal places in both numbers. For this example, 9.45 has 2 decimal places and 120 has no decimal place. Together there are 2 decimal places. The decimal point for the answer will be placed 2 decimal places from

1-5

the right. Therefore, the answer is 1,134.00 watts, or 1,134 watts. 9.45 × 120 000 1890 945 1,134.00

2 decimal places no decimal places

2 decimal places

Division of Decimal Numbers Division of decimal numbers is performed the same way as whole numbers, unless the divisor is a decimal. quotient divisor dividend When the divisor is a decimal, it must be changed to a whole number before dividing. To do this, move the decimal in the divisor to the right until there are no decimal places. At the same time, move the decimal point in the dividend to the right the same number of places. Then divide. The decimal in the quotient will be placed directly above the decimal in the dividend. Example: Divide 0.144 by 0.12 1.2 .12 0.144 = 12. 14.4 12 24 24 0 Move the decimal in the divisor (0.12) two places to the right. Next move the decimal in the dividend (0.144) two places to the right. Then divide. The result is 1.2. Example: The wing area of an airplane is 262.6 square feet and its span is 40.4 feet. Find the mean chord of its wing using the formula: Area ÷ span = mean chord. 6.5 40.4 262.6 = 404. 2626.0 2424 2020 2020 0 Move the decimal in the divisor (40.4) one place to the right. Next move the decimal in the dividend (262.6) one place to the right. Then divide. The mean chord length is 6.5 feet. 1-6

Rounding Off Decimal Numbers Occasionally, it is necessary to round off a decimal number to some value that is practical to use. For example, a measurement is calculated to be 29.4948 inches. To use this measurement, we can use the process of “rounding off.” A decimal is “rounded off” by keeping the digits for a certain number of places and discarding the rest. The degree of accuracy desired determines the number of digits to be retained. When the digit immediately to the right of the last retained digit is 5 or greater, round up by 1. When the digit immediately to the right of the last retained digit is less than 5, leave the last retained digit unchanged. Example: An actuator shaft is 2.1938 inches in diameter. Round to the nearest tenth. The digit in the tenths column is a 1. The digit to the right of the 1 is a 9. Since 9 is greater than or equal to 5, “round up” the 1 to a 2. Therefore, 2.1938 rounded to the nearest tenth is 2.2. Example: The outside diameter of a bearing is 2.1938 inches. Round to the nearest hundredth. The digit in the hundredths column is a 9. The digit to the right of the 9 is a 3. Since 3 is less than 5, do not round up the 9. Therefore, 2.1938 to the nearest hundredth is 2.19. Example: The length of a bushing is 2.1938 inches. Round to the nearest thousandth. The digit in the thousandths column is a 3. The digit to the right of the 3 is an 8. Since 8 is greater than or equal to 5, “round up” the 3 to a 4. Therefore, 2.1938 to the nearest thousandth is 2.194. Converting Decimal Numbers to Fractions To change a decimal number to a fraction, “read” the decimal, and then write it into a fraction just as it is read as shown below. Example: One oversized rivet has a diameter of 0.52 inches. Convert 0.52 to a fraction. The decimal 0.52 is read as “fifty-two hundredths.” Therefore, 0.52 = 52 100

“fifty-two” which can be reduced to 13 25 “hundredths”

A dimension often appears in a maintenance manual or on a blueprint as a decimal instead of a fraction. In order to use the dimension, it may need to be converted

to a fraction. An aviation mechanic frequently uses a steel rule that is calibrated in units of 1⁄64 of an inch. To change a decimal to the nearest equivalent common fraction, multiply the decimal by 64. The product of the decimal and 64 will be the numerator of the fraction and 64 will be the denominator. Reduce the fraction, if needed. Example: The width of a hex head bolt is 0.3123 inches. Convert the decimal 0.3123 to a common fraction to decide which socket would be the best fit for the bolt head. First, multiply the 0.3123 decimal by 64: 0.3123 × 64 = 19.9872 Next, round the product to the nearest whole number: 19.98722 ≈ 20. Use this whole number (20) as the numerator and 64 as the denominator: 20⁄64 . Now, reduce

20⁄ 64

to

5⁄ . 16

Therefore, the correct socket would be the 5⁄16 inch socket ( 20⁄64 reduced). Example: When accurate holes of uniform diameter are required for aircraft structures, they are first drilled approximately 1⁄64 inch undersized and then reamed to the final desired diameter. What size drill bit should be selected for the undersized hole if the final hole is reamed to a diameter of 0.763 inches? First, multiply the 0.763 decimal by 64. 0.763 × 64 = 48.832

Example: .5 1 2 = 1 ÷ 2 = 2 1.0 1.0 0 .375 3 8 3.000 = = 3 ÷ 8 8 – 24 60 – 56 40 – 40 0

49 1 48 3 64 − 64 = 64 = 4 Therefore, a 3⁄4-inch drill bit should be used for the initial undersized holes. Converting Fractions to Decimals To convert any fraction to a decimal, simply divide the top number (numerator) by the bottom number (denominator). Every fraction will have an approximate decimal equivalent.

3 Therefore, 8 = .375

Calculator tip: Numerator (top number) ÷ Denominator (bottom number) = the decimal equivalent of the fraction. Some fractions when converted to decimals produce a repeating decimal. Example: .33 1 3 = 1 ÷ 3 = 3 1.00 = .3 or .33 This decimal can be –9 represented with bar, or can 10 be rounded. (A bar indicates –9 that the number(s) beneath it 1 are repeated to infinity.) Other examples of repeating decimals: .212121… = .21

Next, round the product to the nearest whole number: 48.832 ≈ 49. Use this number (49) as the numerator and 64 as the denominator: 49⁄64 is the closest fraction to the final reaming diameter of 0.763 inches. To determine the drill size for the initial undersized hole, subtract 1⁄64 inch from the finished hole size.

1 Therefore, 2 = .5

.6666… = .7 or .67 .254254… = .254 Decimal Equivalent Chart Figure 1-6 (on the next page) is a fraction to decimal to millimeter equivalency chart. Measurements starting at 1⁄64 inch and up to 23 inches have been converted to decimal numbers and to millimeters.

Ratio A ratio is the comparison of two numbers or quantities. A ratio may be expressed in three ways: as a fraction, with a colon, or with the word “to.” For example, a gear ratio of 5:7 can be expressed as any of the following: 5⁄

7

or 5:7  or 5 to 7

1-7

fraction decimal 1/64 1/32 3/64 1/16 5/64 3/32 7/64 1/8 9/64 5/32 11/64 5/16 13/64 7/32 15/64 1/4 17/64 9/32 19/64 5/16 21/64 11/32 23/64 3/8 25/64 13/32 27/64 7/16 29/64 15/32 31/64 1/2 33/64 17/32 35/64 39341 37/64 19/32 39/64 5/8 41/64 21/32 43/64 11/16 45/64 23/32 47/64 3/4 49/64 25/32 51/64 13/16 53/64 27/32 55/64 7/8 57/64 29/32 59/64 15/16 61/64 31/32 63/64 1

0.015 0.031 0.046 0.062 0.078 0.093 0.109 0.125 0.140 0.156 0.171 0.187 0.203 0.218 0.234 0.25 0.265 0.281 0.296 0.312 0.328 0.343 0.359 0.375 0.390 0.406 0.421 0.437 0.453 0.468 0.484 0.5 0.515 0.531 0.546 0.562 0.578 0.593 0.609 0.625 0.640 0.656 0.671 0.687 0.703 0.718 0.734 0.75 0.765 0.781 0.796 0.812 0.828 0.843 0.859 0.875 0.890 0.906 0.921 0.937 0.953 0.968 0.984 1

mm 0.396 0.793 1.190 1.587 1.984 2.381 2.778 3.175 3.571 3.968 4.365 4.762 5.159 5.556 5.953 6.35 6.746 7.143 7.540 7.937 8.334 8.731 9.128 9.525 9.921 10.318 10.715 11.112 11.509 11.906 12.303 12.7 13.096 13.493 13.890 14.287 14.684 15.081 15.478 15.875 16.271 16.668 17.065 17.462 17.859 18.256 18.653 19.05 19.446 19.843 20.240 20.637 21.034 21.431 21.828 22.225 22.621 23.018 23.415 23.812 24.209 24.606 25.003 25.4

fraction decimal 1 1/64 1 1/32 1 3/64 1 1/16 1 5/64 1 3/32 1 7/64 1 1/8 1 9/64 1 5/32 1 11/64 1 3/16 1 13/64 1 7/32 1 15/64 1 1/4 1 17/64 1 9/32 1 19/64 1 5/16 1 21/64 1 11/32 1 23/64 1 3/8 1 25/64 1 13/32 1 27/64 1 7/16 1 29/64 1 15/32 1 31/64 1 1/2 1 33/64 1 17/32 1 35/64 1 9/16 1 37/64 1 19/32 1 39/64 1 5/8 1 41/64 1 21/32 1 43/64 1 11/16 1 45/64 1 23/32 1 47/64 1 3/4 1 49/64 1 25/32 1 51/64 1 13/16 1 53/64 1 27/32 1 55/64 1 7/8 1 57/64 1 29/32 1 59/64 1 15/16 1 61/64 1 31/32 1 63/64 2

1.015 1.031 1.046 1.062 1.078 1.093 1.109 1.125 1.140 1.156 1.171 1.187 1.203 1.218 1.234 1.25 1.265 1.281 1.296 1.312 1.328 1.343 1.359 1.375 1.390 1.406 1.421 1.437 1.453 1.468 1.484 1.5 1.515 1.531 1.546 1.562 1.578 1.593 1.609 1.625 1.640 1.656 1.671 1.687 1.703 1.718 1.734 1.75 1.765 1.781 1.796 1.812 1.828 1.843 1.859 1.875 1.890 1.906 1.921 1.937 1.953 1.968 1.984 2

mm 25.796 26.193 26.590 26.987 27.384 27.781 28.178 28.575 28.971 29.368 29.765 30.162 30.559 30.956 31.353 31.75 32.146 32.543 32.940 33.337 33.734 34.131 34.528 34.925 35.321 35.718 36.115 36.512 36.909 37.306 37.703 38.1 38.496 38.893 39.290 39.687 40.084 40.481 40.878 41.275 41.671 42.068 42.465 42.862 43.259 43.656 44.053 44.45 44.846 45.243 45.640 46.037 46.434 46.831 47.228 47.625 48.021 48.418 48.815 49.212 49.609 50.006 50.403 50.8

   

 

 

   

   

   

Figure 1-6. Fractions, decimals, and millimeters.

1-8

fraction decimal 2 1/64 2 1/32 2 3/64 2 1/16 2 5/64 2 3/32 2 7/64 2 1/8 2 9/64 2 5/32 2 11/64 2 3/16 2 13/64 2 7/32 2 15/64 2 1/4 2 17/64 2 9/32 2 19/64 2 5/16 2 21/64 2 11/32 2 23/64 2 3/8 2 25/64 2 13/32 2 27/64 2 7/16 2 29/64 2 15/32 2 31/64 2 1/2 2 33/64 2 17/32 2 35/64 2 9/16 2 37/64 2 19/32 2 39/64 2 5/8 2 41/64 2 21/32 2 43/64 2 11/16 2 45/64 2 23/32 2 47/64 2 3/4 2 49/64 2 25/32 2 51/64 2 13/16 2 53/64 2 27/32 2 55/64 2 7/8 2 57/64 2 29/32 2 59/64 2 15/16 2 61/64 2 31/32 2 63/64 3

2.015 2.031 2.046 2.062 2.078 2.093 2.109 2.125 2.140 2.156 2.171 2.187 2.203 2.218 2.234 2.25 2.265 2.281 2.296 2.312 2.328 2.343 2.359 2.375 2.390 2.406 2.421 2.437 2.453 2.468 2.484 2.5 2.515 2.531 2.546 2.562 2.578 2.593 2.609 2.625 2.640 2.656 2.671 2.687 2.703 2.718 2.734 2.75 2.765 2.781 2.796 2.812 2.828 2.843 2.859 2.875 2.890 2.906 2.921 2.937 2.953 2.968 2.984 3

mm 51.196 51.593 51.990 52.387 52.784 53.181 53.578 53.975 54.371 54.768 55.165 55.562 55.959 56.356 56.753 57.15 57.546 57.943 58.340 58.737 59.134 59.531 59.928 60.325 60.721 61.118 61.515 61.912 62.309 62.706 63.103 63.5 63.896 64.293 64.690 65.087 65.484 65.881 66.278 66.675 67.071 67.468 67.865 68.262 68.659 69.056 69.453 69.85 70.246 70.643 71.040 71.437 71.834 72.231 72.628 73.025 73.421 73.818 74.215 74.612 75.009 75.406 75.803 76.2

Aviation Applications Ratios have widespread application in the field of aviation. For example: Compression ratio on a reciprocating engine is the ratio of the volume of a cylinder with the piston at the bottom of its stroke to the volume of the cylinder with the piston at the top of its stroke. For example, a typical compression ratio might be 10:1 (or 10 to 1). Aspect ratio is the ratio of the length (or span) of an airfoil to its width (or chord). A typical aspect ratio for a commercial airliner might be 7:1 (or 7 to 1). Air-fuel ratio is the ratio of the weight of the air to the weight of fuel in the mixture being fed into the cylinders of a reciprocating engine. For example, a typical air-fuel ratio might be 14.3:1 (or 14.3 to 1). Glide ratio is the ratio of the forward distance traveled to the vertical distance descended when an aircraft is operating without power. For example, if an aircraft descends 1,000 feet while it travels through the air for a distance of two linear miles (10,560 feet), it has a glide ratio of 10,560:1,000 which can be reduced to 10.56: 1 (or 10.56 to 1). Gear Ratio is the number of teeth each gear represents when two gears are used in an aircraft component. In Figure 1-7, the pinion gear has 8 teeth and a spur gear has 28 teeth. The gear ratio is 8:28 or 2:7. Speed Ratio. When two gears are used in an aircraft component, the rotational speed of each gear is represented as a speed ratio. As the number of teeth in a gear decreases, the rotational speed of that gear increases, and vice-versa. Therefore, the speed ratio of two gears is the inverse (or opposite) of the gear ratio. If two gears have a gear ratio of 2:9, then their speed ratio is 9:2. Example: A pinion gear with 10 teeth is driving a spur gear with 40 teeth. The spur gear is rotating at 160 rpm. Determine the speed of the pinion gear. Teeth in Pinion Gear Speed of Spur Gear = Teeth in Spur Gear Speed of Pinion Gear 10 teeth 160 rpm = 40 teeth SP (speed of pinion gear) To solve for SP , multiply 40 × 160, then divide by 10. The speed of the pinion gear is 640 rpm. Example: If the cruising speed of an airplane is 200 knots and its maximum speed is 250 knots, what is the ratio of cruising speed to maximum speed? First,

Figure 1-7. Gear ratio.

express the cruising speed as the numerator of a fraction whose denominator is the maximum speed. Ratio =

200 250

Next, reduce the resulting fraction to its lowest terms. 200 4 Ratio = = 250 5 Therefore, the ratio of cruising speed to maximum speed is 4:5. Another common use of ratios is to convert any given ratio to an equivalent ratio with a denominator of 1. Example: Express the ratio 9:5 as a ratio with a denominator of 1. R = 9 = ? 5 1

Since 9 ÷ 5 = 1.8, then 9 = 1.8 5 1

Therefore, 9:5 is the same ratio as 1.8:1. In other words, 9 to 5 is the same ratio as 1.8 to 1.

Proportion A proportion is a statement of equality between two or more ratios. For example, 3 = 6 or 3:4 = 6:8 4 8 This proportion is read as, “3 is to 4 as 6 is to 8.” Extremes and Means The first and last terms of the proportion (the 3 and 8 in this example) are called the extremes. The second and third terms (the 4 and 6 in this example) are called the means. In any proportion, the product of the extremes is equal to the product of the means. In the proportion 2:3 = 4:6, the product of the extremes, 2 × 6, is 12; the product of the means, 3 × 4, is also 12. An inspection of any proportion will show this to be true. 1-9

Solving Proportions Normally when solving a proportion, three quantities will be known, and the fourth will be unknown. To solve for the unknown, multiply the two numbers along the diagonal and then divide by the third number. Example: Solve for X in the proportion given below. 65 = X 80 100 First, multiply 65 × 100: 65 × 100 = 6500 Next, divide by 80: 6500 ÷ 80 = 81.25 Therefore, X = 81.25.

For example: Express the following percentages as decimal numbers: 90% 50% 5% 150%

= = = =

.90 .50 .05 1.5

Expressing a Fraction as a Percentage To express a fraction as a percentage, first change the fraction to a decimal number (by dividing the numerator by the denominator), and then convert the decimal number to a percentage as shown earlier.

Example: An airplane flying a distance of 300 miles used 24 gallons of gasoline. How many gallons will it need to travel 750 miles?

Example: Express the fraction 5⁄8 as a percentage.

The ratio here is: “miles to gallons;” therefore, the proportion is set up as:

Finding a Percentage of a Given Number This is the most common type of percentage calculation. Here are two methods to solve percentage problems: using algebra or using proportions. Each method is shown below to find a percent of a given number.

Miles Gallons

300 = 750 24 G

Solve for G: (750 × 24) ÷ 300 = 60 Therefore, to fly 750 miles, 60 gallons of gasoline will be required.

Percentage Percentage means “parts out of one hundred.” The percentage sign is “%”. Ninety percent is expressed as 90% (= 90 parts out of 100). The decimal 0.90 equals 90⁄ , or 90 out of 100, or 90%. 100 Expressing a Decimal Number as a Percentage To express a decimal number in percent, move the decimal point two places to the right (adding zeros if necessary) and then affix the percent symbol. Example: Express the following decimal numbers as a percent: .90 = 90% .5 = 50% 1.25 = 125% .335 = 33.5% Expressing a Percentage as a Decimal Number Sometimes it may be necessary to express a percentage as a decimal number. To express a percentage as a decimal number, move the decimal point two places to the left and drop the % symbol.

1-10

5 8 = 5 ÷ 8 = 0.625 = 62.5%

Example: In a shipment of 80 wingtip lights, 15% of the lights were defective. How many of the lights were defective? Algebra Method: 15% of 80 lights = N (number of defective lights) 0.15 × 80 = N 12 = N Therefore, 12 defective lights were in the shipment. Proportion Method: N 15 80 = 100 To solve for N: N × 100 = 80 × 15 N × 100 = 1200 N = 1200 ÷ 100 N = 12 or N = (80 × 15) ÷ 100 N = 12 Finding What Percentage One Number Is of Another Example: A small engine rated at 12 horsepower is found to be delivering only 10.75 horsepower. What is the motor efficiency expressed as a percent?

Algebra Method: N% of 12 rated horsepower = 10.75 actual horsepower N% × 12 = 10.75 N% = 10.75 ÷ 12 N% = .8958 N = 89.58 Therefore, the motor efficiency is 89.58%. Proportion Method: 10.75 N 12 = 100 To solve for N: N × 12 = 10.75 × 100 N × 12 = 1075 N = 1075 ÷ 12 N = 89.58 or N = (1075 × 100) ÷ 12 N = 89.58 Therefore, the motor efficiency is 89.58%. Finding a Number When a Percentage of It Is Known Example: Eighty ohms represents 52% of a microphone’s total resistance. Find the total resistance of this microphone. Algebraic Method: 52% of N = 80 ohms 52% × N = 80 N = 80 ÷ .52 N = 153.846 The total resistance of the microphone is 153.846 ohms. Proportion Method: 80 52 N = 100 Solve for N: N × 52 = 80 × 100 N × 52 = 8,000 N = 8,000 ÷ 52 N = 153.846 ohms or N = (80 × 100) ÷ 52 N = 153.846 ohms

Positive and Negative Numbers (Signed Numbers) Positive numbers are numbers that are greater than zero. Negative numbers are numbers less than zero. [Figure 1-8] Signed numbers are also called integers.

Figure 1-8. A scale of signed numbers.

Addition of Positive and Negative Numbers The sum (addition) of two positive numbers is positive. The sum (addition) of two negative numbers is negative. The sum of a positive and a negative number can be positive or negative, depending on the values of the numbers. A good way to visualize a negative number is to think in terms of debt. If you are in debt by $100 (or, −100) and you add $45 to your account, you are now only $55 in debt (or −55). Therefore: −100 + 45 = −55. Example: The weight of an aircraft is 2,000 pounds. A radio rack weighing 3 pounds and a transceiver weighing 10 pounds are removed from the aircraft. What is the new weight? For weight and balance purposes, all weight removed from an aircraft is given a minus sign, and all weight added is given a plus sign. 2,000 + −3 + −10 = 2,000 + −13 = 1987 Therefore, the new weight is 1,987 pounds. Subtraction of Positive and Negative Numbers To subtract positive and negative numbers, first change the “–” (subtraction symbol) to a “+” (addition symbol), and change the sign of the second number to its opposite (that is, change a positive number to a negative number or vice versa). Finally, add the two numbers together. Example: The daytime temperature in the city of Denver was 6° below zero (−6°). An airplane is cruising at 15,000 feet above Denver. The temperature at 15,000 feet is 20° colder than in the city of Denver. What is the temperature at 15,000 feet? Subtract 20 from −6:  −6 – 20 = −6 + −20 = −26 The temperature is −26°, or 26° below zero at 15,000 feet above the city. Multiplication of Positive and Negative Numbers The product of two positive numbers is always positive. The product of two negative numbers is always positive. The product of a positive and a negative number is always negative.

1-11

Examples: 3 × 6 = 18   −3 × 6 = −18   −3 × −6 = 18   3 × −6 = −18

Powers of Ten

Division of Positive and Negative Numbers The quotient of two positive numbers is always positive. The quotient of two negative numbers is always positive. The quotient of a positive and negative number is always negative.

Positive Exponents

Examples: 6 ÷ 3 = 2  −6 ÷ 3 = −2  −6 ÷ −3 = 2  6 ÷ −3 = −2

Powers The power (or exponent) of a number is a shorthand method of indicating how many times a number, called the base, is multiplied by itself. For example, 34 means “3 to the power of 4.” That is, 3 multiplied by itself 4 times. The 3 is the base and 4 is the power. Examples: 23 = 2 × 2 × 2 = 8. Read “two to the third power equals 8.” 105 = 10 × 10 × 10 × 10 × 10 = 100,000 Read “ten to the fifth power equals 100,000.” Special Powers Squared. When a number has a power of 2, it is commonly referred to as “squared.” For example, 72 is read as “seven squared” or “seven to the second power.” To remember this, think about how a square has two dimensions: length and width. Cubed. When a number has a power of 3, it is commonly referred to as “cubed.” For example, 73 is read as “seven cubed” or “seven to the third power.” To remember this, think about how a cube has three dimensions: length, width, and depth. Power of Zero. Any non-zero number raised to the zero power always equals 1. Example: 70 = 1   1810 = 1   (-24)0 = 1 Negative Powers A number with a negative power equals its reciprocal with the same power made positive. Example: The number 2-3 is read as “2 to the negative 3rd power,” and is calculated by: 1 1 1 2-3 = 23 = 2 × 2 × 2 = 8

1-12

Expansion

Value

106

10 × 10 × 10 × 10 × 10 × 10

1,000,000

105

10 × 10 × 10 × 10 × 10

100,000

104

10 × 10 × 10 × 10

10,000

103

10 × 10 × 10

1,000

102

10 × 10

100

101

10

10

100

1

Negative Exponents

10-1

1⁄10

1⁄10 = 0.1

10-2

1⁄(10 × 10)

1⁄100 = 0.01

10-3

1⁄(10 × 10 × 10)

1⁄1,000 = 0.001

10-4

1⁄(10 × 10 × 10 × 10)

1⁄10,000 = 0.0001

10-5

1⁄(10 × 10 × 10 × 10 × 10)

1⁄100,000 = 0.00001

10-6

1⁄(10 × 10 × 10 × 10 × 10 × 10)

1⁄1,000,000 = 0.000001

Figure 1-9. Powers of ten.

When using a calculator to raise a negative number to a power, always place parentheses around the negative number (before raising it to a power) so that the entire number gets raised to the power. Law of Exponents When multiplying numbers with powers, the powers can be added as long as the bases are the same. Example: 32 × 34 = (3 × 3) × (3 × 3 × 3 × 3) = 3 × 3 × 3 × 3 × 3 × 3 = 36 or  32 × 34 = 3(2+4) = 36

When dividing numbers with powers, the powers can be subtracted as long as the bases are the same. Example: 104 ÷ 102 =

10 × 10 × 10 × 10 10 × 10 × 10 × 10 = = 10 × 10 = 102 10 × 10 10 × 10 or 104 ÷ 102 = 10(4 − 2) = 102

Powers of Ten Because we use the decimal system of numbers, powers of ten are frequently seen in everyday applications. For example, scientific notation uses powers of ten. Also, many aircraft drawings are scaled to powers of ten. Figure 1-9 gives more information on the powers of ten and their values. Roots A root is a number that when multiplied by itself a specified number of times will produce a given number.

The two most common roots are the square root and the cube root. For more examples of roots, see the chart in Figure 1-10, Functions of Numbers (on page 1-14). Square Roots The square root of 25, written as √25, equals 5. That is, when the number 5 is squared (multiplied by itself ), it produces the number 25. The symbol √ is called a radical sign. Finding the square root of a number is the most common application of roots. The collection of numbers whose square roots are whole numbers are called perfect squares. The first ten perfect squares are: 1, 4, 9, 16, 25, 36, 49, 64, 81, and 100. The square root of each of these numbers is 1, 2, 3, 4, 5, 6, 7, 8, 9, and 10, respectively. For example, √36 = 6 and √81 = 9 To find the square root of a number that is not a perfect square, use either a calculator or the estimation method. A longhand method does exist for finding square roots, but with the advent of calculators and because of its lengthy explanation, it is no longer included in this handbook. The estimation method uses the knowledge of perfect squares to approximate the square root of a number. Example: Find the square root of 31. Since 31 falls between the two perfect roots 25 and 36, we know that √31 must be between √25 and √36. Therefore,√31 must be greater than 5 and less than 6 because √25 = 5 and √36 = 6. If you estimate the square root of 31 at 5.5, you are close to the correct answer. The square root of 31 is actually 5.568. Cube Roots 3 The cube root of 125, written as √125, equals 5. That is, when the number 5 is cubed (5 multiplied by itself then multiplying the product (25) by 5 again), it produces the number 125. It is common to confuse the “cube” of a number with the “cube root” of a number. For clarification, the cube of 27 = 273 = 27 × 27 × 27 3 = 19,683. However, the cube root of 27 = √27 = 3. Fractional Powers Another way to write a root is to use a fraction as the power (or exponent) instead of the radical sign. The square root of a number is written with a 1⁄2 as the exponent instead of a radical sign. The cube root of a number is written with an exponent of 1⁄3 and the fourth root with an exponent of 1⁄4 and so on. 1

3

1

4

1

Example: √31 = 31 ⁄2     √125 = 125 ⁄3   √16 = 16 ⁄4

Functions of Numbers Chart The Functions of Numbers chart [Figure 1-10] is included in this chapter for convenience in making computations. Each column in the chart is listed below, with new concepts explained. • Number, (N) • N squared, (N2) • N cubed, (N3) • Square root of N, (√N) 3

• Cube root of N, (√N ) • Circumference of a circle with diameter = N. Circumference is the linear measurement of the distance around a circle. The circumference is calculated by multiplying the diameter of the circle by 3.1416 (3.1416 is the number referred to as pi, which has the symbol π). If the diameter of a circle is 10 inches, then the circumference would be 31.416 inches because 10 × 3.1416 = 31.4160. • Area of a circle with diameter = N. Area of a circle is the number of square units of measurement contained in the circle with a diameter of N. The area of a circle equals π multiplied by the radius squared. This is calculated by the formula: A = π × r2. Remember that the radius is equal to one-half of the diameter. Example: A cockpit instrument gauge has a round face that is 3 inches in diameter. What is the area of the face of the gauge? From Figure 1-10 for N = 3, the answer is 7.0686 square inches. This is calculated by: If the diameter of the gauge is 3 inches, then the radius = D⁄2 = 3⁄2 = 1.5 inches. Area = π × r2 = 3.1416 × 1.52 = 3.1416 × 2.25 = 7.0686 square inches.

Scientific Notation Scientific notation is used as a type of shorthand to express very large or very small numbers. It is a way to write numbers so that they do not take up as much space on the page. The format of a number written in scientific notation has two parts. The first part is a number greater than or equal to 1 and less than 10 (for example, 2.35). The second part is a power of 10 (for example, 106). The number 2,350,000 is expressed in scientific notation as 2.35 × 106. It is important that the decimal point is always placed to the right of the first digit. Notice that very large numbers always have

1-13

Number

Square

Cube

Square Root

Cube Root

Circumference

Area

Number (N)

N Squared (N2)

N Cubed (N3)

Square Root of N (√¯¯¯ N)

Cube Root of N ( √3¯¯¯ N)

Circumference of a circle with diameter = N

Area of a circle with diameter = N

1

1

1

1.000

1.000

3.142

0.785

2

4

8

1.414

1.260

6.283

3.142

3

9

27

1.732

1.442

9.425

7.069

4

16

64

2.000

1.587

12.566

12.566

5

25

125

2.236

1.710

15.708

19.635

6

36

216

2.449

1.817

18.850

28.274

7

49

343

2.646

1.913

21.991

38.484

8

64

512

2.828

2.000

25.133

50.265

9

81

729

3.000

2.080

28.274

63.617

10

100

1,000

3.162

2.154

31.416

78.540

11

121

1,331

3.317

2.224

34.558

95.033

12

144

1,728

3.464

2.289

37.699

113.01

13

169

2,197

3.606

2.351

40.841

132.73

14

196

2,744

3.742

2.410

43.982

153.94

15

225

3,375

3.873

2.466

47.124

176.71

16

256

4,096

4.000

2.520

50.265

201.06

17

289

4,913

4.123

2.571

53.407

226.98

18

324

5,832

4.243

2.621

56.549

254.47

19

361

6,859

4.359

2.668

59.690

283.53

20

400

8,000

4.472

2.714

62.832

314.16

21

441

9,261

4.583

2.759

65.973

346.36

22

484

10,648

4.690

2.802

69.115

380.13

23

529

12,167

4.796

2.844

72.257

415.48

24

576

13,824

4.899

2.885

75.398

452.39

25

625

15,625

5.000

2.924

78.540

490.87

26

676

17,576

5.099

2.963

81.681

530.93

27

729

19,683

5.196

3.000

84.823

572.55

28

784

21,952

5.292

3.037

87.965

615.75

29

841

24,389

5.385

3.072

91.106

660.52

30

900

27,000

5.477

3.107

94.248

706.86

31

961

29,791

5.568

3.141

97.389

754.77

32

1,024

32,768

5.657

3.175

100.531

804.25

33

1,089

35,937

5.745

3.208

103.672

855.30

34

1,156

39,304

5.831

3.240

106.814

907.92

35

1,225

42,875

5.916

3.271

109.956

962.11

36

1,296

46,656

6.000

3.302

113.097

1017.88

37

1,369

50,653

6.083

3.332

116.239

1075.21

38

1,444

54,872

6.164

3.362

119.380

1134.11

39

1,521

59,319

6.245

3.391

122.522

1194.59

40

1,600

64,000

6.325

3.420

125.664

1256.64

41

1,681

68,921

6.403

3.448

128.805

1320.25

42

1,764

74,088

6.481

3.476

131.947

1385.44

43

1,849

79,507

6.557

3.503

135.088

1452.20

44

1,936

85,184

6.633

3.530

138.230

1520.53

45

2,025

91,125

6.708

3.557

141.372

1590.43

46

2,116

97,336

6.782

3.583

144.513

1661.90

47

2,209

103,823

6.856

3.609

147.655

1734.94

48

2,304

110,592

6.928

3.634

150.796

1809.56

49

2,401

117,649

7.000

3.659

153.938

1885.74

50

2,500

125,000

7.071

3.684

157.080

1963.49

Figure 1-10. Functions of numbers.

1-14

Number

Square

Cube

Square Root

Cube Root

Circumference

Area

Number (N)

N Squared (N2)

N Cubed (N3)

Square Root of N (√¯¯¯ N)

Cube Root of N ( √3¯¯¯ N)

Circumference of a circle with diameter = N

Area of a circle with diameter = N

51

2,601

132,651

7.141

3.708

160.221

2042.82

52

2,704

140,608

7.211

3.733

163.363

2123.71

53

2,809

148,877

7.280

3.756

166.504

2206.18

54

2,916

157,464

7.348

3.780

169.646

2290.22

55

3,025

166,375

7.416

3.803

172.787

2375.83

56

3,136

175,616

7.483

3.826

175.929

2463.01

57

3,249

185,193

7.550

3.849

179.071

2551.76

58

3,364

195,112

7.616

3.871

182.212

2642.08

59

3,481

205,379

7.681

3.893

185.354

2733.97

60

3,600

216,000

7.746

3.915

188.495

2827.43

61

3,721

226,981

7.810

3.937

191.637

2922.46

62

3,844

238,328

7.874

3.958

194.779

3019.07

63

3,969

250,047

7.937

3.979

197.920

3117.24

64

4,096

262,144

8.000

4.000

201.062

3216.99

65

4,225

274,625

8.062

4.021

204.203

3318.30

66

4,356

287,496

8.124

4.041

207.345

3421.19

67

4,489

300,763

8.185

4.062

210.487

3525.65

68

4,624

314,432

8.246

4.082

213.628

3631.68

69

4,761

328,509

8.307

4.102

216.770

3739.28

70

4,900

343,000

8.367

4.121

219.911

3848.45

71

5,041

357,911

8.426

4.141

223.053

3959.19

72

5,184

373,248

8.485

4.160

226.194

4071.50

73

5,329

389,017

8.544

4.179

229.336

4185.38

74

5,476

405,224

8.602

4.198

232.478

4300.84

75

5,625

421,875

8.660

4.217

235.619

4417.86

76

5,776

438,976

8.718

4.236

238.761

4536.46

77

5,929

456,533

8.775

4.254

241.902

4656.62

78

6,084

474,552

8.832

4.273

245.044

4778.36

79

6,241

493,039

8.888

4.291

248.186

4901.67

80

6,400

512,000

8.944

4.309

251.327

5026.54

81

6,561

531,441

9.000

4.327

254.469

5152.99

82

6,724

551,368

9.055

4.344

257.610

5281.01

83

6,889

571,787

9.110

4.362

260.752

5410.60

84

7,056

592,704

9.165

4.380

263.894

5541.76

85

7,225

614,125

9.220

4.397

267.035

5674.50

86

7,396

636,056

9.274

4.414

270.177

5808.80

87

7,569

658,503

9.327

4.431

273.318

5944.67

88

7,744

681,472

9.381

4.448

276.460

6082.12

89

7,921

704,969

9.434

4.465

279.602

6221.13

90

8,100

729,000

9.487

4.481

282.743

6361.72

91

8,281

753,571

9.539

4.498

285.885

6503.88

92

8,464

778,688

9.592

4.514

289.026

6647.60

93

8,649

804,357

9.644

4.531

292.168

6792.90

94

8,836

830,584

9.695

4.547

295.309

6939.77

95

9,025

857,375

9.747

4.563

298.451

7088.21

96

9,216

884,736

9.798

4.579

301.593

7238.22

97

9,409

912,673

9.849

4.595

304.734

7389.81

98

9,604

941,192

9.900

4.610

307.876

7542.96

99

9,801

970,299

9.950

4.626

311.017

7697.68

100

10,000

1,000,000

10.000

4.642

314.159

7853.98

Figure 1-10. Functions of numbers. (continued)

1-15

a positive power of 10 and very small numbers always have a negative power of 10. Example: The velocity of the speed of light is over 186,000,000 mph. This can be expressed as 1.86 × 108 mph in scientific notation. The mass of an electron is approximately 0.000,000,000,000,000,000,000,000 ,000,911 grams. This can be expressed in scientific notation as 9.11 × 10-28 grams. Converting Numbers from Standard Notation to Scientific Notation Example: Convert 1,244,000,000,000 to scientific notation as follows. First, note that the decimal point is to the right of the last zero. (Even though it is not usually written, it is assumed to be there.)

Conversion

Large numbers with positive powers of 10

Small numbers with negative powers of 10

From standard notation to scientific notation

Move decimal place to the left

Move decimal place to the right

From scientific notation to standard notation

Move decimal place to the right

Move decimal place to the left

Figure 1-11. Converting between scientific and standard notation.

1,244,000,000,000 = 1,244,000,000,000

When converting, remember that large numbers always have positive powers of ten and small numbers always have negative powers of ten. Refer to Figure 1-11 to determine which direction to move the decimal point.

To change to the format of scientific notation, the decimal point must be moved to the position between the first and second digits, which in this case is between the 1 and the 2. Since the decimal point must be moved 12 places to the left to get there, the power of 10 will be 12. Remember that large numbers always have a positive exponent. Therefore, 1,244,000,000,000 = 1.244 × 1012 when written in scientific notation.

Addition, Subtraction, Multiplication, and Division of Scientific Numbers To add, subtract, multiply, or divide numbers in scientific notation, change the scientific notation number back to standard notation. Then add, subtract, multiply or divide the standard notation numbers. After the computation, change the final standard notation number back to scientific notation.

Example: Convert 0.000000457 from standard notation to scientific notation. To change to the format of scientific notation, the decimal point must be moved to the position between the first and second numbers, which in this case is between the 4 and the 5. Since the decimal point must be moved 7 places to the right to get there, the power of 10 will be −7. Remember that small numbers (those less than one) will have a negative exponent. Therefore, 0.000000457 = 4.57 × 10-7 when written in scientific notation. Converting Numbers from Scientific Notation to Standard Notation Example: Convert 3.68 × 107 from scientific notation to standard notation, as follows. To convert from scientific notation to standard notation, move the decimal place 7 places to the right. 3.68 × 107 = 36800000 = 36,800,000. Another way to think about the conversion is 3.68 × 107 = 3.68 × 10,000,000 = 36,800,000. Example: Convert 7.1543 × 10-10 from scientific notation to standard notation. Move the decimal place 10 places to the left: 7.1543 × 10-10 =.00000000071543. Another way to think about the conversion is 7.1543 × 10-10 = 7.1543 × .0000000001 = .00000000071543

1-16

Algebra Algebra is the branch of mathematics that uses letters or symbols to represent variables in formulas and equations. For example, in the equation D = V × T, where Distance = Velocity × Time, the variables are: D, V, and T. Equations Algebraic equations are frequently used in aviation to show the relationship between two or more variables. Equations normally have an equals sign (=) in the expression. Example: The formula A = π × r2 shows the relationship between the area of a circle (A) and the length of the radius (r) of the circle. The area of a circle is equal to π (3.1416) times the radius squared. Therefore, the larger the radius, the larger the area of the circle. Algebraic Rules When solving for a variable in an equation, you can add, subtract, multiply or divide the terms in the equation, you do the same to both sides of the equals sign.

Examples: Solve the following equations for the value N. 3N = 21 To solve for N, divide both sides by 3. 3N ÷ 3 = 21 ÷ 3 N=7 N + 17 = 59 To solve for N, subtract 17 from both sides. N + 17 – 17 = 59 – 17 N = 42 N – 22 = 100 To solve for N, add 22 to both sides. N – 22 + 22 = 100 + 22 N = 122 N 5 = 50 To solve for N, multiply both sides by 5. N 5 × 5 = 50 × 5 N = 250 Solving for a Variable Another application of algebra is to solve an equation for a given variable. Example: Using the formula given in Figure 1-12, find the total capacitance (CT) of the series circuit containing three capacitors with C1 = .1 microfarad C2 = .015 microfarad C3 = .05 microfarad First, substitute the given values into the formula:

Figure 1-12. Total capacitance in a series circuit.

1 1 1 CT = = = 1 1 1 1 1 1 10 + 66.66 + 20 + + + + C1 C2 C3 0.1 0.015 0.05

Therefore, CT = 1⁄96.66 = .01034 microfarad. The microfarad (10-6 farad) is a unit of measurement of capacitance. This will be discussed in greater length beginning on page 10-51 in chapter 10, Electricity.

Use of Parentheses In algebraic equations, parentheses are used to group numbers or symbols together. The use of parentheses helps us to identify the order in which we should apply mathematical operations. The operations inside the parentheses are always performed first in algebraic equations. Example: Solve the algebraic equation N = (4 + 3) 2. First, perform the operation inside the parentheses. That is, 4 + 3 = 7. Then complete the exponent calculation N = (7) 2 = 7 × 7 = 49. When using more complex equations, which may combine several terms and use multiple operations, grouping the terms together helps organize the equation. Parentheses, ( ), are most commonly used in grouping, but you may also see brackets, [ ]. When a term or expression is inside one of these grouping symbols, it means that any operation indicated to be done on the group is done to the entire term or expression. Example: Solve the equation N = 2 × [(9 ÷ 3) + (4 + 3)2]. Start with the operations inside the parentheses ( ), then perform the operations inside the brackets [ ]. N = 2 × [(9 ÷ 3) + (4 + 3)2] N = 2 × [3 + (7)2]  First, complete the operations inside the parentheses ( ). N = 2 × [3 + 49] N = 2 × [52]  Second, complete the operations inside the brackets [ ]. N = 104 Order of Operation In algebra, rules have been set for the order in which operations are evaluated. These same universally accepted rules are also used when programming algebraic equations in calculators. When solving the following equation, the order of operation is given below: N = (62 – 54)2 + 62 – 4 + 3 × [8 + (10 ÷ 2)] + √25 + (42 × 2) ÷ 4 + 3⁄4

1. Parentheses. First, do everything in parentheses, ( ). Starting from the innermost parentheses. If the expression has a set of brackets, [ ], treat these exactly like parentheses. If you are working with a fraction, treat the top as if it were in parentheses and the denominator as if it were in parentheses, even if there are none shown. From the equation

1-17

above, completing the calculation in parentheses gives the following:

N = (8)2 + 62 – 4 + 3 × [8 + (5)] + √25 + (84) ÷ 4 + 3⁄4,



then



N = (8)2 + 62 – 4 + 3 × [13] + √25 + 84 ÷ 4 + 3⁄4

2. Exponents. Next, clear any exponents. Treat any roots (square roots, cube roots, and so forth) as exponents. Completing the exponents and roots in the equation gives the following:

N = 64 + 36 – 4 + 3 × 13 + 5 + 84 ÷ 4 + 3⁄4

3. Multiplication and Division. Evaluate all of the multiplications and divisions from left to right. Multiply and divide from left to right in one step. A common error is to use two steps for this (that is, to clear all of the multiplication signs and then clear all of the division signs), but this is not the correct method. Treat fractions as division. Completing the multiplication and division in the equation gives the following:

N = 64 + 36 – 4 + 39 + 5 + 21 + 3⁄4

4. Addition and Subtraction. Evaluate the additions and subtractions from left to right. Like above, addition and subtraction are computed left to right in one step. Completing the addition and subtraction in the equation gives the following:

X = 161 3⁄4

Computing Area of Two-dimensional Solids Area is a measurement of the amount of surface of an object. Area is usually expressed in such units as square inches or square centimeters for small surfaces or in square feet or square meters for larger surfaces. Rectangle A rectangle is a four-sided figure with opposite sides of equal length and parallel. [Figure 1-13] All of the angles are right angles. A right angle is a 90° angle. The rectangle is a very familiar shape in mechanics. The formula for the area of a rectangle is: Area = Length × Width = L × W Example: An aircraft floor panel is in the form of a rectangle having a length of 24 inches and a width of 12 inches. What is the area of the panel expressed in square inches? First, determine the known values and substitute them in the formula. A = L × W = 24 inches × 12 inches = 288 square inches Square A square is a four-sided figure with all sides of equal length and parallel. [Figure 1-14] All angles are right angles. The formula for the area of a square is: Area = Length × Width = L × W

Order of Operation for Algebraic Equations 1. Parentheses 2. Exponents 3. Multiplication and Division 4. Addition and Subtraction Use the acronym PEMDAS to remember the order of operation in algebra. PEMDAS is an acronym for parentheses, exponents, multiplication, division, addition, and subtraction. To remember it, many use the sentence, “Please Excuse My Dear Aunt Sally.” Always remember, however, to multiply/divide or add/subtract in one sweep from left to right, not separately.

Figure 1-13. Rectangle.

Figure 1-14. Square.

1-18

Since the length and the width of a square are the same value, the formula for the area of a square can also be written as: Area = Side × Side = S2 Example: What is the area of a square access plate whose side measures 25 inches? First, determine the known value and substitute it in the formula.

Figure 1-15. Types of triangles.

A = L × W = 25 inches × 25 inches = 625 square inches Triangle A triangle is a three-sided figure. The sum of the three angles in a triangle is always equal to 180°. Triangles are often classified by their sides. An equilateral triangle has 3 sides of equal length. An isosceles triangle has 2 sides of equal length. A scalene triangle has three sides of differing length. Triangles can also be classified by their angles: An acute triangle has all three angles less than 90°. A right triangle has one right angle (a 90° angle). An obtuse triangle has one angle greater than 90°. Each of these types of triangles is shown in Figure 1-15.

Figure 1-16. Obtuse triangle.

The formula for the area of a triangle is Area = 1⁄2 × (Base × Height) = 1⁄2 × (B × H) Example: Find the area of the obtuse triangle shown in Figure 1-16. First, substitute the known values in the area formula.

Figure 1-17. Parallelogram.

A = 1⁄2 × (B × H) = 1⁄2 × (2'6" × 3'2") Next, convert all dimensions to inches: 2'6" = (2 × 12") + 6" = (24 + 6) = 30 inches 3'2" = (3 × 12") + 2" = (36 + 2) = 38 inches Now, solve the formula for the unknown value: A = 1⁄2 × (30 inches × 38 inches) = 570 square inches

Figure 1-18. Trapezoid.

Parallelogram A parallelogram is a four-sided figure with two pairs of parallel sides. [Figure 1-17] Parallelograms do not necessarily have four right angles. The formula for the area of a parallelogram is:

Trapezoid A trapezoid is a four-sided figure with one pair of parallel sides. [Figure 1-18] The formula for the area of a trapezoid is:

Area = Length × Height = L × H

Area = 1⁄2 (Base1 + Base2) × Height Example: What is the area of a trapezoid in Figure 1-19 whose bases are 14 inches and 10 inches, and whose height (or altitude) is 6 inches? First, substitute the known values in the formula.

1-19

Figure 1-19. Trapezoid, with dimensions.

A = 1⁄2 (b1 + b2) × H = 1⁄2 (14 inches + 10 inches) × 6 inches A = 1⁄2 (24 inches) × 6 inches = 12 inches × 6 inches = 72 square inches. Circle A circle is a closed, curved, plane figure. [Figure 1‑20] Every point on the circle is an equal distance from the center of the circle. The diameter is the distance across the circle (through the center). The radius is the distance from the center to the edge of the circle. The diameter is always twice the length of the radius. The circumference, or distance around, a circle is equal to the diameter times π. Circumference = C = d π The formula for the area of a circle is: Area = π × radius2 = π × r2 Example: The bore, or “inside diameter,” of a certain aircraft engine cylinder is 5 inches. Find the area of the cross section of the cylinder.

Figure 1-21. Ellipse.

First, substitute the known values in the formula: A = π × r2. The diameter is 5 inches, so the radius is 2.5 inches. (diameter = radius × 2) A = 3.1416 × (2.5 inches)2 = 3.1416 × 6.25 square inches = 19.635 square inches Ellipse An ellipse is a closed, curved, plane figure and is commonly called an oval. [Figure 1-21] In a radial engine, the articulating rods connect to the hub by pins, which travel in the pattern of an ellipse (i.e., an elliptical or obital path). Wing Area To describe the shape of a wing [Figure 1-23], several terms are required. To calculate wing area, it will be necessary to know the meaning of the terms “span” and “chord.” The wingspan, S, is the length of the wing from wingtip to wingtip. The chord is the average width

Figure 1-20. Circle.

1-20

Figure 1-22. Wing planform.

Object

Area

Formula

Figure

Rectangle

Length × Width

A=L×W

1-13

Square

Length × Width or Side × Side

A = L × W or A = S2

1-14

Triangle

½ × (Length × Height) or ½ × (Base × Height) or (Base × Height) ÷ 2

A = ½ (L × H) or A = ½ (B × H) or A = (B × H) ÷ 2

1-15

Parallelogram

Length × Height

A=L×H

1-17

Trapezoid

½ (base 1 + base 2 ) × Height

A = ½ (b1 + b2) × H

1-18

Circle

π × radius 2

A = π × r2

1-20

Ellipse

π × semi-axis A × semi-axis B

A=π×A×B

1-21

Figure 1-23. Formulas to compute area.

of the wing from leading edge to trailing edge. If the wing is a tapered wing, the average width, known as the mean chord (C), must be known to find the area. The formula for calculating wing area is: Area of a wing = Span × Mean Chord Example: Find the area of a tapered wing whose span is 50 feet and whose mean chord is 6'8". First, substitute the known values in the formula. A = S × C = 50 feet × 6 feet 8 inches (Note: 8 inches = 8⁄12 feet = .67 feet) = 50 feet × 6.67 feet = 333.5 square feet Units of Area A square foot measures 1 foot by 1 foot. It also measures 12 inches by 12 inches. Therefore, one square foot also equals 144 square inches (that is, 12 × 12 = 144). To convert square feet to square inches, multiply by 144. To convert square inches to square feet, divide by 144. A square yard measures 1 yard by 1 yard. It also measures 3 feet by 3 feet. Therefore, one square yard also equals 9 square feet (that is, 3 × 3 = 9). To convert square yards to square feet, multiply by 9. To convert square feet to square yards, divide by 9. Refer to Figure 1-37, Applied Mathematics Formula Sheet, at the end of the chapter for a comparison of different units of area.

Computing Volume of Three-Dimensional Solids Three-dimensional solids have length, width, and height. There are many three-dimensional solids, but the most common are rectangular solids, cubes, cylinders, spheres, and cones. Volume is the amount of space within a solid. Volume is expressed in cubic units. Cubic inches or cubic centimeters are used for small spaces and cubic feet or cubic meters for larger spaces. Rectangular Solid A rectangular solid is a three-dimensional solid with six rectangle-shaped sides. [Figure 1-24] The volume is the number of cubic units within the rectangular solid. The formula for the volume of a rectangular solid is: Volume = Length × Width × Height = L × W × H In Figure 1-24, the rectangular solid is 3 feet by 2 feet by 2 feet. The volume of the solid in Figure 1-24 is = 3 ft × 2 ft × 2 ft = 12 cubic feet.

Figure 1-23 summarizes the formulas for computing the area of two-dimensional solids. Figure 1-24. Rectangular solid.

1-21

Example: A rectangular baggage compartment measures 5 feet 6 inches in length, 3 feet 4 inches in width, and 2 feet 3 inches in height. How many cubic feet of baggage will it hold? First, substitute the known values into the formula. V = L × W × H = 5'6" × 3'4" × 2'3" = 5.5 ft × 3.33 ft × 2.25 ft = 41.25 cubic feet Cube A cube is a solid with six square sides. [Figure 1-25] A cube is just a special type of rectangular solid. It has the same formula for volume as does the rectangular solid which is Volume = Length × Width × Height = L × W × H. Because all of the sides of a cube are equal, the volume formula for a cube can also be written as: Volume = Side × Side × Side = S3 Example: A large, cube-shaped carton contains a shipment of smaller boxes inside of it. Each of the smaller boxes is 1 ft × 1 ft × 1 ft. The measurement of the large carton is 3 ft × 3 ft × 3 ft. How many of the smaller boxes are in the large carton? First, substitute the known values into the formula. V = L × W × H = 3 ft × 3 ft × 3 ft = 27 cubic feet of volume in the large carton Since each of the smaller boxes has a volume of 1 cubic foot, the large carton will hold 27 boxes.

Figure 1-26. Cylinder.

Cylinder A solid having the shape of a can, or a length of pipe, or a barrel is called a cylinder. [Figure 1-26] The ends of a cylinder are identical circles. The formula for the volume of a cylinder is: Volume = π × radius2 × height of the cylinder = π r2 × H One of the most important applications of the volume of a cylinder is finding the piston displacement of a cylinder in a reciprocating engine. Piston displacement is the total volume (in cubic inches, cubic centimeters, or liters) swept by all of the pistons of a reciprocating engine as they move in one revolution of the crankshaft. The formula for piston displacement is given as: Piston Displacement = π × (bore divided by 2)2 × stroke × (# cylinders) The bore of an engine is the inside diameter of the cylinder. The stroke of the engine is the length the piston travels inside the cylinder. [Figure 1-27] Example: Find the piston displacement of one cylinder in a multi-cylinder aircraft engine. The engine has a cylinder bore of 5.5 inches and a stroke of 5.4 inches. First, substitute the known values in the formula. V = π × r2 × h = (3.1416) × (5.5 ÷ 2)2 × (5.4) V = 23.758 × 5.4 = 128.29 cubic inches

Figure 1-25. Cube.

The piston displacement of one cylinder is 128.29 cubic inches. For an eight cylinder engine, then the total engine displacement would be: Total Displacement for 8 cylinders = 8 × 128.29 = 1026.32 cubic inches of displacement

1-22

Figure 1-29. Cone.

Cone A solid with a circle as a base and with sides that gradually taper to a point is called a cone. [Figure 1-29] The formula for the volume of a cone is given as: Figure 1-27. Cylinder displacement.

Sphere A solid having the shape of a ball is called a sphere. [Figure 1-28] A sphere has a constant diameter. The radius (r) of a sphere is one-half of the diameter (D). The formula for the volume of a sphere is given as: V = 4⁄3 × π × radius3 = 4⁄3 × π × r3  or  V = 1⁄6 × πD3 Example: A pressure tank inside the fuselage of a cargo aircraft is in the shape of a sphere with a diameter of 34 inches. What is the volume of the pressure tank? V = 4⁄3 × π × radius3 = 4⁄3 × (3.1416) × (34⁄2)3 = 1.33 × 3.1416 × 173 = 1.33 × 3.1416 × 4913 V = 20,528.125 cubic inches

V = 1⁄3 × π × radius2 × height = 1⁄3 × π × r2 × H Units of Volume Since all volumes are not measured in the same units, it is necessary to know all the common units of volume and how they are related to each other. For example, the mechanic may know the volume of a tank in cubic feet or cubic inches, but when the tank is full of gasoline, he or she will be interested in how many gallons it contains. Refer to Figure 1-37, Applied Mathematics Formula Sheet, at the end of the chapter for a comparison of different units of volume.

Computing Surface Area of Three-dimensional Solids The surface area of a three-dimensional solid is the sum of the areas of the faces of the solid. Surface area is a different concept from that of volume. For example, surface area is the amount of sheet metal needed to build a rectangular fuel tank while volume is the amount of fuel that the tank can contain. Rectangular Solid The formula for the surface area of a rectangular solid [Figure 1-24] is given as: Surface Area = 2 × [(Width × Length) + (Width × Height) + (Length × Height)] = 2 × [(W × L) + (W × H) + (L × H)]

Figure 1-28. Sphere.

1-23

Cube The formula for the surface area of a cube [Figure 1‑25] is given as: Surface Area = 6 × (Side × Side) = 6 × S2 Example: What is the surface area of a cube with a side measure of 8 inches? Surface Area = 6 × (Side × Side) = 6 × S2 = 6 × 82 = 6 × 64 = 384 square inches Cylinder The formula for the surface area of a cylinder [Figure 1-26] is given as: Surface Area = 2 × π × radius 2 + π × diameter × height = 2 × π × r2 + π × D × H Sphere The formula for the surface area of a sphere [Figure 1-28] is given as: Surface Area = 4 × π × radius2 = 4 × π × r2 Cone The formula for the surface area of a right circular cone [Figure 1-29] is given as: Surface Area = π × radius × [radius + (radius2 + height2)1⁄2] = π × r × [r + (r2 + H2)1⁄2] Figure 1-30 summarizes the formulas for computing the volume and surface area of three-dimensional solids. Solid

Volume

Surface Area

Figure

Rectangular Solid

L×W×H

2 × [(W × L) + (W × H) + (L × H)]

1-23

Cube

S3

6 × S2

1-24

Cylinder

π × r2 × H

2 × π × r2 + π × D × H

1-25

Sphere

4⁄ × π × r3 3

4 × π × r2

1-27

Cone

1⁄ × π × r2 × H 3

π × r × [r + (r2 + H2)1⁄2]

1-28

Trigonometric Functions Trigonometry is the study of the relationship between the angles and sides of a triangle. The word trigonometry comes from the Greek trigonon, which means three angles, and metro, which means measure. Right Triangle, Sides and Angles In Figure 1-31, notice that each angle is labeled with a capital letter. Across from each angle is a corresponding side, each labeled with a lower case letter. This triangle is a right triangle because angle C is a 90° angle. Side a is opposite from angle A, and is sometimes referred to as the “opposite side.” Side b is next to, or adjacent to, angle A and is therefore referred to as the “adjacent side.” Side c is always across from the right angle and is referred to as the “hypotenuse.” Sine, Cosine, and Tangent The three primary trigonometric functions and their abbreviations are: sine (sin), cosine (cos), and tangent (tan). These three functions can be found on most scientific calculators. The three trigonometric functions are actually ratios comparing two of the sides of the triangle as follows: Sine (sin) of angle A =

opposite side (side a) hypotenuse (side c)

Cosine (cos) of angle A =

adjacent side (side b) hypotenuse (side c)

opposite side (side a) Tangent (tan) of angle A = adjacent side (side b) Example: Find the sine of a 30° angle. Calculator Method: Using a calculator, select the “sin” feature, enter the number 30, and press “enter.” The calculator should display the answer as 0.5. This means that when angle

Figure 1-30. Formulas to compute volume and surface area.

Figure 1-31. Right triangle.

1-24

A equals 30°, then the ratio of the opposite side (a) to the hypotenuse (c) equals 0.5 to 1, so the hypotenuse is twice as long as the opposite side for a 30° angle. Therefore, sin 30° = 0.5. Trigonometric Table Method: When using a trigonometry table, find 30° in the first column. Next, find the value for sin 30° under the second column marked “sine” or “sin.” The value for sin 30° should be 0.5. Pythagorean Theorem The Pythagorean Theorem is named after the ancient Greek mathematician, Pythagoras (~500 B.C.). This theorem is used to find the third side of any right triangle when two sides are known. The Pythagorean Theorem states that a2 + b2 = c2. [Figure 1-32] Where c = the hypotenuse of a right triangle, a is one side of the triangle and b is the other side of the triangle. Example: What is the length of the longest side of a right triangle, given the other sides are 7 inches and 9 inches? The longest side of a right triangle is always side c, the hypotenuse. Use the Pythagorean Theorem to solve for the length of side c as follows: a2 + b2 = c2 72 + 92 = c2 49 + 81 = c2 130 = c2 If c2 = 130 then c = √130 = 11.4 inches Therefore, side c = 11.4 inches. Example: The cargo door opening in a military airplane is a rectangle that is 5 1⁄ 2 feet tall by 7 feet wide. A section of square steel plate that is 8 feet wide by 8 feet tall by 1 inch thick must fit inside the airplane. Can the square section of steel plate fit through the cargo

Figure 1-32. Pythagorean Theorem.

door? It is obvious that the square steel plate will not fit horizontally through the cargo door. The steel plate is 8 feet wide and the cargo door is only 7 feet wide. However, if the steel plate is tilted diagonally, will it fit through the cargo door opening? The diagonal distance across the cargo door opening can be calculated using the Pythagorean Theorem where “a” is the cargo door height, “b” is the cargo door width, and “c” is the diagonal distance across the cargo door opening. a2 + b2 = c2 (5.5 ft)2 + (7 ft)2 = c2 30.25 + 49 = c2 79.25 = c2 c = 8.9 ft The diagonal distance across the cargo door opening is 8.9 feet, so the 8-foot wide square steel plate will fit diagonally through the cargo door opening and into the airplane.

Measurement Systems Conventional (U.S. or English) System Our conventional (U.S. or English) system of measurement is part of our cultural heritage from the days when the thirteen colonies were under British rule. It started as a collection of Anglo-Saxon, Roman, and Norman-French weights and measures. For example, the inch represents the width of the thumb and the foot is from the length of the human foot. Tradition holds that King Henry I decreed that the yard should be the distance from the tip of his nose to the end of his thumb. Since medieval times, commissions appointed by various English monarchs have reduced the chaos of measurement by setting specific standards for some of the most important units. Some of the conventional units of measure are: inches, feet, yards, miles, ounces, pints, gallons, and pounds. Because the conventional system was not set up systematically, it contains a random collection of conversions. For example, 1 mile = 5,280 feet and 1 foot = 12 inches. Metric System The metric system, also known as the International System of Units (SI), is the dominant language of measurement used today. Its standardization and decimal features make it well-suited for engineering and aviation work.

1-25

The metric system was first envisioned by Gabriel Mouton, Vicar of St. Paul’s Church in Lyons, France. The meter is the unit of length in the metric system, and it is equal to one ten-millionth of the distance from the equator to the North Pole. The liter is the unit of volume and is equal to one cubic decimeter. The gram is the unit of mass and is equal to one cubic centimeter of water. All of the metric units follow a consistent naming scheme, which consists of attaching a prefix to the unit. For example, since kilo stands for 1,000 one kilometer equals 1,000 meters. Centi is the prefix for one hundredth, so one meter equals one hundred centimeters. Milli is the prefix for one thousandths and one gram equals one thousand milligrams. Refer to Figure 1-33 for the names and definitions of metric prefixes. Measurement Systems and Conversions The United States primarily uses the conventional (U.S. or English) system, although it is slowly integrating the metric system (SI). A recommendation to transition to the metric system within ten years was initiated in the 1970s. However, this movement lost momentum, and the United States continues to use both measurement systems. Therefore, information to convert between the conventional (U.S., or English) system and the metric (SI) system has been included in Figure 1-37, Applied Mathematics Formula Sheet, at the end of this chapter. Examples of its use are as follows: To convert inches to millimeters, multiply the number of inches by 25.4. Example: 20 inches = 20 × 25.4 = 508 mm Prefix

Means

tera (1012)

One trillion times

giga (109)

One billion times

mega (106)

One million times

kilo (103)

One thousand times

hecto (102)

One hundred times

deca (101)

Ten times

deci (10-1)

One tenth of

centi (10-2)

One hundredth of

milli (10-3)

One thousandth of

micro (10-6)

One millionth of

nano (10-9)

One billionth of

pico (10-12)

One trillionth of

Figure 1-33. Names and definitions of metric prefixes.

1-26

To convert ounces to grams, multiply the number of ounces by 28.35. Example: 12 ounces = 12 × 28.35 = 340.2 grams

The Binary Number System The binary number system has only two digits: 0 and 1. The prefix in the word “binary” is a Latin root for the word “two” and its use was first published in the late 1700s. The use of the binary number system is based on the fact that switches or valves have two states: open or closed (on/off). Currently, one of the primary uses of the binary number system is in computer applications. Information is stored as a series of 0s and 1s, forming strings of binary numbers. An early electronic computer, ENIAC (Electronic Numerical Integrator And Calculator), was built in 1946 at the University of Pennsylvania and contained 17,000 vacuum tubes, along with 70,000 resistors, 10,000 capacitors, 1,500 relays, 6,000 manual switches and 5 million soldered joints. Computers obviously have changed a great deal since then, but are still based on the same binary number system. The binary number system is also useful when working with digital electronics because the two basic conditions of electricity, on and off, can be represented by the two digits of the binary number system. When the system is on, it is represented by the digit 1, and when it is off, it is represented by the digit zero. Place Values The binary number system is a base-2 system. That is, the place values in the binary number system are based on powers of 2. An 8-bit binary number system is shown in Figure 1-34 on the next page. Converting Binary Numbers to Decimal Numbers To convert a binary number to a decimal number, add up the place values that have a 1 (place values that have a zero do not contribute to the decimal number conversion). Example: Convert the binary number 10110011 to a decimal number. Using the Place Value chart shown in Figure 1-35, add up the place values of the ‘1s’ in the binary number (ignore the place values with a zero in the binary number). The binary number 10110011 = 128 + 0 + 32 + 16 + 0 + 0 + 2 + 1 = 179 in the decimal number system

Place Value 27

26

25

or 128

or 64

or 32

24 or 16

23 or 8

22 or 4

21 or 2

20 or 1

10011001 shown as

1

0

0

1

1

0

0

1

= 153

00101011 shown as

0

0

1

0

1

0

1

1

= 43

Figure 1-34. Binary system.

Place Value

10110011 shown as

27 or 128

26 or 64

25 or 32

24 or 16

23 or 8

22 or 4

21 or 2

20 or 1

1

0

1

1

0

0

1

1

128

+

0

+

32

+

16

+

0

+

0

+

2

+

1

= 179

Figure 1-35. Conversion from binary number to decimal number.

Place Value 27

26

25

or 128

or 64

or 32

24 or 16

23 or 8

22 or 4

21 or 2

20 or 1

35 shown as

0

0

1

0

0

0

0

1

124 shown as

0

1

1

1

1

1

0

0

96 shown as

0

1

1

0

0

0

0

0

255 shown as

1

1

1

1

1

1

1

1

233 shown as

1

1

1

0

1

0

0

1

Figure 1-36. Conversion from decimal number to binary number.

Converting Decimal Numbers to Binary Numbers To convert a decimal number to a binary number, the place values in the binary system are used to create a sum of numbers that equal the value of the decimal number being converted. Start with the largest binary place value and subtract from the decimal number. Continue this process until all of the binary digits are determined. Example: Convert the decimal number 233 to a binary number. Start by subtracting 128 (the largest place value from the 8-bit binary number) from 233. 233 – 128 = 105 A “1” is placed in the first binary digit space: 1XXXXXXX. Continue the process of subtracting the binary number place values: 105 – 64 = 41 A “1” is placed in the second binary digit space: 11XXXXXX.

41 – 32 = 9 A “1” is placed in the third binary digit space: 111XXXXX. Since 9 is less than 16 (the next binary place value), a “0” is placed in the fourth binary digit space: 1110XXXX. 9–8=1 A “1” is placed in the fifth binary digit space: 11101XXX Since 1 is less than 4 (the next binary place value), a 0 is placed in the sixth binary digit space: 111010XX. Since 1 is less than 2 (the next binary place value), a 0 is placed in the seventh binary digit space: 1110100X. 1–1=0 A “1” is placed in the eighth binary digit space: 11101001. The decimal number 233 is equivalent to the binary number 11101001, as shown in Figure 1-36. Additional decimal number to binary number conversions are shown in Figure 1-36. 1-27

Conversion Factors Length

Area

1 inch

2.54 centimeters

25.4 millimeters

1 square inch

6.45 square centimeters

1 foot

12 inches

30.48 centimeters

1 square foot

144 square inches

0.093 square meters 0.836 square meters

1 yard

3 feet

0.9144 meters

1 square yard

9 square feet

1 mile

5,280 feet

1,760 yards

1 acre

43,560 square feet

1 millimeter

0.0394 inches

1 square mile

640 acres

1 kilometer

0.62 miles

1 square centimeter

0.155 square inches

1 square meter

1.195 square yards

1 square kilometer

0.384 square miles

2.59 square kilometers

Volume 1 fluid ounce

29.57 cubic centimeters

1 cup

8 fluid ounce

1 pint

2 cups

16 fluid ounces

0.473 liters

1 quart

2 pints

4 cups

32 fluid ounces

0.9463 liters

1 gallon

4 quarts

8 pints

16 cups

128 ounces

1 gallon

231 cubic inches

1 liter

0.264 gallons

1 cubic foot

1,728 cubic inches

1 cubic foot

7.5 gallons

1 cubic yard

27 cubic feet

1 board foot

1.057 quarts

1 inch by 12 inches by 12 inches

Weight 1 ounce

28.350 grams

1 pound

16 ounces

1 ton

2,000 pounds

1 milligram

0.001 grams

1 kilogram

1,000 grams

1 gram

0.0353 ounces

Temperature

453.592 grams

0.4536 kilograms

degrees Fahrenheit to degrees Celsius

Celsius = 5⁄9 × (degrees Fahrenheit − 32)

degrees Celsius to degrees Fahrenheit

Fahrenheit = 9⁄5 × (degrees Celsius) + 32

2.2 pounds

Formulas for Area of Two-Dimensional Objects Object

Area

Formula

Figure

Rectangle

Length × Width

A=L×W

1-13

Square

Length × Width or Side × Side

A = L × W or A = S2

1-14

Triangle

½ × (Length × Height) or ½ × (Base × Height) or (Base × Height) ÷ 2

A = ½ (L × H) or A = ½ (B × H) or A = (B × H) ÷ 2

1-15

Parallelogram

Length × Height

A=L×H

1-17

Trapezoid

½ (base1 + base2) x Height

A = ½ (b1 + b2) x H

1-18

Circle

π × radius2

A = π × r2

1-20

Figure 1-37. Applied Mathematics Formula Sheet.

1-28

3.785 liters

Order of Operation for Algebraic Equations

Circumference of a Circle

1. Parentheses 2. Exponents 3. Multiplication and Division 4. Addition and Subtraction Use the acronym PEMDAS to remember the order of operation in algebra. PEMDAS is an acronym for parentheses, exponents, multiplication, division, addition, and subtraction. To remember it, many use the sentence, “Please Excuse My Dear Aunt Sally.” Trigonometric Equations Circumference of an Ellipse

Sine (sin) of angle A =

opposite side (side a) hypotenuse (side c)

Cosine (cos) of angle A =

adjacent side (side b) hypotenuse (side c)

opposite side (side a) Tangent (tan) of angle A = adjacent side (side b) Pythagorean Theorem

Formulas for Surface Area and Volume of Three-Dimensional Solids Volume

Surface Area

Figure

Rectangular Solid

Solid

L×W×H

2 × [(W × L) + (W × H) + (L × H)]

1-23

Cube

S3

6 × S2

1-24

2 × π × r2 + π × D × H

1-25

Sphere

4⁄ × π × r3 3

4 × π × r2

1-27

Cone

1⁄ × π × r2 × H 3

π × r × [r + (r2 + H2)1⁄2]

1-28

Cylinder

π ×

r2 × H

Figure 1-37. Applied Mathematics Formula Sheet. (continued)

1-29

Names and Symbols for Metric Prefixes

Powers of Ten

tera (1012)

One trillion times

Powers of Ten

giga (109)

One billion times

Positive Exponents

mega (106)

One million times

106

10 × 10 × 10 × 10 × 10 × 10

1,000,000

kilo (103)

One thousand times

105

10 × 10 × 10 × 10 × 10

100,000

104

10 × 10 × 10 × 10

10,000

103

10 × 10 × 10

1,000

102

10 × 10

100

10

10

Prefix

Means

Expansion

hecto (102)

One hundred times

deca (101)

Ten times

deci (10-1)

One tenth of

101

One hundredth of

100

milli (10-3)

One thousandth of

Negative Exponents

micro (10-6)

One millionth of

10-1

1⁄10

1⁄10 = 0.1

One billionth of

10-2

1⁄(10 × 10)

1⁄100 = 0.01

10-3

1⁄(10 × 10 × 10)

1⁄1,000 = 0.001

10-4

1⁄(10 × 10 × 10 × 10)

1⁄10,000 = 0.0001

10-5

1⁄(10 × 10 × 10 × 10 × 10)

1⁄100,000 = 0.00001

10-6

1⁄(10 × 10 × 10 × 10 × 10 × 10)

1⁄1,000,000 = 0.000001

centi (10-2)

nano (10-9) pico (10-12)

One trillionth of

1

Figure 1-37. Applied Mathematics Formula Sheet. (continued)

1-30

Value

The exchange of ideas is essential to everyone, regardless of his or her vocation or position. Usually, this exchange is carried on by the oral or written word; but under some conditions, the use of these alone is impractical. Industry discovered that it could not depend entirely upon written or spoken words for the exchange of ideas because misunderstanding and misinterpretation arose frequently. A written description of an object can be changed in meaning just by misplacing a comma; the meaning of an oral description can be completely changed by the use of a wrong word. To avoid these possible errors, industry uses drawings to describe objects. For this reason, drawing is the draftsman’s language. Drawing, as we use it, is a method of conveying ideas concerning the construction or assembly of objects. This is done with the help of lines, notes, abbreviations, and symbols. It is very important that the aviation mechanic who is to make or assemble the object understand the meaning of the different lines, notes, abbreviations, and symbols that are used in a drawing. (See especially the “Lines and Their Meanings” section of this chapter.)

Computer Graphics From the early days of aviation, development of aircraft, aircraft engines, and other components relied heavily on aircraft drawings. For most of the 20th century, drawings were created on a drawing “board” with pen or pencil and paper. However, with the introduction and advancement of computers in the later decades of the 20th century, the way drawings are created changed dramatically. Computers were used not only to create drawings, but they were being used to show items in “virtual reality,” from any possible viewing angle. Further development saw computer software programs with the capability of assembling separately created parts to check for proper fit and possible interferences. Additionally, with nearly instantaneous information sharing capability through computer networking and

the Internet, it became much easier for designers to share their work with other designers and manufacturers virtually anytime, anywhere in the world. Using new computer controlled manufacturing techniques, it literally became possible to design a part and have it precisely manufactured without ever having it shown on paper. New terms and acronyms became commonplace. The more common of these terms are: • Computer Graphics — drawing with the use of a computer, • Computer Aided Design Drafting (CADD) — where a computer is used in the design and drafting process, • Computer Aided Design (CAD) — where a computer is used in the design of a product, • Computer Aided Manufacturing (CAM) — where a computer is used in the manufacturing of a product, and • Computer Aided Engineering (CAE) — where a computer is used in the engineering of a product. As computer hardware and software continue to evolve, there continues to be a greater amount of CAE done in less time at lower cost. In addition to product design, some of the other uses of CAE are product analysis, assembly, simulations and maintenance information. [Figure 2-1]

Purpose and Function of Aircraft Drawings Drawings and prints are the link between the engineers who design an aircraft and the workers who build, maintain, and repair it. A print may be a copy of a working drawing for an aircraft part or group of parts, or for a design of a system or group of systems. They are made by placing a tracing of the drawing over a sheet of chemically treated paper and exposing it to a strong light for a short period of time. When the

2-1

Care and Use of Drawings Drawings are both expensive and valuable; consequently, they should be handled carefully. Open drawings slowly and carefully to prevent tearing the paper. When the drawing is open, smooth out the fold lines instead of bending them backward. To protect drawings from damage, never spread them on the floor or lay them on a surface covered with tools or other objects that may make holes in the paper. Hands should be free of oil, grease, or other unclean matter that can soil or smudge the print. Never make notes or marks on a print as they may confuse other persons and lead to incorrect work. Only authorized persons are permitted to make notes or changes on prints, and they must sign and date any changes they make. Figure 2-1. Computer graphics work station.

exposed paper is developed, it turns blue where the light has penetrated the transparent tracing. The inked lines of the tracing, having blocked out the light, show as white lines on a blue background. Other types of sensitized paper have been developed; prints may have a white background with colored lines or a colored background with white lines. Drawings created using computers may be viewed as they appear on the computer monitor, or they may be printed out in “hard copy” by use of an ink jet or laser printer. Larger drawings may be printed by use of a plotter or large format printer. Large printers can print drawings up to 42 inches high with widths up to 600 inches by use of continuous roll paper. [Figure 2-2]

Figure 2-2. Large format printer.

2-2

When finished with a drawing, fold and return it to its proper place. Prints are folded originally in a proper size for filing, and care should be taken so that the original folds are always used.

Types of Drawings Drawings must give such information as size and shape of the object and all of its parts, specifications for material to be used, how the material is to be finished, how the parts are to be assembled, and any other information essential to making and assembling the particular object. Drawings may be divided into three classes: (1) detail, (2) assembly, and (3) installation. [Figure 2-3] Detail Drawing A detail drawing is a description of a single part, describing by lines, notes, and symbols the specifications for size, shape, material, and methods of manufacture to be used in making the part. Detail drawings are usually rather simple; and, when single parts are small, several detail drawings may be shown on the same sheet or print. (See detail drawing at the top of Figure 2-3.) Assembly Drawing An assembly drawing is a description of an object made up of two or more parts. Examine the assembly drawing in the center of Figure 2-3. It describes the object by stating, in a general way, size and shape. Its primary purpose is to show the relationship of the various parts. An assembly drawing is usually more complex than

a detail drawing, and is often accompanied by detail drawings of various parts. Installation Drawing An installation drawing is one which includes all necessary information for a part or an assembly in the final installed position in the aircraft. It shows the dimensions necessary for the location of specific parts with relation to the other parts and reference dimensions that are helpful in later work in the shop. (See installation drawing at the bottom of Figure 2-3.) Sectional View Drawings A section or sectional view is obtained by cutting away part of an object to show the shape and construction at the cutting plane. The part or parts cut away are shown by the use of section (crosshatching) lines. Types of sections are described in the following paragraphs. Full Section A full section view is used when the interior construction or hidden features of an object cannot be shown clearly by exterior views. For example, Figure 2-4, a sectional view of a coaxial cable connector, shows the internal construction of the connector. Half Section In a half section, the cutting plane extends only halfway across the object, leaving the other half of the object as an exterior view. Half sections are used to advantage with symmetrical objects to show both the interior and exterior. Figure 2-5 is a half sectional view of a quick disconnect used in aircraft fluid systems. Revolved Section A revolved section drawn directly on the exterior view shows the shape of the cross section of a part, such as the spoke of a wheel. An example of a revolved section is shown in Figure 2-6.

Plug body Nut

Washer

Gasket Clamp

Figure 2-3. Types of drawings.

Figure 2-4. Sectional view of a cable connector.

2-3

Quick Disconnect Coupling

Figure 2-5. Half section.

Figure 2-6. Revolved sections.

Removed Section A removed section illustrates particular parts of an object. It is drawn like revolved sections, except it is placed at one side and, to bring out pertinent details, often drawn to a larger scale than the view on which it is indicated. Figure 2-7 is an illustration of removed sections. Section A-A shows the cross-sectional shape of the object at cutting plane line A-A. Section B-B shows the cross-sectional shape at cutting plane line B-B. 2-4

Figure 2-7. Removed sections.

These sectional views are drawn to the same scale as the principal view. Note that they are often drawn to a larger scale to bring out pertinent details.

Title Blocks Every print must have some means of identification. This is provided by a title block. [Figure 2-8] The title block consists of a drawing number and certain

LEWIS AVIATION

ENGINEER

DRAFTER

JOE SMITH

A/C MAKE/MODEL

DASSAULT AVIATION MYSTERE - FALCON 900

DALE LEWIS

REGISTRATION

SERIAL NO.

N32GH

CHECK (SIGNATURE)

Matt Jones

SCALE

FULL

APPROVAL (SIGNATURE)

Roger Lewis SHT

TITLE

All information contained in this document is property of Duncan Aviation and may not be reproduced in whole or part, without permission of Duncan Aviation

017

GALLEY INSTALLATION DRAWING NO.

6384-521

1 OF 2 REV

C

Figure 2-8. Title block.

other data concerning the drawing and the object it represents. This information is grouped in a prominent place on the print, usually in the lower right-hand corner. Sometimes the title block is in the form of a strip extending almost the entire distance across the bottom of the sheet. Although title blocks do not follow a standard form insofar as layout is concerned, all of them present essentially the following information:

detail is shown on a drawing, dash numbers are used. Both parts would have the same drawing number plus an individual number, such as 40267-1 and 40267-2. In addition to appearing in the title block, dash numbers may appear on the face of the drawing near the parts they identify. Dash numbers are also used to identify right-hand and left-hand parts.

5. The name of the firm.

In aircraft, many parts on the left side are like the corresponding parts on the right side but in reverse. The left-hand part is always shown in the drawing. The right-hand part is called for in the title block. Above the title block a notation is found, such as: 470204-1LH shown; 470204-2RH opposite. Both parts carry the same number, but the part called for is distinguished by a dash number. Some prints have odd numbers for left-hand parts and even numbers for right-hand parts.

6. The name of the draftsmen, the checker, and the person approving the drawing.

Universal Numbering System

1. A drawing number to identify the print for filing purposes and to prevent confusing it with any other print. 2. The name of the part or assembly. 3. The scale to which it is drawn. 4. The date.

Drawing or Print Numbers All prints are identified by a number, which appears in a number block in the lower right-hand corner of the title block. It may also be shown in other places — such as near the top border line, in the upper right-hand corner, or on the reverse side of the print at both ends — so that the number will show when the print is folded or rolled. The purpose of the number is quick identification of a print. If a print has more than one sheet and each sheet has the same number, this information is included in the number block, indicating the sheet number and the number of sheets in the series. Reference and Dash Numbers Reference numbers that appear in the title block refer you to the numbers of other prints. When more than one

The universal numbering system provides a means of identifying standard drawing sizes. In the universal numbering system, each drawing number consists of six or seven digits. The first digit is always 1, 2, 4, or 5, and indicates the size of the drawing. The remaining digits identify the drawing. Many firms have modified this basic system to conform to their particular needs. Letters may be used instead of numbers. The letter or number depicting the standard drawing size may be prefixed to the number, separated from it by a dash. Other numbering systems provide a separate box preceding the drawing number for the drawing size identifier. In another modification of this system, the part number of the depicted assembly is assigned as the drawing number.

2-5

Bill of Material A list of the materials and parts necessary for the fabrication or assembly of a component or system is often included on the drawing. The list is usually in ruled columns in which are listed the part number, name of the part, material from which the part is to be constructed, the quantity required, and the source of the part or material. A typical bill of material is shown in Figure 2-9. On drawings that do not have a bill of material, the data may be indicated directly on the drawing. On assembly drawings, each item is identified by a number in a circle or square. An arrow connecting the number with the item assists in locating it in the bill of material.

the identification symbol, the date, the nature of the revision, the authority for the change, and the name of the draftsman who made the change. To distinguish the corrected drawing from its previous version, many firms are including, as part of the title block, a space for entering the appropriate symbol to designate that the drawing has been changed or revised. Notes Notes are added to drawings for various reasons. Some of these notes refer to methods of attachment or construction. Others give alternatives, so that the drawing can be used for different styles of the same object. Still others list modifications that are available. Notes may be found alongside the item to which they refer. If the notes are lengthy, they may be placed elsewhere on the drawing and identified by letters or numbers. Notes are used only when the information cannot be conveyed in the conventional manner or when it is desirable to avoid crowding the drawing. Figure 2-3 illustrates one method of depicting notes. When the note refers to a specific part, a light line with an arrowhead leads from the note to the part. If it applies to more than one part, the note is so worded to eliminate ambiguity as to the parts to which it pertains. If there are several notes, they are generally grouped together and numbered consecutively.

Figure 2-9. A bill of material.

Other Drawing Data Revision Block Revisions to a drawing are necessitated by changes in dimensions, design, or materials. The changes are usually listed in ruled columns either adjacent to the title block or at one corner of the drawing. All changes to approved drawings must be carefully noted on all existing prints of the drawing.

Zone Numbers Zone numbers on drawings are similar to the numbers and letters printed on the borders of a map. They help locate a particular point. To find a point, mentally draw horizontal and vertical lines from the letters and numerals specified; the point where these lines intersect is the area sought.

When drawings contain such corrections, attention is directed to the changes by lettering or numbering them and listing those changes against the symbol in a revision block. [Figure 2-10] The revision block contains

Use the same method to locate parts, sections, and views on large drawings, particularly assembly drawings. Parts numbered in the title block can be located on the drawing by finding the numbers in squares along the lower border. Zone numbers read from right to left.

REV

A B C

ZONE

ALL SHTS PG2 C-2 PG2 A-1

REVISION

DESCRIPTION

DATE

APPR

INITIAL RELEASE

12/05/05

RL

ADDED ADDITIONAL MOUNTING POINTS

12/05/05

RL

ADDED ACCESS PANEL IN BULKHEAD

01/02/06

RL

Figure 2-10. Revision block.

2-6

Station Numbers and Location Identification on Aircraft A numbering system is used on large assemblies for aircraft to locate stations such as fuselage stations. Fuselage station 185 indicates a location that is 185 inches from the datum of the aircraft. The measurement is usually taken from the nose or zero station, but in some instances it may be taken from the firewall or some other point chosen by the manufacturer. Just as forward and aft locations on aircraft are made by reference to the datum, locations left and right of the aircraft’s longitudinal axis are made by reference to the buttock line and are called butt stations. Vertical locations on an airplane are made in reference to the waterline.

Scale Some drawings are made exactly the same size as the drawn part; they have a scale of 1:1. Other scales may be used. However, when drawings are made on a computer, drawing sizes may be easily increased (zoom in) or decreased (zoom out). Some electronic printers have the same capability. Furthermore, when a 1:1 copy of a print is made, the copy size may differ slightly from that of the original. For accurate information, refer to the dimensions shown on the drawing.

The same station numbering system is used for wing and stabilizer frames. The measurement is taken from the centerline or zero station of the aircraft. Figure 2‑11 (on page 2-8) shows use of the fuselage stations (FS), waterline locations (WL), and left and right buttock line locations (RBL and LBL).

Methods of Illustration

Allowances and Tolerances When a given dimension on a print shows an allowable variation, the plus (+) figure indicates the maximum, and the minus (−) figure the minimum allowable variation. The sum of the plus and minus allowance figures is called tolerance. For example, using 0.225 + 0.0025 − 0.0005, the plus and minus figures indicate the part will be acceptable if it is not more than 0.0025 larger than the 0.225 given dimension, or not more than 0.0005 smaller than the 0.225 dimension. Tolerance in this example is 0.0030 (0.0025 max plus 0.0005 min). If the plus and minus allowances are the same, you will find them presented as 0.224 ± 0.0025. The tolerance would then be 0.0050. Allowance can be indicated in either fractional or decimal form. When very accurate dimensions are necessary, decimal allowances are used. Fractional allowances are sufficient when precise tolerances are not required. Standard tolerances of −0.010 or −1/32 may be given in the title block of many drawings, to apply throughout the drawing. Finish Marks Finish marks are used to indicate the surface that must be machine finished. Such finished surfaces have a better appearance and allow a closer fit with adjoining parts. During the finishing process, the required limits and tolerances must be observed. Do not confuse machined finishes with those of paint, enamel, chromium plating, and similar coating.

Application When shown near the title block, application may refer to the aircraft, assembly, sub-assembly or next installation on which the part would be used.

Applied Geometry Geometry is the branch of mathematics that deals with lines, angles, figures and certain assumed properties in space. Applied geometry, as used in drawing, makes use of these properties to accurately and correctly represent objects graphically. In the past, draftsmen utilized a variety of instruments with various scales, shapes and curves to make their drawings. Today, computer software graphics programs showing drawings provide nearly any scale, shape and curve imaginable, outdating the need for additional instruments. A number of methods are used to illustrate objects graphically. The most common are orthographic projections, pictorial drawings, diagrams, and flowcharts. Orthographic Projection Drawings In order to show the exact size and shape of all the parts of complex objects, a number of views are necessary. This is the system used in orthographic projection. In orthographic projection, there are six possible views of an object, because all objects have six sides — front, top, bottom, rear, right side, and left side. Figure 2‑12(a) shows an object placed in a transparent box, hinged at the edges. The projections on the sides of the box are the views as seen looking straight at the object through each side. If the outlines of the object are drawn on each surface and the box opened as shown in (b), then laid flat as shown in (c), the result is a sixview orthographic projection. It is seldom necessary to show all six views to portray an object clearly; therefore, only those views necessary to illustrate the required characteristics of the object 2-7

Figure 2-11. Station numbers and location identification on aircraft.

2-8

(a) Object

(b) Rotated

(c) Flat

Figure 2-12. Orthographic projection.

are drawn. One-, two-, and three-view drawings are the most common. Regardless of the number of views used, the arrangement is generally as shown in Figure 2-12, with the front view as principal view. If the right side view is shown, it will be to the right of the front view. If the left side view is shown, it will be to the left of the front view. The top and bottom views, if included, will be shown in their respective positions relative to the front view. One-view drawings are commonly used for objects of uniform thickness such as gaskets, shims, and plates. A dimensional note gives the thickness as shown in Figure 2-13. One-view drawings are also commonly used for cylindrical, spherical, or square parts if all

the necessary dimensions can be properly shown in one view. When space is limited and two views must be shown, symmetrical objects are often represented by half views, as illustrated in Figure 2-14. Aircraft drawings seldom show more than two principal or complete views of an object. Instead, there will be usually one complete view and one or more detail views or sectional views. Detail View A detail view shows only a part of the object but in greater detail and to a larger scale than the principal 2-9

Figure 2-14. Symmetrical object with exterior half view.

Figure 2-13. One view drawing.

Figure 2-15. Detail view.

2-10

view. The part that is shown in detail elsewhere on the drawing is usually encircled by a heavy line on the principal view. Figure 2-15 is an example of the use of detail views. The principal view shows the complete control wheel, while the detail view is an enlarged drawing of a portion of the control wheel. Pictorial Drawings A pictorial drawing [Figure 2-16] is similar to a photograph. It shows an object as it appears to the eye, but it is not satisfactory for showing complex forms and shapes. Pictorial drawings are useful in showing the general appearance of an object and are used extensively with orthographic projection drawings. Pictorial drawings are used in maintenance, overhaul, and part numbers. Three types of pictorial drawings are used frequently by aircraft engineers and technicians: (1) perspective, (2) isometric, and (3) oblique.

Figure 2-16. Pictorial drawing.

Oblique Drawings An oblique view [Figure 2-17(c)] is similar to an isometric view except for one distinct difference. In an oblique drawing, two of the three drawing axes are always at right angles to each other. Exploded View Drawings An exploded view drawing is a pictorial drawing of two or more parts that fit together as an assembly. The view shows the individual parts and their relative position to the other parts before they are assembled.

Perspective Drawings A perspective view [Figure 2-17(a)] shows an object as it appears to an observer. It most closely resembles the way an object would look in a photograph. Because of perspective, some of the lines of an object are not parallel and therefore the actual angles and dimensions are not accurate.

Diagrams A diagram may be defined as a graphic representation of an assembly or system, indicating the various parts and expressing the methods or principles of operation.

Isometric Drawings An isometric view [Figure 2-17(b)] uses a combination of the views of an orthographic projection and tilts the object forward so that portions of all three views can be seen in one view. This provides the observer with a three-dimensional view of the object. Unlike a perspective drawing where lines converge and dimensions are not true, lines in an isometric drawing are parallel and dimensioned as they are in an orthographic projection.

a

There are many types of diagrams; however, those with which the aviation mechanic will be concerned during the performance of his or her job may be grouped into four classes or types: (1) installation, (2) schematic, (3) block, and (4) wiring diagrams. Installation Diagrams Figure 2-18 is an example of an installation diagram. This is a diagram of the installation of the flight guid-

b

c

Figure 2-17. (a) Perspective, (b) isometric, and (c) oblique drawings.

2-11

Figure 2-18. Example of installation diagram (flight guidance components).

ance control components of an aircraft. It identifies each of the components in the systems and shows their location in the aircraft. Each number (1, 2, 3, and 4) on the detail shows the location of the individual flight guidance system components within the cockpit of the 2-12

aircraft. Installation diagrams are used extensively in aircraft maintenance and repair manuals, and are invaluable in identifying and locating components and understanding the operation of various systems.

System pressure

RH engine pump

Engine pump suction Idling circuit pump pressure Return flow Hand pump pressure Hand pump suction Check valve Accum. air gauges

Gen. system accumulator Brake accumulator To brake system

Thermal relief valve From emergency sel. valve (brake)

Unloading and relief valve Snubber

Vent line

R. and L. engine cowl flaps sel. val.

Hand pump

Reservoir

Hydraulic pressure gauge

Normal

Land gear sel. valve Emer. selector valve

LH engine pump

Figure 2-19. Aircraft hydraulic system schematic.

Schematic Diagrams Schematic diagrams do not indicate the location of individual components in the aircraft, but locate components with respect to each other within the system. Figure 2-19 illustrates a schematic diagram of an aircraft hydraulic system. The hydraulic pressure gauge is not necessarily located above the landing gear selector valve in the aircraft. It is, however, connected to the pressure line that leads to the selector valve. Schematic diagrams of this type are used mainly in troubleshooting. Note that each line is coded for ease

of reading and tracing the flow. Each component is identified by name, and its location within the system can be ascertained by noting the lines that lead into and out of the unit. Schematic diagrams and installation diagrams are used extensively in aircraft manuals. Block Diagrams Block diagrams [Figure 2-20] are used to show a simplified relationship of a more complex system of components. Individual components are drawn as 2-13

AC Input Phase A

Signal Conditioning and Balancing

Reference Voltage PU

DO

AC Input Phase B

Signal Conditioning and Balancing

Voltage Selector

Comparator

Output Relay

AC Input Phase C

Signal Conditioning and Balancing

Figure 2-20. Block diagram.

Figure 2-21. Wiring diagram.

2-14

Power Amplifier

a rectangle (block) with lines connecting it to other components (blocks) that it interfaces with during operation. Wiring Diagrams Wiring diagrams [Figure 2-21] show the electrical wiring and circuitry, coded for identification, of all the electrical appliances and devices used on aircraft. These diagrams, even for relatively simple circuits, can be quite complicated. For technicians involved with electrical repairs and installations, a thorough knowledge of wiring diagrams and electrical schematics is essential. Flowcharts Flowcharts are used to illustrate a particular sequence, or flow of events. Troubleshooting Flowchart Troubleshooting flowcharts are frequently used for the detection of faulty components. They often consist

of a series of yes or no questions. If the answer to a question is yes, one course of action is followed. If the answer is no, a different course of action is followed. In this simple manner, a logical solution to a particular problem may be achieved. Another type of flowchart, developed specifically for analysis of digitally controlled components and systems, is the logic flowchart. Logic Flowchart A logic flowchart [Figure 2-22] uses standardized symbols to indicate specific types of logic gates and their relationship to other digital devices in a system. Since digital systems make use of binary mathematics consisting of 1s and 0s, voltage or no voltage, a light pulse or no light pulse, and so forth, logic flowcharts consist of individual components that take an input and provide an output which is either the same as the input or opposite. By analyzing the input or multiple inputs, it is possible to determine the digital output or outputs.

Figure 2-22. Logic flowchart.

2-15

Lines and Their Meanings Every drawing is composed of lines. Lines mark the boundaries, edges, and intersection of surfaces. Lines are used to show dimensions and hidden surfaces and to indicate centers. Obviously, if the same kind of line is used to show all of these variations, a drawing becomes a meaningless collection of lines. For this reason, various kinds of standardized lines are used on aircraft drawings. These are illustrated in Figure 2-23, and their correct uses are shown in Figure 2-24. Most drawings use three widths, or intensities, of lines: thin, medium, or thick. These lines may vary somewhat on different drawings, but there will always be a noticeable difference between a thin and a thick line, with the width of the medium line somewhere between the two. Center line Dimension Extension Line Break (Long) Break (Long) Phantom Sectioning Hidden Stitch Line Visible Line Satum Line Cutting Plane

Thin Thin Thin Thin Thick Thin Thin Medium Medium Thick Thick Extra Thick

Cutting Plane

Extra Thick

Complex Cutting Plane

Extra Thick

Figure 2-23. The meaning of lines.

Centerlines Centerlines are made up of alternate long and short dashes. They indicate the center of an object or part of an object. Where centerlines cross, the short dashes intersect symmetrically. In the case of very small circles, the centerlines may be shown unbroken. Dimension Lines A dimension line is a light solid line, broken at the midpoint for insertion of measurement indications, and having opposite pointing arrowheads at each end to show origin and termination of a measurement. They are generally parallel to the line for which the dimension is given, and are usually placed outside the outline of the object and between views if more than one view is shown. All dimensions and lettering are placed so that they will read from left to right. The dimension of an angle is indicated by placing the degree of the angle in its arc. The dimensions of circular parts are always given in terms of the diameter of the circle and are usually marked with the letter D or the abbreviation DIA following the dimension. The dimension of an arc is given in terms of its radius and is marked with the letter R following the dimension. Parallel dimensions are placed so that the longest dimension is farthest from the outline and the shortest dimension is closest to the outline of the object. On a drawing showing several views, the dimensions will be placed upon each view to show its details to the best advantage. In dimensioning distances between holes in an object, dimensions are usually given from center to center rather than from outside to outside of the holes. When a Extension line

Dimension line

Phantom line

Leader line

Center line Sectioning line

Break line

Outline

Section AA

Hidden Line Cutting plane line

Figure 2-24. Correct use of lines.

2-16

number of holes of various sizes are shown, the desired diameters are given on a leader followed by notes indicating the machining operations for each hole. If a part is to have three holes of equal size, equally spaced, this information is explicitly stated. For precision work, sizes are given in decimals. Diameters and depths are given for counterbored holes. For countersunk holes, the angle of countersinking and the diameters are given. Study the examples shown in Figure 2-25. The dimensions given for tolerances signifies the amount of clearance allowable between moving parts. A positive allowance is indicated for a part that is to slide or revolve upon another part. A negative allowance is one given for a force fit. Whenever possible, the tolerance and allowances for desired fits conform to those set up in the American Standard for Tolerances, Allowances, and Gauges for Metal Fits. The classes of fits specified in the standard may be indicated on assembly drawings. Extension Lines Extensions are used to extend the line showing the side or edge of a figure for the purpose of placing a dimension to that side or edge. They are very narrow and have a short break where they extend from the object and extend a short distance past the arrow of the dimensioning line. Sectioning Lines Sectioning lines indicate the exposed surfaces of an object in sectional view. They are generally thin full lines but may vary with the kind of material shown in section. Phantom Lines Phantom lines, composed of one long and two short evenly spaced dashes, indicate the alternate position of parts of the object or the relative position of a missing part. Break Lines Break lines indicate that a portion of the object is not shown on the drawing. Short breaks are made by solid, freehand lines. For long breaks, solid ruled lines with zigzags are used. Shafts, rods, tubes, and other such parts which have a portion of their length broken out have the ends of the break drawn as indicated in Figure 2-24. Leader Lines Leader lines are solid lines with one arrowhead and indicate a part or portion to which a note, number, or other reference applies.

Figure 2-25. Dimensioning holes.

Hidden Lines Hidden lines indicate invisible edges or contours. Hidden lines consist of short dashes evenly spaced and are frequently referred to as dash lines. Outline or Visible Lines The outline or visible line is used for all lines on the drawing representing visible lines on the object. Stitch Lines Stitch lines indicate stitching or sewing lines and consist of a series of evenly spaced dashes. Cutting Plane and Viewing Plane Lines Cutting plane lines indicate the plane in which a sectional view of the object is taken. In Figure 2-24, plane line A-A indicates the plane in which section A-A is taken. Viewing plane lines indicate the plane from which a surface is viewed.

Drawing Symbols The drawings for a component are composed largely of symbols and conventions representing its shape and material. Symbols are the shorthand of drawing. They graphically portray the characteristics of a component with a minimal amount of drawing. 2-17



CAST IRON

MAGNESIUM, ALUMINUM, AND ALUMINUM ALLOYS



STEEL

RUBBER, PLASTIC ELECTRICAL INSULATION

BRASS, BRONZE, AND COPPER

BABBITT, LEAD, ZINC, AND ALLOYS

WOOD — ACROSS GRAIN

WOOD — WITH GRAIN

Material Symbols Section line symbols show the kind of material from which the part is to be constructed. The material may not be indicated symbolically if its exact specification is shown elsewhere on the drawing. In this case, the more easily drawn symbol for cast iron is used for the sectioning, and the material specification is listed in the bill of materials or indicated in a note. Figure 2-26 illustrates a few standard material symbols. Shape Symbols Symbols can be used to excellent advantage when needed to show the shape of an object. Typical shape symbols used on aircraft drawings are shown in Figure 2-27. Shape symbols are usually shown on a drawing as a revolved or removed section. Electrical Symbols Electrical symbols [Figure 2-28] represent various electrical devices rather than an actual drawing of the units. Having learned what the various symbols indicate, it becomes relatively simple to look at an electrical diagram and determine what each unit is, what function it serves, and how it is connected in the system.

Reading and Interpreting Drawings

CORK, FELT, FABRIC, ASBESTOS, LEATHER, AND FIBRE

Figure 2-26. Standard material symbols.

Aircraft technicians do not necessarily need to be accomplished in making drawings. However, they must have a working knowledge of the information that is to be conveyed to them. They most frequently encounter drawings for construction and assembly of

Figure 2-27. Shape symbols.

2-18

Figure 2-28. Electrical symbols.

2-19

new aircraft and components, during modifications, and for making repairs.

the other view must be consulted to determine what each circle represents.

A drawing cannot be read all at once any more than a whole page of print can be read at a glance. Both must be read a line at a time. To read a drawing effectively, follow a systematic procedure.

A glance at the other view tells us that the smaller circle represents a hole, and the larger circle represents a protruding boss. In the same way, the top view must be consulted to determine the shape of the hole and the protruding boss.

Upon opening a drawing, read the drawing number and the description of the article. Next, check the model affected, the latest change letter, and the next assembly listed. Having determined that the drawing is the correct one, proceed to read the illustration(s). In reading a multiview drawing, first get a general idea of the shape of the object by scanning all the views; then select one view for a more careful study. By referring back and forth to the adjacent view, it will be possible to determine what each line represents. Each line on a view represents a change in the direction of a surface but another view must be consulted to determine what the change is. For example, a circle on one view may mean either a hole or a protruding boss, as in the top view of the object in Figure 2-29. Looking at the top view, we see two circles; however,

It can be seen from this example that one cannot read a print by looking at a single view when more than one view is given. Two views will not always describe an object and when three views are given, all three must be consulted to be sure the shape has been read correctly. After determining the shape of an object, determine its size. Information on dimensions and tolerances is given so that certain design requirements may be met. Dimensions are indicated by figures either with or without the inch mark. If no inch mark is used, the dimension is in inches. It is customary to give part dimensions and an overall dimension that gives the greatest length of the part. If the overall dimension is missing, it can be determined by adding the separate part dimensions. Drawings may be dimensioned in decimals or fractions. This is especially true in reference to tolerances. Instead of using plus and minus signs for tolerances, many figures give the complete dimension for both tolerances. For example, if a dimension is 2 inches with a plus or minus tolerance of 0.01, the drawing would show the total dimensions as: 2.01 1.99 A print tolerance (usually found in the title block) is a general tolerance that can be applied to parts where the dimensions are noncritical. Where a tolerance is not shown on a dimension line, the print tolerance applies. To complete the reading of a drawing, read the general notes and the contents of the material block, check and find the various changes incorporated, and read the special information given in or near views and sections.

Drawing Sketches A sketch is a simple rough drawing that is made rapidly and without much detail. Sketches may take many forms — from a simple pictorial presentation to a multi-view orthographic projection. Figure 2-29. Reading views.

2-20

Just as aircraft technicians need not be highly skilled in making drawings, they need not be accomplished artists. However, in many situations, they will need to prepare a drawing to present an idea for a new design, a modification, or a repair method. The medium of sketching is an excellent way of accomplishing this. The rules and conventional practices for making mechanical drawings are followed to the extent that all views needed to portray an object accurately are shown in their proper relationship. It is also necessary to observe the rules for correct line use [Figures 2-23 and 2-24] and dimensioning. Sketching Techniques To make a sketch, first determine what views are necessary to portray the object; then block in the views, using light construction lines. Next, complete the details, darken the object outline, and sketch extension and dimension lines. Complete the drawing by adding notes, dimensions, title, date, and when necessary, the sketcher’s name. The steps in making a sketch of an object are illustrated in Figure 2-30. Basic Shapes Depending on the complexity of the sketch, basic shapes such as circles and rectangles may be drawn in freehand or by use of templates. If the sketch is quite complicated or the technician is required to make frequent sketches, use of a variety of templates and other drafting tools is highly recommended. Repair Sketches A sketch is frequently drawn for repairs or for use in manufacturing a replacement part. Such a sketch must provide all necessary information to those persons who must make the repair or manufacture the part. The degree to which a sketch is complete will depend on its intended use. Obviously, a sketch used only to represent an object pictorially need not be dimensioned. If a part is to be manufactured from the sketch, it should show all the necessary construction details.

Care of Drafting Instruments Good drawing instruments are expensive precision tools. Reasonable care given to them during their use and storage will prolong their service life. T-squares, triangles, and scales should not be used or placed where their surfaces or edges may be damaged. Use a drawing board only for its intended purpose and not in a manner that will mar the working surface.

Block in

Add Detail

Add Dimensions

Darken Views ⁄"

11⁄2 "

1 2

1"

11⁄2 "

⁄"

3 4

3"

1-27-06 WEDGE

RJA

Figure 2-30. Steps in sketching.

Compasses, dividers, and pens will provide better results with less annoyance, if they are correctly shaped and sharpened and are not damaged by careless handling. Store drawing instruments in a place where they are not likely to be damaged by contact with other tools or equipment. Protect compass and divider points by inserting them into a piece of soft rubber or similar material. Never store ink pens without first cleaning and drying them thoroughly.

Graphs and Charts Graphs and charts are frequently used to convey information graphically or information given certain conditions. They often utilize values shown on the x and y axes that can be projected up and across to arrive at a specific result. Also, when data is entered into a computer database, software programs can create a variety of different bar graphs, pie charts, and so forth, to graphically represent that data. Reading and Interpreting Graphs and Charts When interpreting information shown on graphs and charts, it is extremely important that all the notes and legend information be carefully understood in order to eliminate any misinterpretation of the information presented. Nomograms A nomogram is a graph that usually consists of three sets of data. Knowledge of any two sets of data enables the interpreter to obtain the value for the third unknown corresponding value. One type of nomogram consists of three parallel scales graduated for different variables so that when a straight edge connects any two values, 2-21

the third can be read directly. Other types may use values on the x and y axes of a graph with the third corresponding value determined by the intersection of the x and y values with one of a series of curved lines. Figure 2-31 is an example of a nomogram that shows the relationship between aviation fuels, specific weight, and temperature.

Microfilm and Microfiche

Most modern aircraft manufacturers have replaced microfilm and microfiche with digital storage methods utilizing CDs, DVDs and other data storage devices. A great deal of service and repair information for older aircraft has been transferred to digital storage devices. However, there may still be a need to access information using the old methods. A well-equipped shop should have available, both the old microfilm and microfiche equipment, as well as new computer equipment.

The practice of recording drawings, parts catalogs, and maintenance and overhaul manuals on microfilms was utilized extensively in the past. Microfilm is available as regular 16 mm or 35 mm film. Since 35 mm film is larger, it provides a better reproduction of drawings. Microfiche is a card with pages laid out in a grid format. Microfilm and microfiche require use of special devices for both reading and printing the information.

Digital Images Though not a drawing, a digital image created by a digital camera can be extremely helpful to aviation maintenance technicians in evaluating and sharing information concerning the airworthiness or other information about aircraft. Digital images can be rap-

Density Variation of Aviation Fuel Based on Average Specific Gravity Fuel

Average Specific Gravity ar 15 °C (59 °F)

Aviation Kerosene Jet A and Jet A1

.812

Jet B (JP-4)

.785

AV Gas Grade 100/130

.703

Note: The fuel quantity indicator is calibrated for correct indication when using Aviation Kerosene Jet A and Jet A1. When using other fuels, multiply the indicated fuel quantity in pounds by .99 for Jet B (JP-4) or by .98 for Aviation Gasoline (100/130) to obtain actual fuel quantity in pounds.

Specific Weight (Lb/US Gal)

7.5

Aviation Ke rosene

7.0

Jet A & Jet

A1

Jet B (JP-4) 6.5

Aviatio nG

asoline

6.0

Grade

100/13

0

5.5 − 40

−30

−20

−10

0

Temperature (°C) Figure 2-31. Nomogram.

2-22

10

20

30

40

Figure 2-32. Digital image of impact damage.

idly transmitted over the World Wide Web as attachments to e-mail messages. Images of structural fatigue cracks, failed parts, or other flaws, as well as desired design and paint schemes, are just a few examples of the types of digital images that might be shared by any number of users over the Internet. Figure 2-32 is a digital image of impact damage to a composite structure

taken with a simple digital camera. To provide information about the extent of the damage, a measurement scale, or other object, such as a coin, can be placed near the area of concern before the picture is taken. Also, within the text of the e-mail, the technician should state the exact location of the damage, referenced to fuselage station, wing station, and so forth.

2-23

2-24

Physical science, or physics as it is most often called, is a very interesting and exciting topic. For the individual who likes technical things and is a hands-on type person, it is an invaluable tool. Physics allows us to explain how engines work, both piston and gas turbine; how airplanes and helicopters fly; and countless other things related to the field of aviation and aerospace. In addition to allowing us to explain the operation of the things around us, it also allows us to quantify them. For example, through the use of physics we can explain what the concept of thrust means for a jet engine, and then follow it up by mathematically calculating the pounds of thrust being created. Physics is the term applied to that area of knowledge regarding the basic and fundamental nature of matter and energy. It does not attempt to determine why matter and energy behave as they do in their relation to physical phenomena, but rather how they behave. The people who maintain and repair aircraft should have a knowledge of basic physics, which is sometimes called the science of matter and energy.

Matter Matter is the foundation or the building blocks for any discussion of physics. According to the dictionary, matter is what all things are made of; whatever occupies space, has mass, and is perceptible to the senses in some way. According to the Law of Conservation, matter cannot be created or destroyed, but it is possible to change its physical state. When liquid gasoline vaporizes and mixes with air, and then burns, it might seem that this piece of matter has disappeared and no longer exists. Although it no longer exists in the state of liquid gasoline, the matter still exists in the form of the gases given off by the burning fuel. Characteristics of Matter Mass and Weight Mass is a measure of the quantity of matter in an object. In other words, how many molecules are in the object,

or how many atoms, or to be more specific, how many protons, neutrons, and electrons. The mass of an object does not change regardless of where you take it in the universe, and it does not change with a change of state. The only way to change the mass of an object is to add or take away atoms. Mathematically, mass can be stated as follows: Mass = Weight ÷ Acceleration due to gravity The acceleration due to gravity here on earth is 32.2 feet per second per second (32.2 fps/s). An object weighing 32.2 pounds (lb) here on earth is said to have a mass of 1 slug. A slug is a quantity of mass that will accelerate at a rate of 1 ft/s2 when a force of 1 pound is applied. In other words, under standard atmospheric conditions (gravity equal to 32.2) a mass of one slug is equal to 32.2 lb. Weight is a measure of the pull of gravity acting on the mass of an object. The more mass an object has, the more it will weigh under the earth’s force of gravity. Because it is not possible for the mass of an object to go away, the only way for an object to be weightless is for gravity to go away. We view astronauts on the space shuttle and it appears that they are weightless. Even though the shuttle is quite a few miles above the surface of the earth, the force of gravity has not gone away, and the astronauts are not weightless. The astronauts and the space shuttle are in a state of free fall, so relative to the shuttle the astronauts appear to be weightless. Mathematically, weight can be stated as follows: Weight = Mass × Gravity Attraction Attraction is the force acting mutually between particles of matter, tending to draw them together. Sir Isaac Newton called this the “Law of Universal Gravitation.” He showed how each particle of matter attracts every other particle, how people are bound to the earth, and how the planets are attracted in the solar system. 3-1

Porosity Porosity means having pores or spaces where smaller particles may fit when a mixture takes place. This is sometimes referred to as granular — consisting or appearing to consist of small grains or granules. Impenetrability Impenetrability means that no two objects can occupy the same place at the same time. Thus, two portions of matter cannot at the same time occupy the same space. Density The density of a substance is its weight per unit volume. The unit volume selected for use in the English system of measurement is 1 cubic foot (ft3 ). In the metric system, it is 1 cubic centimeter (cm3 ). Therefore, density is expressed in pounds per cubic foot ( lb⁄ft3) or in grams per cubic centimeter ( g ⁄cm3 ). To find the density of a substance, its weight and volume must be known. Its weight is then divided by its volume to find the weight per unit volume. For example, the liquid which fills a certain container weighs 1,497.6 lb. The container is 4 ft long, 3 ft wide, and 2 ft deep. Its volume is 24 ft3 (4 ft × 3 ft × 2 ft). If 24 ft3 of liquid weighs 1,497.6 lb, then 1 ft3 weighs 1,497.6 ÷ 24, or 62.4 lb. Therefore, the density of the liquid is 62.4 lb/ft3. This is the density of water at 4°C (Centigrade) and is usually used as the standard for comparing densities of other substances. In the metric system, the density of water is 1 g ⁄cm3 . The standard temperature of 4°C is used when measuring the density of liquids and solids. Changes in temperature will not change the weight of a substance, but will change the volume of the substance by expansion or contraction, thus changing its weight per unit volume. The procedure for finding density applies to all substances; however, it is necessary to consider the pressure when finding the density of gases. Pressure is more critical when measuring the density of gases than it is for other substances. The density of a gas increases in direct proportion to the pressure exerted on it. Standard conditions for the measurement of the densities of gases have been established at 0°C for temperature and a pressure of 76 cm of mercury (Hg). (This is the average pressure of the atmosphere at sea level.) Density is computed based on these conditions for all gases. Specific Gravity It is often necessary to compare the density of one substance with that of another. For this purpose, a standard 3-2

is needed. Water is the standard that physicists have chosen to use when comparing the densities of all liquids and solids. For gases, air is most commonly used. However, hydrogen is sometimes used as a standard for gases. In physics, the word “specific” implies a ratio. Thus, specific gravity is calculated by comparing the weight of a definite volume of the given substance with the weight of an equal volume of water. The terms “specific weight” or “specific density” are sometimes used to express this ratio. The following formulas are used to find the specific gravity of liquids and solids. Specific Gravity =

Weight of the substance Weight of an equal volume of water OR

Specific Gravity =

Density of the substance Density of water

The same formulas are used to find the density of gases by substituting air or hydrogen for water. Specific gravity is not expressed in units, but as pure numbers. For example, if a certain hydraulic fluid has a specific gravity of 0.8, 1 ft3 of the liquid weighs 0.8 times as much as 1 ft3 of water: 62.4 times 0.8, or 49.92 lb. Specific gravity and density are independent of the size of the sample under consideration and depend only upon the substance of which it is made. See Figure 3-1 for typical values of specific gravity for various substances. A device called a hydrometer is used for measuring specific gravity of liquids. This device consists of a tubular glass float contained in a larger glass tube. [Figure 3-2] The larger glass tube provides the container for the liquid. A rubber suction bulb draws the liquid up into the container. There must be enough liquid to raise the float and prevent it from touching the bottom. The float is weighted and has a vertically graduated scale. To determine specific gravity, the scale is read at the surface of the liquid in which the float is immersed. An indication of 1000 is read when the float is immersed in pure water. When immersed in a liquid of greater density, the float rises, indicating a greater specific gravity. For liquids of lesser density the float sinks, indicating a lower specific gravity. An example of the use of the hydrometer is to determine the specific gravity of the electrolyte (battery

0.0695

0.785

Aluminum

2.7

Helium

0.138

0.789

Titanium

4.4

Acetylene

0.898

0.82

Zinc

7.1

Nitrogen

0.967

Kerosene

0.82

Iron

7.9

Air

1.000

Lube Oil

0.89

Brass

8.4

Oxygen

1.105

Synthetic Oil

0.928

Copper

8.9

Carbon Dioxide

1.528

Water

1.000

Lead

11.4

Sulfuric Acid

1.84

Gold

19.3

Mercury

13.6

Platinum

21.5

1150 Discharged 1275 Charged

11 50 12 00 13 00

Figure 3-1. Specific gravity of various substances.

12 50

Jet Fuel Jp-4 Ethyl Alcohol Jet Fuel Jp-5

11 00

Hydrogen

111 50

0.917

12 00

Ice

12 50

0.72

13 00

Gasoline

11 00

Specific Gravity 11 00

Gas

11 50

Specific Gravity

12 00

Solid

12 50

Specific Gravity

13 00

Liquid

liquid) in an aircraft battery. When a battery is discharged, the calibrated float immersed in the electrolyte will indicate approximately 1150. The indication of a charged battery is between 1275 and 1310. The values 1150, 1275, and 1310 actually represent 1.150, 1.275, and 1.310. The electrolyte in a discharged battery is 1.15 times denser than water, and in a charged battery 1.275 to 1.31 times denser than water.

Energy Energy is typically defined as something that gives us the capacity to perform work. As individuals, saying that we feel full of energy is probably indicating that we can perform a lot of work. Energy can be classified as one of two types: either potential or kinetic. Potential Energy Potential energy is defined as being energy at rest, or energy that is stored. Potential energy may be classified into three groups: (1) that due to position, (2) that due to distortion of an elastic body, and (3) that which produces work through chemical action. Water in an elevated reservoir, and an airplane raised off the ground sitting on jacks are examples of the first group; a stretched bungee chord on a Piper Tri-Pacer or compressed spring are examples of the second group; and energy in aviation gasoline, food, and storage batteries are examples of the third group.

Figure 3-2. Hydrometer for checking battery specific gravity.

To calculate the potential energy of an object due to its position, as in height, the following formula is used: Potential Energy = Weight × Height A calculation based on this formula will produce an answer that has units of foot-pounds (ft-lb) or inchpounds (in-lb), which are the same units that apply to work. Work, which is covered later in this chapter, is described as a force being applied over a measured distance, with the force being pounds and the distance being feet or inches. It can be seen that potential energy and work have a lot in common.

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Example: A Boeing 747 weighing 450,000 pounds needs to be raised 4 feet in the air so maintenance can be done on the landing gear. How much potential energy does the airplane possess because of this raised position? Potential Energy = Weight × Height PE = 450,000 lb × 4 ft PE = 1,800,000 ft-lb As mentioned previously, aviation gasoline possesses potential energy because of its chemical nature. Gasoline has the potential to release heat energy, based on its British thermal unit (BTU) content. One pound of aviation gas contains 18,900 BTU of heat energy, and each BTU is capable of 778 ft-lb of work. So if we multiply 778 by 18,900, we find that one pound of aviation gas is capable of 14,704,200 ft-lb of work. Imagine the potential energy in the completely serviced fuel tanks of an airplane. Kinetic Energy Kinetic energy is defined as being energy in motion. An airplane rolling down the runway or a rotating flywheel on an engine are both examples of kinetic energy. Kinetic energy has the same units as potential energy, namely foot-pounds or inch-pounds. To calculate the kinetic energy for something in motion, the following formula is used: Kinetic Energy = 1⁄2 Mass × Velocity 2 To use the formula, we will show the mass as weight ÷ gravity and the velocity of the object will be in feet per second. This is necessary to end up with units in foot-pounds. Example: A Boeing 777 weighing 600,000 lb is moving down the runway on its takeoff roll with a velocity of 200 fps. How many foot-pounds of kinetic energy does the airplane possess? [Figure 3-3] Kinetic Energy = 1⁄2 Mass × Velocity 2 Kinetic Energy = 1⁄2 × 600,000 ÷ 32.2 × 200 2 KE = 372,670,000 ft-lb

Force, Work, Power, and Torque Force Before the concept of work, power, or torque can be discussed, we must understand what force means. According to the dictionary, force is the intensity of an impetus, or the intensity of an input. For example, if we apply a force to an object, the tendency will be 3-4

Figure 3-3. Kinetic energy (Boeing 777 taking off).

for the object to move. Another way to look at it is that for work, power, or torque to exist, there has to be a force that initiates the process. The unit for force in the English system of measurement is pounds, and in the metric system it is newtons. One pound of force is equal to 4.448 newtons. When we calculate the thrust of a turbine engine, we use the formula “Force = Mass × Acceleration,” and the thrust of the engine is expressed in pounds. The GE90-115 turbofan engine (powerplant for the Boeing 777-300), for example, has 115,000 pounds of thrust. Work The study of machines, both simple and complex, is in one sense a study of the energy of mechanical work. This is true because all machines transfer input energy, or the work done on the machine, to output energy, or the work done by the machine. Work, in the mechanical sense of the term, is done when a resistance is overcome by a force acting through a measurable distance. Two factors are involved: (1) force and (2) movement through a distance. As an example, suppose a small aircraft is stuck in the snow. Two men push against it for a period of time, but the aircraft does not move. According to the technical definition, no work was done in pushing against the aircraft. By definition, work is accomplished only when an object is displaced some distance against a resistive force. To calculate work, the following formula is used: Work = Force (F) × distance (d) In the English system, the force will be identified in pounds and the distance either in feet or inches, so the units will be foot-pounds or inch-pounds. Notice

Figure 3-4. Airbus A-320 being jacked.

these are the same units that were used for potential and kinetic energy. In the metric system, the force is identified in newtons (N) and the distance in meters, with the resultant units being joules. One pound of force is equal to 4.448 N and one meter is equal to 3.28 feet. One joule is equal to 1.36 ft-lb. Example: How much work is accomplished by jacking a 150,000-lb Airbus A-320 airplane a vertical height of 3 ft? [Figure 3-4] Work = Force × Distance = 150,000 lb × 4 ft = 600,000 ft-lb Example: How much work is accomplished when a tow tractor is hooked up to a tow bar and a Boeing 737-800 airplane weighing 130,000 lb is pushed 80 ft into the hangar? The force on the tow bar is 5,000 lb. Work = Force × Distance = 5,000 lb × 80 ft = 400,000 ft-lb In this last example, notice the force does not equal the weight of the airplane. This is because the airplane is being moved horizontally and not lifted vertically. In virtually all cases, it takes less work to move something horizontally than it does to lift it vertically. Most people can push their car a short distance if it runs out of gas, but they cannot get under their car and lift it off the ground.

Friction and Work In calculating work done, the actual resistance overcome is measured. This is not necessarily the weight of the object being moved. [Figure 3-5] A 900-lb load is being pulled a distance of 200 ft. This does not mean that the work done (force × distance) is 180,000 ft-lb (900 lb × 200 ft). This is because the person pulling the load is not working against the total weight of the load, but rather against the rolling friction of the cart, which may be no more than 90 lb. Friction is an important aspect of work. Without friction it would be impossible to walk. One would have to shove oneself from place to place, and would have to bump against some obstacle to stop at a destination. Yet friction is a liability as well as an asset, and

Force 90 lb

Work = force × distance = 90 lb × 200 ft = 18,000 ft-lb

Gravity

200 ft

Resistance

Figure 3-5. The effect of friction on work.

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requires consideration when dealing with any moving mechanism. In experiments relating to friction, measurement of the applied forces reveals that there are three kinds of friction. One force is required to start a body moving, while another is required to keep the body moving at constant speed. Also, after a body is in motion, a definitely larger force is required to keep it sliding than to keep it rolling. Thus, the three kinds of friction may be classified as: (1) starting (static) friction, (2) sliding friction, and (3) rolling friction. Static Friction When an attempt is made to slide a heavy object along a surface, the object must first be broken loose or started. Once in motion, it slides more easily. The “breaking loose” force is, of course, proportional to the weight of the body. The force necessary to start the body moving slowly is designated “F,” and “F'” is the normal force pressing the body against the surface (usually its weight). Since the nature of the surfaces rubbing against each other is important, they must be considered. The nature of the surfaces is indicated by the coefficient of starting friction which is designated by the letter “k.” This coefficient can be established for various materials and is often published in tabular form. Thus, when the load (weight of the object) is known, starting friction can be calculated by using the following formula: F = kF' For example, if the coefficient of sliding friction of a smooth iron block on a smooth, horizontal surface is 0.3, the force required to start a 10 lb block would be 3 lb; a 40-lb block, 12 lb. Starting friction for objects equipped with wheels and roller bearings is much smaller than that for sliding objects. Nevertheless, a locomotive would have difficulty getting a long train of cars in motion all at one time. Therefore, the couples between the cars are purposely made to have a few inches of play. When starting the train, the engineer backs the engine until all the cars are pushed together. Then, with a quick start forward the first car is set in motion. This technique is employed to overcome the static friction of each wheel (as well as the inertia of each car). It would be impossible for the engine to start all of the cars at the same instant, for static friction, which is the resistance

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of being set in motion, would be greater than the force exerted by the engine. Once the cars are in motion, however, static friction is greatly reduced and a smaller force is required to keep the train in motion than was required to start it. Sliding Friction Sliding friction is the resistance to motion offered by an object sliding over a surface. It pertains to friction produced after the object has been set in motion, and is always less than starting friction. The amount of sliding resistance is dependent on the nature of the surface of the object, the surface over which it slides, and the normal force between the object and the surface. This resistive force may be computed by using the following formula. F = mN In the formula above, “F” is the resistive force due to friction expressed in pounds; “N” is the force exerted on or by the object perpendicular (normal) to the surface over which it slides; and “m” (mu) is the coefficient of sliding friction. On a horizontal surface, N is equal to the weight of the object in pounds. The area of the sliding object exposed to the sliding surface has no effect on the results. A block of wood, for example, will not slide any easier on one of the broad sides than it will on a narrow side, (assuming all sides have the same smoothness). Therefore, area does not enter into the equation above. Rolling Friction Resistance to motion is greatly reduced if an object is mounted on wheels or rollers. The force of friction for objects mounted on wheels or rollers is called rolling friction. This force may be computed by the same equation used in computing sliding friction, but the values of “m” will be much smaller. For example, the value of “m” for rubber tires on concrete or macadam is about 0.02. The value of “m” for roller bearings is very small, usually ranging from 0.001 to 0.003 and is often disregarded. Example: An aircraft with a gross weight of 79,600 lb is towed over a concrete ramp. What force must be exerted by the towing vehicle to keep the airplane rolling after once set in motion? F = mN = 0.02 mu × 79,600 lb = 1,592 lb

Power The concept of power involves the previously discussed topic of work, which was a force being applied over a measured distance, but adds one more consideration — time. In other words, how long does it take to accomplish the work. If someone asked the average person if he or she could lift one million pounds 5 feet off the ground, the answer most assuredly would be no. This person would probably assume that he or she is to lift it all at once. What if he or she is given 365 days to lift it, and could lift small amounts of weight at a time? The work involved would be the same, regardless of how long it took to lift the weight, but the power required is different. If the weight is to be lifted in a shorter period of time, it will take more power. The formula for power is as follows: Power = Force × distance ÷ time The units for power will be foot-pounds per minute, foot-pounds per second, inch-pounds per minute or second, and possibly mile-pounds per hour. The units depend on how distance and time are measured. Many years ago there was a desire to compare the power of the newly evolving steam engine to that of horses. People wanted to know how many horses the steam engine was equivalent to. Because of this, the value we currently know as one horsepower (hp) was developed, and it is equal to 550 foot-pounds per second (ft-lb/s). It was found that the average horse could lift a weight of 550 lb, one foot off the ground, in one second. The values we use today, in order to convert power to horsepower, are as follows: 1 hp = 550 ft-lb/s 1 hp = 33,000 ft-lb/min. 1 hp = 375 mile pounds per hour (mi-lb/hr) 1 hp = 746 watts (electricity conversion) To convert power to horsepower, divide the power by the appropriate conversion based on the units being used. Example: What power would be needed, and also horsepower, to raise the GE-90 turbofan engine into position to install it on a Boeing 777-300 airplane? The engine weighs 19,000 lb, and it must be lifted 4 ft in 2 minutes.

Power = Force × distance ÷ time = 19,000 lb × 4 ft ÷ 2 min. = 38,000 ft-lb/min. Hp = 38,000 ft-lb/min. ÷ 33,000 ft-lb/min. Hp = 1.15 The hoist that will be used to raise this engine into position will need to be powered by an electric motor because the average person will not be able to generate 1.15 hp in their arms for the necessary 2 minutes. Torque Torque is a very interesting concept and occurrence, and it is definitely something that needs to be discussed in conjunction with work and power. Whereas work is described as a force acting through a distance, torque is described as a force acting along a distance. Torque is something that creates twisting and tries to make something rotate. If we push on an object with a force of 10 lb and it moves 10 inches in a straight line, we have done 100 in‑lb of work. By comparison, if we have a wrench 10 inches long that is on a bolt, and we push down on it with a force of 10 lb, a torque of 100 lb-in is applied to the bolt. If the bolt was already tight and did not move as we pushed down on the wrench, the torque of 100 lb‑in would still exist. The formula for torque is: Torque = Force × distance Even though the formula looks the same as the one for calculating work, recognize that the distance value in this formula is not the linear distance an object moves, but rather the distance along which the force is applied. Notice that with torque nothing had to move, because the force is being applied along a distance and not through a distance. Notice also that although the units of work and torque appear to be the same, they are not. The units of work were inch-pounds and the units of torque were pound-inches, and that is what differentiates the two. Torque is very important when thinking about how engines work, both piston engines and gas turbine engines. Both types of engines create torque in advance of being able to create work or power. With a piston engine, a force in pounds pushes down on the top of the piston and tries to make it move. The piston is attached to the connecting rod, which is attached to the crankshaft at an offset. That offset would be like

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the length of the wrench discussed earlier, and the force acting along that length is what creates torque. [Figure 3-6]

Example: A Cessna 172R has a Lycoming IO-360 engine that creates 180 horsepower at 2,700 rpm. How many pound-feet of torque is the engine producing?

For the cylinder in Figure 3-6, there is a force of 500 lb pushing down on the top of the piston. The connecting rod attaches to the crankshaft at an offset distance of 4 in. The product of the force and the offset distance is the torque, in this case 2,000 lb-in.

Torque = 180 × 5,252 ÷ 2,700 = 350 lb-ft

In a turbine engine, the turbine blades at the back of the engine extract energy from the high velocity exhaust gases. The energy extracted becomes a force in pounds pushing on the turbine blades, which happen to be a certain number of inches from the center of the shaft they are trying to make rotate. The number of inches from the turbine blades to the center of the shaft would be like the length of the wrench discussed earlier. Mathematically, there is a relationship between the horsepower of an engine and the torque of an engine. The formula that shows this relationship is as follows: Torque = Horsepower × 5,252 ÷ rpm

Simple Machines A machine is any device with which work may be accomplished. In application, machines can be used for any of the following purposes, or combinations of these purposes. 1. Machines are used to transform energy, as in the case of a generator transforming mechanical energy into electrical energy. 2. Machines are used to transfer energy from one place to another, as in the examples of the connecting rods, crankshaft, and reduction gears transferring energy from an aircraft’s engine to its propeller. 3. Machines are used to multiply force; for example, a system of pulleys may be used to lift a heavy load. The pulley system enables the load to be raised by exerting a force that is smaller than the weight of the load. 4. Machines can be used to multiply speed. A good example is the bicycle, by which speed can be gained by exerting a greater force.

Torque = F × d Torque = 500 × 4 Torque = 2,000 lb-in Torque = 166.7 lb-ft

Force of 500 lb

Piston

5. Machines can be used to change the direction of a force. An example of this use is the flag hoist. A downward force on one side of the rope exerts an upward force on the other side, raising the flag toward the top of the pole. There are only six simple machines. They are the lever, the pulley, the wheel and axle, the inclined plane, the screw, and the gear. Physicists, however, recognize only two basic principles in machines: the lever and the inclined plane. The pulley (block and tackle), the wheel and axle, and gears operate on the machine principle of the lever. The wedge and the screw use the principle of the inclined plane.

Connecting rod

Crankshaft

An understanding of the principles of simple machines provides a necessary foundation for the study of compound machines, which are combinations of two or more simple machines. 4"

Figure 3-6. Piston engine and torque.

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Mechanical Advantage of Machines As identified in statements 3 and 4 under simple machines, a machine can be used to multiply force or to multiply speed. It cannot, however, multiply force

and speed at the same time. In order to gain one, it must lose the other. To do otherwise would mean the machine has more power going out than coming in, and that is not possible. In reference to machines, mechanical advantage is a comparison of the output force to the input force, or the output distance to the input distance. If there is a mechanical advantage in terms of force, there will be a fractional disadvantage in terms of distance. The following formulas can be used to calculate mechanical advantage. Mechanical Advantage = Force Out ÷ Force In Or Mechanical Advantage = Distance Out ÷ Distance In The Lever The simplest machine, and perhaps the most familiar one, is the lever. A seesaw is a familiar example of a lever, with two people sitting on either end of a board and a pivoting point in the middle. There are three basic parts in all levers. They are the fulcrum “F,” a force or effort “E,” and a resistance “R.” Shown in Figure 3-7 are the pivot point “F” (fulcrum), the effort “E” which is applied at a distance “L” from the fulcrum, and a resistance “R” which acts at a distance “l” from the fulcrum. Distances “L” and “l” are the lever arms. The concept of torque was discussed earlier in this chapter, and torque is very much involved in the operation of a lever. When a person sits on one end of a seesaw, that person applies a downward force in pounds which acts along the distance to the center of the seesaw. This combination of force and distance creates torque, which tries to cause rotation. First Class Lever In the first class lever, the fulcrum is located between the effort and the resistance. As mentioned earlier, the seesaw is a good example of a lever, and it happens to be a first class lever. The amount of weight and the distance from the fulcrum can be varied to suit the need. Increasing the distance from the applied effort to the

Resistance “R”

Effort “E” “I”

“L”

Fulcrum “F”

fulcrum, compared to the distance from the fulcrum to the weight being moved, increases the advantage provided by the lever. Crowbars, shears, and pliers are common examples of this class of lever. The proper balance of an airplane is also a good example, with the center of lift on the wing being the pivot point (fulcrum) and the weight fore and aft of this point being the effort and the resistance. When calculating how much effort is required to lift a specific weight, or how much weight can be lifted by a specific effort, the following formula can be used. Effort (E) × Effort Arm (L) = Resistance (R) × Resistance Arm (l)

What this formula really shows is the input torque (effort times effort arm) equals the output torque (resistance times resistance arm). This formula and concept apply to all three classes of levers, and to all simple machines in general. Example: A first class lever is to be used to lift a 500-lb weight. The distance from the weight to the fulcrum is 12 inches and from the fulcrum to the applied effort is 60 inches. How much force is required to lift the weight? Effort (E) × Effort Arm (L) = Resistance (R) × Resistance Arm (l) E × 60 in = 500 lb × 12 in E = 500 lb × 12 in ÷ 60 in E = 100 lb

The mechanical advantage of the lever in this example would be: Mechanical Advantage = Force Out ÷ Force In = 500 lb ÷ 100 lb = 5, or 5 to 1 An interesting thing to note with this example lever is if the applied effort moved down 10 inches, the weight on the other end would only move up 2 inches. The weight being lifted would only move one-fifth as far. The reason for this is the concept of work. Because a lever cannot have more work output than input, if it allows you to lift 5 times more weight, you will only move it 1⁄5 as far as you move the effort. Second Class Lever The second class lever has the fulcrum at one end and the effort is applied at the other end. The resistance is somewhere between these points. A wheelbarrow is a good example of a second class lever, with the wheel at one end being the fulcrum, the handles at the opposite end being the applied effort, and the bucket

Figure 3-7. First class lever.

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Effort “E”

Effort “E”

“L” “I”

“L” “I”

Fulcrum “F” Resistance “R”

Fulcrum “F” Resistance “R”

Figure 3-8. Second class lever.

Figure 3-9. Third class lever.

in the middle being where the weight or resistance is placed. [Figure 3-8]

and brake assembly at the bottom of the landing gear is the resistance. The hydraulic actuator that makes the gear retract is attached somewhere in the middle, and that is the applied effort.

Both first and second class levers are commonly used to help in overcoming big resistances with a relatively small effort. The first class lever, however, is more versatile. Depending on how close or how far away the weight is placed from the fulcrum, the first class lever can be made to gain force or gain distance, but not both at the same time. The second class lever can only be made to gain force. Example: The distance from the center of the wheel to the handles on a wheelbarrow is 60 inches. The weight in the bucket is 18 inches from the center of the wheel. If 300 lb is placed in the bucket, how much force must be applied at the handles to lift the wheelbarrow? Effort (E) × Effort Arm (L) = Resistance (R) × Resistance Arm (l) E × 60 inches = 300 lb × 18 in E = 300 lb × 18 in ÷ 60 in E = 90 lb

The mechanical advantage of the lever in this example would be: Mechanical Advantage = Force Out ÷ Force In = 300 lb ÷ 90 lb = 3.33, or 3.33 to 1 Third Class Lever There are occasions when it is desirable to speed up the movement of the resistance even though a large amount of effort must be used. Levers that help accomplish this are third class levers. As shown in Figure 3-9, the fulcrum is at one end of the lever and the weight or resistance to be overcome is at the other end, with the effort applied at some point between. Third class levers are easily recognized because the effort is applied between the fulcrum and the resistance. The retractable main landing gear on an airplane is a good example of a third class lever. The top of the landing gear, where it attaches to the airplane, is the pivot point. The wheel

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The Pulley Pulleys are simple machines in the form of a wheel mounted on a fixed axis and supported by a frame. The wheel, or disk, is normally grooved to accommodate a rope. The wheel is sometimes referred to as a “sheave” (sometimes “sheaf”). The frame that supports the wheel is called a block. A block and tackle consists of a pair of blocks. Each block contains one or more pulleys and a rope connecting the pulley(s) of each block. Single Fixed Pulley A single fixed pulley is really a first class lever with equal arms. In Figure 3-10, the arm from point “R” to point “F” is equal to the arm from point “F” to point “E” (both distances being equal to the radius of the pulley). When a first class lever has equal arms, the mechanical advantage is 1. Thus, the force of the pull on the rope must be equal to the weight of the object being lifted. The only advantage of a single fixed pulley is to change the direction of the force, or pull on the rope. Single Movable Pulley A single pulley can be used to magnify the force exerted. In Figure 3-11, the pulley is movable, and both ropes extending up from the pulley are sharing in the support of the weight. This single movable pulley acts like a second class lever, with the effort arm (EF) being the diameter of the pulley and the resistance arm (FR) being the radius of the pulley. This type of pulley would have a mechanical advantage of two because the diameter of the pulley is double the radius of the pulley. In use, if someone pulled in 4 ft of the effort rope, the weight would only rise off the floor 2 ft. If the weight was 100 lb, the effort applied would only need to be 50 lb. With this type of pulley, the effort will always be one-half of the weight being lifted.

Effort

E R

F

E

R F

Weight Effort “E”

Figure 3-10. Single fixed pulley.

Block and Tackle A block and tackle is made up of multiple pulleys, some of them fixed and some movable. In Figure 3-12, the block and tackle is made up of four pulleys, the top two being fixed and the bottom two being movable. Viewing the figure from right to left, notice there are four ropes supporting the weight and a fifth rope where the effort is applied. The number of weight supporting ropes determines the mechanical advantage of a block and tackle, so in this case the mechanical advantage is four. If the weight was 200 lb, it would require a 50 lb effort to lift it. The Gear Two gears with teeth on their outer edges, as shown in Figure 3-13, act like a first class lever when one gear drives the other. The gear with the input force is called the drive gear, and the other is called the driven gear. The effort arm is the diameter of the driven gear, and the resistance arm is the diameter of the drive gear.

Weight

Figure 3-11. Single movable pulley.

Notice that the two gears turn in opposite directions (the bottom one clockwise and the top one counterclockwise). The gear on top (yellow) is 9 inches in diameter and has 45 teeth, and the gear on the bottom (blue) is 12 inches in diameter and has 60 teeth. Imagine that the blue gear is driving the yellow one (blue is the drive, yellow is the driven). The mechanical advantage in terms of force would be the effort arm divided by the resistance arm, or 9 ÷ 12, which is 0.75. This would actually be called a fractional disadvantage, because there would be less force out than force in.

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Effort Support Ropes

Weight

Figure 3-13. Spur gears.

Figure 3-12. Block and tackle.

The mechanical advantage in terms of distance (rpm in this case), would be 12 ÷ 9, or 1.33. This analysis tells us that when a large gear drives a small one, the small one turns faster and has less available force. In order to be a force gaining machine, the small gear needs to turn the large one. When the terminology reduction gearbox is used, such as a propeller reduction gearbox, it means that there is more rpm going in than is coming out. The end result is an increase in force, and ultimately torque. Bevel gears are used to change the plane of rotation, so that a shaft turning horizontally can make a vertical shaft rotate. The size of the gears and their number of teeth determine the mechanical advantage, and whether force is being increased or rpm is being increased. If each gear

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has the same number of teeth, there would be no change in force or rpm. [Figure 3-14] The worm gear has an extremely high mechanical advantage. The input force goes into the spiral worm gear, which drives the spur gear. One complete revolution of the worm gear only makes the spur gear turn an amount equal to one tooth. The mechanical advantage is equal to the number of teeth on the spur gear, which in this case is 25. This is a force gaining machine, to the tune of 25 times more output force. [Figure 3-15] The planetary sun gear system is typical of what would be found in a propeller reduction gearbox. The power output shaft of the engine would drive the sun gear in the middle, which rotates the planetary gears and ultimately the ring gear. In this example, the sun gear has 28 teeth, each planet gear has 22 teeth, and the ring gear has 82 teeth. To figure how much gear reduction is taking place, the number of teeth on the

Sun Gear

Ring Gear

Figure 3-14. Bevel gears.

Planetary Gears

Figure 3-16. Planetary sun gear.

Cessna 172 right main gear is sitting on an electronic scale. The airplane was pushed up the ramps to get it on the scales. With an inclined plane, the length of the incline is the effort arm and the vertical height of the incline is the resistance arm. If the length of the incline is five times greater than the height, there will be a force advantage, or mechanical advantage, of five. The Cessna 172 in Figure 3-17 weighed 1,600 lb on the day of the weighing. The ramp it is sitting on is 6 inches tall (resistance Figure 3-15. Worm gear.

ring gear is divided by the number of teeth on the sun gear. In this case, the gear reduction is 2.93, meaning the engine has an rpm 2.93 times greater than the propeller. [Figure 3-16] Inclined Plane The inclined plane is a simple machine that facilitates the raising or lowering of heavy objects by application of a small force over a relatively long distance. Some familiar examples of the inclined plane are mountain highways and a loading ramp on the back of a moving truck. When weighing a small airplane, like a Cessna 172, an inclined plane (ramp) can be used to get the airplane on the scales by pushing it, rather than jacking it. A ramp can be seen in Figure 3-17, where a

Figure 3-17. Ramp in use with a Cessna 172.

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like a weight pushing on the end of a lever, a reaction will occur inside the object which is known as stress. Stress is typically measured in pounds per square foot or pounds per square inch (psi). An external force acting on an object causes the stress to manifest itself in one of five forms, or combination of those five. The five forms are tension, compression, torsion, bending, and shear.

Figure 3-18. A bolt and nut as an inclined plane.

arm) and the length of the ramp is 24 inches (effort arm). To calculate the force needed to push the airplane up the ramps, use the same formula introduced earlier when levers were discussed, as follows: Effort (E) × Effort Arm (L) = Resistance (R) × Resistance Arm (l) E × 24 in = 1,600 lb × 6 in E = 1,600 lb × 6 in ÷ 24 in E = 400 lb

Bolts, screws, and wedges are also examples of devices that operate on the principle of the inclined plane. A bolt, for example, has a spiral thread that runs around its circumference. As the thread winds around the bolt’s circumference, it moves a vertical distance equal to the space between the threads. The circumference of the bolt is the effort arm and the distance between the threads is the resistance arm. [Figure 3-18] Based on this analysis, it can be seen that a fine threaded bolt (more threads per inch) has a greater mechanical advantage than a coarse threaded bolt. A chisel is a good example of a wedge. A chisel might be 8 inches long and only 1⁄2 inch wide, with a sharp tip and tapered sides. The 8-inch length is the effort arm and the 1⁄2-inch width is the resistance arm. This chisel would provide a force advantage (mechanical advantage) of 16.

Stress Whenever a machine is in operation, be it a simple machine like a lever or a screw, or a more complex machine like an aircraft piston engine or a hydraulically operated landing gear, the parts and pieces of that machine will experience something called stress. Whenever an external force is applied to an object,

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Tension Tension is a force that tries to pull an object apart. In the block and tackle system discussed earlier in this chapter, the upper block that housed the two fixed pulleys was secured to an overhead beam. The movable lower block and its two pulleys were hanging by ropes, and the weight was hanging below the entire assembly. The weight being lifted would cause the ropes and the blocks to be under tension. The weight is literally trying to pull the rope apart, and ultimately would cause the rope to break if the weight was too great. Compression Compression is a force that tries to crush an object. An excellent example of compression is when a sheet metal airplane is assembled using the fastener known as a rivet. The rivet passes through a hole drilled in the pieces of aluminum, and then a rivet gun on one side and a bucking bar on the other apply a force. This applied force tries to crush the rivet and makes it expand to fill the hole and securely hold the aluminum pieces together. [Figure 3-19] Torsion Torsion is the stress an object experiences when it is twisted, which is what happens when torque is applied to a shaft. Torsion is actually made up of two other stresses: tension and compression. When a shaft is twisted, tension is experienced at a diagonal to the shaft and compression acts 90 degrees to the tension. [Figure 3-20] The turbine shaft on a turbofan engine, which connects to the compressor in order to drive it, is under a torsion stress. The turbine blades extract energy from the high velocity air as a force in pounds. This force in pounds acts along the length from the blades to the center of the shaft, and creates the torque that causes rotation. [Figure 3-21] Bending An airplane in flight experiences a bending force on the wing as aerodynamic lift tries to raise the wing. This force of lift actually causes the skin on the top of the

Torque applied to shaft

Force from rivet gun

Shaft experiences torsion

Turbine blades applied force

Figure 3-21. Turbofan engine, torque creating torsion in the shaft.

Rivet head

Rivet shank

Bucking bar

wing to compress and the skin on the bottom of the wing to be under tension. When the airplane is on the ground sitting on its landing gear, the force of gravity tries to bend the wing downward, subjecting the bottom of the wing to compression and the top of the wing to tension. [Figure 3-22] During the testing that occurs prior to FAA certification, an airplane manufacturer intentionally bends the wing up and down to make sure it can take the stress without failing. Shear When a shear stress is applied to an object, the force tries to cut or slice through, like a knife cutting through butter. A clevis bolt, which is often used to secure a cable to a part of the airframe, has a shear stress acting on it. As shown in Figure 3-23, a fork fitting is secured

Wing top is under Tension

Figure 3-19. A rivet fastener and compression.

Wing bottom is under Compression

Figure 3-22. Airplane on the ground, wing under tension and compression. Tension stress

Clevis Bolt Force

Rotation

Compression

Figure 3-20. Torsion on a rotating shaft, made up of tension and compression.

Figure 3-23. Clevis bolt, red arrows show opposing forces trying to shear the bolt.

3-15

to the end of the cable, and the fork attaches to an eye on the airframe with the clevis bolt. When the cable is put under tension, the fork tries to slide off the eye by cutting through the clevis bolt. This bolt would be designed to take very high shear loads. Strain If the stress acting on an object is great enough, it can cause the object to change its shape or to become distorted. One characteristic of matter is that it tends to be elastic, meaning it can be forced out of shape when a force is applied, and then return to its original shape when the force is removed. When an object becomes distorted by an applied force, the object is said to be strained. On turbine engine test cells, the thrust of the engine is typically measured by what are called strain gages. When the force (thrust) of the engine is pulling out against the strain gages, the amount of distortion is measured and then translated into the appropriate thrust reading. A deflecting beam style of torque wrench uses the strain on the drive end of the wrench and the resulting distortion of the beam to indicate the amount of torque on a bolt or nut. [Figure 3-24]

Motion The study of the relationship between the motion of bodies or objects and the forces acting on them is often called the study of “force and motion.” In a more specific sense, the relationship between velocity, acceleration, and distance is known as kinematics. Uniform Motion Motion may be defined as a continuing change of position or place, or as the process in which a body undergoes displacement. When an object is at different points in space at different times, that object is said to be in motion, and if the distance the object moves remains the same for a given period of time, the motion may be described as uniform. Thus, an object in uniform motion always has a constant speed. Speed and Velocity In everyday conversation, speed and velocity are often used as if they mean the same thing. In physics they have definite and distinct meanings. Speed refers to how fast an object is moving, or how far the object will travel in a specific time. The speed of an object tells nothing about the direction an object is moving. For example, if the information is supplied that an airplane leaves New York City and travels 8 hours at a speed of 150 mph, this information tells nothing about the direction in which the airplane is moving. At the end of 8 hours, it might be in Kansas City, or if it traveled in a circular route, it could be back in New York City. Velocity is that quantity in physics which denotes both the speed of an object and the direction in which the object moves. Velocity can be defined as the rate of motion in a particular direction. Velocity is also described as being a vector quantity, a vector being a line of specific length, having an arrow on one end or the other. The length of the line indicates the number value and the arrow indicates the direction in which that number is acting.

Figure 3-24. Deflecting beam torque wrench, measures strain by distortion.

3-16

Two velocity vectors, such as one representing the velocity of an airplane and one representing the velocity of the wind, can be added together in what is called vector analysis. Figure 3-25 demonstrates this, with vectors “A” and “B” representing the velocity of the airplane and the wind, and vector “C” being the resultant. With no wind, the speed and direction of the airplane would be that shown by vector “A.” When accounting for the wind direction and speed, the airplane ends up flying at the speed and direction shown by vector “C.”

fps, multiply by 1.467. Velocity is calculated the same way, the only difference being it must be recalculated every time the direction changes.

Vector B = Wind

To calculate acceleration, the following formula is used.

Vec tor

C=

Mov

eme nt o f

airp

lane

Vector A = Velocity of airplane

Acceleration Acceleration is defined as the rate of change of velocity. If the velocity of an object is increased from 20 mph to 30 mph, the object has been accelerated. If the increase in velocity is 10 mph in 5 seconds, the rate of change in velocity is 10 mph in 5 seconds, or 2 mph per second. If this were multiplied by 1.467, it could also be expressed as an acceleration of 2.93 feet per second per second (fps/s). By comparison, the acceleration due to gravity is 32.2 fps/s.

Acceleration (A) =

Velocity Final (Vf) − Velocity Initial (Vi) Time (t)

Example: An Air Force F-15 fighter is cruising at 400 mph. The pilot advances the throttles to full afterburner and accelerates to 1,200 mph in 20 seconds. What is the average acceleration in mph/s and fps/s? A=

Vf − Vi t

A=

1200 − 400 20

A = 40 mph⁄s , or by multiplying by 1.467, 58.7 fps⁄s In the example just shown, the acceleration was found to be 58.7 fps/s. Since 32.2 fps/s is equal to the acceleration due to gravity, divide the F-15’s acceleration by 32.2 to find out how many G forces the pilot is experiencing. In this case, it would be 1.82 Gs. Figure 3-25. Vector analysis for airplane velocity and wind velocity.

Imagine that an airplane is flying in a circular pattern at a constant speed. Because of the circular pattern, the airplane is constantly changing direction, which means the airplane is constantly changing velocity. The reason for this is the fact that velocity includes direction. To calculate the speed of an object, the distance it travels is divided by the elapsed time. If the distance is measured in miles and the time in hours, the units of speed will be miles per hour (mph). If the distance is measured in feet and the time in seconds, the units of speed will be feet per second (fps). To convert mph to

Newton’s Law of Motion First Law When a magician snatches a tablecloth from a table and leaves a full setting of dishes undisturbed, he is not displaying a mystic art; he is demonstrating the principle of inertia. Inertia is responsible for the discomfort felt when an airplane is brought to a sudden halt in the parking area and the passengers are thrown forward in their seats. Inertia is a property of matter. This property of matter is described by Newton’s first law of motion, which states: Objects at rest tend to remain at rest and objects in motion tend to remain in motion at the same speed and in the same direction, unless acted on by an external force.

3-17

When a force acts upon a body, the momentum of that body is changed. The rate of change of momentum is proportional to the applied force. Based on Newton’s second law, the formula for calculating thrust is derived, which states that force equals mass times acceleration (F = MA). Earlier in this chapter, it was determined that mass equals weight divided by gravity, and acceleration equals velocity final minus velocity initial divided by time. Putting all these concepts together, the formula for thrust is: Force =

Weight (Velocity final − Velocity initial) Gravity (Time)

Force =

W (Vf − Vi) Gt

Example: A turbojet engine is moving 150 lb of air per second through the engine. The air enters going 100 fps and leaves going 1,200 fps. How much thrust, in pounds, is the engine creating? F=

W (Vf − Vi) Gt

F=

150 (1200 − 100) 32.2 (1)

F = 5,124 lb of thrust Third Law Newton’s third law of motion is often called the law of action and reaction. It states that for every action there is an equal and opposite reaction. This means that if a force is applied to an object, the object will supply a resistive force exactly equal to and in the opposite direction of the force applied. It is easy to see how this might apply to objects at rest. For example, as a man stands on the floor, the floor exerts a force against his feet exactly equal to his weight. But this law is also applicable when a force is applied to an object in motion. Forces always occur in pairs. The term “acting force” means the force one body exerts on a second body, and reacting force means the force the second body exerts on the first. 3-18

When an aircraft propeller pushes a stream of air backward with a force of 500 lb, the air pushes the blades forward with a force of 500 lb. This forward force causes the aircraft to move forward. A turbofan engine exerts a force on the air entering the inlet duct, causing it to accelerate out the fan duct and the tailpipe. The air accelerating to the rear is the action, and the force inside the engine that makes it happen is the reaction, also called thrust. Circular Motion Circular motion is the motion of an object along a curved path that has a constant radius. For example, if one end of a string is tied to an object and the other end is held in the hand, the object can be swung in a circle. The object is constantly deflected from a straight (linear) path by the pull exerted on the string, as shown in Figure 3-26. When the weight is at point A, due to inertia it wants to keep moving in a straight line and end up at point B. Because of the force being exerted on the string, it is forced to move in a circular path and end up at point C. The string exerts a centripetal force on the object, and the object exerts an equal but opposite force on the string, obeying Newton’s third law of motion. The

B

WT “C” Centripetal Force

Second Law Bodies in motion have the property called momentum. A body that has great momentum has a strong tendency to remain in motion and is therefore hard to stop. For example, a train moving at even low velocity is difficult to stop because of its large mass. Newton’s second law applies to this property. It states:

A

e orc lF

ga ifu ntr e C

Figure 3-26. Circular motion.

force that is equal to centripetal force, but acting in an opposite direction, is called centrifugal force. Centripetal force is always directly proportional to the mass of the object in circular motion. Thus, if the mass of the object in Figure 3-26 is doubled, the pull on the string must be doubled to keep the object in its circular path, provided the speed of the object remains constant. Centripetal force is inversely proportional to the radius of the circle in which an object travels. If the string in Figure 3-26 is shortened and the speed remains constant, the pull on the string must be increased since the radius is decreased, and the string must pull the object from its linear path more rapidly. Using the same reasoning, the pull on the string must be increased if the object is swung more rapidly in its orbit. Centripetal force is thus directly proportional to the square of the velocity of the object. The formula for centripetal force is: Centripetal Force = Mass (Velocity 2) ÷ Radius For the formula above, mass would typically be converted to weight divided by gravity, velocity would be in feet per second, and the radius would be in feet. Example: What would the centripetal force be if a 10 pound weight was moving in a 3-ft radius circular path at a velocity of 500 fps? Centripetal Force = Mass (Velocity 2) ÷ Radius Centripetal Force = 10 (500 2) ÷ 32.2 (3) = 25,880 lb In the condition identified in the example, the object acts like it weighs 2,588 times more than it actually does. It can also be said that the object is experiencing 2,588 Gs (force of gravity). The fan blades in a large turbofan engine, when the engine is operating at maximum rpm, are experiencing many thousands of Gs for the same reason.

Heat Heat is a form of energy. It is produced only by the conversion of one of the other forms of energy. Heat may also be defined as the total kinetic energy of the molecules of any substance. Some forms of energy which can be converted into heat energy are as follows: • Mechanical Energy. This includes all methods of producing increased motion of molecules such

as friction, impact of bodies, or compression of gases. • Electrical Energy. Electrical energy is converted to heat energy when an electric current flows through any form of resistance such as an electric iron, electric light, or an electric blanket. • Chemical Energy. Most forms of chemical reaction convert stored potential energy into heat. Some examples are the explosive effects of gunpowder, the burning of oil or wood, and the combining of oxygen and grease. • Radiant Energy. Electromagnetic waves of certain frequencies produce heat when they are absorbed by the bodies they strike such as x-rays, light rays, and infrared rays. • Nuclear Energy. Energy stored in the nucleus of atoms is released during the process of nuclear fission in a nuclear reactor or atomic explosion. • The Sun. All heat energy can be directly or indirectly traced to the nuclear reactions occurring in the sun. When a gas is compressed, work is done and the gas becomes warm or hot. Conversely, when a gas under high pressure is allowed to expand, the expanding gas becomes cool. In the first case, work was converted into energy in the form of heat; in the second case heat energy was expended. Since heat is given off or absorbed, there must be a relationship between heat energy and work. Also, when two surfaces are rubbed together, the friction develops heat. However, work was required to cause the heat, and by experimentation, it has been shown that the work required and the amount of heat produced by friction are proportional. Thus, heat can be regarded as a form of energy. According to this theory of heat as a form of energy, the molecules, atoms, and electrons in all bodies are in a continual state of motion. In a hot body, these small particles possess relatively large amounts of kinetic energy, but in cooler bodies they have less. Because the small particles are given motion, and hence kinetic energy, work must be done to slide one body over the other. Mechanical energy apparently is transformed, and what we know as heat is really kinetic energy of the small molecular subdivisions of matter. Heat Energy Units Two different units are used to express quantities of heat energy. They are the calorie and the BTU. One calorie is equal to the amount of heat required to change the temperature of 1 gram of water 1 degree Centigrade.

3-19

This term “calorie” (spelled with a lower case c) is 1/1,000 of the Calorie (spelled with a capital C) used in the measurement of the heat energy in foods. One BTU is defined as the amount of heat required to change the temperature of 1 lb of water 1 degree Fahrenheit (1°F). The calorie and the gram are seldom used in discussing aviation maintenance. The BTU, however, is commonly referred to in discussions of engine thermal efficiencies and the heat content of aviation fuel. A device known as the calorimeter is used to measure quantities of heat energy. For example, it may be used to determine the quantity of heat energy available in 1 pound of aviation gasoline. A given weight of the fuel is burned in the calorimeter, and the heat energy is absorbed by a large quantity of water. From the weight of the water and the increase in its temperature, it is possible to compute the heat yield of the fuel. A definite relationship exists between heat and mechanical energy. This relationship has been established and verified by many experiments which show that: One BTU of heat energy = 778 ft-lb of work As discussed earlier in this chapter under the topic “Potential Energy,” one pound of aviation gasoline contains 18,900 BTU of heat energy. Since each BTU is capable of 778 ft-lb of work, 1 lb of aviation gasoline is capable of 14,704,200 ft-lb of work. Heat Energy and Thermal Efficiency Thermal efficiency is the relationship between the potential for power contained in a specific heat source, and how much usable power is actually created when that heat source is used. The formula for calculating thermal efficiency is: Thermal Efficiency = Horsepower Produced ÷ Potential Horsepower in Fuel For example, consider the piston engine used in a small general aviation airplane, which typically consumes 0.5 lb of fuel per hour for each horsepower it creates. Imagine that the engine is creating 200 hp. If we multiply 0.5 by the horsepower of 200, we find the engine is consuming 100 lb of fuel per hour, or 1.67 lb per minute. Earlier in this chapter, one horsepower was found to be 33,000 ft-lb of work per minute. The potential horsepower in the fuel burned for this example engine would be: Hp =

1.67 lb per minute × 18,900 BTU per lb × 778 ft lb per BTU

Hp = 744

3-20

33,000 ft-lb/min

The example engine is burning enough fuel that it has the potential to create 744 horsepower, but it is only creating 200. The thermal efficiency of the engine would be: Thermal Efficiency = Hp Produced ÷ Hp in Fuel = 200 ÷ 744 = .2688 or 26.88% More than 70 percent of the energy in the fuel is not being used to create usable horsepower. The wasted energy is in the form of friction and heat. A tremendous amount of heat is given up to the atmosphere and not used inside the engine to create power. Heat Transfer There are three methods by which heat is transferred from one location to another or from one substance to another. These three methods are conduction, convection, and radiation. Conduction Heat transfer always takes place by areas of high heat energy migrating to areas of low heat energy. Heat transfer by conduction requires that there be physical contact between an object that has a large amount of heat energy and one that has a smaller amount of heat energy. Everyone knows from experience that the metal handle of a heated pan can burn the hand. A plastic or wood handle, however, remains relatively cool even though it is in direct contact with the pan. The metal transmits the heat more easily than the wood because it is a better conductor of heat. Different materials conduct heat at different rates. Some metals are much better conductors of heat than others. Aluminum and copper are used in pots and pans because they conduct heat very rapidly. Woods and plastics are used for handles because they conduct heat very slowly. Figure 3-27 illustrates the different rates of conduction of various metals. Of those listed, silver is the best conductor and lead is the poorest. As previously mentioned, copper and aluminum are used in pots and pans because they are good conductors. It is interesting to note that silver, copper, and aluminum are also excellent conductors of electricity. Liquids are poorer conductors of heat than metals. Notice that the ice in the test tube shown in Figure 328 is not melting rapidly even though the water at the top is boiling. The water conducts heat so poorly that not enough heat reaches the ice to melt it.

1.10

Steam 1.00

1.00

Boiling water

.94

.90 Water

.80 .70

.40

MAGNESIUM

.22 NICKEL

LEAD

.08

IRON

.18

.20

SILVER

ALUMINUM

.37

.30

0

COPPER

.50

.10

Ice

.57

.60

Figure 3-27. Conductivity of various metals.

Gases are even poorer conductors of heat than liquids. It is possible to stand quite close to a stove without being burned because air is such a poor conductor. Since conduction is a process whereby the increase in molecular energy is passed along by actual contact, gases are poor conductors. At the point of application of the heat source, the molecules become violently agitated. These molecules strike adjacent molecules causing them to become agitated. This process continues until the heat energy is distributed evenly throughout the substance. Because molecules are farther apart in gases than in solids, the gases are much poorer conductors of heat. Materials that are poor conductors are used to prevent the transfer of heat and are called heat insulators. A wooden handle on a pot or a soldering iron serves as a heat insulator. Certain materials, such as finely spun glass or asbestos, are particularly poor heat conductors. These materials are therefore used for many types of insulation. Convection Convection is the process by which heat is transferred by movement of a heated fluid (gas or liquid). For example, an incandescent light bulb will, when heated, become increasingly hotter until the air surrounding it begins to move. The motion of the air is upward. This upward motion of the heated air carries the heat away from the hot light bulb by convection. Transfer of heat by convection may be hastened by using a ventilating

Metal ring to keep ice from rising

Figure 3-28. Water as a poor conductor.

fan to move the air surrounding a hot object. The rate of cooling of a hot electronics component, such as the CPU in a computer, can be increased if it is provided with copper fins that conduct heat away from the hot surface. The fins provide large surfaces against which cool air can be blown. A convection process may take place in a liquid as well as in a gas. A good example of this is a pan of water sitting on the stove. The bottom of the pan becomes hot because it conducts heat from the surface it is in contact with. The water on the bottom of the pan also heats up because of conduction. As the heated water starts to rise and cooler water moves in to take its place, the convection process begins. When the circulation of gas or liquid is not rapid enough to remove sufficient heat, fans or pumps are used to accelerate the motion of the cooling material. In some installations, pumps are used to circulate water or oil to help cool large equipment. In airborne installations, electric fans and blowers are used to aid convection. An aircraft air-cooled piston engine is a good example of convection being used to transfer heat. The engine shown in Figure 3-29 is a Continental IO-520, with six heavily finned air-cooled cylinders. This engine does not depend on natural convection for cooling, but rather forced air convection coming from the propeller on the engine. The heat generated inside the engine finds its way to the cylinder cooling fins by conduction, meaning transfer within the metal of the cylinder. Once the heat gets to the fins, forced air flowing around the cylinders carries the heat away.

3-21

Figure 3-29. Aircraft piston engine cooled by convection.

Radiation Conduction and convection cannot wholly account for some of the phenomena associated with heat transfer. For example, the heat one feels when sitting in front of an open fire cannot be transferred by convection because the air currents are moving toward the fire. It cannot be transferred through conduction because the conductivity of the air is very small, and the cooler currents of air moving toward the fire would more than overcome the transfer of heat outward. Therefore, there must be some way for heat to travel across space other than by conduction and convection. The existence of another process of heat transfer is still more evident when the heat from the sun is considered. Since conduction and convection take place only through some medium, such as a gas or a liquid, heat from the sun must reach the earth by another method, since space is an almost perfect vacuum. Radiation is the name given to this third method of heat transfer. The term “radiation” refers to the continual emission of energy from the surface of all bodies. This energy is known as radiant energy. It is in the form of electromagnetic waves, radio waves, or x-rays, which are all alike except for a difference in wave length. These waves travel at the velocity of light and are transmitted through a vacuum more easily than through air because air absorbs some of them. Most forms of energy can 3-22

be traced back to the energy of sunlight. Sunlight is a form of radiant heat energy that travels through space to reach the earth. These electromagnetic heat waves are absorbed when they come in contact with nontransparent bodies. The result is that the motion of the molecules in the body is increased as indicated by an increase in the temperature of the body. The differences between conduction, convection, and radiation may now be considered. First, although conduction and convection are extremely slow, radiation takes place at the speed of light. This fact is evident at the time of an eclipse of the sun when the shutting off of the heat from the sun takes place at the same time as the shutting off of the light. Second, radiant heat may pass through a medium without heating it. For example, the air inside a greenhouse may be much warmer than the glass through which the sun’s rays pass. Third, although heat transfer by conduction or convection may travel in roundabout routes, radiant heat always travels in a straight line. For example, radiation can be cut off with a screen placed between the source of heat and the body to be protected. Specific Heat One important way in which substances differ is in the requirement of different quantities of heat to produce the same temperature change in a given mass of the substance. Each substance requires a quantity of heat,

called its specific heat capacity, to increase the temperature of a unit of its mass 1°C. The specific heat of a substance is the ratio of its specific heat capacity to the specific heat capacity of water. Specific heat is expressed as a number which, because it is a ratio, has no units and applies to both the English and the metric systems. It is fortunate that water has a high specific heat capacity. The larger bodies of water on the earth keep the air and solid matter on or near the surface of the earth at a fairly constant temperature. A great quantity of heat is required to change the temperature of a large lake or river. Therefore, when the temperature falls below that of such bodies of water, they give off large quantities of heat. This process keeps the atmospheric temperature at the surface of the earth from changing rapidly. The specific heat values of some common materials are listed in Figure 3-30. Temperature Temperature is a dominant factor affecting the physical properties of fluids. It is of particular concern when calculating changes in the state of gases. The four temperature scales used extensively are the Centigrade, the Fahrenheit, the absolute or Kelvin, and the Rankine scales. The Centigrade scale is constructed by using the freezing and boiling points of water, under standard conditions, as fixed points of zero and 100, respectively, with 100 equal divisions between. The Fahrenheit scale uses 32° as the freezing point of water and 212° as the boiling point, and has 180 equal divisions between. The absolute or Kelvin

Material

Specific Heat

Lead

0.031

Mercury

0.033

Brass

0.094

Copper

0.095

Iron or Steel

0.113

Glass

0.195

Alcohol

0.547

Aluminum

0.712

Water

1.000

scale is constructed with its zero point established as minus 273°C, meaning 273° below the freezing point of water. The relationships of the other fixed points of the scales are shown in Figure 3-31. When working with temperatures, always make sure which system of measurement is being used and know how to convert from one to another. The conversion formulas are as follows: Degrees Fahrenheit = (1.8 × Degrees Celsius) + 32 Degrees Celsius = (Degrees Fahrenheit – 32) × 5⁄9 Degrees Kelvin = Degrees Celsius + 273 Degrees Rankine = Degrees Fahrenheit + 460 For purposes of calculations, the Rankine scale is commonly used to convert Fahrenheit to absolute. For Fahrenheit readings above zero, 460° is added. Thus, 72°F equals 460° plus 72°, or 532° absolute. If the Fahrenheit reading is below zero, it is subtracted from 460°. Thus −40°F equals 460° minus 40°, or 420° absolute. It should be stressed that the Rankine scale does not indicate absolute temperature readings in accordance with the Kelvin scale, but these conversions may be used for the calculations of changes in the state of gases. The Kelvin and Centigrade scales are used more extensively in scientific work; therefore, some technical manuals may use these scales in giving directions and operating instructions. The Fahrenheit scale is commonly used in the United States, and most people are familiar with it. Therefore, the Fahrenheit scale is used in most areas of this book.

100

373

212

672 Pure water boils

0

273

32

492 Pure water freezes

−273

0

−460

0

Celsius (Centigrade)

Kelvin

Fahrenheit

Molecular motion ceases at absolute zero

Rankine

Figure 3-31. Comparison of temperature scales.

Figure 3-30. Specific heat value for various substances.

3-23

Thermal Expansion/Contraction Thermal expansion takes place in solids, liquids, and gases when they are heated. With few exceptions, solids will expand when heated and contract when cooled. Because the molecules of solids are much closer together and are more strongly attracted to each other, the expansion of solids when heated is very slight in comparison to the expansion in liquids and gases. The expansion of fluids is discussed in the study of Boyle’s law. Thermal expansion in solids must be explained in some detail because of its close relationship to aircraft metals and materials. Because some substances expand more than others, it is necessary to measure experimentally the exact rate of expansion of each one. The amount that a unit length of any substance expands for a one degree rise in temperature is known as the coefficient of linear expansion for that substance. The coefficient of linear expansion for various materials is shown in Figure 3-32. To estimate the expansion of any object, such as a steel rail, it is necessary to know three things about it: its length, the rise in temperature to which it is subjected, and its coefficient of expansion. This relationship is expressed by the equation: Expansion = (coefficient) × (length) × (rise in temperature)

If a steel rod measures exactly 9 ft at 21°C, what is its length at 55°C? The coefficient of expansion for steel is 11 × 10−6. Expansion = (11 × 10 −6 ) × (9 feet) × 34° Expansion = 0.003366 feet Substance

Coefficient of Expansion Per Degree Centigrade

Aluminum

25 × 10− 6

Brass or Bronze

19 × 10− 6

Brick

9 × 10− 6

Copper

17 × 10− 6

Glass (Plate)

9 × 10− 6

Glass (Pyrex)

3 × 10− 6

Ice

51 × 10− 6

Iron or Steel

11 × 10− 6

Lead

29 × 10− 6

Quartz

0.4 × 10− 6

Silver

19 × 10− 6

Figure 3-32. Coefficient of expansion for various materials.

3-24

This amount, when added to the original length of the rod, makes the rod 9.003366 ft long. Its length has only increased by 4⁄100 of an inch. The increase in the length of the rod is relatively small, but if the rod were placed where it could not expand freely, there would be a tremendous force exerted due to thermal expansion. Thus, thermal expansion must be taken into consideration when designing airframes, powerplants, or related equipment.

Pressure Pressure is the amount of force acting on a specific amount of surface area. The force is typically measured in pounds and the surface area in square inches, making the units of pressure pounds per square inch or psi. If a 100-lb weight was placed on top of a block with a surface area of 10 in2, the average weight distribution would be 10 lb for each of the square inches (100 ÷ 10), or 10 psi. When atmospheric pressure is being measured, in addition to psi, other means of pressure measurement can be used. These include inches or millimeters of mercury, and millibars. Standard day atmospheric pressure is equal to 14.7 psi, 29.92 inches of mercury ("Hg), 760 millimeters of mercury (mm Hg), or 1013.2 millibars. The relationship between these units of measure is as follows: 1 psi = 2.04 "Hg 1 psi = 51.7 mm Hg 1 psi = 68.9 millibars The concept behind measuring pressure in inches of mercury involves filling a test tube with the liquid mercury and then covering the top. The test tube is then turned upside down and placed in an open container of mercury, and the top is uncovered. Gravity acting on the mercury in the test tube will try to make the mercury run out. Atmospheric pressure pushing down on the mercury in the open container tries to make the mercury stay in the test tube. At some point these two forces (gravity and atmospheric pressure) will equal out and the mercury will stabilize at a certain height in the test tube. Under standard day atmospheric conditions, the air in a 1-in2 column extending all the way to the top of the atmosphere would weigh 14.7 lb. A 1 in2 column of mercury, 29.92 inches tall, would also weigh 14.7 lb. That is why 14.7 psi is equal to 29.92 "Hg. Figure 3-33 demonstrates this point.

Vacuum

40

50 60

30 14.7 psi Atmospheric pressure

760 mm 29.92 in

70

20

80

10

90

0

100

Fuel Pressure psi

Figure 3-34. Psig read on a fuel pressure gauge. Figure 3-33. Atmospheric pressure as inches of mercury.

Gauge Pressure When an instrument, such as an oil pressure gauge, fuel pressure gauge, or hydraulic system pressure gauge, displays pressure which is over and above ambient, the reading is referred to as gauge pressure (psig). This can be seen on the fuel pressure gauge shown in Figure 3-34. When the oil, fuel, or hydraulic pump is not turning, and there is no pressure being created, the gauge will read zero. Absolute Pressure A gauge that includes atmospheric pressure in its reading is measuring what is known as absolute pressure, or psia. Absolute pressure is equal to gauge pressure plus atmospheric pressure. If someone hooked up a psia indicating instrument to an engine’s oil system, the gauge would read atmospheric pressure when the engine was not running. Since this would not make good sense to the typical operator, psia gauges are not used in this type of application. For the manifold pressure on a piston engine, a psia gauge does make good sense. Manifold pressure on a piston engine can read anywhere from less than atmospheric pressure if the engine is not supercharged, to more than atmospheric if it is supercharged. The only gauge that has the flexibility to show this variety of readings is the absolute pressure gauge. Figure 3-35 shows a manifold pressure gauge, with a readout that ranges from 10 "Hg to 35 "Hg. Remember that 29.92 "Hg is standard day atmospheric.

35 30 25

30 28 29.5 psi

Man. Press. ("Hg)

26 24 22 20 18

Fuel Flow ( Gal⁄Hr )

14

20

10 4.0 psi

15

6

10

Figure 3-35. Manifold pressure gauge indicating absolute pressure.

Differential Pressure Differential pressure, or psid, is the difference between pressures being read at two different locations within a system. For example, in a turbine engine oil system the pressure is read as it enters the oil filter, and also as it leaves the filter. These two readings are sent to a transmitter which powers a light located on the flight deck. Across anything that poses a resistance to flow, like an oil filter, there will be a drop in pressure. If the filter starts to clog, the pressure drop will become 3-25

a long drive on a hot day, the pressure in the tires of an automobile increases, and a tire which appeared to be somewhat “soft” in cool morning temperature may appear normal at a higher midday temperature. Such phenomena as these have been explained and set forth in the form of laws pertaining to gases and tend to support the kinetic theory.

Figure 3-36. Differential pressure gauge.

greater, eventually causing the advisory light on the flight deck to come on. Figure 3-36 shows a differential pressure gauge for the pressurization system on a Boeing 737. In this case, the difference in pressure is between the inside and the outside of the airplane. If the pressure difference becomes too great, the structure of the airplane could become overstressed.

Gas Laws The simple structure of gases makes them readily adaptable to mathematical analysis from which has evolved a detailed theory of the behavior of gases. This is called the kinetic theory of gases. The theory assumes that a body of gas is composed of identical molecules which behave like minute elastic spheres, spaced relatively far apart and continuously in motion. The degree of molecular motion is dependent upon the temperature of the gas. Since the molecules are continuously striking against each other and against the walls of the container, an increase in temperature with the resulting increase in molecular motion causes a corresponding increase in the number of collisions between the molecules. The increased number of collisions results in an increase in pressure because a greater number of molecules strike against the walls of the container in a given unit of time. If the container were an open vessel, the gas would expand and overflow from the container. However, if the container is sealed and possesses elasticity (such as a rubber balloon), the increased pressure causes the container to expand. For instance, when making

3-26

Boyle’s Law As previously stated, compressibility is an outstanding characteristic of gases. The English scientist, Robert Boyle, was among the first to study this characteristic that he called the “springiness of air.” By direct measurement he discovered that when the temperature of a combined sample of gas was kept constant and the absolute pressure doubled, the volume was reduced to half the former value. As the applied absolute pressure was decreased, the resulting volume increased. From these observations, he concluded that for a constant temperature the product of the volume and absolute pressure of an enclosed gas remains constant. Boyle’s law is normally stated: “The volume of an enclosed dry gas varies inversely with its absolute pressure, provided the temperature remains constant.” The following formula is used for Boyle’s law calculations. Remember, pressure needs to be in the absolute. Volume 1 × Pressure 1 = Volume 2 × Pressure 2 Or V1P1 = V2P2 Example: 10 ft 3 of nitrogen is under a pressure of 500 psia. If the volume is reduced to 7 ft 3, what will the new pressure be? [Figure 3-37] V1P1 = V2P2 10 (500) = 7 (P2) 10 (500) ÷ 7 = P2 P2 = 714.29 psia The useful applications of Boyle’s law are many and varied. Some applications more common to aviation are: (1) the carbon dioxide (CO2) bottle used to inflate life rafts and life vests; (2) the compressed oxygen and the acetylene tanks used in welding; (3) the compressed air brakes and shock absorbers; and (4) the use of oxygen tanks for high altitude flying and emergency use. Charles’ Law The French scientist, Jacques Charles, provided much of the foundation for the modern kinetic theory of gases. He found that all gases expand and contract in direct proportion to the change in the absolute tem-

General Gas Law By combining Boyle’s and Charles’ laws, a single expression can be derived which states all the information contained in both. The formula which is used to express the general gas law is as follows:

Force pushing down on gas

Pressure 1 (Volume 1) Pressure 2 (Volume 2) = Temperature 2 Temperature 1 Or P1 (V1) (T2) = P2 (V2) (T1) When using the general gas law formula, temperature and pressure must be in the absolute.

Pressure increasing

Temperature held constant

Figure 3-37. Boyle’s law example.

perature, provided the pressure is held constant. As a formula, this law is shown as follows: Volume 1 × Absolute Temperature 2 = Volume 2 × Absolute Temperature 1 Or V1T2 = V2T1 Charles’ law also works if the volume is held constant, and pressure and temperature are the variables. In this case, the formula would be as follows: P1T2 = P2T1 For this second formula, pressure and temperature must be in the absolute. Example: A 15-ft 3 cylinder of oxygen is at a temperature of 70°F and a pressure of 750 psig. The cylinder is placed in the sun and the temperature of the oxygen increases to 140°F. What would be the new pressure in psig? 70 degrees Fahrenheit = 530 degrees Rankine 140 degrees Fahrenheit = 600 degrees Rankine 750 psig + 14.7 = 764.7 psia P1T2 = P2T1 764.7 (600) = P2 (530) P2 = 764.7 (600) ÷ 530 P2 = 865.7 psia P2 = 851 psig

Example: 20 ft 3 of the gas argon is compressed to 15 ft 3. The gas starts out at a temperature of 60°F and a pressure of 1,000 psig. After being compressed, its temperature is 90°F. What would its new pressure be in psig? 60 degrees Fahrenheit = 520 degrees Rankine 90 degrees Fahrenheit = 550 degrees Rankine 1,000 psig + 14.7 = 1,014.7 psia P1 (V1) (T2) = P2 (V2) (T1) 1,014.7 (20) (550) = P2 (15) (520) P2 = 1,431 psia P2 = 1,416.3 psig Dalton’s Law If a mixture of two or more gases that do not combine chemically is placed in a container, each gas expands throughout the total space and the absolute pressure of each gas is reduced to a lower value, called its partial pressure. This reduction is in accordance with Boyle’s law. The pressure of the mixed gases is equal to the sum of the partial pressures. This fact was discovered by Dalton, an English physicist, and is set forth in Dalton’s law: “A mixture of several gases which do not react chemically exerts a pressure equal to the sum of the pressures which the several gases would exert separately if each were allowed to occupy the entire space alone at the given temperature.”

Fluid Mechanics A fluid, by definition, is any substance that is able to flow if it is not in some way confined or restricted. Liquids and gases are both classified as fluids, and often act in a very similar way. One significant difference comes into play when a force is applied to these fluids. In this case, liquids tend to be incompressible and gases are highly compressible. Many of the principles that aviation is based on, such as the theory of lift on a wing and the force generated by a hydraulic 3-27

system, can be explained and quantified by using the laws of fluid mechanics.

exactly equals the buoyant force of the water, which the scale shows to be 7 lb.

Buoyancy A solid body submerged in a liquid or a gas weighs less than when weighed in free space. This is because of the upward force, called buoyant force, which any fluid exerts on a body submerged in it. An object will float if this upward force of the fluid is greater than the weight of the object. Objects denser than the fluid, even though they sink readily, appear to lose a part of their weight when submerged. A person can lift a larger weight under water than he or she can possibly lift in the air.

Archimedes (287–212 B.C.) performed similar experiments. As a result, he discovered that the buoyant force which a fluid exerts upon a submerged body is equal to the weight of the fluid the body displaces. This statement is referred to as Archimedes’ principle. This principle applies to all fluids, gases as well as liquids. Just as water exerts a buoyant force on submerged objects, air exerts a buoyant force on objects submerged in it.

The following experiment is illustrated in Figure 3-38. The overflow can is filled to the spout with water. The heavy metal cube is first weighed in still air and weighs 10 lb. It is then weighed while completely submerged in the water and it weighs 3 lb. The difference between the two weights is the buoyant force of the water. As the cube is lowered into the overflow can, the water is caught in the catch bucket. The volume of water which overflows equals the volume of the cube. (The volume of irregular shaped objects can be measured by this method.) If this experiment is performed carefully, the weight of the water displaced by the metal cube

The amount of buoyant force available to an object can be calculated by using the following formula: Buoyant Force = Volume of Object × Density of Fluid Displaced

If the buoyant force is more than the object weighs, the object will float. If the buoyant force is less than the object weighs, the object will sink. For the object that sinks, its measurable weight will be less by the weight of the displaced fluid. Example: A 10-ft 3 object weighing 700 lb is placed in pure water. Will the object float? If the object sinks, what is its measurable weight in the submerged condition? If the object floats, how many cubic feet of its volume is below the water line? Buoyant Force = Volume of Object × Density of Fluid Displaced = 10 (62.4) = 624 lb

3

Because the buoyant force is less than the object weighs, the object will sink. The difference between the buoyant force and the object’s weight will be its measurable weight, or 76 lb.

10 lbs.

Catch bucket

0.11 cu'

0.11 cu'

Overflow can 7

Figure 3-38. Example of buoyancy.

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Two good examples of buoyancy are a helium filled airship and a seaplane on floats. An airship is able to float in the atmosphere and a seaplane is able to float on water. That means both have more buoyant force than weight. Figure 3-39 is a DeHavilland Twin Otter seaplane, with a gross takeoff weight of 12,500 lb. At a minimum, the floats on this airplane must be large enough to displace a weight in water equal to the airplane’s weight. According to Title 14 of the Code of Federal Regulations (14 CFR) part 23, the floats must be 80 percent larger than the minimum needed to support the airplane. For this airplane, the necessary size of the floats would be calculated as follows:

Divide the airplane weight by the density of water. 12,500 ÷ 62.4 = 200.3 ft 3 Multiply this volume by 80%. 200.3 × 80% = 160.2 ft 3 Add the two volumes together to get the total volume of the floats. 200.3 + 160.2 = 360.5 ft 3

By looking at the Twin Otter in Figure 3-39, it is obvious that much of the volume of the floats is out of the water. This is accomplished by making sure the floats have at least 80 percent more volume than the minimum necessary. Some of the large Goodyear airships have a volume of 230,000 ft 3. Since the fluid they are submerged in is air, to find the buoyant force of the airship, the volume of the airship is multiplied by the density of air (.07651 lb⁄ ft3). For this Goodyear airship, the buoyant force is 17,597 lb. Figure 3-40 shows an inside view of the Goodyear airship. The ballonets, items 2 and 4 in the picture, are air chambers within the airship. Through the air scoop, item 9, air can be pumped into the ballonets or evacuated from the ballonets in order to control the weight of the airship. Controlling the weight of the airship controls how much positive or negative lift it has. Although the airship is classified as a lighter-than-air aircraft, it is in fact flown in a condition slightly heavier than air. Fluid Pressure The pressure exerted on the bottom of a container by a liquid is determined by the height of the liquid and not by the shape of the container. This can be seen in Figure 3-41, where three different shapes and sizes of containers are full of colored water. Even though they are different shapes and have different volumes of liquid, each one has a height of 231 inches. Because of this height, each one would exert a pressure on the bottom of 8.34 psi. The container on the left, with a surface area of 1 in 2, contains a volume of 231 in 3 (one gallon). One gallon of water weighs 8.34 lb, which is why the pressure on the bottom is 8.34 psi. Still thinking about Figure 3-41, if the pressure was measured half way down, it would be half of 8.34, or 4.17 psi. In other words, the pressure is adjustable by varying the height of the column. Pressure based on the column height of a fluid is known as static pressure. With liquids, such as gasoline, it is sometimes referred to as a head of pressure. For example, if a carburetor needs to have 2 psi supplied to its inlet (head of pressure), this could be accomplished by having the

Figure 3-39. De Havilland Twin Otter seaplane.

Nose cone support

Suspension cables

Light sign

Neoprene cover Control surfaces

Air valves Forward Aft balloonet balloonet Passenger car Engines Air scoops

Figure 3-40. The Goodyear Airship and buoyancy.

Each container is filled with colored water to a height of 231 inches.

1 in2

100 in2

150 in2

Each pressure gauge reads 8.34 psi

Figure 3-41. Fluid pressure based on column height.

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fuel tank positioned the appropriate number of inches higher than the carburetor. As identified in the previous paragraph, pressure due to the height of a fluid column is known as static pressure. When a fluid is in motion, and its velocity is converted to pressure, that pressure is known as ram. When ram pressure and static pressure are added together, the result is known as total pressure. In the inlet of a gas turbine engine, for example, total pressure is often measured to provide a signal to the fuel metering device or to provide a signal to a gauge on the flight deck. Pascal’s Law The foundations of modern hydraulics and pneumatics were established in 1653 when Pascal discovered that pressure set up in a fluid acts equally in all directions. This pressure acts at right angles to containing surfaces. When the pressure in the fluid is caused solely by the fluid’s height, the pressure against the walls of the container is equal at any given level, but it is not equal if the pressure at the bottom is compared to the pressure half way down. The concept of the pressure set up in a fluid, and how it relates to the force acting on the fluid and the surface area through which it acts, is Pascal’s law. In Figure 3-41, if a piston is placed at the top of the cylinder and an external force pushes down on the piston, additional pressure will be created in the liquid. If the additional pressure is 100 psi, this 100 psi will act equally and undiminished from the top of the cylinder all the way to the bottom. The gauge at the bottom will now read 108.34 psi, and if a gauge were positioned half way down the cylinder, it would read 104.17 psi (100 plus half of 8.34). Pascal’s law, when dealing with the variables of force, pressure, and area, is dealt with by way of the following formula.

F A

P

Figure 3-42. Force, area, pressure relationship.

An easy and convenient way to remember the formulas for Pascal’s law, and the relationship between the variables, is with the triangle shown in Figure 3-42. If the variable we want to solve for is covered up, the position of the remaining two variables shows the proper math relationship. For example, if the “A” (area) is covered up, what remains is the “F” on the top and the “P” on the bottom, meaning force divided by pressure. The simple hydraulic system in Figure 3-43 has a 5‑lb force acting on a piston with a 1⁄2 -in2 surface area. Based on Pascal’s law, the pressure in the system would be equal to the force applied divided by the area of the piston, or 10 psi. As shown in Figure 3-43, the pressure of 10 psi is present everywhere in the fluid. The hydraulic system in Figure 3-44 is a little more complex than the one in Figure 3-43. In Figure 3-44, the input force of 5 lb is acting on a 1⁄2 -in2 piston, creating a pressure of 10 psi. The input cylinder and piston is connected to a second cylinder, which contains a 5‑in2 piston. The pressure of 10 psi created by the

Force = Pressure × Area In this formula, the force is in units of pounds, the pressure is in pounds per square inch (psi), and the area is in square inches. By transposing the original formula, we have two additional formulas, as follows: Pressure = Force ÷ Area And Area = Force ÷ Pressure

10 psi 10 psi

Force = 5 lb

Piston area = 1⁄2 in2 Pressure = Force ÷ Area Pressure = 5 ÷ 1⁄2 Pressure = 10 psi

Figure 3-43. Pressure created in a hydraulic system.

3-30

Distance moved = 1 in

Force = 50 lb

10 psi

50 lb

Force = 5 lb

1

Piston area = ⁄2 in

2

Piston area = 5 in2

Force = Pressure × Area Force = 10 psi × 5 in2 Force = 50 lb

Distance moved = 20 in

Input piston area = 1⁄4 in2

10 psi

Output piston area = 15 in2

Piston area (distance) = Piston area (distance) 1 ⁄4 in2 (20 in) = 5 in (distance) 5 ÷ 5 = Distance Distance = 1 in

Figure 3-44. Output force created in a hydraulic system.

Figure 3-45. Piston movement in a hydraulic system.

input piston pushes on the piston in the second cylinder, creating an output force of 50 pounds.

to move. In a simple two-piston hydraulic system, the relationship between the piston area and the distance moved is shown by the following formula.

More often than not, the purpose of a hydraulic system is to generate a large output force, with the input force being much less. In Figure 3-44, the input force is 5 lb and the output force is 50 lb, or 10 times greater. The relationship between the output force and the input force, as discussed earlier in this chapter, is known as mechanical advantage. The mechanical advantage in Figure 3-44 would be 50 divided by 5, or 10. The following formulas can be used to calculate mechanical advantage. Mechanical Advantage = Force Out ÷ Force In Or Mechanical Advantage = Distance Out ÷ Distance In Earlier in this chapter when simple machines, such as levers and gears were discussed, it was identified that no machine allows us to gain work. The same statement holds true for a hydraulic system, that we get no more work out of a hydraulic system than we put in. Since work is equal to force times distance, if we gain force with a hydraulic system, we must lose distance. We only get the same work out, if the system is 100 percent efficient. In order to think about the distance that the output piston will move in response to the movement of the input piston, the volume of fluid displaced must be considered. In the study of geometry, one learns that the volume of a cylinder is equal to the cylinder’s surface area multiplied by its height. So when a piston of 2 in2 moves down in a cylinder a distance of 10 in, it displaces a volume of fluid equal to 20 in3 (2 in2 × 10 in). The 20 in3 displaced by the first piston is what moves over to the second cylinder and causes its piston

Input Piston Area (Distance Moved) = Output Piston Area (Distance Moved) In essence, this formula shows that the volume in is equal to the volume out. This concept is shown in Figure 3-45, where a small input piston moves a distance of 20 inches, and the larger output piston only moves a distance of 1 inch. Example: A two-piston hydraulic system, like that shown in Figure 3-45, has an input piston with an area of 1⁄4 in2 and an output piston with an area of 15 in2. An input force of 50 lb is applied, and the input piston moves 30 inches. What is the pressure in the system, how much force is generated by the output piston, how far would the output piston move, and what is the mechanical advantage? Pressure = Force ÷ Area = 50 ÷ 1⁄4 = 200 psi Force = Pressure × Area = 200 × 15 = 3,000 lb Mechanical Advantage = Force Out ÷ Force In = 3,000 ÷ 50 = 60 Input Piston Area (Distance Moved) = Output Piston Area (Distance Moved) 1⁄ (30) = 15 (Distance Moved) 4 1⁄ (30) ÷ 15 = Distance Moved 4 Distance Moved = 1⁄2 in

3-31

Part of understanding Pascal’s law and hydraulics involves utilizing formulas, and recognizing the relationship between the individual variables. Before the numbers are plugged into the formulas, it is often possible to analyze the variables in the system and come to a realization about what is happening. For example, look at the variables in Figure 3-45 and notice that the output piston is 20 times larger than the input piston (5 in 2 compared to 1⁄4 in 2). That comparison tells us that the output force will be 20 times greater than the input force, and also that the output piston will only move 1/20 as far. Without doing any formula based calculations, we can conclude that the hydraulic system in question has a mechanical advantage of 20. Bernoulli’s Principle Bernoulli’s principle was originally stated to explain the action of a liquid flowing through the varying cross-sectional areas of tubes. In Figure 3-46 a tube is shown in which the cross-sectional area gradually decreases to a minimum diameter in its center section. A tube constructed in this manner is called a “venturi,” or “venturi tube.” Where the cross-sectional area is decreasing, the passageway is referred to as a converging duct. As the passageway starts to spread out, it is referred to as a diverging duct. As a liquid (fluid) flows through the venturi tube, the gauges at points “A,” “B,” and “C” are positioned to register the velocity and the static pressure of the liquid. The venturi in Figure 3-46 can be used to illustrate Bernoulli’s principle, which states that: The static pressure of a fluid (liquid or gas) decreases at points where the velocity of the fluid increases, provided no energy is added to nor taken away from the fluid. The velocity of the air is kinetic energy and the static pressure of the air is potential energy.

Velocity

Pressure

Velocity

Pressure

Low High

Low High

Low High

Low High

“A”

“B”

“C”

Low High

Low High

Velocity

Pressure

Figure 3-46. Bernoulli’s principle and a venturi.

3-32

In the wide section of the venturi (points A and C of Figure 3-46), the liquid moves at low velocity, producing a high static pressure, as indicated by the pressure gauge. As the tube narrows in the center, it must contain the same volume of fluid as the two end areas. In this narrow section, the liquid moves at a higher velocity, producing a lower pressure than that at points A and C, as indicated by the velocity gauge reading high and the pressure gauge reading low. A good application for the use of the venturi principle is in a float-type carburetor. As the air flows through the carburetor on its way to the engine, it goes through a venturi, where the static pressure is reduced. The fuel in the carburetor, which is under a higher pressure, flows into the lower pressure venturi area and mixes with the air. Bernoulli’s principle is extremely important in understanding how some of the systems used in aviation work, including how the wing of an airplane generates lift or why the inlet duct of a turbine engine on a subsonic airplane is diverging in shape. The wing on a slow moving airplane has a curved top surface and a relatively flat bottom surface. The curved top surface acts like half of the converging shaped middle of a venturi. As the air flows over the top of the wing, the air speeds up, and its static pressure decreases. The static pressure on the bottom of the wing is now greater than the pressure on the top, and this pressure difference creates the lift on the wing. Bernoulli’s principle and the concept of lift on a wing is covered in greater depth in “Aircraft Theory of Flight” located in this chapter.

Sound Sound has been defined as a series of disturbances in matter that the human ear can detect. This definition can also be applied to disturbances which are beyond the range of human hearing. There are three elements which are necessary for the transmission and reception of sound. These are the source, a medium for carrying the sound, and the detector. Anything which moves back and forth (vibrates) and disturbs the medium around it may be considered a sound source. An example of the production and transmission of sound is the ring of a bell. When the bell is struck and begins to vibrate, the particles of the medium (the surrounding air) in contact with the bell also vibrate. The vibrational disturbance is transmitted from one particle of the medium to the next, and the vibrations travel in a “wave” through the medium until they reach the ear. The eardrum, acting as detector, is set in motion by the vibrating particles of air, and the brain interprets

the eardrum’s vibrations as the characteristic sound associated with a bell. Wave Motion Since sound is a wave motion in matter, it can best be understood by first considering water waves. When an object is thrown into a pool, a series of circular waves travel away from the disturbance. In Figure 3-47 such waves are seen from a top perspective, with the waves traveling out from the center. In the cross-section perspective in Figure 3-47, notice that the water waves are a succession of crests and troughs. The wavelength is the distance from the crest of one wave to the crest of the next. Water waves are known as transverse waves because the motion of the water molecules is up and down, or at right angles to the direction in which the waves are traveling. This can be seen by observing a cork on the water, bobbing up and down as the waves pass by. Sound travels through matter in the form of longitudinal wave motions. These waves are called longitudinal waves because the particles of the medium vibrate back and forth longitudinally in the direction of propagation. [Figure 3-48] When the tine of a tuning fork moves in an outward direction, the air immediately in front of the tine is compressed so that its momentary pressure is raised above that at other points in the surrounding medium. Because air is elastic, this disturbance is transmitted progressively in an outward direction from the tine in the form of a compression wave.

Tuning fork

Rarefaction

Compression

Amplitude

Wave length

Figure 3-48. Sound propagation by a tuning fork.

When the tine returns and moves in an inward direction, the air in front of the tine is rarefied so that its momentary pressure is reduced below that at other points in the surrounding medium. This disturbance is transmitted in the form of a rarefaction (expansion) wave and follows the compression wave through the medium. The progress of any wave involves two distinct motions: (1) The wave itself moves forward with constant speed, and (2) simultaneously, the particles of the medium that convey the wave vibrate harmonically. Examples of harmonic motion are the motion of a clock pendulum, the balance wheel in a watch, and the piston in a reciprocating engine. Speed of Sound In any uniform medium, under given physical conditions, sound travels at a definite speed. In some substances, the velocity of sound is higher than in others. Even in the same medium under different conditions of temperature, pressure, and so forth, the velocity of sound varies. Density and elasticity of a medium are the two basic physical properties which govern the velocity of sound.

Pressure waves traveling outward

Round object dropped into the water

Crest

Crest Trough

Figure 3-47. Relationship between sound and waves in water.

In general, a difference in density between two substances is sufficient to indicate which one will be the faster transmission medium for sound. For example, sound travels faster through water than it does through air at the same temperature. However, there are some surprising exceptions to this rule of thumb. An outstanding example among these exceptions involves comparison of the speed of sound in lead and aluminum at the same temperature. Sound travels at 16,700 fps in 3-33

Mach Number In the study of aircraft that fly at supersonic speeds, it is customary to discuss aircraft speed in relation to the velocity of sound (approximately 760 miles per hour (mph) at 59°F). The term “Mach number” has been given to the ratio of the speed of an aircraft to the speed of sound, in honor of Ernst Mach, an Austrian scientist. If the speed of sound at sea level is 760 mph, an aircraft flying at a Mach number of 1.2 at sea level would be traveling at a speed of 760 mph × 1.2 = 912 mph. Frequency of Sound The term “pitch” is used to describe the frequency of a sound. The outstanding recognizable difference between the tones produced by two different keys on a piano is a difference in pitch. The pitch of a tone is proportional to the number of compressions and rarefactions received per second, which in turn, is determined by the vibration frequency of the sounding source. A good example of frequency is the noise generated by a turbofan engine on a commercial airliner. The high tip speeds of the fan in the front of the engine creates a high frequency sound, and the hot exhaust creates a low frequency sound. Loudness When a bell rings, the sound waves spread out in all directions and the sound is heard in all directions. When a bell is struck lightly, the vibrations are of small amplitude and the sound is weak. A stronger blow produces vibrations of greater amplitude in the bell, and the sound is louder. It is evident that the amplitude of the air vibrations is greater when the amplitude of the vibrations of the source is increased. Hence, the loudness of the sound depends on the amplitude of the vibrations of the sound waves. As the distance from the source increases, the energy in each wave spreads out, and the sound becomes weaker.

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Measurement of Sound Intensity Sound intensity is measured in decibels, with a decibel being the ratio of one sound to another. One decibel (dB) is the smallest change in sound intensity the human ear can detect. A faint whisper would have an intensity of 20 dB, and a pneumatic drill would be 80 dB. The engine on a modern jetliner, at takeoff thrust, would have a sound intensity of 90 dB when heard by someone standing 150 ft away. A 110 dB noise, by comparison, would sound twice as loud as the jetliner’s engine. Figure 3-49 shows the sound intensity from a variety of different sources. Doppler Effect When sound is coming from a moving object, the object’s forward motion adds to the frequency as sensed from the front and takes away from the frequency as sensed from the rear. This change in frequency is known as the Doppler effect, and it explains Lethal Level Turbojet engine at 50 ft 50 horsepower siren at 50 ft

THRESHOLD OF FEELING

Using density as a rough indication of the speed of sound in a given substance, it can be stated as a general rule that sound travels fastest in solid materials, slower in liquids, and slowest in gases. The velocity of sound in air at 0°C (32°F) is 1,087 fps and increases by 2 fps for each Centigrade degree of temperature rise (1.1 fps for each degree Fahrenheit).

As the sound wave advances, variations in pressure occur at all points in the transmitting medium. The greater the pressure variations, the more intense the sound wave. The intensity is proportional to the square of the pressure variation regardless of the frequency. Thus, by measuring pressure changes, the intensities of sounds having different frequencies can be compared directly.

THRESHOLD OF AUDIBILITY

aluminum at 20°C, and only 4,030 fps in lead at 20°C, despite the fact that lead is much more dense than aluminum. The reason for such exceptions is found in the fact, mentioned above, that sound velocity depends on elasticity as well as density.

180 160 140 130 120 110 100 90 80 70 60 50 40 30 20 10 0

Turbojet engine at takeoff at 150 ft Automobile horn Riveting machine, as heard by operator An express train passing close by Modern turbo engine at takeoff at 150 ft Subway train at 20 ft

(1⁄2 the loudness of 110 dB noise)

Pneumatic drill at 50 ft Vacuum cleaner at 50 ft Nearby freeway auto traffic Private business office Residential area in the evening Soft whisper at 5 ft Radio studio Faint whisper

Figure 3-49. Sound intensity from different sources.

why the sound from an airplane seems different as it approaches compared to how it sounds as it flies overhead. As it approaches, it becomes both louder and higher pitched. As it flies away, the loudness and pitch both decrease noticeably. If an airplane is flying at or higher than the speed of sound, the sound energy cannot travel out ahead of the airplane, because the airplane catches up to it the instant it tries to leave. The sound energy being created by the airplane piles up, and attaches itself to the structure of the airplane. As the airplane approaches, a person standing on the ground will not be able to hear it until it gets past their position, because the sound energy is actually trailing behind the airplane. When the sound of the airplane is heard, it will be in the form of what is called a sonic boom. Resonance All types of matter, regardless of whether it is a solid, liquid, or gas, have a natural frequency at which the atoms within that matter vibrate. If two pieces of matter have the same natural frequency, and one of them starts to vibrate, it can transfer its wave energy to the other one and cause it to vibrate. This transfer of energy is known as resonance. Some piston engine powered airplanes have an rpm range that they are placarded to avoid, because spinning the prop at that rpm can cause vibration problems. The difficulty lies in the natural frequency of the metal in the prop, and the frequency of vibration that will be set up with a particular tip speed for the prop. At that particular rpm, stresses can be set up that could lead to the propeller coming apart.

The Atmosphere Aviation is so dependent upon that category of fluids called gases and the effect of forces and pressures acting upon gases, that a discussion of the subject of the atmosphere is important to the persons maintaining and repairing aircraft. Data available about the atmosphere may determine whether a flight will succeed, or whether it will even become airborne. The various components of the air around the earth, the changes in temperatures and pressures at different levels above the earth, the properties of weather encountered by aircraft in flight, and many other detailed data are considered in the preparation of flight plans. Pascan and Torricelli have been credited with developing the barometer, an instrument for measuring atmospheric pressure. The results of their experiments are

still used today with very little improvement in design or knowledge. They determined that air has weight which changes as altitude is changed with respect to sea level. Today scientists are also interested in how the atmosphere affects the performance of the aircraft and its equipment. Composition of the Atmosphere The atmosphere is a complex and ever changing mixture. Its ingredients vary from place to place and from day to day. In addition to a number of gases, it contains quantities of foreign matter such as pollen, dust, bacteria, soot, volcanic ash, spores, and dust from outer space. The composition of the air remains almost constant from sea level up to its highest level, but its density diminishes rapidly with altitude. Six miles up, for example, it is too thin to support respiration, and 12 miles up, there is not enough oxygen to support combustion, except in some specially designed turbine engine powered airplanes. At a point several hundred miles above the earth, some gas particles spray out into space, some are dragged by gravity and fall back into the ocean of air below, while others never return. Physicists disagree as to the boundaries of the outer fringes of the atmosphere. Some think it begins 240 miles above the earth and extends to 400 miles; others place its lower edge at 600 miles and its upper boundary at 6,000 miles. There are also certain nonconformities at various levels. Between 12 and 30 miles, high solar ultraviolet radiation reacts with oxygen molecules to produce a thin curtain of ozone, a very poisonous gas without which life on earth could not exist. This ozone filters out a portion of the sun’s lethal ultraviolet rays, allowing only enough to come through to give us sunburn, kill bacteria, and prevent rickets. At 50 to 65 miles up, most of the oxygen molecules begin to break down under solar radiation into free atoms, and to form hydroxy ions (OH) from water vapor. Also in this region, all the atoms become ionized. Studies of the atmosphere have revealed that the temperature does not decrease uniformly with increasing altitude; instead it gets steadily colder up to a height of about 7 miles, where the rate of temperature change slows down abruptly and remains almost constant at −55° Centigrade (218° Kelvin) up to about 20 miles. Then the temperature begins to rise to a peak value of 77° Centigrade (350° Kelvin) at the 55 mile level. Thereafter it climbs steadily, reaching 2,270° Centigrade (2,543° Kelvin) at a height of 250 to 400 miles. From the 50 mile level upward, a man or any other living creature, without the protective cover of the 3-35

atmosphere, would be broiled on the side facing the sun and frozen on the other.

above an area at 15,000 ft would be less than the total weight of the air above an area at 10,000 ft.

The atmosphere is divided into concentric layers or levels. Transition through these layers is gradual and without sharply defined boundaries. However, one boundary, the tropopause, exists between the first and second layer. The tropopause is defined as the point in the atmosphere at which the decrease in temperature (with increasing altitude) abruptly ceases. The four atmosphere layers are the troposphere, stratosphere, ionosphere, and the exosphere. The upper portion of the stratosphere is often called the chemosphere or ozonosphere, and the exosphere is also known as the mesosphere.

Atmospheric pressure is often measured by a mercury barometer. A glass tube somewhat over 30 inches in length is sealed at one end and then filled with mercury. It is then inverted and the open end placed in a dish of mercury. Immediately, the mercury level in the inverted tube will drop a short distance, leaving a small volume of mercury vapor at nearly zero absolute pressure in the tube just above the top of the liquid mercury column. Gravity acting on the mercury in the tube will try to make the mercury run out. Atmospheric pressure pushing down on the mercury in the open container tries to make the mercury stay in the tube. At some point these two forces (gravity and atmospheric pressure) will equilibrate and the mercury will stabilize at a certain height in the tube. Under standard day atmospheric conditions, the air in a 1-in2 column extending to the top of the atmosphere would weigh 14.7 lb. A 1-in 2 column of mercury, 29.92 inches tall, would also weigh 14.7 lb. That is why 14.7 psi is equal to 29.92 "Hg. Figure 3-50 demonstrates this point.

The troposphere extends from the earth’s surface to about 35,000 ft at middle latitudes, but varies from 28,000 ft at the poles to about 54,000 ft at the equator. The troposphere is characterized by large changes in temperature and humidity and by generally turbulent conditions. Nearly all cloud formations are within the troposphere. Approximately three-fourths of the total weight of the atmosphere is within the troposphere. The stratosphere extends from the upper limits of the troposphere (and the tropopause) to an average altitude of 60 miles. The ionosphere ranges from the 50 mile level to a level of 300 to 600 miles. Little is known about the characteristics of the ionosphere, but it is thought that many electrical phenomena occur there. Basically, this layer is characterized by the presence of ions and free electrons, and the ionization seems to increase with altitude and in successive layers. The exosphere (or mesosphere) is the outer layer of the atmosphere. It begins at an altitude of 600 miles and extends to the limits of the atmosphere. In this layer, the temperature is fairly constant at 2,500° Kelvin, and propagation of sound is thought to be impossible due to lack of molecular substance. Atmospheric Pressure The human body is under pressure, since it exists at the bottom of a sea of air. This pressure is due to the weight of the atmosphere. On a standard day at sea level, if a 1-in 2 column of air extending to the top of the atmosphere was weighed, it would weigh 14.7 lb. That is why standard day atmospheric pressure is said to be 14.7 pounds per square inch (14.7 psi). Since atmospheric pressure at any altitude is due to the weight of air above it, pressure decreases with increased altitude. Obviously, the total weight of air

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A second means of measuring atmospheric pressure is with an aneroid barometer. This mechanical instrument is a much better choice than a mercury barometer for use on airplanes. Aneroid barometers (altimeters) are used to indicate altitude in flight. The calibrations are made in thousands of feet rather than in psi or inches of mercury. For example, the standard pressure at sea

Vacuum

14.7 psi Atmospheric pressure

760 mm 29.92 in

Figure 3-50. Atmospheric pressure as inches of mercury.

same density as standard air at 10,000 ft. Density altitude is a calculated altitude obtained by correcting pressure altitude for temperature. Water Content of the Atmosphere In the troposphere, the air is rarely completely dry. It contains water vapor in one of two forms: (1) fog or (2) water vapor. Fog consists of minute droplets of water held in suspension by the air. Clouds are composed of fog. The height to which some clouds extend is a good indication of the presence of water in the atmosphere almost up to the stratosphere. The presence of water vapor in the air is quite evident in Figure 3‑52, with a military F-18 doing a high-speed fly-by at nearly Mach 1. The temperature and pressure changes that occur as the airplane approaches supersonic flight cause the water vapor in the air to condense and form the vapor cloud that is visible. Figure 3-51. An airplane’s altimeter is an aneroid barometer.

level is 29.92 "Hg, or 14.7 psi. At 10,000 feet above sea level, standard pressure is 20.58 "Hg, or 10.10 psi. Altimeters are calibrated so that if the pressure exerted by the atmosphere is 10.10 psi, the altimeter will point to 10,000 ft. [Figure 3-51] Atmospheric Density Since both temperature and pressure decrease with altitude, it might appear that the density of the atmosphere would remain fairly constant with increased altitude. This is not true, however, because pressure drops more rapidly with increased altitude than does the temperature. The result is that density decreases with increased altitude. By use of the general gas law, studied earlier, it can be shown that for a particular gas, pressure and temperature determine the density. Since standard pressure and temperatures have been associated with each altitude, the density of the air at these standard temperatures and pressures must also be considered standard. Thus, a particular atmospheric density is associated with each altitude. This gives rise to the expression “density altitude,” symbolized “Hd.” A density altitude of 15,000 ft is the altitude at which the density is the same as that considered standard for 15,000 ft. Remember, however, that density altitude is not necessarily true altitude. For example, on a day when the atmospheric pressure is higher than standard and the temperature is lower than standard, the density which is standard at 10,000 ft might occur at 12,000 ft. In this case, at an actual altitude of 12,000 ft, we have air that has the

As a result of evaporation, the atmosphere always contains some moisture in the form of water vapor. The moisture in the air is called the humidity of the air. Moisture does not consist of tiny particles of liquid held in suspension in the air as in the case of fog, but is an invisible vapor truly as gaseous as the air with which it mixes. Fog and humidity both affect the performance of an aircraft. In flight, at cruising power, the effects are small and receive no consideration. During takeoff, however, humidity has important effects. Two things are done to compensate for the effects of humidity on takeoff performance. Since humid air is less dense than dry air, the allowable takeoff gross weight of an aircraft is generally reduced for operation in areas that are

Figure 3-52. F-18 high-speed fly-by and a vapor cloud.

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consistently humid. Second, because the power output of reciprocating engines is decreased by humidity, the manifold pressure may need to be increased above that recommended for takeoff in dry air in order to obtain the same power output. Engine power output is calculated on dry air. Since water vapor is incombustible, its pressure in the atmosphere is a total loss as far as contributing to power output. The mixture of water vapor and air is drawn through the carburetor, and fuel is metered into it as though it were all air. This mixture of water vapor, air, and fuel enters the combustion chamber where it is ignited. Since the water vapor will not burn, the effective fuel/air ratio is enriched and the engine operates as though it were on an excessively rich mixture. The resulting horsepower loss under humid conditions can therefore be attributed to the loss in volumetric efficiency due to displaced air, and the incomplete combustion due to an excessively rich fuel⁄air mixture. The reduction in power that can be expected from humidity is usually given in charts in the flight manual. There are several types of charts in use. Some merely show the expected reduction in power due to humidity; others show the boost in manifold pressure necessary to restore full takeoff power. The effect of fog on the performance of an engine is very noticeable, particularly on engines with high compression ratios. Normally, some detonation will occur during acceleration, due to the high BMEP (brake mean effective pressures) developed. However, on a foggy day it is difficult to cause detonation to occur. The explanation of this lies in the fact that fog consists of particles of water that have not vaporized. When these particles enter the cylinders, they absorb a tremendous amount of heat energy in the process of vaporizing. The temperature is thus lowered, and the decrease is sufficient to prevent detonation. Fog will generally cause a decrease in horsepower output. However, with a supercharged engine, it will be possible to use higher manifold pressures without danger of detonation. Absolute Humidity Absolute humidity is the actual amount of the water vapor in a mixture of air and water. It is expressed either in grams per cubic meter or pounds per cubic foot. The amount of water vapor that can be present in the air is dependent upon the temperature and pressure. The higher the temperatures, the more water vapor the air is capable of holding, assuming constant pressure. When 3-38

air has all the water vapor it can hold at the prevailing temperature and pressure, it is said to be saturated. Relative Humidity Relative humidity is the ratio of the amount of water vapor actually present in the atmosphere to the amount that would be present if the air were saturated at the prevailing temperature and pressure. This ratio is usually multiplied by 100 and expressed as a percentage. Suppose, for example, that a weather report includes the information that the temperature is 75°F and the relative humidity is 56 percent. This indicates that the air holds 56 percent of the water vapor required to saturate it at 75°F. If the temperature drops and the absolute humidity remains constant, the relative humidity will increase. This is because less water vapor is required to saturate the air at the lower temperature. Dew Point The dew point is the temperature to which humid air must be cooled at constant pressure to become saturated. If the temperature drops below the dew point, condensation occurs. People who wear eyeglasses have experience going from cold outside air into a warm room and having moisture collect quickly on their glasses. This happens because the glasses were below the dew point temperature of the air in the room. The air immediately in contact with the glasses was cooled below its dew point temperature, and some of the water vapor was condensed out. This principle is applied in determining the dew point. A vessel is cooled until water vapor begins to condense on its surface. The temperature at which this occurs is the dew point. Vapor Pressure Vapor pressure is the portion of atmospheric pressure that is exerted by the moisture in the air (expressed in tenths of an inch of mercury). The dew point for a given condition depends on the amount of water pressure present; thus, a direct relationship exists between the vapor pressure and the dew point. Standard Atmosphere If the performance of an aircraft is computed, either through flight tests or wind tunnel tests, some standard reference condition must be determined first in order to compare results with those of similar tests. The conditions in the atmosphere vary continuously, and it is generally not possible to obtain exactly the same set of conditions on two different days or even on two successive flights. For this reason, a set group of standards must be used as a point of reference. The set of

standard conditions presently used in the United States is known as the U.S. Standard Atmosphere.

Lift

The standard atmosphere approximates the average conditions existing at 40° latitude, and is determined on the basis of the following assumptions. The standard sea level conditions are: Pressure at 0 altitude (P0) = 29.92 "Hg Temperature at 0 altitude (T0) = 15°C or 59°F Gravity at 0 altitude (G0) = 32.174 fps/s The U.S. Standard Atmosphere is in agreement with the International Civil Aviation Organization (ICAO) Standard Atmosphere over their common altitude range. The ICAO Standard Atmosphere has been adopted as standard by most of the principal nations of the world.

Aircraft Theory of Flight Before a technician can consider performing maintenance on an aircraft, it is necessary to understand the pieces that make up the aircraft. Names like fuselage, empennage, wing, and so many others, come into play when describing what an airplane is and how it operates. For helicopters, names like main rotor, anti-torque rotor, and autorotation come to mind as a small portion of what needs to be understood about rotorcraft. The study of physics, which includes basic aerodynamics, is a necessary part of understanding why aircraft operate the way they do. Four Forces of Flight During flight, there are four forces acting on an airplane. These forces are lift, weight, thrust, and drag. [Figure 3-53] Lift is the upward force created by the wing, weight is the pull of gravity on the airplane’s mass, thrust is the force created by the airplane’s propeller or turbine engine, and drag is the friction caused by the air flowing around the airplane. All four of these forces are measured in pounds. Any time the forces are not in balance, something about the airplane’s condition is changing. The possibilities are as follows: 1. When an airplane is accelerating, it has more thrust than drag. 2. When an airplane is decelerating, it has less thrust than drag. 3. When an airplane is at a constant velocity, thrust and drag are equal.

Thrust

Drag

Weight

Figure 3-53. Four forces acting on an airplane.

4. When an airplane is climbing, it has more lift than weight. 5. When an airplane is descending, it has more weight than lift. 6. When an airplane is at a constant altitude, lift and weight are equal. Bernoulli’s Principle and Subsonic Flow The basic concept of subsonic airflow and the resulting pressure differentials was discovered by Daniel Bernoulli, a Swiss physicist. Bernoulli’s principle, as we refer to it today, states that “as the velocity of a fluid increases, the static pressure of that fluid will decrease, provided there is no energy added or energy taken away.” A direct application of Bernoulli’s principle is the study of air as it flows through either a converging or a diverging passage, and to relate the findings to some aviation concepts. A converging shape is one whose cross-sectional area gets progressively smaller from entry to exit. A diverging shape is just the opposite, with the cross-sectional area getting larger from entry to exit. Figure 3-54 shows a converging shaped duct, with the air entering on the left at subsonic velocity and exiting on the right. Looking at the pressure and velocity gauges, and the indicated velocity and pressure, notice that the air exits at an increased velocity and a decreased static pressure. Because a unit of air must exit the duct when another unit enters, the unit leaving must increase its velocity as it flows into a smaller space. In a diverging duct, just the opposite would happen. From the entry point to the exit point, the duct is spreading out and the area is getting larger. [Figure 3-55] With the increase in cross-sectional area, the velocity of the air decreases and the static pressure increases. The total energy in the air has not changed. What has 3-39

300 mph

Velocity

Pressure

Low High

Low High

14 psi Airflow

Subsonic

Velocity increase

Figure 3-56. Venturi with a superimposed wing.

Airflow

400 mph

Low High

Low High

Velocity

Pressure

10 psi

Figure 3-54. Bernoulli’s principle and a converging duct.

300 mph

Velocity

Pressure

Low High

Low High

14 psi

Airflow

Low High

Low High

Velocity

Pressure

10 psi

Figure 3-55. Bernoulli’s principle and a diverging duct.

been lost in velocity (kinetic energy) is gained in static pressure (potential energy). In the discussion of Bernoulli’s principle earlier in this chapter, a venturi was shown in Figure 3-46. In Figure 3-56, a venturi is shown again, only this time a wing is shown tucked up into the recess where the venturi’s converging shape is. There are two arrows showing airflow. The large arrow shows airflow within the venturi, and the small arrow shows airflow on the outside heading toward the leading edge of the wing.

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In Figure 3-56, the air reaching the leading edge of the wing separates into two separate flows. Some of the air goes over the top of the wing and some travels along the bottom. The air going over the top, because of the curvature, has farther to travel. With a greater distance to travel, the air going over the top must move at a greater velocity. The higher velocity on the top causes the static pressure on the top to be less than it is on the bottom, and this difference in static pressures is what creates lift. For the wing shown in Figure 3-56, imagine it is 5 ft wide and 15 ft long, for a surface area of 75 ft 2 (10,800 in2). If the difference in static pressure between the top and bottom is 0.1 psi, there will be 1⁄10 lb of lift for each square inch of surface area. Since there are 10,800 in 2 of surface area, there would be 1,080 lb of lift (0.1 × 10,800).

Subsonic

400 mph

In the converging part of the venturi, velocity would increase and static pressure would decrease. The same thing would happen to the air flowing around the wing, with the velocity over the top increasing and static pressure decreasing.

Lift and Newton’s Third Law Newton’s third law identifies that for every force there is an equal and opposite reacting force. In addition to Bernoulli’s principle, Newton’s third law can also be used to explain the lift being created by a wing. As the air travels around a wing and leaves the trailing edge, the air is forced to move in a downward direction. Since a force is required to make something change direction, there must be an equal and opposite reacting force. In this case, the reacting force is what we call lift. In order to calculate lift based on Newton’s third law, Newton’s second law and the formula “Force = Mass × Acceleration” would be used. The mass would be the weight of air flowing over the wing every second, and the acceleration would be the change in velocity the wing imparts to the air.

The lift on the wing as described by Bernoulli’s principle, and lift on the wing as described by Newton’s third law, are not separate or independent of each other. They are just two different ways to describe the same thing, namely the lift on a wing. Airfoils An airfoil is any device that creates a force, based on Bernoulli’s principles or Newton’s laws, when air is caused to flow over the surface of the device. An airfoil can be the wing of an airplane, the blade of a propeller, the rotor blade of a helicopter, or the fan blade of a turbofan engine. The wing of an airplane moves through the air because the airplane is in motion, and generates lift by the process previously described. By comparison, a propeller blade, helicopter rotor blade, or turbofan engine fan blade rotates through the air. These rotating blades could be referred to as rotating wings, as is common with helicopters when they are called rotary wing aircraft. The rotating wing can be viewed as a device that creates lift, or just as correctly, it can be viewed as a device that creates thrust. In Figure 3-57 an airfoil (wing) is shown, with some of the terminology that is used to describe a wing. The terms and their meaning are as follows: Camber  The camber of a wing is the curvature which is present on top and bottom surfaces. The camber on the top is much more pronounced, unless the wing is a symmetrical airfoil, which has the same camber top and bottom. The bottom of the wing, more often than not, is relatively flat. The increased camber on top is what causes the velocity of the air to increase and the static pressure to decrease. The bottom of the wing has less velocity and more static pressure, which is why the wing generates lift. Chord Line  The chord line is an imaginary straight line running from the wing’s leading edge to its trailing edge. The angle between the chord line and the longitudinal axis of the airplane is known as the angle of incidence. Relative Wind  Whatever direction the airplane is flying, the relative wind is in the opposite direction. If the airplane is flying due north, and someone in the airplane is not shielded from the elements, that person will feel like the wind is coming directly from the south.

Leading edge

Chord line Upper camber

Airplane path Relative wind

Angle o

f attack

Lower camber

Trailing edge

Figure 3-57. Wing terminology.

Angle of Attack  The angle between the chord line and the relative wind is the angle of attack. As the angle of attack increases, the lift on the wing increases. If the angle of attack becomes too great, the airflow can separate from the wing and the lift will be destroyed. When this occurs, a condition known as a stall takes place. There are a number of different shapes, known as planforms, that a wing can have. A wing in the shape of a rectangle is very common on small general aviation airplanes. An elliptical shape or tapered wing can also be used, but these do not have as desirable a stall characteristic. For airplanes that operate at high subsonic speeds, sweptback wings are common, and for supersonic flight, a delta shape might be used. The aspect ratio of a wing is the relationship between its span (wingtip to wingtip measurement) and the chord of the wing. If a wing has a long span and a very narrow chord, it is said to have a high aspect ratio. A higher aspect ratio produces less drag for a given flight speed, and is typically found on glider type aircraft. The angle of incidence of a wing is the angle formed by the intersection of the wing chord line and the horizontal plane passing through the longitudinal axis of the aircraft. Many airplanes are designed with a greater angle of incidence at the root of the wing than at the tip, and this is referred to as washout. This feature causes the inboard part of the wing to stall before the outboard part, which helps maintain aileron control during the initial stages of a wing stall. Boundary Layer Airflow The boundary layer is a very thin layer of air lying over the surface of the wing and, for that matter, all other surfaces of the airplane. Because air has viscosity, this layer of air tends to adhere to the wing. As the wing moves forward through the air, the boundary layer at first flows smoothly over the streamlined shape of the airfoil. Here the flow is called the laminar layer.

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A wing leading edge slot is a duct that allows air to flow from the bottom of the wing, through the duct, to the top of the wing. As the air flows to the top of the wing, it is directed along the wing’s surface at a high velocity and helps keep the boundary layer from becoming turbulent and separating from the wing’s surface. View A

View B

Figure 3-58. Wing boundary layer separation.

As the boundary layer approaches the center of the wing, it begins to lose speed due to skin friction and it becomes thicker and turbulent. Here it is called the turbulent layer. The point at which the boundary layer changes from laminar to turbulent is called the transition point. Where the boundary layer becomes turbulent, drag due to skin friction is relatively high. As speed increases, the transition point tends to move forward. As the angle of attack increases, the transition point also tends to move forward. With higher angles of attack and further thickening of the boundary layer, the turbulence becomes so great the air breaks away from the surface of the wing. At this point, the lift of the wing is destroyed and a condition known as a stall has occurred. In Figure 3-58, view A shows a normal angle of attack and the airflow staying in contact with the wing. View B shows an extreme angle of attack and the airflow separating and becoming turbulent on the top of the wing. In view B, the wing is in a stall. Boundary Layer Control One way of keeping the boundary layer air under control, or lessening its negative effect, is to make the wing’s surface as smooth as possible and to keep it free of dirt and debris. As the friction between the air and the surface of the wing increases, the boundary layer thickens and becomes more turbulent and eventually a wing stall occurs. With a smooth and clean wing surface, the onset of a stall is delayed and the wing can operate at a higher angle of attack. One of the reasons ice forming on a wing can be such a serious problem is because of its effect on boundary layer air. On a high-speed airplane, even a few bugs splattered on the wing’s leading edge can negatively affect boundary layer air. Other methods of controlling boundary layer air include wing leading edge slots, air suction through small holes on the wing’s upper surface, and the use of devices called vortex generators.

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Another way of controlling boundary layer air is to create a suction on the top of the wing through a large number of small holes. The suction on the top of the wing draws away the slow-moving turbulent air, and helps keep the remainder of the airflow in contact with the wing. Vortex generators are used on airplanes that fly at high subsonic speed, where the velocity of the air on the top of the wing can reach Mach 1. As the air reaches Mach 1 velocity, a shock wave forms on the top of the wing, and the subsequent shock wave causes the air to separate from the wing’s upper surface. Vortex generators are short airfoils, arranged in pairs, located on the wing’s upper surface. They are positioned such that they pull high-energy air down into the boundary layer region and prevent airflow separation. Wingtip Vortices Wingtip vortices are caused by the air beneath the wing, which is at the higher pressure, flowing over the wingtip and up toward the top of the wing. The end result is a spiral or vortex that trails behind the wingtip anytime lift is being produced. This vortex is also referred to as wake turbulence, and is a significant factor in determining how closely one airplane can follow behind another on approach to land. The wake turbulence of a large airplane can cause a smaller airplane, if it is following too closely, to be thrown out of control. Vortices from the wing and from the horizontal stabilizer are quite visible on the MD-11 shown in Figure 3-59. Upwash and downwash refer to the effect an airfoil has on the free airstream. Upwash is the deflection of the oncoming airstream, causing it to flow up and over the wing. Downwash is the downward deflection of the airstream after it has passed over the wing and is leaving the trailing edge. This downward deflection is what creates the action and reaction described under lift and Newton’s third law. Axes of an Aircraft An airplane in flight is controlled around one or more of three axes of rotation. These axes of rotation are the longitudinal, lateral, and vertical. On the airplane, all three axes intersect at the center of gravity (CG). As the airplane pivots on one of these axes, it is in essence

Figure 3-59. Wing and horizontal stabilizer vortices on an MD-11.

pivoting around the center of gravity (CG). The center of gravity is also referred to as the center of rotation. On the brightly colored airplane shown in Figure 3‑60, the three axes are shown in the colors red (vertical axis), blue (longitudinal axis), and orange (lateral axis). The flight control that makes the airplane move around the axis is shown in a matching color. The rudder, in red, causes the airplane to move around the vertical axis and this movement is described as being a yaw. The elevator, in orange, causes the airplane to move around the lateral axis and this movement is described as being a pitch. The ailerons, in blue, cause the airplane to move around the longitudinal axis and this movement is described as being a roll.

Longitudinal Axis

Vertical Axis

Lateral Axis

The 3 axes intersect at the airplane’s center of gravity. The flight control that produces motion around the indicated axis is a matching color.

Figure 3-60. The three axes of an airplane.

Aircraft Stability When an airplane is in straight-and-level flight at a constant velocity, all the forces acting on the airplane are in equilibrium. If that straight-and-level flight is disrupted by a disturbance in the air, such as wake turbulence, the airplane might pitch up or down, yaw left or right, or go into a roll. If the airplane has what is characterized as stability, once the disturbance goes away, the airplane will return to a state of equilibrium.

has the nose pitch up because of wake turbulence, the tendency will be for the nose to continue to pitch up even after the turbulence goes away. If an airplane tends to remain in a displaced position after the force is removed, but does not continue to move toward even greater displacement, its static stability is described as being neutral.

Static Stability The initial response that an airplane displays after its equilibrium is disrupted is referred to as its static stability. If the static stability is positive, the airplane will tend to return to its original position after the disruptive force is removed. If the static stability is negative, the airplane will continue to move away from its original position after the disruptive force is removed. If an airplane with negative static stability

Dynamic Stability The dynamic stability of an airplane involves the amount of time it takes for it to react to its static stability after it has been displaced from a condition of equilibrium. Dynamic stability involves the oscillations that typically occur as the airplane tries to return to its original position or attitude. Even though an airplane may have positive static stability, it may have dynamic stability which is positive, neutral, or negative.

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(A) Positive static and positive dynamic stability

Time

(B) Positive static and negative dynamic stability

(C) Positive static and negative dynamic stability

Time

Forward CG limit

Aft CG limit

Center of gravity

Center of lift

Time

Figure 3-61. Dynamic stability.

Figure 3-62. Longitudinal stability and balance.

Imagine that an airplane in straight-and-level flight is disturbed and pitches nose up. If the airplane has positive static stability, the nose will pitch back down after the disturbance is removed. If it immediately returns to straight-and-level flight, it is also said to have positive dynamic stability. The airplane, however, may pass through level flight and remain pitched down, and then continue the recovery process by pitching back up. This pitching up and then down is known as an oscillation. If the oscillations lessen over time, the airplane is still classified as having positive dynamic stability. If the oscillations increase over time, the airplane is classified as having negative dynamic stability. If the oscillations remain the same over time, the airplane is classified as having neutral dynamic stability.

airplane is concentrated. For an airplane to have good longitudinal stability, the center of gravity is typically located forward of the center of lift. This gives the airplane a nose-down pitching tendency, which is balanced out by the force generated at the horizontal stabilizer and elevator. The center of gravity has limits within which it must fall. If it is too far forward, the forces at the tail might not be able to compensate and it may not be possible to keep the nose of the airplane from pitching down.

Figure 3-61 shows the concept of dynamic stability. In view A, the displacement from equilibrium goes through three oscillations and then returns to equilibrium. In view B, the displacement from equilibrium is increasing after two oscillations, and will not return to equilibrium. In view C, the displacement from equilibrium is staying the same with each oscillation. Longitudinal Stability Longitudinal stability for an airplane involves the tendency for the nose to pitch up or pitch down, rotating around the lateral axis (wingtip to wingtip). If an airplane is longitudinally stable, it will return to a properly trimmed angle of attack after the force that upset its flightpath is removed. The weight and balance of an airplane, which is based on both the design characteristics of the airplane and the way it is loaded, is a major factor in determining longitudinal stability. There is a point on the wing of an airplane, called the center of pressure or center of lift, where all the lifting forces concentrate. In flight, the airplane acts like it is being lifted from or supported by this point. This center of lift runs from wingtip to wingtip. There is also a point on the airplane, called the center of gravity, where the mass or weight of the 3-44

In Figure 3-62, the center of lift, center of gravity, and center of gravity limits are shown. It can be seen that the center of gravity is not only forward of the center of lift, it is also forward of the center of gravity limit. At the back of the airplane, the elevator trailing edge is deflected upward to create a downward force on the tail, to try and keep the nose of the airplane up. This airplane would be highly unstable longitudinally, especially at low speed when trying to land. It is especially dangerous if the center of gravity is behind the aft limit. The airplane will now have a tendency to pitch nose up, which can lead to the wing stalling and possible loss of control of the airplane. Lateral Stability Lateral stability of an airplane takes place around the longitudinal axis, which is from the airplane’s nose to its tail. If one wing is lower than the other, good lateral stability will tend to bring the wings back to a level flight attitude. One design characteristic that tends to give an airplane good lateral stability is called dihedral. Dihedral is an upward angle for the wings with respect to the horizontal, and it is usually just a few degrees. Imagine a low wing airplane with a few degrees of dihedral experiencing a disruption of its flightpath such that the left wing drops. When the left wing drops, this will cause the airplane to experience a sideslip toward the low wing. The sideslip causes the low wing to experience a higher angle of attack, which increases

Dihedral

Figure 3-63. The dihedral of a wing.

Pivot point (CG)

Distance to vertical stabilizer creates stability

Figure 3-64. Directional stability caused by distance to vertical stabilizer.

its lift and raises it back to a level flight attitude. The dihedral on a wing is shown in Figure 3-63. Directional Stability Movement of the airplane around its vertical axis, and the airplane’s ability to not be adversely affected by a force creating a yaw type of motion, is called directional stability. The vertical fin gives the airplane this stability, causing the airplane to align with the relative wind. In flight, the airplane acts like the weather vane we use around our home to show the direction the wind is blowing. The distance from the pivot point on a weather vane to its tail is greater than the distance from its pivot point to the nose. So when the wind blows, it creates a greater torque force on the tail and forces it to align with the wind. On an airplane, the same is true. With the CG being the pivot point, it is a greater distance from the CG to the vertical stabilizer than it is from the CG to the nose. [Figure 3-64] Dutch Roll The dihedral of the wing tries to roll the airplane in the opposite direction of how it is slipping, and the vertical fin will try to yaw the airplane in the direction of the slip. These two events combine in a way that affects lateral and directional stability. If the wing dihedral has the greatest effect, the airplane will have

a tendency to experience a Dutch roll. A Dutch roll is a small amount of oscillation around both the longitudinal and vertical axes. Although this condition is not considered dangerous, it can produce an uncomfortable feeling for passengers. Commercial airliners typically have yaw dampers that sense a Dutch roll condition and cancel it out. Flight Control Surfaces The purpose of flight controls is to allow the pilot to maneuver the airplane, and to control it from the time it starts the takeoff roll until it lands and safely comes to a halt. Flight controls are typically associated with the wing and the vertical and horizontal stabilizers, because these are the parts of the airplane that flight controls most often attach to. In flight, and to some extent on the ground, flight controls provide the airplane with the ability to move around one or more of the three axes. Flight controls function by changing the shape or aerodynamic characteristics of the surface they are attached to. Flight Controls and the Lateral Axis The lateral axis of an airplane is a line that runs below the wing, from wingtip to wingtip, passing through the airplane’s center of gravity. Movement around this axis is called pitch, and control around this axis is called longitudinal control. The flight control that handles this job is the elevator attached to the horizontal stabilizer, a fully moving horizontal stabilizer, or on a v-tail configured airplane, it is called ruddervators. An elevator on a Cessna 182 can be seen in Figure 3-65. In Figure 3-66, a fully moving horizontal stabilizer, known as a stabilator, can be seen on a Piper Cherokee Arrow, and Figure 3-67 shows a ruddervator on a Beechcraft Bonanza. Depending on the airplane being discussed, movement around the lateral axis happens as a result of the pilot moving the control wheel or yoke, the control stick,

Elevator

Figure 3-65. Elevator on a Cessna 182 provides pitch control.

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Anti-servo tab

Moving horizontal stabilizer (stabilator)

Figure 3-66. Moving horizontal stabilizer, known as a stabilator, on a Piper Cherokee Arrow provides pitch control.

Ruddervators

Figure 3-67. Ruddervators on a Beechcraft Bonanza provide pitch control.

Control wheel or yoke

Rudder pedals

Figure 3-68. Cessna 182 control wheel and rudder pedals.

or on some airplanes, a side stick. On the airplanes shown in Figures 3-65, 3-66, and 3-67, a control wheel or yoke is used. On the Cessna 182 shown in Figure 3-65, pulling back on the control wheel causes the trailing edge of the elevator to deflect upward, causing an increased downward force that raises the nose of the airplane. Movement of the elevator causes the nose of the airplane to pitch up or pitch down by rotating around the lateral axis. The Cessna 182 control wheel can be seen in Figure 3-68.

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On the Piper Cherokee Arrow shown in Figure 3-66, pulling back on the control wheel causes the entire horizontal surface (stabilator) to move, with the trailing edge deflecting upward. The anti-servo tab seen on the Cherokee provides a control feel similar to what would be experienced by moving an elevator. Without this tab, the stabilator might be too easy to move and a pilot could overcontrol the airplane. The ruddervators shown on the Beechcraft Bonanza in Figure 3-67 are also moved by the control wheel, with their trailing edges deflecting upward when the control wheel is pulled back. As the name implies, these surfaces also act as the rudder for this airplane. Flight Controls and the Longitudinal Axis The longitudinal axis of the airplane runs through the middle of the airplane, from nose to tail, passing through the center of gravity. Movement around this axis is known as roll, and control around this axis is called lateral control. Movement around this axis is controlled by the ailerons, and on jet transport airplanes, it is aided by surfaces on the wing known as spoilers. The ailerons move as a result of the pilot rotating the control wheel to the left or to the right, much the same as turning the steering wheel on an automobile. [Figure 3-68] When a pilot turns the control wheel to the left, the airplane is being asked to turn or bank to the left. Turning the control wheel to the left causes the trailing edge of the aileron on the left wing to rise up into the airstream, and the aileron on the right wing lowers down into the airstream. This increases the lift on the right wing and decreases the lift on the left wing, causing the right wing to move up and the airplane to bank to the left. In Figure 3-69, an Air Force F-15 can be seen doing an aileron roll. Notice that the left aileron is up and the right aileron is down, which would cause the airplane to roll around the longitudinal axis in a counterclockwise direction. Flight Controls and the Vertical Axis The vertical axis of an airplane runs from top to bottom through the middle of the airplane, passing through the center of gravity. Movement around this axis is known as yaw, and control around this axis is called directional control. Movement around this axis is controlled by the rudder, or in the case of the Beechcraft Bonanza in Figure 3-67, by the ruddervators.

Aileron Up

Aileron Down

Figure 3-69. Air Force F-15 doing an aileron roll.

The feet of the pilot are on the rudder pedals, and pushing on the left or right rudder pedal makes the rudder move left or right. When the right rudder pedal is pushed, the trailing edge of the rudder moves to the right, and the nose of the airplane yaws to the right. The rudder pedals of a Cessna 182 can be seen in Figure 3-68. Even though the rudder of the airplane will make the nose yaw to the left or the right, the rudder is not what turns the airplane. For what is called a coordinated turn to occur, both the ailerons and rudder come into play. Let’s say we want to turn the airplane to the right. We start by turning the control wheel to the right, which raises the right aileron and lowers the left aileron and initiates the banking turn. The increased lift on the left wing also increases the induced drag on the left wing, which tries to make the nose of the airplane yaw to the left. To counteract this, when the control wheel is moved to the right, a small amount of right rudder is used to keep the nose of the airplane from yawing to the left. Once the nose of the airplane is pointing in the right direction, pressure on the rudder is no longer needed. The rudder of a Piper Cherokee Arrow can be seen in Figure 3-70.

Tabs Trim Tabs Trim tabs are small movable surfaces that attach to the trailing edge of flight controls. These tabs can be controlled from the flight deck, and their purpose is to create an aerodynamic force that keeps the flight control in a deflected position. Trim tabs can be installed on any of the primary flight controls. A very common flight control to find fitted with a trim tab is the elevator. In order to be stable in flight, most airplanes have the center of gravity located forward of the center of lift on the wing. This causes a nose heavy condition, which needs to be balanced out by having the elevator deflect upwards and create a downward force. To relieve the pilot of the need to hold back pressure on the control wheel, a trim tab on the elevator can be adjusted to hold the elevator in a slightly deflected position. An elevator trim tab for a Cessna 182 is shown in Figure 3-71. Anti-servo Tab

Some airplanes, like a Piper Cherokee Arrow, do not have a fixed horizontal stabilizer and movable elevator. The Cherokee uses a moving horizontal surface known as a stabilator. Because of the location of the pivot point for this movable surface, it has a tendency to be extremely sensitive to pilot input. To reduce the sensitivity, a full length anti-servo tab is installed on the trailing edge of the stabilator. As the trailing edge of the stabilator moves down, the anti-servo tab moves down and creates a force trying to raise the trailing edge. With this force acting against the movement of the stabilator, it reduces the sensitivity to pilot input. The anti-servo tab on a Piper Cherokee Arrow is shown in Figure 3-70.

Rudder

Elevator trim tab

Anti-servo tab

Figure 3-71. Elevator trim tab on a Cessna 182. Figure 3-70. Rudder on a Piper Cherokee Arrow.

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Balance Tab

Plain flap

On some airplanes, the force needed to move the flight controls can be excessive. In these cases, a balance tab can be used to generate a force that assists in the movement of the flight control. Just the opposite of anti-servo tabs, balance tabs move in the opposite direction of the flight control’s trailing edge, providing a force that helps the flight control move.

Split flap

Slotted flap

Servo Tab

On large airplanes, because the force needed to move the flight controls is beyond the capability of the pilot, hydraulic actuators are used to provide the necessary force. In the event of a hydraulic system malfunction or failure, some of these airplanes have servo tabs on the trailing edge of the primary flight controls. When the control wheel is pulled back in an attempt to move the elevator, the servo tab moves and creates enough aerodynamic force to move the elevator. The servo tab is acting like a balance tab, but rather than assisting the normal force that moves the elevator, it becomes the sole force that makes the elevator move. Like the balance tab, the servo tab moves in the opposite direction of the flight control’s trailing edge. The Boeing 727 has servo tabs that back up the hydraulic system in the event of a failure. During normal flight, the servo tabs act like balance tabs. [Figure 3-73] Supplemental Lift-Modifying Devices If the wing of an airplane was designed to produce the maximum lift possible at low airspeed, to accommodate takeoffs and landings, it would not be suited for higher speed flight because of the enormous amount of drag it would produce. To give the wing the ability to produce maximum low speed lift without being drag prohibitive, retractable high lift devices, such as flaps and slats, are utilized. Flaps

The most often used lift-modifying device, for small airplanes and large, is the wing flap. Flaps can be installed on the leading edge or trailing edge, with the leading edge versions used only on larger airplanes. Flaps change the camber of the wing, and they increase both the lift and the drag for any given angle of attack. The four different types of flaps in use are the plain, split, slotted, and Fowler. [Figure 3-72] Plain flaps attach to the trailing edge of the wing, inboard of the ailerons, and form part of the wing’s overall surface. When deployed downward, they

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Fowler flap

Figure 3-72. Four types of wing flaps.

increase the effective camber of the wing and the wing’s chord line. Both of these factors cause the wing to create more lift and more drag. The split flap attaches to the bottom of the wing, and deploys downward without changing the top surface of the wing. This type of flap creates more drag than the plain flap because of the increase in turbulence. The slotted flap is similar to the plain flap, except when it deploys, the leading edge drops down a small amount. By having the leading edge drop down slightly, a slot opens up, which lets some of the high pressure air on the bottom of the wing flow over the top of the flap. This additional airflow over the top of the flap produces additional lift. The Fowler flap attaches to the back of the wing using a track and roller system. When it deploys, it moves aft in addition to deflecting downward. This increases the total wing area, in addition to increasing the wing camber and chord line. This type of flap is the most effective of the four types, and it is the type used on commercial airliners and business jets. Leading Edge Slots

Leading edge slots are ducts or passages in the leading edge of a wing that allow high pressure air from the bottom of the wing to flow to the top of the wing. This ducted air flows over the top of the wing at a high velocity and helps keep the boundary layer air from becoming turbulent and separating from the wing. Slots are often placed on the part of the wing ahead of the ailerons, so during a wing stall, the inboard part of the wing stalls first and the ailerons remain effective.

Stabilizer

Elevator

Upper rudder

Elevator tab

Lower rudder

Anti-balance tabs

Ground spoilers Inboard flap Inboard aileron Inboard aileron tab

Leading edge flaps (extended)

Outboard flap Balance tab

Leading edge slats (extended)

Outboard aileron Flight spoilers

Figure 3-73. Boeing 727 flight controls.

Leading Edge Slats

Leading edge slats serve the same purpose as slots, the difference being that slats are movable and can be retracted when not needed. On some airplanes, leading edge slats have been automatic in operation, deploying in response to the aerodynamic forces that come into play during a high angle of attack. On most of today’s commercial airliners, the leading edge slats deploy when the trailing edge flaps are lowered. The flight controls of a large commercial airliner are shown in Figure 3-73. The controls by color are as follows:

High-Speed Aerodynamics Compressibility Effects When air is flowing at subsonic speed, it acts like an incompressible fluid. As discussed earlier in this chapter, when air at subsonic speed flows through a diverging shaped passage, the velocity decreases and the static pressure rises, but the density of the air does not change. In a converging shaped passage, subsonic air speeds up and its static pressure decreases. When supersonic air flows through a converging passage, its velocity decreases and its pressure and density both increase. [Figure 3-74] At supersonic flow, air acts like a compressible fluid. Because air behaves differently

1. All aerodynamic tabs are shown in green. 2. All leading and trailing edge high lift devices are shown in red (leading edge flaps and slats, trailing edge inboard and outboard flaps). 3. The tail mounted primary flight controls are in yellow (rudder and elevator). 4. The wing mounted primary flight controls are in purple (inboard and outboard aileron).

Supersonic Airflow

Converging

Diverging

Decreasing velocity Increasing pressure Increasing density

Increasing velocity Decreasing pressure Decreasing density

Figure 3-74. Supersonic airflow through a venturi.

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when flowing at supersonic velocity, airplanes that fly supersonic must have wings with a different shape. The Speed of Sound Sound, in reference to airplanes and their movement through the air, is nothing more than pressure disturbances in the air. As discussed earlier in this chapter, it is like dropping a rock in the water and watching the waves flow out from the center. As an airplane flies through the air, every point on the airplane that causes a disturbance creates sound energy in the form of pressure waves. These pressure waves flow away from the airplane at the speed of sound, which at standard day temperature of 59°F, is 761 mph. The speed of sound in air changes with temperature, increasing as temperature increases. Figure 3-75 shows how the speed of sound changes with altitude. Altitude in Feet

Temperature (°F)

Speed of Sound (mph)

0 1,000 2,000 3,000 4,000 5,000 6,000 7,000 8,000 9,000 10,000 15,000 20,000 25,000 30,000 35,000 * 36,089 40,000 45,000 50,000 55,000 60,000 65,000 70,000 75,000 80,000 85,000 90,000 95,000 100,000

59.00 55.43 51.87 48.30 44.74 41.17 37.60 34.04 30.47 26.90 23.34 5.51 –12.32 –30.15 –47.98 –65.82 –69.70 –69.70 –69.70 –69.70 –69.70 –69.70 –69.70 –69.70 –69.70 –69.70 –64.80 –56.57 –48.34 –40.11

761 758 756 753 750 748 745 742 740 737 734 721 707 692 678 663 660 660 660 660 660 660 660 660 660 660 664 671 678 684

* Altitude at which temperature stops decreasing

Figure 3-75. Altitude and temperature versus speed of sound.

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Subsonic, Transonic, and Supersonic Flight When an airplane is flying at subsonic speed, all of the air flowing around the airplane is at a velocity of less than the speed of sound (known as Mach 1). Keep in mind that the air accelerates when it flows over certain parts of the airplane, like the top of the wing, so an airplane flying at 500 mph could have air over the top of the wing reach a speed of 600 mph. How fast an airplane can fly and still be considered in subsonic flight varies with the design of the wing, but as a Mach number, it will typically be just over Mach 0.8. When an airplane is flying at transonic speed, part of the airplane is experiencing subsonic airflow and part is experiencing supersonic airflow. Over the top of the wing, probably about halfway back, the velocity of the air will reach Mach 1 and a shock wave will form. The shock wave forms 90 degrees to the airflow and is known as a normal shock wave. Stability problems can be encountered during transonic flight, because the shock wave can cause the airflow to separate from the wing. The shock wave also causes the center of lift to shift aft, causing the nose to pitch down. The speed at which the shock wave forms is known as the critical Mach number. Transonic speed is typically between Mach 0.80 and 1.20. When an airplane is flying at supersonic speed, the entire airplane is experiencing supersonic airflow. At this speed, the shock wave which formed on top of the wing during transonic flight has moved all the way aft and has attached itself to the wing trailing edge. Supersonic speed is from Mach 1.20 to 5.0. If an airplane flies faster than Mach 5, it is said to be in hypersonic flight. Shock Waves Sound coming from an airplane is the result of the air being disturbed as the airplane moves through it, and the resulting pressure waves that radiate out from the source of the disturbance. For a slow moving airplane, the pressure waves travel out ahead of the airplane, traveling at the speed of sound. When the speed of the airplane reaches the speed of sound, however, the pressure waves (sound energy) cannot get away from the airplane. At this point the sound energy starts to pile up, initially on the top of the wing, and eventually attaching itself to the wing leading and trailing edges. This piling up of sound energy is called a shock wave. If the shock waves reach the ground, and cross the path of a person, they will be heard as a sonic boom. Figure 3-76A shows a wing in slow speed flight, with many

Normal shock wave Subsonic air

Supersonic air

Subsonic air

(A) Figure 3-77. Normal shock wave.

Expansion wave

Oblique shock

Oblique shock

Airflow

(B) Figure 3-76. Sound energy in subsonic and supersonic flight.

disturbances on the wing generating sound pressure waves that are radiating outward. View B is the wing of an airplane in supersonic flight, with the sound pressure waves piling up toward the wing leading edge. Normal Shock Wave

When an airplane is in transonic flight, the shock wave that forms on top of the wing, and eventually on the bottom of the wing, is called a normal shock wave. If the leading edge of the wing is blunted, instead of being rounded or sharp, a normal shock wave will also form in front of the wing during supersonic flight. Normal shock waves form perpendicular to the airstream. The velocity of the air behind a normal shock wave is subsonic, and the static pressure and density of the air are higher. Figure 3-77 shows a normal shock wave forming on the top of a wing. Oblique Shock Wave

An airplane that is designed to fly supersonic will have very sharp edged surfaces, in order to have the least amount of drag. When the airplane is in supersonic flight, the sharp leading edge and trailing edge of the wing will have shock waves attach to them. These shock waves are known as oblique shock waves. Behind an oblique shock wave the velocity of the air is lower, but still supersonic, and the static pressure and density are higher. Figure 3-78 shows an oblique

Figure 3-78. Supersonic airfoil with oblique shock waves and expansion waves.

shock wave on the leading and trailing edges of a supersonic airfoil. Expansion Wave

Earlier in the discussion of high-speed aerodynamics, it was stated that air at supersonic speed acts like a compressible fluid. For this reason, supersonic air, when given the opportunity, wants to expand outward. When supersonic air is flowing over the top of a wing, and the wing surface turns away from the direction of flow, the air will expand and follow the new direction. At the point where the direction of flow changes, an expansion wave will occur. Behind the expansion wave the velocity increases, and the static pressure and density decrease. An expansion wave is not a shock wave. Figure 3-78 shows an expansion wave on a supersonic airfoil. High-Speed Airfoils Transonic flight is the most difficult flight regime for an airplane, because part of the wing is experiencing subsonic airflow and part is experiencing supersonic airflow. For a subsonic airfoil, the aerodynamic center (the point of support) is approximately 25 percent of the way back from the wing leading edge. In supersonic flight, the aerodynamic center moves back to 50 percent of the wing’s chord, causing some significant changes in the airplane’s control and stability.

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Airflow over the entire wing is subsonic

Mach number = .60 View A Airflow over the wing reaches Mach .99

View B

C. The velocity has surpassed the critical Mach number, and a normal shock wave has formed on the top of the wing. Some airflow separation starts to occur behind the shock wave.

Normal shock Subsonic

Mach number = .85 View C

Supersonic flow Normal shock Separation

Mach number = .88 View D

Normal shock

Normal shock Mach number = .95 View E Bow wave

Subsonic flow Mach number = 1.05 View F

Figure 3-79. Airflow with progressively greater Mach numbers.

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D. The velocity has continued to increase beyond the critical Mach number, and the normal shock wave has moved far enough aft that serious airflow separation is occurring. A normal shock wave is now forming on the bottom of the wing as well. Behind the normal shock waves, the velocity of the air is subsonic and the static pressure has increased. E. The velocity has increased to the point that both shock waves on the wing (top and bottom) have moved to the back of the wing and attached to the trailing edge. Some airflow separation is still occurring.

Normal shock

Supersonic flow

A. The Mach number is fairly low, and the entire wing is experiencing subsonic airflow. B. The velocity has reached the critical Mach number, where the airflow over the top of the wing is reaching Mach 1 velocity.

Mach number = .82 (Critical Mach no.)

Supersonic flow

If an airplane designed to fly subsonic, perhaps at a Mach number of 0.80, flies too fast and enters transonic flight, some noticeable changes will take place with respect to the airflow over the wing. Figure 3-79 shows six views of a wing, with each view showing the Mach number getting higher. The scenario for the six views is as follows:

F. The forward velocity of the airfoil is greater than Mach 1, and a new shock wave has formed just forward of the leading edge of the wing. If the wing has a sharp leading edge, the shock wave will attach itself to the sharp edge. The airfoil shown in Figure 3-79 is not properly designed to handle supersonic airflow. The bow wave in front of the wing leading edge of view F would be attached to the leading edge, if the wing was a double wedge or biconvex design. These two wing designs are shown in Figure 3-80. Aerodynamic Heating One of the problems with airplanes and high-speed flight is the heat that builds up on the airplane’s surface because of air friction. When the SR-71 Blackbird airplane is cruising at Mach 3.5, skin temperatures on its surface range from 450°F to over 1,000°F. To withstand this high temperature, the airplane was constructed of titanium alloy, instead of the traditional alu-

Expansion wave

Oblique shock

Oblique shock

Main rotor

Transmission

Tail rotor

Airflow

Cabin

Landing gear

Powerplant

Tail boom

Double wedge

Figure 3-81. Main components of a helicopter. Oblique shock

Oblique shock

Expansion wave Biconvex

Figure 3-80. Double wedge and biconvex supersonic wing design.

minum alloy. The supersonic transport Concorde was originally designed to cruise at Mach 2.2, but its cruise speed was reduced to Mach 2.0 because of structural problems that started to occur because of aerodynamic heating. If airplanes capable of hypersonic flight are going to be built in the future, one of the obstacles that will have to be overcome is the stress on the airplane’s structure caused by heat. Helicopter Aerodynamics The helicopter, as we know it today, falls under the classification known as rotorcraft. Rotorcraft are also known as rotary wing aircraft, because instead of their wing being fixed like it is on an airplane, the wing rotates. The rotating wing of a rotorcraft can be thought of as a lift producing device, like the wing of an airplane, or as a thrust producing device, like the propeller on a piston engine. Helicopter Structures and Airfoils The main parts that make up a helicopter are the cabin, landing gear, tail boom, powerplant, transmission, main rotor, and tail rotor. [Figure 3-81]

Main Rotor Systems

The classification of main rotor systems is based on how the blades move relative to the main rotor hub. The principal classifications are known as fully articulated, semi-rigid, and rigid. In the fully articulated rotor system, the blades are attached to the hub with multiple hinges. The blades are hinged in a way that allows them to move up and down and fore and aft, and bearings provide for motion around the pitch change axis. Rotor systems using this type of arrangement typically have three or more blades. The hinge that allows the blades to move up and down is called the flap hinge, and movement around this hinge is called flap. The hinge that allows the blades to move fore and aft is called a drag or lag hinge. Movement around this hinge is called dragging, lead/lag, or hunting. These hinges and their associated movement are shown in Figure 3-82. The main rotor head of a Eurocopter model 725 is shown in Figure 3-83, with the drag hinge and pitch change rods visible. The semi-rigid rotor system is used with a two blade main rotor. The blades are rigidly attached to the hub, with the hub and blades able to teeter like a seesaw. The teetering action allows the blades to flap, with one blade dropping down while the other blade rises. The blades are able to change pitch independently of each other. Figure 3-84 shows a Bell Jet Ranger helicopter in flight. This helicopter uses a semi-rigid rotor system, which is evident because of the way the rotor is tilted forward when the helicopter is in forward flight. With a rigid rotor system, the blades are not hinged for movement up and down (flapping) or for movement fore and aft (drag). The blades are able to move around the pitch change axis, with each blade being able to independently change its blade angle. The rigid 3-53

Lead or lag

Flap Drag hinge

Pitch Flap hinge

Pitch change rod Axis of rotation

Figure 3-82. Fully articulated main rotor head.

rotor system uses blades that are very strong and yet flexible. They are flexible enough to bend when they need to, without the use of hinges or a teetering rotor, to compensate for the uneven lift that occurs in forward flight. The Eurocopter model 135 uses a rigid rotor system. [Figure 3-85] Anti-Torque Systems Drag hinge

Pitch change rod

Figure 3-83. Eurocopter 725 main rotor head.

Semi-rigid main rotor

Figure 3-84. Bell Jet Ranger with semi-rigid main rotor.

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Any time a force is applied to make an object rotate, there will be an equal force acting in the opposite direction. If the helicopter’s main rotor system rotates clockwise when viewed from the top, the helicopter will try to rotate counterclockwise. Earlier in this chapter, it was discovered that torque is what tries to make something rotate. For this reason, a helicopter uses what is called an anti-torque system to counteract the force trying to make it rotate. One method that is used on a helicopter to counteract torque is to place a spinning set of blades at the end of the tail boom. These blades are called a tail rotor or anti-torque rotor, and their purpose is to create a force (thrust) that acts in the opposite direction of the way the helicopter is trying to rotate. The tail rotor force, in pounds, multiplied by the distance from the tail rotor to the main rotor, in feet, creates a torque in pound-feet that counteracts the main rotor torque.

Figure 3-85. Eurocopter Model 135 rigid rotor system.

Tail rotor

Figure 3-86. Aerospatiale helicopter tail rotor.

Figure 3-86 shows a three bladed tail rotor on an Aerospatiale AS-315B helicopter. This tail rotor has open tipped blades that are variable pitch, and the helicopter’s anti-torque pedals (positioned like rudder pedals on an airplane) control the amount of thrust they create. Some potential problems with this tail rotor system are as follows: • The spinning blades are deadly if someone walks into them.

Figure 3-87. Fenestron on a Eurocopter Model 135.

volume of air at low pressure, which comes from a fan driven by the helicopter’s engine. The fan forces air into the tail boom, where a portion of it exits out of slots on the right side of the boom and, in conjunction with the main rotor downwash, creates a phenomenon called the “Coanda effect.” The air coming out of the slots on the right side of the boom causes a higher velocity, and therefore lower pressure, on that side of the boom. The higher pressure on the left side of the boom creates the primary force that counteracts the torque of the main rotor. The remainder of the air travels back to a controllable rotating nozzle in the helicopter’s tail. The air exits the nozzle at a high velocity, and creates an additional force (thrust) that helps counteracts the torque of the main rotor. A NOTAR system is shown in Figures 3-88 and 3-89. For helicopters with two main rotors, such as the Chinook that has a main rotor at each end, no anti-torque

• When the helicopter is in forward flight and a vertical fin may be in use to counteract torque, the tail rotor robs engine power and creates drag. An alternative to the tail rotor seen in Figure 3-86 is a type of anti-torque rotor known as a fenestron, or “fan-in-tail” design as seen in Figure 3-87. Because the rotating blades in this design are enclosed in a shroud, they present less of a hazard to personnel on the ground and they create less drag in flight. A third method of counteracting the torque of the helicopter’s main rotor is a technique called the “no tail rotor” system, or NOTAR. This system uses a high

Figure 3-88. McDonnell Douglas 520 NOTAR.

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Low pressure side

Air exit slots

High pressure side

Rotating nozzle

Figure 3-89. Airflow for a NOTAR.

rotor is needed. For this type of helicopter, the two main rotors turn in opposite directions, and each one cancels out the torque of the other. Helicopter Axes of Flight Helicopters, like airplanes, have a vertical, lateral, and longitudinal axis that passes through the helicopter’s center of gravity. Helicopters yaw around the vertical axis, pitch around the lateral axis, and rotate around the longitudinal axis. Figure 3-90 shows the three axes of a helicopter and how they relate to the helicopter’s movement. All three axes will intersect at the helicopter’s center of gravity, and the helicopter pivots around this point. Notice in the figure that the vertical axis passes

Longitudinal axis

Lateral axis Vertical axis

Figure 3-90. Three axes of rotation for a helicopter.

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almost through the center of the main rotor, because the helicopter’s center of gravity needs to be very close to this point. Control Around the Vertical Axis

For a single main rotor helicopter, control around the vertical axis is handled by the anti-torque rotor (tail rotor) or from the fan’s airflow on a NOTAR type helicopter. Like in an airplane, rotation around this axis is known as yaw. The pilot controls yaw by pushing on the anti-torque pedals located on the cockpit floor, in the same way the airplane pilot controls yaw by pushing on the rudder pedals. To make the nose of the helicopter yaw to the right, the pilot pushes on the right anti-torque pedal. When viewed from the top, if the helicopter tries to spin in a counterclockwise direction because of the torque of the main rotor, the pilot will also push on the right anti-torque pedal to counteract the main rotor torque. By using the anti-torque pedals, the pilot can intentionally make the helicopter rotate in either direction around the vertical axis. The anti-torque pedals can be seen in Figure 3-91. Some helicopters have a vertical stabilizer, such as those shown in Figures 3-90 and 3-92. In forward flight, the vertical stabilizer creates a force that helps counteract the torque of the main rotor, thereby reducing the power needed to drive the anti-torque system located at the end of the tail boom.

Cyclic pitch control Collective pitch control

Anti-torque pedals

Figure 3-91. Helicopter cockpit controls.

Control Around the Longitudinal and Lateral Axes

Movement around the longitudinal and lateral axes is handled by the helicopter’s main rotor. In the cockpit, there are two levers that control the main rotor, known as the collective and cyclic pitch controls. The collective pitch lever is on the side of the pilot’s seat, and the cyclic pitch lever is at the front of the seat in the middle. [Figure 3-91] When the collective pitch control lever is raised, the blade angle of all the rotor blades increases uniformly and they create the lift that allows the helicopter to take off vertically. The grip on the end of the collective pitch control is the throttle for the engine, which is rotated to increase engine power as the lever is raised. On many helicopters, the throttle automatically rotates and increases engine power as the collective lever is raised. The collective pitch lever may have adjustable friction built into it, so the pilot does not have to hold upward pressure on it during flight. The cyclic pitch control lever, like the yoke of an airplane, can be pulled back or pushed forward, and can be moved left and right. When the cyclic pitch lever is pushed forward, the rotor blades create more lift as they pass through the back half of their rotation and less lift as they pass through the front half. The difference in lift is caused by changing the blade angle (pitch) of the rotor blades. The pitch change rods that were seen earlier, in Figures 3-82 and 3-83, are controlled by the cyclic pitch lever and they are what change the pitch of the rotor blades. The increased lift in the back either causes the main rotor to tilt forward, the nose

of the helicopter to tilt downward, or both. The end result is the helicopter moves in the forward direction. If the cyclic pitch lever is pulled back, the rotor blade lift will be greater in the front and the helicopter will back up. If the cyclic pitch lever is moved to the left or the right, the helicopter will bank left or bank right. For the helicopter to bank to the right, the main rotor blades must create more lift as they pass by the left side of the helicopter. Just the opposite is true if the helicopter is banking to the left. By creating more lift in the back than in the front, and more lift on the left than on the right, the helicopter can be in forward flight and banking to the right. In Figure 3-92, an Agusta A-109 can be seen in forward flight and banking to the right. The

Figure 3-92. Agusta A-109 banking to the right.

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Figure 3-93. Air Force CH-53 in a hover.

rotor blade in the rear and the one on the left are both in an upward raised position, meaning they have both experienced the condition called flap.

Force CH-53 is seen in a hover, with all the rotor blades flapping up as a result of creating equal lift.

Some helicopters use a horizontal stabilizer, similar to what is seen on an airplane, to help provide additional stability around the lateral axis. A horizontal stabilizer can be seen on the Agusta A-109 in Figure 3-92.

In the early days of helicopter development, the ability to hover was mastered before there was success in attaining forward flight. The early attempts at forward flight resulted in the helicopter rolling over when it tried to depart from the hover and move in any direction. The cause of the rollover is what we now refer to as dissymmetry of lift.

Helicopters in Flight Hovering For a helicopter, hovering means that it is in flight at a constant altitude, with no forward, aft, or sideways movement. In order to hover, a helicopter must be producing enough lift in its main rotor blades to equal the weight of the aircraft. The engine of the helicopter must be producing enough power to drive the main rotor, and also to drive whatever type of anti-torque system is being used. The ability of a helicopter to hover is affected by many things, including whether or not it is in ground effect, the density altitude of the air, the available power from the engine, and how heavily loaded it is. For a helicopter to experience ground effect, it typically needs to be no higher off the ground than one half of its main rotor system diameter. If a helicopter has a main rotor diameter of 40 ft, it will be in ground effect up to an altitude of approximately 20 ft. Being close to the ground affects the velocity of the air through the rotor blades, causing the effective angle of attack of the blades to increase and the lift to increase. So, if a helicopter is in ground effect, it can hover at a higher gross weight than it can when out of ground effect. On a windy day, the positive influence of ground effect is lessened, and at a forward speed of 5 to 10 mph the positive influence becomes less. In Figure 3-93, an Air

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Forward Flight

When a helicopter is in a hover, all the rotor blades are experiencing the same velocity of airflow. When the helicopter starts to move, the velocity of airflow seen by the rotor blades changes. For helicopters built in the United States, the main rotor blades turn in a counterclockwise direction when viewed from the top. Viewed from the top, as the blades move around the right side of the helicopter, they are moving toward the nose; as they move around the left side of the helicopter, they are moving toward the tail. When the helicopter starts moving forward, the blade on the right side is moving toward the relative wind, and the blade on the left side is moving away from the relative wind. This causes the blade on the right side to create more lift and the blade on the left side to create less lift. Figure 3-94 shows how this occurs. In Figure 3-94, blade number 2 would be called the advancing blade, and blade number 1 would be called the retreating blade. The advancing blade is moving toward the relative wind, and therefore experiences a greater velocity of airflow. The increased lift created by the blade on the right side will try to roll the helicopter to the left. If this condition is allowed to exist, it will ultimately lead to the helicopter crashing.

Blade rotation

Direction of relative wind #1

#2 Direction of flight (100 mph) Blade tip speed— 400 mph

Blade #1 experiences 300 mph airflow (Tip speed – airspeed) Blade #2 experiences 500 mph airflow (Tip speed + airspeed)

Figure 3-94. Dissymmetry of lift for rotor blades.

Blade Flapping

To solve the problem of dissymmetry of lift, helicopter designers came up with a hinged design that allows the rotor blade to flap up when it experiences increased lift, and to flap down when it experiences decreased lift. When a rotor blade advances toward the front of the helicopter and experiences an increased velocity of airflow, the increase in lift causes the blade to flap up. This upward motion of the blade changes the direction of the relative wind in relation to the chord line of the blade, and causes the angle of attack to decrease. The decrease in the angle of attack decreases the lift on the blade. The retreating blade experiences a reduced velocity of airflow and reduced lift, and flaps down. By flapping down, the retreating blade ends up with an increased angle of attack and an increase in lift. The end result is the lift on the blades is equalized, and the tendency for the helicopter to roll never materializes. The semi-rigid and fully articulated rotor systems have flapping hinges that automatically allow the blades to move up or down with changes in lift. The rigid type of rotor system has blades that are flexible enough to bend up or down with changes in lift. Advancing Blade and Retreating Blade Problems

As a helicopter flies forward at higher and higher speeds, the blade advancing toward the relative wind sees the airflow at an ever increasing velocity. Eventually, the velocity of the air over the rotor blade will

reach sonic velocity, much like the critical Mach number for the wing of an airplane. When this happens, a shock wave will form and the air will separate from the rotor blade, resulting in a high-speed stall. As the helicopter’s forward speed increases, the relative wind over the retreating blade decreases, resulting in a loss of lift. The loss of lift causes the blade to flap down and the effective angle of attack to increase. At a high enough forward speed, the angle of attack will increase to a point that the rotor blade stalls. The tip of the blade stalls first, and then progresses in toward the blade root. When approximately 25 percent of the rotor system is stalled, due to the problems with the advancing and retreating blades, control of the helicopter will be lost. Conditions that will lead to the rotor blades stalling include high forward speed, heavy gross weight, turbulent air, high-density altitude, and steep or abrupt turns. Autorotation

The engine on a helicopter drives the main rotor system by way of a clutch and a transmission. The clutch allows the engine to be running and the rotor system not to be turning, while the helicopter is on the ground, and it also allows the rotor system to disconnect from the engine while in flight, if the engine fails. Having the rotor system disconnect from the engine in the event of an engine failure is necessary if the helicopter is to be capable of a flight condition called autorotation. Autorotation is a flight condition where the main rotor blades are driven by the force of the relative wind passing through the blades, rather than by the engine. This flight condition is similar to an airplane gliding if its engine fails while in flight. As long as the helicopter maintains forward airspeed, while decreasing altitude, and the pilot lowers the blade angle on the blades with the collective pitch, the rotor blades will continue to rotate. The altitude of the helicopter, which equals potential energy, is given up in order to have enough energy (kinetic energy) to keep the rotor blades turning. As the helicopter nears the ground, the cyclic pitch control is used to slow the forward speed and to flare the helicopter for landing. With the airspeed bled off, and the helicopter now close to the ground, the final step is to use the collective pitch control to cushion the landing. The airflow through the rotor blades in normal forward flight and in an autorotation flight condition are shown in Figure 3-95. In Figure 3-96, a Bell Jet Ranger is shown approaching the ground in the final stage of an autorotation.

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Normal forward flight under power

Direction of airflow

Forward flight in autorotation

Figure 3-97. Weight-shift control aircraft in level flight.

Direction of airflow

Figure 3-95. Rotor blade airflow during normal flight and during autorotation.

Figure 3-96. Bell Jet Ranger in final stage of autorotation.

Weight-Shift Control, Flexible Wing Aircraft Aerodynamics A weight-shift control, flexible wing type aircraft consists of a fabric-covered wing, often referred to as the sail, attached to a tubular structure that has wheels, seats, and an engine and propeller. The wing structure is also tubular, with the fabric covering creating the airfoil shape. The shape of the wing varies among the

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different models of weight-shift control aircraft being produced, but a delta shaped wing is a very popular design. Within the weight-shift control aircraft community, these aircraft are typically referred to as trikes. [Figure 3-97] In Figure 3-97, the trike’s mast is attached to the wing at the hang point on the keel of the wing with a hang point bolt and safety cable. There is also a support tube, known as a king post, extending up from the top of the wing, with cables running down and secured to the tubular wing structure. The cables running down from the king post as part of the upper rigging are there to support the wing when the aircraft is on the ground, and to handle negative loads when in flight. The lines that run from the king post to the trailing edge of the wing are known as reflex cables. These cables maintain the shape of the wing when it is in a stalled state by holding the trailing edge of the wing up which helps raise the nose during recovery from the stall. If the aircraft goes into an inadvertent stall, having the trailing edge of the wing in a slightly raised position helps raise the nose of the aircraft and get it out of the stall. The passenger seat is centered under the wing’s aerodynamic center, with the weight of the pilot being forward of this point and the weight of the engine and propeller being aft. Unlike a traditional airplane, the trike does not have a rudder, elevator, or ailerons. Instead, it has a wing that can be pivoted forward or aft, and left or right. In Figure 3-98, the pilot’s hand is on a control bar that is connected to a pivot point just forward of where the wing attaches. There are cables attached to the ends of the bar that extend up to the wing’s leading and

Wing attach point

Crossbar

Wing batten

Wing keel Mast Nose strut

Control bar with cables attached to the wing

Brakes Throttle

Figure 3-98. Weight-shift control aircraft getting ready for flight.

trailing edge, and to the left and right side of the cross bar. Running from the wing leading edge to trailing edge are support pieces known as battens. The battens fit into pockets, and they give the wing its cambered shape. The names of some of the primary parts of the trike are shown in Figure 3-98, and these parts will be referred to when the flight characteristics of the trike are described in the paragraphs that follow. In order to fly the trike, engine power is applied to get the aircraft moving. As the groundspeed of the aircraft reaches a point where flight is possible, the pilot pushes forward on the control bar, which causes the wing to pivot where it attaches to the mast and the leading edge of the wing tilts up. When the leading edge of the wing tilts up, the angle of attack and the lift of the wing increase. With sufficient lift, the trike rotates and starts climbing. Pulling back on the bar reduces the angle of attack, and allows the aircraft to stop climbing and to fly straight and level. Once the trike is in level flight, airspeed can be increased or decreased by adding or taking away engine power by use of the throttle. Stability in flight along the longitudinal axis (nose to tail), for a typical airplane, is achieved by having the horizontal stabilizer and elevator generate a force that balances out the airplane’s nose heavy tendency.

Because the trike does not have a horizontal stabilizer or elevator, it must create stability along the longitudinal axis in a different way. The trike has a sweptback delta wing, with the trailing edge of the wingtips located well aft of the aircraft center of gravity. Pressure acting on the tips of the delta wing creates the force that balances out the nose heavy tendency. The wings of weight-shift control aircraft are designed in a way that allows them to change their shape when subjected to an external force. This is possible because the frame leading edges and the sail are flexible, which is why they are sometimes referred to as flexible wing aircraft. This produces somewhat different aerodynamic effects when compared with a normal fixed-wing aircraft. A traditional small airplane, like a Cessna 172, turns or banks by using the ailerons, effectively altering the camber of the wing and thereby generating differential lift. By comparison, weight shift on a trike actually causes the wing to twist, which changes the angle of attack on the wing and causes the differential lift to exist that banks the trike. The cross-bar (wing spreader) of the wing frame is allowed to float slightly with respect to the keel, and this, along with some other geometric considerations allows the sail to “billow shift.” Billow shift can be demonstrated on the ground by grabbing the trailing edge of one end of the 3-61

power will make it descend. The throttle is typically controlled with a foot pedal, like a gas pedal in an automobile.

Direction of turn

Figure 3-100. Weight shift to the left causing a left-hand turn.

A trike lands in a manner very similar to an airplane. When it is time to land, the pilot reduces engine power with the foot-operated throttle, causing airspeed and wing lift to decrease. As the trike descends, the rate of descent can be controlled by pushing forward or pulling back on the bar, and varying engine power. When the trike is almost to the point of touchdown, the engine power will be reduced and the angle of attack of the wing will be increased, to cushion the descent and provide a smooth landing. If the aircraft is trying to land in a very strong crosswind, the landing may not be so smooth. When landing in a cross wind, the pilot will land in a crab to maintain direction down the runway. Touchdown is done with the back wheels first, then letting the front wheel down.

wing and lifting up on it. If this was done, the fabric on the other end of the wing would become slightly flatter and tighter, and the wing’s angle of attack would increase.

A trike getting ready to touch down can be seen in Figure 3-101. The control cables coming off the control bar can be seen, and the support mast and the cables on top of the wing, including the luff lines, can also be seen.

Push out

Figure 3-99. Direction of turn based on weight shift.

Aircraft banks to the left because of increased lift on the right wing. Billow

Pushing the bar to the right shifts the weight to the left.

If the pilot pushes the bar to the right, the wing pivots with the left wingtip dropping down and the right wingtip rising up, causing the aircraft to bank to the left. This motion is depicted in Figure 3-99, showing a hang glider as an example. The shift in weight to the left increases the wing loading on the left, and lessens it on the right. The increased loading on the left wing increases its washout and reduces its angle of attack and lift. The increased load on the left wing causes the left wing to billow, which causes the fabric to tighten on the right wing and the angle of attack and lift to increase. The change in lift is what banks the aircraft to the left. Billow on the left wing is depicted in Figure 3-100. Shifting weight to the right causes the aircraft to bank right. The weight of the trike and its occupants acts like a pendulum, and helps keep the aircraft stable in flight. Pushing or pulling on the bar while in flight causes the weight hanging below the wing to shift its position relative to the wing, which is why the trike is referred to as a weight-shift aircraft. Once the trike is in flight and flying straight and level, the pilot only needs to keep light pressure on the bar that controls the wing. If the trike is properly balanced and there is no air turbulence, the aircraft will remain stable even if the pilot’s hands are removed from the bar. The same as with any airplane, increasing engine power will make the aircraft climb and decreasing 3-62

Powered Parachute Aerodynamics A powered parachute has a carriage very similar to the weight-shift control aircraft. Its wing, however, has no support structure or rigidity and only takes on the shape of an airfoil when it is inflated by the blast of air from the propeller and the forward speed of the aircraft. In Figure 3-102, a powered parachute is on its approach to land with the wing fully inflated and rising up above the aircraft. Each colored section of the inflated wing is made up of cells that are open in the front to allow air to ram in, and closed in the back to keep the air trapped inside. In between all the cells there are holes that allow the air to flow from one cell to the next, in order to equalize the pressure within the inflated wing. The wing is attached to the carriage of the aircraft by a large number of nylon or kevlar lines that run from the tips of the wing all the way to the center. The weight of the aircraft acting on these lines and their individual lengths cause the inflated wing to take its shape. The lines attach to the carriage of the aircraft at a location very close to where the center of gravity is located, and this attachment point is adjustable to account for balance changes with occupants of varying weights. As in weight-shift control aircraft, the powered parachute does not have the traditional flight controls of a fixed-wing airplane. When the wing of the aircraft

Figure 3-101. Weight-shift control aircraft landing.

is inflated and the aircraft starts moving forward, the wing starts generating lift. Once the groundspeed is sufficient for the wing’s lift greater than the weight of the aircraft, the aircraft lifts off the ground. Unlike an airplane, where the pilot has a lot of control over when the airplane rotates by deciding when to pull back on the yoke, the powered parachute will not take off until it reaches a specific airspeed. The powered parachute will typically lift off the ground at a speed somewhere between 28 and 30 mph, and will have an airspeed in flight of approximately 30 mph.

Once the powered parachute is in flight, control over climbing and descending is handled with engine power. Advancing the throttle makes the aircraft climb, and retarding the throttle makes it descend. The inflated wing creates a lot of drag in flight, so reducing the engine power creates a very controllable descent of the aircraft. The throttle, for controlling engine power, is typically located on the right-hand side of the pilot. [Figure 3-103] Turning of the powered parachute in flight is handled by foot-operated pedals (steering bars) located at the front of the aircraft. These bars can be seen in Figure 3-103. Each foot-operated pedal controls a set of lines, usually made out of nylon, that runs up to the trailing edge of each wingtip. When the right foot pedal is pushed, the line pulls down on the trailing edge of the right wingtip. As the trailing edge of the right wing drops down, drag is increased on the right side and the aircraft turns right. When pressure is taken off the foot Wing attach point adjustable for CG

Steering bars to control turning

Figure 3-102. Powered parachute with the wing inflated.

Nosewheel steering

Throttle

Figure 3-103. Two seat powered parachute.

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Figure 3-104. Powered parachute wing trailing edge.

pedal, the drag on the entire airfoil equalizes and the aircraft resumes its straight-and-level flight.

the wing leading edge and increases the angle of attack and lift.

To land a powered parachute, the first action the pilot takes is to reduce engine power and allow the aircraft to descend. With the power reduced to idle, the aircraft will descend at a rate of approximately 5 to 10 fps. As the aircraft approaches the ground, the descent rate can be lessened by increasing the engine power. Just before touchdown, the pilot pushes on both foot-operated pedals to drop the trailing edges on both sides of the wing. This action increases the drag on the wing uniformly, causing the wing to pivot aft, which raises

In Figure 3-104, the pilot is pushing on both foot pedals and the left and right wing trailing edges are deflected downward. The aircraft has just touched down and the wing is trailing behind the aircraft, caused by the high angle of attack and the additional drag on the wing. The increase in lift reduces the descent rate to almost nothing, and provides for a gentle landing. If the pilot pushes on the foot pedals too soon, the wing may pivot too far aft before touchdown, resulting in an unacceptable descent rate. In that case, it might be a relatively hard landing.

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The weight of an aircraft and its balance are extremely important for operating an aircraft in a safe and efficient manner. When a manufacturer designs an aircraft and the Federal Aviation Administration (FAA) certifies it, the specifications identify the aircraft’s maximum weight and the limits within which it must balance. The maximum allowable weight is based on the surface area of the wing, and how much lift it will generate at a safe and appropriate airspeed. If a small general aviation airplane, for example, required a takeoff speed of 200 miles per hour (mph) to generate enough lift to support its weight, that would not be safe. Taking off and landing at lower airspeeds is certainly safer than doing so at higher speeds. Where an aircraft balances is also a significant factor in determining if the aircraft is safe to operate. An aircraft that does not have good balance can exhibit poor maneuverability and controllability, making it difficult or impossible to fly. This could result in an accident, causing damage to the aircraft and injury to the people on board. Safety is the primary reason for concern about an aircraft’s weight and balance. A secondary reason for concern about weight and balance, but also a very important one, is the efficiency of the aircraft. Improper loading reduces the efficiency of an aircraft from the standpoint of ceiling, maneuverability, rate of climb, speed, and fuel consumption. If an airplane is loaded in such a way that it is extremely nose heavy, higher than normal forces will need to be exerted at the tail to keep the airplane in level flight. The higher than normal forces at the tail will create additional drag, which will require additional engine power and therefore additional fuel flow in order to maintain airspeed. The most efficient condition for an aircraft is to have the point where it balances fall very close to, or perhaps exactly at, the aircraft’s center of lift. If this were the case, little or no flight control force would be needed to keep the aircraft flying straight and level. In terms

of stability and safety, however, this perfectly balanced condition might not be desirable. All of the factors that affect aircraft safety and efficiency, in terms of its weight and balance, are discussed in detail in this chapter.

Need and Requirements for Aircraft Weighing Every aircraft type certificated by the FAA, before leaving the factory for delivery to its new owner, receives a weight and balance report as part of its required aircraft records. The weight and balance report identifies the empty weight of the aircraft and the location at which the aircraft balances, known as the center of gravity. If the manufacturer chooses to do so, it can weigh every aircraft it produces and issue the weight and balance report based on that weighing. As an alternative, the manufacturer is permitted to weigh an agreed upon percentage of a particular model of aircraft produced, perhaps 10 to 20 percent, and apply the average to all the aircraft. After the aircraft leaves the factory and is delivered to its owner, the need or requirement for placing the aircraft on scales and reweighing it varies depending on the type of aircraft and how it is used. For a small general aviation airplane being used privately, such as a Cessna 172, there is no FAA requirement that it be periodically reweighed. There is, however, an FAA requirement that the airplane always have a current and accurate weight and balance report. If the weight and balance report for an aircraft is lost, the aircraft must be weighed and a new report must be created. If the airplane has new equipment installed, such as a radio or a global positioning system, a new weight and balance report must be created. If the installer of the equipment wants to place the airplane on scales and weigh it after the installation, that is a perfectly acceptable way of creating the new report. If the installer knows the exact weight and location of the new equipment, it is also possible to create a new report by doing a series of mathematical calculations. 4-1

Over a period of time, almost all aircraft have a tendency to gain weight. Examples of how this can happen include an airplane being repainted without the old paint being removed, and the accumulation of dirt, grease, and oil in parts of the aircraft that are not easily accessible for cleaning. When new equipment is installed, and its weight and location are mathematically accounted for, some miscellaneous weight might be overlooked, such as wire and hardware. For this reason, even if the FAA does not require it, it is a good practice to periodically place an aircraft on scales and confirm its actual empty weight and empty weight center of gravity. Some aircraft are required to be weighed and have their center of gravity calculated on a periodic basis, typically every 3 years. Examples of aircraft that fall under this requirement are: 1. Air taxi and charter twin-engine airplanes operating under Title 14 of the Code of Federal Regulations (14 CFR) part 135, section (§)135.185(a). 2. Airplanes with a seating capacity of 20 or more passengers or a maximum payload of 6,000 pounds or more, as identified in 14 CFR part 125, §125.91(b). This paragraph applies to most airplanes operated by the airlines, both main line and regional, and to many of the privately operated business jets.

Weight and Balance Terminology Datum The datum is an imaginary vertical plane from which all horizontal measurements are taken for balance purposes, with the aircraft in level flight attitude. If the datum was viewed on a drawing or photograph of an aircraft, it would appear as a vertical line which is perpendicular (90 degrees) to the aircraft’s horizontal axis. For each aircraft make and model, the location of all items is identified in reference to the datum. For example, the fuel in a tank might be 60 inches (60") behind the datum, and a radio on the flight deck might be 90" forward of the datum. There is no fixed rule for the location of the datum, except that it must be a location that will not change during the life of the aircraft. For example, it would not be a good idea to have the datum be the tip of the propeller spinner or the front edge of a seat, because changing to a new design of spinner or moving the seat would cause the datum to change. It might be located at or near the nose of the aircraft, a specific number of inches forward of the nose, at the engine firewall, 4-2

Datum ( leading edge of wing) Negative arm

Positive arm

Center of lift

Figure 4-1. Datum location and its effect on positive and negative arms.

at the center of the main rotor shaft of a helicopter, or any place that can be imagined. The manufacturer has the choice of locating the datum where it is most convenient for measurement, equipment location, and weight and balance computation. Figure 4-1 shows an aircraft with the leading edge of the wing being the datum. The location of the datum is identified in the Aircraft Specifications or Type Certificate Data Sheet. Aircraft certified prior to 1958 fell under the Civil Aeronautics Administration, and had their weight and balance information contained in a document known as Aircraft Specifications. Aircraft certified since 1958 fall under the FAA and have their weight and balance information contained in a document known as a Type Certificate Data Sheet. The Aircraft Specifications typically included the aircraft equipment list. For aircraft with a Type Certificate Data Sheet, the equipment list is a separate document. Arm The arm is the horizontal distance that a part of the aircraft or a piece of equipment is located from the datum. The arm’s distance is always given or measured in inches, and, except for a location which might be exactly on the datum, it is preceded by the algebraic sign for positive (+) or negative (−). The positive sign indicates an item is located aft of the datum and the negative sign indicates an item is located forward of the datum. If the manufacturer chooses a datum that is at the most forward location on an aircraft (or some distance forward of the aircraft), all the arms will be positive numbers. Location of the datum at any other point on the aircraft will result in some arms being positive numbers, or aft of the datum, and some arms being negative numbers, or forward of the datum. Figure 4-1 shows an aircraft where the datum is the leading edge of the wing. For this aircraft, any item (fuel, seat, radio, and so forth) located forward of the wing leading edge will have a negative arm, and any item located aft of

the wing leading edge will have a positive arm. If an item was located exactly at the wing leading edge, its arm would be zero, and mathematically it would not matter whether its arm was considered to be positive or negative. The arm of each item is usually included in parentheses immediately after the item’s name or weight in the Aircraft Specifications, Type Certificate Data Sheet, or equipment list for the aircraft. In a Type Certificate Data Sheet, for example, the fuel quantity might be identified as 150 gallons (gal) (+138) and the nose baggage limit as 200 pounds (lb) (−55). These numbers indicate that the fuel is located 138" aft of the datum and the nose baggage is located 55" forward of the datum. If the arm for a particular piece of equipment is not known, its exact location must be accurately measured. When the arm for a piece of equipment is being determined, the measurement is taken from the datum to the piece of equipment’s own center of gravity. Moment A moment is the product of a weight multiplied by its arm. The moment for a piece of equipment is in fact a torque value, measured in units of inch-pounds (in-lb). To obtain the moment of an item with respect to the datum, multiply the weight of the item by its horizontal distance from the datum. Likewise, the moment of an item with respect to the center of gravity (CG) of an aircraft can be computed by multiplying its weight by the horizontal distance from the CG. A 5 lb radio located 80" from the datum would have a moment of 400 inch-pounds (in-lb) (5 lb × 8"). Whether the value of 400 in-lb is preceded by a positive (+) or negative (−) sign depends on whether the moment is the result of a weight being removed or added and its location in relation to the datum. This situation is shown in Figure 4-2, where the moment ends up being a positive number because the weight and arm are both positive. The algebraic sign of the moment, based on the datum location and whether weight is being installed or removed, would be as follows: • Weight being added aft of the datum produces a positive moment (+ weight, + arm).

Datum 40" forward of the firewall

Radio (5 lb)

Arm = 80"

Center of lift Moment = Weight (Arm) = 5 lb (80") = 400 inch-pounds

Figure 4-2. Moment of a radio located aft of the datum.

When dealing with positive and negative numbers, remember that the product of like signs produces a positive answer and the product of unlike signs produces a negative answer. Center of Gravity The center of gravity (CG) of an aircraft is a point about which the nose heavy and tail heavy moments are exactly equal in magnitude. It is the balance point for the aircraft. An aircraft suspended from this point would have no tendency to rotate in either a nose-up or nose-down attitude. It is the point about which the weight of an airplane or any object is concentrated. Figure 4-3 shows a first class lever with the pivot point (fulcrum) located at the center of gravity for the lever. Even though the weights on either side of the fulcrum are not equal, and the distances from each weight to the fulcrum are not equal, the product of the weights and arms (moments) are equal, and that is what produces a balanced condition. Maximum Weight The maximum weight is the maximum authorized weight of the aircraft and its contents, and is indicated in the Aircraft Specifications or Type Certificate Data Sheet. For many aircraft, there are variations to the maximum allowable weight, depending on the purForce

Force Distance = 90"

Distance = 70"

• Weight being added forward of the datum produces a negative moment (+ weight, − arm). • Weight being removed aft of the datum produces a negative moment (− weight, + arm). • Weight being removed forward of the datum produces a positive moment (− weight, − arm)

Moment = 700 lb (90") = 63,000 in-lb

Fulcrum and center of gravity

Moment = 900 lb (70") = 63,000 in-lb

Figure 4-3. Center of gravity and a first class lever.

4-3

pose and conditions under which the aircraft is to be flown. For example, a certain aircraft may be allowed a maximum gross weight of 2,750 lb when flown in the normal category, but when flown in the utility category, which allows for limited aerobatics, the same aircraft’s maximum allowable gross weight might only be 2,175 lb. There are other variations when dealing with the concept of maximum weight, as follows:

included in the value, so it generally involves only aircraft certified prior to 1978. Standard empty weight would be a value supplied by the aircraft manufacturer, and it would not include any optional equipment that might be installed in a particular aircraft. For most people working in the aviation maintenance field, the basic empty weight of the aircraft is the most important one.

• Maximum Ramp Weight — the heaviest weight to which an aircraft can be loaded while it is sitting on the ground. This is sometimes referred to as the maximum taxi weight.

Empty Weight Center of Gravity The empty weight center of gravity for an aircraft is the point at which it balances when it is in an empty weight condition. The concepts of empty weight and center of gravity were discussed earlier in this chapter, and now they are being combined into a single concept.

• Maximum Takeoff Weight — the heaviest weight an aircraft can have when it starts the takeoff roll. The difference between this weight and the maximum ramp weight would equal the weight of the fuel that would be consumed prior to takeoff. • Maximum Landing Weight — the heaviest weight an aircraft can have when it lands. For large wide body commercial airplanes, it can be 100,000 lb less than maximum takeoff weight, or even more. • Maximum Zero Fuel Weight — the heaviest weight an aircraft can be loaded to without having any usable fuel in the fuel tanks. Any weight loaded above this value must be in the form of fuel. Empty Weight The empty weight of an aircraft includes all operating equipment that has a fixed location and is actually installed in the aircraft. It includes the weight of the airframe, powerplant, required equipment, optional or special equipment, fixed ballast, hydraulic fluid, and residual fuel and oil. Residual fuel and oil are the fluids that will not normally drain out because they are trapped in the fuel lines, oil lines, and tanks. They must be included in the aircraft’s empty weight. For most aircraft certified after 1978, the full capacity of the engine oil system is also included in the empty weight. Information regarding residual fluids in aircraft systems that must be included in the empty weight, and whether or not full oil is included, will be indicated in the Aircraft Specifications or Type Certificate Data Sheet. Other terms that are sometimes used when describing empty weight include basic empty weight, licensed empty weight, and standard empty weight. The term “basic empty weight” typically applies when the full capacity of the engine oil system is included in the value. The term “licensed empty weight” typically applies when only the weight of residual oil is 4-4

One of the most important reasons for weighing an aircraft is to determine its empty weight center of gravity. All other weight and balance calculations, including loading the aircraft for flight, performing an equipment change calculation, and performing an adverse condition check, begin with knowing the empty weight and empty weight center of gravity. This crucial information is part of what is contained in the aircraft weight and balance report. Useful Load To determine the useful load of an aircraft, subtract the empty weight from the maximum allowable gross weight. For aircraft certificated in both normal and utility categories, there may be two useful loads listed in the aircraft weight and balance records. An aircraft with an empty weight of 900 lb will have a useful load of 850 lb, if the normal category maximum weight is listed as 1,750 lb. When the aircraft is operated in the utility category, the maximum gross weight may be reduced to 1,500 lb, with a corresponding decrease in the useful load to 600 lb. Some aircraft have the same useful load regardless of the category in which they are certificated. The useful load consists of fuel, any other fluids that are not part of empty weight, passengers, baggage, pilot, copilot, and crewmembers. Whether or not the weight of engine oil is considered to be a part of useful load depends on when the aircraft was certified, and can be determined by looking at the Aircraft Specifications or Type Certificate Data Sheet. The payload of an aircraft is similar to the useful load, except it does not include fuel. A reduction in the weight of an item, where possible, may be necessary to remain within the maximum weight allowed for the category in which an aircraft

is operating. Determining the distribution of these weights is called a weight check. Minimum Fuel There are times when an aircraft will have a weight and balance calculation done, known as an extreme condition check. This is a pencil and paper check in which the aircraft is loaded in as nose heavy or tail heavy a condition as possible to see if the center of gravity will be out of limits in that situation. In a forward adverse check, for example, all useful load in front of the forward CG limit is loaded, and all useful load behind this limit is left empty. An exception to leaving it empty is the fuel tank. If the fuel tank is located behind the forward CG limit, it cannot be left empty because the aircraft cannot fly without fuel. In this case, an amount of fuel is accounted for, which is known as minimum fuel. Minimum fuel is typically that amount needed for 30 minutes of flight at cruise power. For a piston engine powered aircraft, minimum fuel is calculated based on the METO (maximum except take-off) horsepower of the engine. For each METO horsepower of the engine, one-half pound of fuel is used. This amount of fuel is based on the assumption that the piston engine in cruise flight will burn 1 lb of fuel per hour for each horsepower, or 1⁄2 lb for 30 minutes. The piston engines currently used in small general aviation aircraft are actually more efficient than that, but the standard for minimum fuel has remained the same. Minimum fuel is calculated as follows: Minimum Fuel (pounds) = Engine METO Horsepower ÷ 2 For example, if a forward adverse condition check was being done on a piston engine powered twin, with each engine having a METO horsepower of 500, the minimum fuel would be 250 lb (500 METO Hp ÷ 2). For turbine engine powered aircraft, minimum fuel is not based on engine horsepower. If an adverse condition check is being performed on a turbine engine powered aircraft, the aircraft manufacturer would need to supply information on minimum fuel. Tare Weight When aircraft are placed on scales and weighed, it is sometimes necessary to use support equipment to aid in the weighing process. For example, to weigh a tail dragger airplane, it is necessary to raise the tail in order to get the airplane level. To level the airplane, a jack might be placed on the scale and used to raise

the tail. Unfortunately, the scale is now absorbing the weight of the jack in addition to the weight of the airplane. This extra weight is known as tare weight, and must be subtracted from the scale reading. Other examples of tare weight are wheel chocks placed on the scales and ground locks left in place on retractable landing gear.

Procedures for Weighing an Aircraft General Concepts The most important reason for weighing an aircraft is to find out its empty weight (basic empty weight), and to find out where it balances in the empty weight condition. When an aircraft is to be flown, the pilot in command must know what the loaded weight of the aircraft is, and where its loaded center of gravity is. In order for the loaded weight and center of gravity to be calculated, the pilot or dispatcher handling the flight must first know the empty weight and empty weight center of gravity. Earlier in this chapter it was identified that the center of gravity for an object is the point about which the nose heavy and tail heavy moments are equal. One method that could be used to find this point would involve lifting an object off the ground twice, first suspending it from a point near the front, and on the second lift suspending it from a point near the back. With each lift, a perpendicular line (90 degrees) would be drawn from the suspension point to the ground. The two perpendicular lines would intersect somewhere in the object, and the point of intersection would be the center of gravity. This concept is shown in Figure 4-4, where an irregular shaped object is suspended from two different points. The perpendicular line from the first suspension point is shown in red, and the new

Suspended from this point first, with red line dropping perpendicular to the ground

Center of gravity

Second suspension, with blue line dropping perpendicular to the ground

Figure 4-4. Center of gravity determined by two suspension points.

4-5

suspension point line is shown in blue. Where the red and blue lines intersect is the center of gravity. If an airplane were suspended from two points, one at the nose and one at the tail, the perpendicular drop lines would intersect at the center of gravity the same way they do for the object in Figure 4-4. Suspending an airplane from the ceiling by two hooks, however, is clearly not realistic. Even if it could be done, determining where in the airplane the lines intersect would not be possible. A more realistic way to find the center of gravity for an object, especially an airplane, is to place it on a minimum of two scales and to calculate the moment value for each scale reading. In Figure 4-5, there is a plank that is 200" long, with the left end being the datum (zero arm), and 6 weights placed at various locations along the length of the plank. The purpose of Figure 4-5 is to show how the center of gravity can be calculated when the arms and weights for an object are known. To calculate the center of gravity for the object in Figure 4-5, the moments for all the weights need to be calculated and then summed, and the weights need to be summed. In the four column table in Figure 4-6, the item, weight, and arm are listed in the first three columns, with the information coming from Figure 4-5. The moment value in the fourth column is the 50 lb

0"

30"

125 lb

60"

80 lb

50 lb

95" C.G. 125" 106.9"

product of the weight and arm. The weight and moment columns are summed, with the center of gravity being equal to the total moment divided by the total weight. The arm column is not summed. The number appearing at the bottom of that column is the center of gravity. The calculation would be as shown in Figure 4-6. For the calculation shown in Figure 4-6, the total moment is 52,900 in-lb, and the total weight is 495 lb. The center of gravity is calculated as follows: Center of Gravity = Total Moment ÷ Total Weight = 52,900 in-lb ÷ 495 lb = 106.9" (106.87 rounded off to tenths) An interesting characteristic exists for the problem presented in Figure 4-5, and the table showing the center of gravity calculation. If the datum (zero arm) for the object was in the middle of the 200" long plank, with 100" of negative arm to the left and 100" of positive arm to the right, the solution would show the center of gravity to be in the same location. The arm for the center of gravity would not be the same number, but its physical location would be the same. Figure 4-7 and Figure 4-8 show the new calculation. Center of Gravity = Total Moment ÷ Total Weight = 3,400 in-lb ÷ 495 lb = 6.9" (6.87 rounded off to tenths)

90 lb 100 lb

145"

170" 200"

–100"

Figure 4-5. Center of gravity for weights on a plank, datum at one end.

Item

Weight (lb)

Arm (inches)

Moment (in-lb)

50 lb

125 lb

–70"

–40"

80 lb

C.G. 6.9"

–5" 0"

50 lb

25"

90 lb 100 lb

45"

70" 100"

Figure 4-7. Center of gravity for weights on a plank, datum in the middle.

Item

Weight (lb)

Arm (inches)

Moment (in-lb)

50 pound weight

50

+30

1,500

50 pound weight

50

–70

–3,500

125 pound weight

125

+60

7,500

125 pound weight

125

–40

–5,000

80 pound weight

80

+95

7,600

80 pound weight

80

–5

–400

50 pound weight

50

+125

6,250

50 pound weight

50

+25

1,250

90 pound weight

90

+145

13,050

90 pound weight

90

+45

4,050

100 pound weight

100

+170

17,000

100 pound weight

100

+70

7,000

Total

495

+106.9

52,900

Total

495

+6.9

3,400

Figure 4-6. Center of gravity calculation for weights on a plank with datum at one end.

4-6

Figure 4-8. Center of gravity calculation for weights on a plank with datum in the middle.

In Figure 4-7, the center of gravity is 6.9" to the right of the plank’s center. Even though the arm is not the same number, in Figure 4-5 the center of gravity is also 6.9" to the right of center (CG location of 106.9 with the center being 100). Because both problems are the same in these two figures, except for the datum location, the center of gravity must be the same. The definition for center of gravity states that it is the point about which all the moments are equal. We can prove that the center of gravity for the object in Figure 4-7 is correct by showing that the total moments on either side of this point are equal. Using 6.87 as the CG location for slightly greater accuracy, instead of the rounded off 6.9 number, the moments to the left of the CG would be as shown in Figure 4-9. The moments to the right of the CG, as shown in Figure 4-7, would be as shown in Figure 4-10. Disregarding the slightly different decimal value, the moment in both of the previous calculations is 10,651 in-lb. Showing that the moments are equal is a good way of proving that the center of gravity has been properly calculated. Weight and Balance Data In order to weigh an aircraft and calculate its empty weight and empty weight center of gravity, a technician must have access to weight and balance information

Item

Weight (lb)

Arm (inches)

Moment (in-lb)

50 pound weight

50

76.87

3,843.50

125 pound weight

125

46.87

5,858.75

80 pound weight

80

11.87

949.60

255

135.61

10,651.85

Total

Figure 4-9. Moments to the left of the center of gravity.

about the aircraft. Possible sources of weight and balance data are as follows: • Aircraft Specifications — applies primarily to aircraft certified under the Civil Aeronautics Administration, when the specifications also included a list of equipment with weights and arms. • Aircraft Operating Limitations — supplied by the aircraft manufacturer. • Aircraft Flight Manual — supplied by the aircraft manufacturer. • Aircraft Weight and Balance Report — supplied by the aircraft manufacturer when the aircraft is new, and by the technician when an aircraft is reweighed in the field. • Aircraft Type Certificate Data Sheet — applies primarily to aircraft certified under the FAA and the Federal Aviation Regulations, where the equipment list with weights and arms is a separate document. The document in Figure 4-11 is a Type Certificate Data Sheet (TCDS) for a Piper twin-engine airplane known as the Seneca (PA-34-200). The main headings for the information typically contained in a TCDS are included, but much of the information contained under these headings has been removed if it did not directly pertain to weight and balance. Information on only one model of Seneca is shown, because to show all the different models would make the document excessively long. The portion of the TCDS that has the most direct application to weight and balance is highlighted in yellow. Some of the important weight and balance information found in a Type Certificate Data Sheet is as follows: 1. Center of gravity range 2. Maximum weight 3. Leveling means 4. Number of seats and location

Item

Weight (lb)

Arm (inches)

Moment (in-lb)

5. Baggage capacity 6. Fuel capacity 7. Datum location

50 pound weight

50

18.13

906.50

90 pound weight

90

38.13

3,431.70

8. Engine horsepower

100 pound weight

100

63.13

6,313.00

9. Oil capacity

Total

240

119.39

10,651.25

10. Amount of fuel in empty weight 11. Amount of oil in empty weight

Figure 4-10. Moments to the right of the center of gravity.

4-7

Department of Transportation Federal Aviation Administration Type Certificate Data Sheet No. A7so – Revision 14 PIPER PA-34-200 PA-34-200T PA-34-220T June 1, 2001 This data sheet, which is a part of type certificate No. A7SO, prescribes conditions and limitations under which the product for which the type certificate was issued meets the airworthiness requirements of the Federal Aviation Regulations. Type Certificate Holder: The New Piper Aircraft, Inc. 2926 Piper Drive Vero Beach, Florida 32960 I.   Model PA-34-200 (Seneca), 7 PCLM (Normal Category), Approved 7 May 1971. Engines

S/N 34-E4, 34-7250001 through 34-7250214: 1 Lycoming LIO-360-C1E6 with fuel injector, Lycoming P/N LW-10409 or LW-12586 (right side); and 1 Lycoming IO-360-C1E6 with fuel injector, Lycoming P/N LW-10409 or LW 12586 (left side). S/N 34-7250215 through 34-7450220: 1 Lycoming LIO-360-C1E6 with fuel injector, Lycoming P/N LW-12586 (right side); and 1 Lycoming IO-360-C1E6 with fuel injector, Lycoming P/N LW-12586 (left side).

Fuel

100/130 minimum grade aviation gasoline

Engine Limits

For all operations, 2,700 RPM (200 hp)

Propeller and Propeller Limits

Left Engine 1 Hartzell, Hub Model HC-C2YK-2 ( ) E, Blade Model C7666A-0; 1 Hartzell, Hub Model HC-C2YK-2 ( ) EU, Blade Model C7666A-0; 1 Hartzell, Hub Model HC-C2YK-2 ( ) EF, Blade Model FC7666A-0; 1 Hartzell, Hub Model HC-C2YK-2 ( ) EFU, Blade Model FC7666A-0; 1 Hartzell, Hub Model HC-C2YK-2CG (F), Blade Model (F) C7666A (This model includes the Hartzell damper); or 1 Hartzell, Hub Model HC-C2YK-2CGU (F), Blade Model (F) C7666A (This model includes the Hartzell damper). Note: HC-( )2YK-( ) may be substituted for HC-( )2YR-( ) per Hartzell Service Advisory 61.



Right Engine 1 Hartzell, Hub Model HC-C2YK-2 ( ) LE, Blade Model JC7666A-0; 1 Hartzell, Hub Model HC-C2YK-2 ( ) LEU, Blade Model JC7666A-0; 1 Hartzell, Hub Model HC-C2YK-2 ( ) LEF, Blade Model FJC7666A-0; 1 Hartzell, Hub Model HC-C2YK-2 ( ) LEFU, Blade Model FJC7666A-0; 1 Hartzell, Hub Model HC-C2YK-2CLG (F), Blade Model (F) JC7666A (This model includes the Hartzell damper); or Figure 4-11. Type Certificate Data Sheet.

4-8



1 Hartzell, Hub Model HC-C2YK-2CLGU (F), Blade Model (F) JC7666A (This model includes the Hartzell damper.) Note: HC-( )2YK-( ) may be substituted for HC-( )2YR-( ) per Hartzell Service Advisory 61. Pitch setting: High 79° to 81°, Low 13.5° at 30" station. Diameter: Not over 76", not under 74". No further reduction permitted.



Spinner: Piper P/N 96388 Spinner Assembly and P/N 96836 Cap Assembly, or P/N 78359-0 Spinner Assembly and P/N 96836-2 Cap Assembly (See NOTE 4)



Governor Assembly: 1 Hartzell hydraulic governor, Model F-6-18AL (Right); 1 Hartzell hydraulic governor, Model F-6-18A (Left). Avoid continuous operation between 2,200 and 2,400 RPM unless aircraft is equipped with Hartzell propellers which incorporate a Hartzell damper on both left and right engine as noted above.

Airspeed Limits

VNE (Never exceed) VNO (Maximum structural cruise) VA (Maneuvering, 4,200 lb) VA (Maneuvering, 4,000 lb) VA (Maneuvering, 2,743 lb) VFE (Flaps extended) VLO (Landing gear operating) Extension Retract VLE (Landing gear extended) VMC (Minimum control speed)

CG Range (Gear Extended)

S/N 34-E4, 34-7250001 through 34-7250214 (See NOTE 3): (+86.4) to (+94.6) at 4,000 lb (+82.0) to (+94.6) at 3,400 lb (+80.7) to (+94.6) at 2,780 lb S/N 34-7250215 through 34-7450220: (+87.9) to (+94.6) at 4,200 lb (+82.0) to (+94.6) at 3,400 lb (+80.7) to (+94.6) at 2,780 lb Straight line variation between points given. Moment change due to gear retracting landing gear (-32 in-lb)

Empty Weight CG Range

None

Maximum Weight

S/N 34-E4, 34-7250001 through 34-7250214: 4,000 lb – Takeoff 4,000 lb – Landing See NOTE 3.

Maximum Weight

S/N 34-7250215 through 34-7450220: 4,200 lb – Takeoff 4,000 lb – Landing

No. of Seats

7 (2 at +85.5, 3 at +118.1, 2 at +155.7)

217 mph 190 mph 146 mph 146 mph 133 mph 125 mph

(188 knots) (165 knots) (127 knots) (127 knots) (115 knots) (109 knots)

150 mph 125 mph 150 mph 80 mph

(130 knots) (109 knots) (130 knots) (69 knots)

Maximum Baggage 200 lb (100 lb at +22.5, 100 lb at +178.7) Figure 4-11. Type Certificate Data Sheet. (continued)

4-9

Fuel Capacity

98 gallons (2 wing tanks) at (+93.6) (93 gallons usable) See NOTE 1 for data on system fuel.

Oil Capacity

8 qt per engine (6 qt per engine usable) See NOTE 1 for data on system oil.

Control Surface Movements Rudder

Ailerons (±2°) Up 30° Down 15° Stabilator Up 12.5° (+0,-1°) Down 7.5° (±1°) (±1°) Left 35° Right 35° Stabilator Trim Tab (±1°) Down 10.5° Up 6.5° (Stabilator neutral) Wing Flaps (±2°) Up 0° Down 40° Rudder Trim Tab (±1°) Left 17° Right 22° (Rudder neutral) Nosewheel S/N 34-E4, 34-7250001 through 34-7350353: Travel (±1°) Left 21° Right 21° Nosewheel S/N 34-7450001 through 34-7450220: Travel (±1°) Left 27° Right 27°

Manufacturer's Serial Numbers

3449001 and up.

Data Pertinent To All Models Datum

78.4" forward of wing leading edge from the inboard edge of the inboard fuel tank.

Leveling Means

Two screws left side fuselage below window.

Certification Basis

Type Certificate No. A7SO issued May 7, 1971, obtained by the manufacturer under the delegation option authorization. Date of Type Certificate application July 23, 1968.



Model PA-34-200 (Seneca I): 14 CFR part 23 as amended by Amendment 23-6 effective August 1, 1967; 14 CFR part 23.959 as amended by Amendment 23-7 effective September 14, 1969; and 14 CFR part 23.1557(c)(1) as amended by Amendment 23-18 effective May 2, 1977. Compliance with 14 CFR part 23.1419 as amended by Amendment 23-14 effective December 20, 1973, has been established with optional ice protection provisions.

Production Basis

Production Certificate No. 206. Production Limitation Record issued and the manufacturer is authorized to issue an airworthiness certificate under the delegation option provisions of 14 CFR part 21.

Equipment

The basic required equipment as prescribed in the applicable airworthiness regulations (see Certification Basis) must be installed in the aircraft for certification. In addition, the following items of equipment are required:

Figure 4-11. Type Certificate Data Sheet. (continued)

4-10

Model

Afm/poh Report #

Approved

PA-34-200(Seneca) AFM VB-353 7/2/71 AFM VB-423 5/20/72 AFM VB-563 5/14/73 AFM Supp. VB-588 7/20/73 AFM Supp. VB-601 11/9/73

Serial Effectivity 34-E4, 34-7250001 through 34-7250214 34-7250001 through 34-7250189 when Piper Kit 760-607 is installed; 34-7250190 through 34-7250214 when Piper Kit 760-611 is installed; and 34-7250215 through 34-7350353 34-7450001 through 34-7450220 34-7250001 through 34-7450039 when propeller with dampers are installed 34-7250001 through 34-745017 when ice protection system is installed

NOTES NOTE 1

Current Weight and Balance Report, including list of equipment included in certificated empty weight, and loading instructions when necessary, must be provided for each aircraft at the time of original certification. The certificated empty weight and corresponding center of gravity locations must include undrainable system oil (not included in oil capacity) and unusable fuel as noted below:



Fuel: 30.0 lb at (+103.0) for PA-34 series, except Model PA-34-220T (Seneca V), S/N 3449001 and up Fuel: 36.0 lb at (+103.0) for Model PA-34-220T (Seneca V), S/N 3449001 and up Oil: 6.2 lb at (+ 39.6) for Model PA-34-200 Oil: 12.0 lb at (+ 43.7) for Models PA-34-200T and PA-34-220T

NOTE 2

All placards required in the approved Airplane Flight Manual or Pilot’s Operating Handbook and approved Airplane Flight Manual or Pilot’s Operating Handbook supplements must be installed in the appropriate location.

NOTE 3

The Model PA-34-200; S/N 34-E4, 34-7250001 through 34-7250189, may be operated at a maximum takeoff weight of 4,200 lb when Piper Kit 760-607 is installed. S/N 34-7250190 through 34-7250214 may be operated at a maximum takeoff weight of 4,200 lb when Piper Kit 760-611 is installed.

NOTE 4

The Model PA-34-200; S/N 34-E4, 34-7250001 through 34-7250189, may be operated without spinner domes or without spinner domes and rear bulkheads when Piper Kit 760-607 has been installed. ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

NOTE 5

The Model PA-34-200 may be operated in known icing conditions when equipped with spinner assembly and the following kits: ------------------------- --------------------------------------

NOTE 6

Model PA-34-200T; S/N 34-7570001 through 34-8170092, may be operated in known icing conditions when equipped with deicing equipment installed per Piper Drawing No. 37700 and spinner assembly. Figure 4-11. Type Certificate Data Sheet. (continued)

4-11

NOTE 7

The following serial numbers are not eligible for import certification to the United States: ------------------------------------------------------------------------------------------------

NOTE 8

Model PA-34-200; S/N 34-E4, S/N 34-7250001 through 34-7450220, and Model PA-34-200T; S/N 34-7570001 through 34-8170092, and Model PA-34-220T may be operated subject to the limitations listed in the Airplane Flight Manual or Pilot’s Operating Handbook with rear cabin and cargo door removed.

NOTE 9

In the following serial numbered aircraft, rear seat location is farther aft as shown and the center seats may be removed and replaced by CLUB SEAT INSTALLATION, which has a more aft CG location as shown in “No. of Seats,” above:PA-34-200T: S/N 34-7770001 through 34-8170092.

NOTE 10

These propellers are eligible on Teledyne Continental L/TSIO-360-E only.

NOTE 11

With Piper Kit 764-048V installed weights are as follows: 4,407 lb – Takeoff 4,342 lb Landing (All weight in excess of 4,000 lb must be fuel) Zero fuel weight may be increased to a maximum of 4,077.7 lb when approved wing options are installed (See POH VB-1140).

NOTE 12

With Piper Kit 764-099V installed, weights are as follows: 4,430 lb - Ramp 4,407 lb Takeoff, Landing, and Zero Fuel (See POH VB-1150).

NOTE 13

With Piper Kit 766-203 installed, weights are as follows: 4,430 lb - Ramp 4,407 lb - Takeoff, Landing and Zero Fuel (See POH VB-1259).

NOTE 14

With Piper Kit 766-283 installed, weights are as follows: 4,430 lb - Ramp 4,407 lb - Takeoff, Landing and Zero Fuel (See POH VB-1558).

NOTE 15

With Piper Kit 766-608 installed, weights are as follows: 4,430 lb - Ramp 4,407 lb - Takeoff, Landing and Zero Fuel (See POH VB-1620).

NOTE 16

With Piper Kit 766-632 installed, weights are as follows: 4,430 lb - Ramp 4,407 lb - Takeoff, Landing and Zero Fuel (See POH VB-1649).

NOTE 17

The bolt and stack-up that connect the upper drag link to the nose gear trunnion are required to be replaced every 500 hours’ time-in-service. -----------------------------------------------------------------------------------------------------------------------------------------------------------------------

Figure 4-11. Type Certificate Data Sheet. (continued)

4-12

Weight and Balance Equipment Scales Two types of scales are typically used to weigh aircraft: those that operate mechanically with balance weights or springs, and those that operate electronically with what are called load cells. The balance weight type of mechanical scale, known as a beam scale, is similar to that found in a doctor’s office, in which a bar rises up when weight is put on the scale. A sliding weight is then moved along the bar until the bar is centered between a top and bottom stop. The sliding weight provides the capability to measure up to 50 lb, and the cup holds fixed weights that come in 50 lb equivalent units. As an example of this scale in use, let’s say the nosewheel of a small airplane is placed on the scale with an applied weight of 580 lb. To find out what the applied weight is, a technician would place 550 lb of equivalent weight in the cup, and then slide the weight on the beam out to the 30 lb point. The 580 lb applied by the nosewheel would now be balanced by the 580 lb of equivalent weight, and the end of the beam would be centered between the top and bottom stop. A mechanical scale based on springs is like the typical bathroom scale. When weight is applied to the scale, a spring compresses, which causes a wheel that displays the weight to rotate. It would be difficult to use this type of scale to weigh anything other than a very small aircraft, because these scales typically measure only up to 300 lb. The accuracy of this type of scale is also an issue. Electronic scales that utilize load cells come in two varieties: the platform type and the type that mounts to the top of a jack. The platform type of electronic scale sits on the ground, with the tire of the airplane sitting on top of the platform. Built into the platform is an electronic load cell, which senses the weight being applied to it and generates a corresponding electrical signal. Inside the load cell there is an electronic grid that experiences a proportional change in electrical resistance as the weight being applied to it increases. An electrical cable runs from the platform scale to a display unit, which interprets the resistance change of the load cell and equates it to a specific number of pounds. A digital readout on the display typically shows the weight. In Figure 4-12, a Piper Archer is being weighed using platform scales that incorporate electronic load cells. In this case, the platform scales are secured to the hangar floor and stay permanently in place.

Figure 4-12. Weighing a Piper Archer using electronic platform scales.

In Figure 4-13, a Mooney M20 airplane is being weighed with portable electronic platform scales. Notice in the picture of the Mooney that its nose tire is deflated (close-up shown in the lower right corner of the photo). This was done to get the airplane in a level flight attitude. This type of scale is easy to transport and can be powered by household current or by a battery contained in the display unit. The display unit for these scales is shown in Figure 4-14. The display unit for the portable scales is very simple to operate. [Figure 4-14] In the lower left corner is the power switch, and in the lower right is the switch for selecting pounds or kilograms. The red, green, and yellow knobs are potentiometers for zeroing the three scales, and next to them are the on/off switches for the scales. Before the weight of the airplane is placed on the scales, each scale switch is turned on and the potentiometer knob turned until the digital display reads zero. In Figure 4-14, the nose scale is turned on and the readout of 546 lb is for the Mooney airplane in Figure 4-13. If all three scale switches are turned on at the same time, the total weight of the airplane will be displayed. The second type of electronic scale utilizes a load cell that attaches to the top of a jack. The top of the load cell has a concave shape that matches up with the jack pad on the aircraft, with the load cell absorbing all the weight of the aircraft at each jacking point. Each load cell has an electrical cable attached to it, which connects to the display unit that shows the weight being absorbed by each load cell. An important advantage of weighing an aircraft this way is that it allows the technician to level the aircraft. An aircraft needs to be in a flight level attitude when it is weighed. If an aircraft is sitting on floor scales, the only way to level the aircraft might be to deflate tires and landing gear

4-13

Figure 4-13. Mooney M20 being weighed with portable electronic platform scales.

struts. When an aircraft is weighed using load cells on jacks, leveling the aircraft is easy by simply adjusting the height with the jacks. Figure 4-15 shows a regional jet on jacks with the load cells in place. Spirit Level Before an aircraft can be weighed and reliable readings obtained, it must be in a level flight attitude. One method that can be used to check for a level condition is to use a spirit level, sometimes thought of as a carpenter’s level, by placing it on or against a specified place on the aircraft. Spirit levels consist of a vial full of liquid, except for a small air bubble. When the air bubble is centered between the two black lines, a level condition is indicated.

Figure 4-14. Display unit showing nosewheel weight for Mooney M20.

4-14

In Figure 4-16, a spirit level is being used on a Mooney M20 to check for a flight level attitude. By looking in the Type Certificate Data Sheet, it is determined that the leveling means is two screws on the left side of the airplane fuselage, in line with the trailing edge of the wing. Plumb Bob A plumb bob is a heavy metal object, cylinder or cone shape, with a sharp point at one end and a string attached to the other end. If the string is attached to a given point on an aircraft, and the plumb bob is allowed to hang down so the tip just touches the ground, the point where the tip touches will be perpendicular to where the string is attached. An example of the use of

Figure 4-15. Airplane on jacks with load cells in use.

Figure 4-16. Spirit level being used on a Mooney M20.

Figure 4-17. Plumb bob dropped from a wing leading edge.

a plumb bob would be measuring the distance from an aircraft’s datum to the center of the main landing gear axle. If the leading edge of the wing was the datum, a plumb bob could be dropped from the leading edge and a chalk mark made on the hangar floor. The plumb bob could also be dropped from the center of the axle on the main landing gear, and a chalk mark made on the floor. With a tape measure, the distance between the two chalk marks could be determined, and the arm for the main landing gear would be known. Plumb bobs can also be used to level an aircraft, described on page 4-27 of the Helicopter Weight and Balance section of this chapter. Figure 4-17 shows a plumb bob being dropped from the leading edge of an aircraft wing.

of pounds per gallon. When placed in a flask with fuel in it, the glass tube floats at a level dependent on the density of the fuel. Where the fuel intersects the markings on the side of the tube indicates the pounds per gallon.

Hydrometer When an aircraft is weighed with full fuel in the tanks, the weight of the fuel must be accounted for by mathematically subtracting it from the scale readings. To subtract it, its weight, arm, and moment must be known. Although the standard weight for aviation gasoline is 6.0 lb/gal and jet fuel is 6.7 lb/gal, these values are not exact for all conditions. On a hot day versus a cold day, these values can vary dramatically. On a hot summer day in the state of Florida, aviation gasoline checked with a hydrometer typically weighs between 5.85 and 5.9 lb/gal. If 100 gallons of fuel were involved in a calculation, using the actual weight versus the standard weight would make a difference of 10 to 15 lb. When an aircraft is weighed with fuel in the tanks, the weight of fuel per gallon should be checked with a hydrometer. A hydrometer consists of a weighted glass tube which is sealed, with a graduated set of markings on the side of the tube. The graduated markings and their corresponding number values represent units

Preparing an Aircraft for Weighing Weighing an aircraft is a very important and exacting phase of aircraft maintenance, and must be carried out with accuracy and good workmanship. Thoughtful preparation saves time and prevents mistakes. To begin, assemble all the necessary equipment, such as: 1. Scales, hoisting equipment, jacks, and leveling equipment. 2. Blocks, chocks, or sandbags for holding the airplane on the scales. 3. Straightedge, spirit level, plumb bobs, chalk line, and a measuring tape. 4. Applicable Aircraft Specifications and weight and balance computation forms. If possible, aircraft should be weighed in a closed building where there are no air currents to cause incorrect scale readings. An outside weighing is permissible if wind and moisture are negligible. Fuel System When weighing an aircraft to determine its empty weight, only the weight of residual (unusable) fuel should be included. To ensure that only residual fuel is accounted for, the aircraft should be weighed in one of the following three conditions. 1. Weigh the aircraft with absolutely no fuel in the aircraft tanks or fuel lines. If an aircraft is weighed 4-15

in this condition, the technician can mathematically add the proper amount of residual fuel to the aircraft, and account for its arm and moment. The proper amount of fuel can be determined by looking at the Aircraft Specifications or Type Certificate Data Sheet. 2. Weigh the aircraft with only residual fuel in the tanks and lines. 3. Weigh the aircraft with the fuel tanks completely full. If an aircraft is weighed in this condition, the technician can mathematically subtract the weight of usable fuel, and account for its arm and moment. A hydrometer can be used to determine the weight of each gallon of fuel, and the Aircraft Specifications or Type Certificate Data Sheet can be used to identify the fuel capacity. If an aircraft is to be weighed with load cells attached to jacks, the technician should check to make sure it is permissible to jack the aircraft with the fuel tanks full. It is possible that this may not be allowed because of stresses that would be placed on the aircraft. Never weigh an aircraft with the fuel tanks partially full, because it will be impossible to determine exactly how much fuel to account for. Oil System For aircraft certified since 1978, full engine oil is typically included in an aircraft’s empty weight. This can be confirmed by looking at the Type Certificate Data Sheet. If full oil is to be included, the oil level needs to be checked and the oil system serviced if it is less than full. If the Aircraft Specifications or Type Certificate Data Sheet specifies that only residual oil is part of empty weight, this can be accommodated by one of the following two methods. 1. Drain the engine oil system to the point that only residual oil remains. 2. Check the engine oil quantity, and mathematically subtract the weight of the oil that would leave only the residual amount. The standard weight for lubricating oil is 7.5 lb/gal (1.875 pounds per quart (lb/qt)), so if 7 qt of oil needed to be removed, the technician would subtract 13.125 lb at the appropriate arm. Miscellaneous Fluids Unless otherwise noted in the Aircraft Specifications or manufacturer’s instructions, hydraulic reservoirs and systems should be filled, drinking and washing water

4-16

reservoirs and lavatory tanks should be drained, and constant speed drive oil tanks should be filled. Flight Controls The position of such items as spoilers, slats, flaps, and helicopter rotor systems is an important factor when weighing an aircraft. Always refer to the manufacturer’s instructions for the proper position of these items. Other Considerations Inspect the aircraft to see that all items included in the certificated empty weight are installed in the proper location. Remove items that are not regularly carried in flight. Also look in the baggage compartments to make sure they are empty. Replace all inspection plates, oil and fuel tank caps, junction box covers, cowling, doors, emergency exits, and other parts that have been removed. All doors, windows, and sliding canopies should be in their normal flight position. Remove excessive dirt, oil, grease, and moisture from the aircraft. Some aircraft are not weighed with the wheels on the scales, but are weighed with the scales placed either at the jacking points or at special weighing points. Regardless of what provisions are made for placing the aircraft on the scales or jacks, be careful to prevent it from falling or rolling off, thereby damaging the aircraft and equipment. When weighing an aircraft with the wheels placed on the scales, release the brakes to reduce the possibility of incorrect readings caused by side loads on the scales. All aircraft have leveling points or lugs, and care must be taken to level the aircraft, especially along the longitudinal axis. With light, fixed-wing airplanes, the lateral level is not as critical as it is with heavier airplanes. However, a reasonable effort should be made to level the light airplanes along the lateral axis. Helicopters must be level longitudinally and laterally when they are weighed. Accuracy in leveling all aircraft longitudinally cannot be overemphasized. Weighing Points When an aircraft is being weighed, the arms must be known for the points where the weight of the aircraft is being transferred to the scales. If a tricycle gear small airplane has its three wheels sitting on floor scales, the weight transfer to each scale happens through the center of the axle for each wheel. If an airplane is weighed while it is on jacks, the weight transfer happens through the center of the jack pad. For a helicopter

with skids for landing gear, determining the arm for the weighing points can be difficult if the skids are sitting directly on floor scales. The problem is that the skid is in contact with the entire top portion of the scale, and it is impossible to know exactly where the center of weight transfer is occurring. In such a case, place a piece of pipe between the skid and the scale, and the center of the pipe will now be the known point of weight transfer. The arm for each of the weighing points is the distance from the center of the weight transfer point to the aircraft’s datum. If the arms are not known, based on previous weighing of the aircraft or some other source of data, they must be measured when the aircraft is weighed. This involves dropping a plumb bob from the center of each weighing point and from the aircraft datum, and putting a chalk mark on the hangar floor representing each point. The perpendicular distance between the datum and each of the weighing points can then be measured. In Figure 4-18, the distance from the nosewheel centerline to the datum is being measured on a Cessna 310 airplane. Notice the chalk lines on the hangar floor, which came as a result of dropping a plumb bob from the nosewheel axle centerline and from the datum. The nosewheel sitting on an electronic scale can be seen in the background. Center of Gravity Range The center of gravity range for an aircraft is the limits within which the aircraft must balance. It is identified as a forward most limit (arm) and an aft most limit (arm). In the Type Certificate Data Sheet for the Piper Seneca airplane, shown earlier in this chapter, the range is given as follows: CG Range: (Gear Extended)

S/N 34-E4, 34-7250001 through 34- 7250214 (See NOTE 3):



(+86.4") to (+94.6") at 4,000 lb



(+82.0") to (+94.6") at 3,400 lb



(+80.7") to (+94.6") at 2,780 lb



Straight line variation between points given.



Moment change due to gear retracting landing gear (–32 in-lb)

Because the Piper Seneca is a retractable gear airplane, the specifications identify that the range applies when the landing gear is extended, and that the airplane’s total moment will be decreased by 32 when the gear retracts. To know how much the center of gravity will

Figure 4-18. Measuring the nosewheel arm on a Cessna 310.

change when the gear is retracted, the moment of 32 in-lb would need to be divided by the loaded weight of the airplane. For example, if the airplane weighed 3,500 lb, the center of gravity would move forward 0.009" (32 ÷ 3500). Based on the numbers given, up to a loaded weight of 2,780 lb, the forward CG limit is +80.7" and the aft CG limit is +94.6". As the loaded weight of the airplane increases to 3,400 lb and eventually to the maximum of 4,000 lb, the forward CG limit moves aft. In other words, as the loaded weight of the airplane increases, the CG range gets smaller. The range gets smaller as a result of the forward limit moving back, while the aft limit stays in the same place. The data sheet identifies that there is a straight line variation between the points given. The points being referred to are the forward and aft center of gravity limits. From a weight of 2,780 lb to a weight of 3,400 lb, the forward limit moves from +80.7" to +82.0", and if plotted on a graph, that change would form a straight line. From 3,400 lb to 4,000 lb, the forward limit moves from +82 to +86.4", again forming a straight line. Plotted on a graph, the CG limits would look like what is shown in Figure 4-19. When graphically plotted, the CG limits form what is known as the CG envelope. In Figure 4-19, the red line represents the forward limit up to a weight of 2,780 lb. The blue and green lines represent the straight line variation that occurs for the forward limit as the weight increases up to a maximum of 4,000 lb. The yellow line represents the maximum weight for the airplane, and the purple line represents the aft limit.

4-17

4,200 4,100 4,000 3,900 3,800

86.4 at 4,000 lb

Weight (lb)

3,700 3,600 3,500 3,400

82 at 3,400 lb

3,200

Forward

Aft

3,000

CG limit

CG limit

2,900 2,800

80.7 at 2,780 lb

2,700 2,600 78 79

80

81

82

83

84

85

86

87

88

89

90

91

92

93

94

95

96

97

98

Center of Gravity (in) Figure 4-19. Center of gravity envelope for the Piper Seneca.

Empty Weight Center of Gravity Range For some aircraft, a center of gravity range is given for the aircraft in the empty weight condition. This practice is not very common with airplanes, but is often done for helicopters. This range would only be listed for an airplane if it was very small and had limited positions for people and fuel. If the empty weight CG of an aircraft falls within the empty weight CG limits, it is known that the loaded CG of the aircraft will be within limits if standard loading is used. This information will be listed in the Aircraft Specifications or Type Certificate Data Sheet, and if it does not apply, it will be identified as “none.” Operating Center of Gravity Range All aircraft will have center of gravity limits identified for the operational condition, with the aircraft loaded and ready for flight. If an aircraft can operate in more than one category, such as normal and utility, more than one set of limits might be listed. As shown earlier for the Piper Seneca airplane, the limits can change as the weight of the aircraft increases. In order to legally fly, the center of gravity for the aircraft must fall within the CG limits. Standard Weights Used for Aircraft Weight and Balance Unless the specific weight for an item is known, the standard weights used in aircraft weight and balance are as follows: 4-18

• Aviation gasoline

6 lb/gal

• Turbine fuel

6.7 lb/gal

• Lubricating oil

7.5 lb/gal

• Water

8.35 lb/gal

• Crew and passengers 170 lb per person Example Weighing of an Airplane In Figure 4-20, a tricycle gear airplane is being weighed by using three floor scales. The specifications on the airplane and the weighing specific data are as follows: • Aircraft Datum:

Leading edge of the wing

• Leveling Means:

Two screws, left side of fuselage below window

• Wheelbase:

100"

• Fuel Capacity:

30 gal aviation gasoline at +95"

• Unusable Fuel:

6 lb at +98"

• Oil Capacity:

8 qt at –38"

• Note 1:

Empty weight includes unusable fuel and full oil

• Left Main Scale Reading:

650 lb

• Right Main Scale Reading:

640 lb

• Nose Scale Reading: 225 lb

Datum

Main wheel centerline

Right scale reading 640 lb

70"

Left scale reading 650 lb

100" Wheel base

Nose scale reading 225 lb

Chocks

Nose wheel centerline

Figure 4-20. Example airplane being weighed.

• Tare Weight:

5 lb chocks on left main 5 lb chocks on right main 2.5 lb chock on nose

• During Weighing:

Fuel tanks full and oil full Hydrometer check on fuel shows 5.9 lb/gal

By analyzing the data identified for the airplane being weighed in Figure 4-20, the following needed information is determined. • Because the airplane was weighed with the fuel tanks full, the full weight of the fuel must be subtracted and the unusable fuel added back in. The weight of the fuel being subtracted is based on the pounds per gallon determined by the hydrometer check (5.9 lb/gal).

• Because wheel chocks are used to keep the airplane from rolling off the scales, their weight must be subtracted from the scale readings as tare weight. • Because the main wheel centerline is 70" behind the datum, its arm is a +70". • The arm for the nosewheel is the difference between the wheelbase (100") and the distance from the datum to the main wheel centerline (70"). Therefore, the arm for the nosewheel is −30". To calculate the airplane’s empty weight and empty weight center of gravity, a six column chart is used. Figure 4-21 shows the calculation for the airplane in Figure 4-20.

Item

Weight (lb)

Tare (lb)

Net Wt. (lb)

Nose

225

–2.5

222.5

–30

–6,675

Left Main

650

–5

645

+70

45,150

Right Main

640

–5

635

+70

44,450

1,515

–12.5

1,502.5

Subtotal Fuel Total

Arm (inches)

Moment (in-lb)

82,925

–177

+95

–16,815

Fuel Unuse

+6

+98

588

Oil

Full +50.1

66,698

Total

1,331.5

Figure 4-21. Center of gravity calculation for airplane being weighed.

4-19

Based on the calculation shown in the chart, the center of gravity is at +50.1", which means it is 50.1" aft of the datum. This places the center of gravity forward of the main landing gear, which must be the case for a tricycle gear airplane. This number is the result of dividing the total moment of 66,698 in-lb by the total weight of 1,331.5 lb.

Loading an Aircraft for Flight The ultimate test of whether or not there is a problem with an airplane’s weight and balance is when it is loaded and ready to fly. The only real importance of an airplane’s empty weight and empty weight center of gravity is how it affects the loaded weight and balance of the airplane, since an airplane doesn’t fly when it is empty. The pilot in command is responsible for the weight and balance of the loaded airplane, and he or she makes the final decision on whether or not the airplane is safe to fly. Example Loading of an Airplane As an example of an airplane being loaded for flight, the Piper Seneca twin will be used. The Type Certificate Data Sheet for this airplane was shown earlier in this chapter, and its center of gravity range and CG envelope were also shown. The information from the Type Certificate Data Sheet that pertains to this example loading is as follows: CG Range (Gear Extended) S/N 34-7250215 through 34-7450220:

(+87.9") to (+94.6") at 4,200 lb



(+82.0") to (+94.6") at 3,400 lb



(+80.7") to (+94.6") at 2,780 lb



Straight line variation between points given.



−32 in‑lb moment change due to gear retracting landing gear

Empty Weight CG Range

None

Maximum Weight

S/N 34-7250215 through 34-7450220:



4,200 lb — Takeoff



4,000 lb — Landing

No. of Seats

7 (2 at +85.5", 3 at +118.1", 2 at +155.7")

Maximum Baggage

200 lb (100 lb at +22.5, 100 lb at +178.7)

Fuel Capacity

98 gal (2 wing tanks) at (+93.6") (93 gal usable). See NOTE 1 for data on system fuel.

For the example loading of the airplane, the following information applies: • Airplane Serial Number:

34-7250816

• Airplane Empty Weight:

2,650 lb

• Airplane Empty Weight CG: +86.8" For today’s flight, the following useful load items will be included: • 1 pilot at 180 lb at an arm of +85.5" • 1 passenger at 160 lb at an arm of +118.1" • 1 passenger at 210 lb at an arm of +118.1" • 1 passenger at 190 lb at an arm of +118.1" • 1 passenger at 205 lb at an arm of +155.7" • 50 lb of baggage at +22.5" • 100 lb of baggage at +178.7" • 80 gal of fuel at +93.6" To calculate the loaded weight and CG of this airplane, a four column chart will be used, as shown in Figure 4-22. Based on the information in the Type Certificate Data Sheet, the maximum takeoff weight of this airplane is 4,200 lb, and the aft-most CG limit is +94.6". The loaded airplane in the chart above is 25 lb too heavy, and the CG is 1.82" too far aft. To make the airplane Weight (lb)

Arm (inches)

Moment (in-lb)

2,650

+86.80

230,020.0

Pilot

180

+85.50

15,390.0

Passenger

160

+118.10

18,896.0

Passenger

210

+118.10

24,801.0

Passenger

190

+118.10

22,439.0

Passenger

205

155.70

31,918.5

Baggage

50

+178.70

1,125.0

Baggage

100

+93.60

17,870.0

Fuel

480

11.87

44,928.0

4,255

+96.42

407,387.5

Item Empty Weight

Total

Figure 4-22. Center of gravity calculation for Piper Seneca.

4-20

Weight (lb)

Arm (inches)

Moment (in-lb)

2,650

+86.8

230,020.0

Pilot

180

+85.5

15,390.0

Passenger

210

+85.5

17,955.0

Passenger

160

+118.1

24,801.0

Passenger

190

+118.1

22,439.0

Passenger

205

+118.1

24,210.5

Baggage

100

+22.5

2,250.0

Baggage

25

+178.7

4,467.5

480

93.6

44,928.0

4,200

+92.0

386,461.0

Item Empty Weight

Fuel Total

Figure 4-23. Center of gravity calculation for Piper Seneca with weights shifted.

safe to fly, the load needs to be reduced by 25 lb and some of the load needs to be shifted forward. For example, the baggage can be reduced by 25 lb, and a full 100 lb of it can be placed in the more forward compartment. One passenger can be moved to the forward seat next to the pilot, and the aft-most passenger can then be moved forward. If these changes are made, the four column calculation will be as shown in Figure 4-23. With the changes made, the loaded weight is now at the maximum allowable of 4,200 lb, and the CG has moved forward 4.41". The airplane is now safe to fly.

Weight and Balance Extreme Conditions A weight and balance extreme condition check, sometimes called an adverse condition check, involves loading the aircraft in as nose heavy or tail heavy a condition as possible, and seeing if the center of gravity falls outside the allowable limits. This check is done with pencil and paper. In other words, the aircraft is not actually loaded in an adverse way and an attempt made to fly it. On what is called a forward extreme condition check, all useful load items in front of the forward CG limit are loaded, and all useful load items behind the forward CG limit are left empty. So if there are two seats and a baggage compartment located in front of the forward CG limit, two people weighing 170 lb each will be put in the seats, and the maximum allowable baggage will be put in the baggage compartment. Any seat or baggage compartment located behind the forward CG limit will be left empty. If the fuel is located behind

the forward CG limit, minimum fuel will be shown in the tank. Minimum fuel is calculated by dividing the engine’s METO horsepower by 2. On an aft extreme condition check, all useful load items behind the aft CG limit are loaded, and all useful load items in front of the aft CG limit are left empty. Even though the pilot’s seat will be located in front of the aft CG limit, the pilot’s seat cannot be left empty. If the fuel tank is located forward of the aft CG limit, minimum fuel will be shown. Example Forward and Aft Extreme Condition Checks Using the stick airplane in Figure 4-24 as an example, adverse forward and aft checks will be calculated. Some of the data for the airplane is shown in the figure, such as seat, baggage, and fuel information. The center of gravity limits are shown, with arrows pointing in the direction where maximum and minimum weights will be loaded. On the forward check, any useful load item located in front of 89" will be loaded, and anything behind that location will be left empty. On the aft check, maximum weight will be added behind 99" and minimum weight in front of that location. For either of the checks, if fuel is not located in a maximum weight location, minimum fuel must be accounted for. Notice that the front seats show a location of 82" to 88", meaning they are adjustable fore and aft. In a forward check, the pilot’s seat will be shown at 82", and in the aft check it will be at 88". Additional specifications for the airplane shown in Figure 4-24 are as follows: • Airplane Empty Weight: 1,850 lb • Empty Weight CG:

+92.45"

• CG Limits:

+89" to +99"

• Maximum Weight:

3,200 lb

• Fuel Capacity:

45 gal at +95" (44 usable)



40 gal at +102" (39 usable)

In evaluating the two extreme condition checks, the following key points should be recognized. • The total arm is the airplane center of gravity, and is found by dividing the total moment by the total weight. • For the forward check, the only thing loaded behind the forward limit was minimum fuel. • For the forward check, the pilot and passenger seats were shown at the forward position of 82". 4-21

2 at 82"– 88"

2 at 105"

375 HP

2 at 125"

100 lb at 140"

FUEL

FUEL

95"

102"

75 lb at 60" Maximum weight

Minimum weight

Forward limit 89"

Minimum weight

Aft limit 99"

Maximum weight

Figure 4-24. Example airplane for extreme condition checks.

• For the forward check, the CG was within limits, so the airplane could be flown this way. • For the aft check, the only thing loaded in front of the aft limit was the pilot, at an arm of 88". Extreme Condition Forward Check Item

Weight (lb)

Arm (inches)

Moment (in-lb)

Empty Weight

1,850.0

+92.45

171,032.5

Pilot

170.0

+82.00

13,940.0

Passenger

170.0

+82.00

13,940.0

Baggage Fuel Total

75.0

+60.00

4,500.0

187.5

+95.00

17,812.5

2,452.5

+90.20

221,225.0

Extreme Condition Aft Check Item Empty Weight

Weight (lb)

Arm (inches)

Moment (in-lb)

1,850

+92.45

171,032.5

Pilot

170

+88.00

14,960.0

2 Passengers

340

+105.00

35,700.0

2 Passengers

340

+125.00

42,500.0

Baggage

100

+140.00

14,000.0

Fuel

234

+102.00

23,868.0

3,034

+99.60

302,060.5

Total

Figure 4-25. Center of gravity extreme condition checks.

4-22

• For the aft check, the fuel tank at 102" was filled, which more than accounted for the required minimum fuel. • For the aft check, the CG was out of limits by 0.6", so the airplane should not be flown this way.

Equipment Change and Aircraft Alteration When the equipment in an aircraft is changed, such as the installation of a new radar system or ground proximity warning system, or the removal of a radio or seat, the weight and balance of an aircraft will change. An alteration performed on an aircraft, such as a cargo door being installed or a reinforcing plate being attached to the spar of a wing, will also change the weight and balance of an aircraft. Any time the equipment is changed or an alteration is performed, the new empty weight and empty weight center of gravity must be determined. This can be accomplished by placing the aircraft on scales and weighing it, or by mathematically calculating the new weight and balance. The mathematical calculation is acceptable if the exact weight and arm of all the changes are known. Example Calculation After an Equipment Change A small twin-engine airplane has some new equipment installed, and some of its existing equipment removed. The details of the equipment changes are as follows:

• The total arm is the airplane’s center of gravity, and is found by dividing the total moment by the total weight.

• Airplane Empty Weight:

2,350 lb

• Airplane Empty Weight CG:

+24.7"

• Airplane Datum:

Leading edge of the wing

• Radio Installed:

5.8 lb at an arm of –28"

• Global Positioning System Installed:

7.3 lb at an arm of –26"

• Emergency Locater Transmitter Installed: 2.8 lb at an arm of +105" • Strobe Light Removed:

1.4 lb at an arm of +75"

• Automatic Direction Finder (ADF) removed: 3 lb at an arm of –28" • Seat Removed:

34 lb at an arm of +60"

To calculate the new empty weight and empty weight center of gravity, a four column chart is used. The calculation would be as shown in Figure 4-26. In evaluating the weight and balance calculation shown in Figure 4-26, the following key points should be recognized. • The weight of the equipment needs to be identified with a plus or minus to signify whether it is being installed or removed. • The sign of the moment (plus or minus) is determined by the signs of the weight and arm. • The strobe and the ADF are both being removed (negative weight), but only the strobe has a negative moment. This is because the arm for the ADF is also negative, and two negatives multiplied together produce a positive result.

• The result of the equipment change is that the airplane’s weight was reduced by 22.5 lb and the center of gravity has moved forward 0.67".

The Use of Ballast Ballast is used in an aircraft to attain the desired CG balance, when the center of gravity is not within limits or is not at the location desired by the operator. It is usually located as far aft or as far forward as possible to bring the CG within limits, while using a minimum amount of weight. Ballast that is installed to compensate for the removal or installation of equipment items and that is to remain in the aircraft for long periods is called permanent ballast. It is generally lead bars or plates bolted to the aircraft structure. It may be painted red and placarded: PERMANENT BALLAST — DO NOT REMOVE. The installation of permanent ballast results in an increase in the aircraft empty weight, and it reduces the useful load. Temporary ballast, or removable ballast, is used to meet certain loading conditions that may vary from time to time. It generally takes the form of lead shot bags, sand bags, or other weight items that are not permanently installed. Temporary ballast should be placarded: BALLAST, XX LB. REMOVAL REQUIRES WEIGHT AND BALANCE CHECK. The baggage compartment is usually the most convenient location for temporary ballast. Whenever permanent or temporary ballast is installed, it must be placed in an approved location and secured in an appropriate manner. If permanent ballast is being bolted to the structure of the aircraft, the location must be one that was previously approved and designed for the installation, or it must be approved by the FAA as a major alteration before the aircraft is returned to service. When temporary ballast is placed in a baggage compartment, it must be secured in a way that prevents it from becoming a projectile if the aircraft encounters turbulence or an unusual flight attitude.

Item

Weight (lb)

Arm (inches)

Moment (in-lb)

Empty Weight

2,350.0

+24.70

58,045.0

Radio Install

+5.8

−28.00

−162.4

GPS Install

+7.3

−26.00

−189.8

ELT Install

+2.8

+105.00

294.0

Strobe Remove

−1.4

+75.00

−105.0

ADF Remove

−3.0

−28.00

84.0

Seat Remove

−34.0

+60.00

−2,040.0

To calculate how much ballast is needed to bring the center of gravity within limits, the following formula is used.

2,327.5

24.03

55,925.8

Ballast Needed =

Total

Figure 4-26. Center of gravity calculation after equipment change.

Loaded weight of aircraft (distance CG is out of limits) Arm from ballast location to affected limit

4-23

Item Empty Weight

Weight (lb)

Arm (inches)

Moment (in-lb)

1,850

+92.45

171,032.5

Pilot

170

+88.00

14,960.0

2 Passengers

340

+105.00

35,700.0

2 Passengers

340

+125.00

42,500.0

Baggage

100

+140.00

14,000.0

Fuel

234

+102.00

23,868.0

3,034

+99.60

302,060.5

Total

Figure 4-27. Extreme condition check.

Figure 4-24 on page 4-22 and Figure 4-27 show an aft extreme condition check being performed on an airplane. In this previously shown example, the airplane’s center of gravity was out of limits by 0.6". If there were a need or a desire to fly the airplane loaded this way, one way to make it possible would be the installation of temporary ballast in the front of the airplane. The logical choice for placement of this ballast is the forward baggage compartment. The center of gravity for this airplane is 0.6" too far aft. If the forward baggage compartment is used as a temporary ballast location, the ballast calculation will be as follows: Ballast Needed = Loaded weight of aircraft (distance CG is out of limits) Arm from ballast location to affected limit =

3,034 lb (0.6") 39" = 46.68 lb

When ballast is calculated, the answer should always be rounded up to the next higher whole pound, or in this case, 47 lb of ballast would be used. To ensure the ballast calculation is correct, the weight of the ballast should be plugged back into the four column calculation and a new center of gravity calculated.

The aft limit for the airplane was 99", and the new CG is at 98.96", which puts it within acceptable limits. The new CG did not fall exactly at 99" because the amount of needed ballast was rounded up to the next higher whole pound. If the ballast could have been placed farther forward, such as being bolted to the engine firewall, less ballast would have been needed. That is why ballast is always placed as far away from the affected limit as possible. In evaluating the ballast calculation shown above, the following key points should be recognized. • The loaded weight of the aircraft, as identified in the formula, is what the airplane weighed when the CG was out of limits. • The distance the CG is out of limits is the difference between the CG location and the CG limit, in this case 99.6" minus 99". • The affected limit identified in the formula is the CG limit which has been exceeded. If the CG is too far aft, it is the aft limit that has been exceeded. • The aft limit for this example airplane is 99", and the ballast is being placed in the baggage compartment at an arm of 60". The difference between the two is 39", the quantity divided by in the formula. Viewed as a first class lever problem, Figure 4-29 shows what this ballast calculation would look like. A ballast weight of 46.68 lb on the left side of the lever multiplied by the arm of 39" (99 minus 60) would equal the aircraft weight of 3,034 lb multiplied by the distance the CG is out of limits, which is 0.6" (99.6 minus 99).

Ballast weight of 46.68 lb at an arm of 60"

Distance from Aft limit to Ballast = 39"

Item Loaded Weight Ballast Total

Weight (lb)

Arm (inches)

Moment (in-lb)

3,034

+99.60

302,060.5

47

+60.00

2,820.0

3,081

+98.96

304,880.5

Aircraft weight of 3,034 lb at a CG of of 99.6"

0.6" Distance out of limits

In order to balance at the aft limit of 99", the moment to the left of the fulcrum must equal the moment to the right of the fulcrum. The moment to the right is the weight of the airplane multiplied by 0.6". The moment to the left is the ballast weight multiplied by 39".

Figure 4-29. Ballast calculation as a first class lever. Figure 4-28. Ballast calculation.

4-24

and model aircraft at the time of original certification. The graphs become a permanent part of the aircraft records, and are typically found in the Airplane Flight Manual or Pilot’s Operating Handbook (AFM/POH). These graphs, used in conjunction with the empty weight and empty weight CG data found in the weight and balance report, allow the pilot to plot the CG for the loaded aircraft.

Loading Graphs and CG Envelopes The weight and balance computation system, commonly called the loading graph and CG envelope system, is an excellent and rapid method for determining the CG location for various loading arrangements. This method can be applied to any make and model of aircraft, but is more often seen with small general aviation aircraft.

The loading graph illustrated in Figure 4-30 is used to determine the index number (moment value) of any item or weight that may be involved in loading the aircraft. To use this graph, find the point on the verti-

Aircraft manufacturers using this method of weight and balance computation prepare graphs similar to those shown in Figures 4-30 and 4-31 for each make

360 320

ge

r

er

ng

Fro nt Pa ss

en

240 200

Fu

120

Re

el

Pil o

t&

160

ar

sse Pa

e

gag

Bag

80 40 0

Oil

−2

0

2

4

6

8

10

12

14

16

18

20

22

24

26

28

Moment Index (Moment /1,000) Figure 4-30. Aircraft loading graph.

2,400 2,300

Normal Category

2,200 2,100 2,000

ate go r

y

1,900

yC

1,800 1,700

Ut ilit

Load Aircraft Weight (lb)

Load Weight (lb)

280

1,600 1,500

50

55

60

65

70

75

80

85

90

95

100 105 110 115

Loaded Aircraft Moment/1,000 (in-lb) Figure 4-31. CG envelope.

4-25

cal scale that represents the known weight. Project a horizontal line to the point where it intersects the proper diagonal weight line (i.e., pilot, copilot, baggage). Where the horizontal line intersects the diagonal, project a vertical line downward to determine the loaded moment (index number) for the weight being added.

for the aircraft in the Normal Category and one for the aircraft in the Utility Category. The loading graph and CG envelope shown in Figures 4-30 and 4-31 are for an airplane with the following specifications and weight and balance data. • Number of Seats:

After the moment for each item of weight has been determined, all weights are added and all moments are added. The total weight and moment is then plotted on the CG envelope. [Figure 4-31] The total weight is plotted on the vertical scale of the graph, with a horizontal line projected out from that point. The total moment is plotted on the horizontal scale of the graph, with a vertical line projected up from that point. Where the horizontal and vertical plot lines intersect on the graph is the center of gravity for the loaded aircraft. If the point where the plot lines intersect falls inside the CG envelope, the aircraft CG is within limits. In Figure 4-31, there are actually two CG envelopes, one Item

Weight (lb)

Moment (in-lb)

1,400

53,900

Pilot

180

6,000

Front Passengers

140

4,500

Rear Passengers

210

15,000

Baggage

100

9,200

Fuel

228

10,800

Total

2,258

99,400

Aircraft Empty Wt.

4

• Fuel Capacity (Usable): 38 gal of Av Gas • Oil Capacity:

8 qt (included in empty weight)

• Baggage:

120 lb

• Empty Weight:

1,400 lb

• Empty Weight CG:

38.5"

• Empty Weight Moment: 53,900 in-lb An example of loading the airplane for flight and calculating the total loaded weight and the total loaded moment is shown in Figures 4-32 and 4-33. The use of the loading graph to determine the moment for each of the useful load items is shown in Figure 4-33. The color used for each useful load item in Figure 4-32 matches the color used for the plot on the loading graph. The total loaded weight of the airplane is 2,258 pounds and the total loaded moment is 99,400 in-lb. These two numbers can now be plotted on the CG envelope to see if the airplane is within CG limits. Figure 4-34 shows the CG envelope, with the loaded weight and moment of the airplane plotted. The CG location shown falls

&F ron tP ass en g

er

Figure 4-32. Aircraft load chart.

360 320

240 200

ge

en

Pil ot

Load Weight (lb)

280

Re

a

ass rP

r

l

e Fu

160

e

gag

Bag

120 80 40 0

Oil

−2

0

2

4

6

8

10

12

14

16

18

20

Moment Index (Moment/1,000) Figure 4-33. Example plots on a loading graph.

4-26

22

24

26

28

2,400

Normal Category

2,200

CG Location

2,100

Forward Limit

ry

2,000

yC ate go

1,900 1,800

Ut ilit

Load Aircraft Weight (lb)

2,300

1,700

Aft Limit

1,600 1,500 50

55

60

65

70

75

80

85

90

95

100 105 110 115

Loaded Aircraft Moment/1,000 (in-lb) Figure 4-34. CG envelope example plot.

within the normal category envelope, so the airplane is within CG limits for this category. It is interesting to note that the lines that form the CG envelope are actually graphic plots of the forward and aft CG limits. In Figure 4-34, the red line is a graphic plot of the forward limit, and the blue and green lines are graphic plots of the aft limit for the two different categories.

Helicopter Weight and Balance General Concepts All of the terminology and concepts that apply to airplane weight and balance also apply generally to helicopter weight and balance. There are some specific differences, however, which need to be identified. Most helicopters have a much more restricted CG range than do airplanes. In some cases, this range is less than 3". The exact location and length of the CG range is specified for each helicopter, and usually extends a short distance fore and aft of the main rotor mast or centered between the main rotors of a dual rotor system. Whereas airplanes have a center of gravity range only along the longitudinal axis, helicopters have both longitudinal and lateral center of gravity ranges. Because the wings extend outward from the center of gravity, airplanes tend to have a great deal of lateral stability. A helicopter, on the other hand, acts like a pendulum, with the weight of the helicopter hanging from the main rotor shaft. Ideally, the helicopter should have such perfect balance that the fuselage remains horizontal while in a hover.

If the helicopter is too nose heavy or tail heavy while it is hovering, the cyclic pitch control will be used to keep the fuselage horizontal. If the CG location is too extreme, it may not be possible to keep the fuselage horizontal or maintain control of the helicopter. Helicopter Weighing When a helicopter is being weighed, the location of both longitudinal and lateral weighing points must be known to determine its empty weight and empty weight CG. This is because helicopters have longitudinal and lateral CG limits. As with the airplane, the longitudinal arms are measured from the datum, with locations behind the datum being positive arms and locations in front of the datum being negative arms. Laterally, the arms are measured from the butt line, which is a line from the nose to the tail running through the middle of the helicopter. When facing forward, arms to the right of the butt line are positive; to the left they are negative. Before a helicopter is weighed, it must be leveled longitudinally and laterally. This can be done with a spirit level, but more often than not it is done with a plumb bob. For example, the Bell JetRanger has a location inside the aft cabin where a plumb can be attached, and allowed to hang down to the cabin floor. On the cabin floor is a plate bearing cross hairs, with the cross hairs corresponding to the horizontal and lateral axis of the helicopter. When the point of the plumb bob falls in the middle of the cross hairs, the helicopter is level along both axes. If the tip of the plumb bob falls forward of this point, the nose of the helicopter is too low; if it falls to the left of this point, the left side of the helicopter is 4-27

Forward Ballast Station +13"

Leveling plate with cross hairs

Plumb Bob

Datum

Forward Jack Point +55.16" +25" and −25" Laterally

Leveling Plate +117.7"

Aft Jack Point +204.92"

e Sid ht g i R se No

l Tai t Lef

Sid

e

Aft Ballast Station +377"

Figure 4-35. Bell JetRanger.

too low. In other words, the tip of the plumb bob will always move toward the low point. A Bell JetRanger helicopter is shown in Figure 4-35, with the leveling plate depicted on the bottom right of the figure. The helicopter has three jack pads, two at the front and one in the back. To weigh this helicopter, three jacks would be placed on floor scales, and the helicopter would be raised off the hangar floor. To level the helicopter, the jacks would be adjusted until the plumb bob point falls exactly in the middle of the cross hairs. As an example of weighing a helicopter, consider the Bell JetRanger in Figure 4-35, and the following specifications and weighing data. • Datum: 55.16" forward of the front jack point centerline • Leveling Means:

Plumb line from ceiling left rear cabin to index plate on floor

• Longitudinal CG Limits:

+106" to +111.4" at 3,200 lb +106" to +112.1" at 3,000 lb +106" to +112.4" at 2,900 lb +106" to +113.4" at 2,600 lb +106" to +114.2" at 2,350 lb +106" to +114.2" at 2,100 lb



Straight line variation between points given

4-28

• Lateral CG Limits:

2.3" left to 3.0" right at longitudinal CG +106.0"



3.0" left to 4.0" right at longitudinal CG +108" to +114.2"



Straight line variation between points given

• Fuel and Oil:

Empty weight includes unusable fuel and unusable oil

• Left Front Scale Reading: 650 lb • Left Front Jack Point:

Longitudinal arm of +55.16" Lateral arm of –25"

• Right Front Scale Reading: 625 lb • Right Front Jack Point:

Longitudinal arm of +55.16" Lateral arm of +25"

• Aft Scale Reading:

710 lb

• Aft Jack Point:

Longitudinal arm of +204.92" Lateral arm of 0.0"

• Notes:

The helicopter was weighed with unusable fuel and oil. Electronic scales were used, which were zeroed with the jacks in place, so no tare weight needs to be accounted for.

Longitudinal CG Calculation Item

Scale (lb)

Tare Wt. (lb)

Left Front

650

0

650

+55.16

35,854.0

Right Front

625

0

625

+55.16

34,475.0

Aft

710

0

710

+204.92

145,493.2

1,985

+108.73

215,822.2

Arm (inches)

Moment (in-lb)

Total

Nt. Wt. (lb)

1,985

Arm (inches)

Moment (in-lb)

Lateral CG Calculation Item

Scale (lb)

Tare Wt. (lb)

Nt. Wt. (lb)

Left Front

650

0

650

−25

−16,250

Right Front

625

0

625

+25

+15,625

Aft

710

0

710

0

0

1,985

+.31

−625

Total

1,985

Figure 4-36. Center of gravity calculation for Bell JetRanger.

Using six column charts for the calculations, the empty weight and the longitudinal and lateral center of gravity for the helicopter would be as shown in Figure 4-36. Based on the calculations in Figure 4-36, it has been determined that the empty weight of the helicopter is 1,985 lb, the longitudinal CG is at +108.73", and the lateral CG is at –0.31".

Weight and Balance — Weight-Shift Control Aircraft and Powered Parachutes The terminology, theory, and concepts of weight and balance that applies to airplanes also applies to weightshift aircraft and powered parachutes. Weight is still weight, and the balance point is still the balance point. There are, however, a few differences that need to be discussed. Before reading about the specifics of weight and balance on weight-shift control and powered parachute aircraft, be sure to read about their aerodynamic characteristics in Chapter 3, Physics. Weight-shift control aircraft and powered parachutes do not fall under the same Code of Federal Regulations that govern certified airplanes and helicopters and, therefore, do not have Type Certificate Data Sheets or the same type of FAA mandated weight and balance reports. Weight and balance information and guidelines are left to the individual owners and the companies with which they work in acquiring this type of aircraft. Overall, the industry that is supplying these aircraft is regulating itself well, and the safety record is good for those aircraft being operated by experienced pilots.

The FAA has recently (2005) accepted a new classification of aircraft, known as Light Sport Aircraft (LSA). A new set of standards is being developed which will have an impact on the weight-shift control and powered parachute aircraft and how their weight and balance is handled. Weight-Shift Control Aircraft Weight-shift control aircraft, commonly known by the name “trikes,” have very few options for loading because they have very few places to put useful load items. Some trikes have only one seat and a fuel tank, so the only variables for a flight are amount of fuel and weight of the pilot. Some trikes have two seats and a small storage bin, in addition to the fuel tank. The most significant factor affecting the weight and balance of a trike is the weight of the pilot; if the aircraft has two seats, the weight of the passenger must be considered. The trike acts somewhat like a single main rotor helicopter because the weight of the aircraft is hanging like a pendulum under the wing. Figure 4-37 shows a two-place trike, in which the mast and the nose strut come together slightly below the wing attach point. When the trike is in flight, the weight of the aircraft is hanging from the wing attach point. The weight of the engine and fuel is behind this point, the passenger is almost directly below this point, and the pilot is forward of this point. The balance of the aircraft is determined by how all these weights compare. The wing attach point, with respect to the wing keel, is an adjustable location. The attach point can be loosened

4-29

Figure 4-38. Wing attach point for a weight-shift aircraft.

Figure 4-37. Weight and balance for a weight-shift aircraft.

and moved slightly forward or slightly aft, depending on the weight of the occupants. For example, if the aircraft is flown by a heavy person, the attach point can be moved a little farther aft, bringing the wing forward, to compensate for the change in center of gravity. Figure 4-38 shows a close-up of the wing attach point, and the small amount of forward and aft movement that is available. Powered Parachutes Powered parachutes have many of the same characteristics as weight-shift control aircraft when it comes to weight and balance. They have the same limited loading, with only one or two seats and a fuel tank. They also act like a pendulum, with the weight of the aircraft hanging beneath the inflated wing (parachute).

Figure 4-39. Powered parachute structure with wing attach points.

a large airplane. The jacks and scales will be larger, and it may take more personnel to handle the equipment, but the concepts and processes are the same.

Weight and Balance for Large Airplanes

Built-In Electronic Weighing One difference that may be found with large airplanes is the incorporation of electronic load cells in the aircraft’s landing gear. With this type of system, the airplane is capable of weighing itself as it sits on the tarmac. The load cells are built into the axles of the landing gear, or the landing gear strut, and they work in the same manner as load cells used with jacks. This system is currently in use on the Boeing 747-400, Boeing 777, Boeing 787, McDonnell Douglas MD-11, and the wide body Airbus airplanes like the A-330, A-340, and A-380.

Weight and balance for large airplanes is almost identical to what it is for small airplanes, on a much larger scale. If a technician can weigh a small airplane and calculate its empty weight and empty weight center of gravity, that same technician should be able to do it for

The Boeing 777, utilizes two independent systems that provide information to the airplane’s flight management computer. If the two systems agree on the weight and center of gravity of the airplane, the data being provided are considered accurate and the airplane can

The point at which the inflated wing attaches to the structure of the aircraft is adjustable to compensate for pilots and passengers of varying weights. With a very heavy pilot, the wing attach point would be moved forward to prevent the aircraft from being too nose heavy. Figure 4-39 illustrates the structure of a powered parachute with the adjustable wing attach points.

4-30

be dispatched based on that information. The flight crew has access to the information on the flight deck by accessing the flight management computer and bringing up the weight and balance page.

decreases toward the tip. In relation to the aerodynamics of the wing, the average length of the chord on these tapered swept-back wings is known as the mean aerodynamic chord (MAC).

Mean Aerodynamic Chord On small airplanes and on all helicopters, the center of gravity location is identified as being a specific number of inches from the datum. The center of gravity range is identified the same way. On larger airplanes, from private business jets to large jumbo jets, the center of gravity and its range are typically identified in relation to the width of the wing.

On these larger airplanes, the CG is identified as being at a location that is a specific percent of the mean aerodynamic chord (% MAC). For example, imagine that the MAC on a particular airplane is 100", and the CG falls 20" behind the leading edge of the MAC. That means it falls one-fifth of the way back, or at 20% of the MAC.

The width of the wing on an airplane is known as the chord. If the leading edge and trailing edge of a wing are parallel to each other, the chord of the wing is the same along the wing’s length. Business jets and commercial transport airplanes have wings that are tapered and that are swept back, so the width of their wings is different along their entire length. The width is greatest where the wing meets the fuselage and progressively

Figure 4-40 shows a large twin-engine commercial transport airplane. The datum is forward of the nose of the airplane, and all the arms shown in the figure are being measured from that point. The center of gravity for the airplane is shown as an arm measured in inches. In the lower left corner of the figure, a cross section of the wing is shown, with the same center of gravity information being presented.

Center of gravity 945" Datum MAC 180" Leading edge of MAC 900"

LEMAC 900" 45"

Trailing edge of MAC 1,080"

CG at 945" Chord line MAC = 180"

Figure 4-40. Center of gravity location on a large commercial transport.

4-31

To convert the center of gravity location from inches to a percent of MAC, for the airplane shown in Figure 4-40, the steps are as follows: 1. Identify the center of gravity location, in inches from the datum. 2. Identify the leading edge of the MAC (LEMAC), in inches from the datum. 3. Subtract LEMAC from the CG location. 4. Divide the difference by the length of the MAC. 5. Convert the result in decimals to a percentage by multiplying by 100. As a formula, the solution to solve for the percent of MAC would be: CG − LEMAC Percent of MAC = × 100 MAC The result using the numbers shown in Figure 4-40 would be: Percent of MAC =

CG − LEMAC × 100 MAC



=

945 − 900 × 100 180



= 25%

If the center of gravity is known in percent of MAC, and there is a need to know the CG location in inches from the datum, the conversion would be done as follows: 1. Convert the percent of MAC to a decimal by dividing by 100. 2. Multiply the decimal by the length of the MAC. 3. Add this number to LEMAC. As a formula, the solution to convert a percent of MAC to an inch value would be: CG in inches = MAC % ÷ 100 × MAC + LEMAC

4-32

For the airplane in Figure 4-40, if the CG was at 32.5% of the MAC, the solution would be: CG in inches = MAC % ÷ 100 × MAC + LEMAC

= 32.5 ÷ 100 × 180 + 900 = 958.5

Weight and Balance Records When a technician gets involved with the weight and balance of an aircraft, it almost always involves a calculation of the aircraft’s empty weight and empty weight center of gravity. Only on rare occasions will the technician be involved in calculating extreme conditions, how much ballast is needed, or the loaded weight and balance of the aircraft. Calculating the empty weight and empty weight CG might involve putting the aircraft on scales and weighing it, or a pencil and paper exercise after installing a new piece of equipment. The FAA requires that a current and accurate empty weight and empty weight center of gravity be known for an aircraft. This information must be included in the weight and balance report, which is a part of the aircraft permanent records. The weight and balance report must be in the aircraft when it is being flown. There is no required format for this report, but Figure 4-41 is a good example of recording the data obtained from weighing an aircraft. As it is currently laid out, the form would accommodate either a tricycle gear or tail dragger airplane. Depending on the gear type, either the nose or the tail row would be used. If an airplane is being weighed using jacks and load cells, or if a helicopter is being weighed, the item names must be changed to reflect the weight locations. If an equipment change is being done on an aircraft, and the new weight and balance is calculated mathematically instead of weighing the aircraft, the same type of form shown in Figure 4-41 can be used. The only change would be the use of a four column solution, instead of six columns, and there would be no tare weight or involvement with fuel and oil.

Aircraft Weight and Balance Report Results of Aircraft Weighing Make



Serial #

Model N#

Datum Location Leveling Means Scale Arms:

Nose

Tail

Left Main

Right Main

Scale Weights: Nose

Tail

Left Main

Right Main

Tare Weights: Nose

Tail

Left Main

Right Main

Weight and Balance Calculation Item

Scale (lb)

Tare Wt. (lb)

Net Wt. (lb)

Arm (inches)

Moment (in-lb)

Nose Tail Left Main Right Main Subtotal Fuel Oil Misc. Total

Aircraft Current Empty Weight: Aircraft Current Empty Weight CG: Aircraft Maximum Weight: Aircraft Useful Load: Computed By:

(print name)



(signature)

Certificate #:

(A&P, Repair Station, etc.)

Date:

Figure 4-41. Aircraft weight and balance report.

4-33

4-34

Aircraft Metals Knowledge and understanding of the uses, strengths, limitations, and other characteristics of structural metals is vital to properly construct and maintain any equipment, especially airframes. In aircraft maintenance and repair, even a slight deviation from design specification, or the substitution of inferior materials, may result in the loss of both lives and equipment. The use of unsuitable materials can readily erase the finest craftsmanship. The selection of the correct material for a specific repair job demands familiarity with the most common physical properties of various metals. Properties of Metals Of primary concern in aircraft maintenance are such general properties of metals and their alloys as hardness, malleability, ductility, elasticity, toughness, density, brittleness, fusibility, conductivity contraction and expansion, and so forth. These terms are explained to establish a basis for further discussion of structural metals. Hardness Hardness refers to the ability of a material to resist abrasion, penetration, cutting action, or permanent distortion. Hardness may be increased by cold working the metal and, in the case of steel and certain aluminum alloys, by heat treatment. Structural parts are often formed from metals in their soft state and are then heat treated to harden them so that the finished shape will be retained. Hardness and strength are closely associated properties of metals. Strength One of the most important properties of a material is strength. Strength is the ability of a material to resist deformation. Strength is also the ability of a material to resist stress without breaking. The type of load or stress on the material affects the strength it exhibits.

Density Density is the weight of a unit volume of a material. In aircraft work, the specified weight of a material per cubic inch is preferred since this figure can be used in determining the weight of a part before actual manufacture. Density is an important consideration when choosing a material to be used in the design of a part in order to maintain the proper weight and balance of the aircraft. Malleability A metal which can be hammered, rolled, or pressed into various shapes without cracking, breaking, or leaving some other detrimental effect, is said to be malleable. This property is necessary in sheet metal that is worked into curved shapes, such as cowlings, fairings, or wingtips. Copper is an example of a malleable metal. Ductility Ductility is the property of a metal which permits it to be permanently drawn, bent, or twisted into various shapes without breaking. This property is essential for metals used in making wire and tubing. Ductile metals are greatly preferred for aircraft use because of their ease of forming and resistance to failure under shock loads. For this reason, aluminum alloys are used for cowl rings, fuselage and wing skin, and formed or extruded parts, such as ribs, spars, and bulkheads. Chrome molybdenum steel is also easily formed into desired shapes. Ductility is similar to malleability. Elasticity Elasticity is that property that enables a metal to return to its original size and shape when the force which causes the change of shape is removed. This property is extremely valuable because it would be highly undesirable to have a part permanently distorted after an applied load was removed. Each metal has a point known as the elastic limit, beyond which it cannot be loaded without causing permanent distortion. In aircraft construction, members and parts are so designed that 5-1

the maximum loads to which they are subjected will not stress them beyond their elastic limits. This desirable property is present in spring steel.

Among the common materials used are ferrous metals. The term “ferrous” applies to the group of metals having iron as their principal constituent.

Toughness A material which possesses toughness will withstand tearing or shearing and may be stretched or otherwise deformed without breaking. Toughness is a desirable property in aircraft metals.

Iron If carbon is added to iron, in percentages ranging up to approximately 1 percent, the product is vastly superior to iron alone and is classified as carbon steel. Carbon steel forms the base of those alloy steels produced by combining carbon steel with other elements known to improve the properties of steel. A base metal (such as iron) to which small quantities of other metals have been added is called an alloy. The addition of other metals changes or improves the chemical or physical properties of the base metal for a particular use.

Brittleness Brittleness is the property of a metal which allows little bending or deformation without shattering. A brittle metal is apt to break or crack without change of shape. Because structural metals are often subjected to shock loads, brittleness is not a very desirable property. Cast iron, cast aluminum, and very hard steel are examples of brittle metals. Fusibility Fusibility is the ability of a metal to become liquid by the application of heat. Metals are fused in welding. Steels fuse around 2,600 °F and aluminum alloys at approximately 1,100 °F. Conductivity Conductivity is the property which enables a metal to carry heat or electricity. The heat conductivity of a metal is especially important in welding because it governs the amount of heat that will be required for proper fusion. Conductivity of the metal, to a certain extent, determines the type of jig to be used to control expansion and contraction. In aircraft, electrical conductivity must also be considered in conjunction with bonding, to eliminate radio interference. Thermal Expansion Thermal expansion refers to contraction and expansion that are reactions produced in metals as the result of heating or cooling. Heat applied to a metal will cause it to expand or become larger. Cooling and heating affect the design of welding jigs, castings, and tolerances necessary for hot rolled material. Ferrous Aircraft Metals Many different metals are required in the repair of aircraft. This is a result of the varying needs with respect to strength, weight, durability, and resistance to deterioration of specific structures or parts. In addition, the particular shape or form of the material plays an important role. In selecting materials for aircraft repair, these factors plus many others are considered in relation to the mechanical and physical properties. 5-2

Steel and Steel Alloys To facilitate the discussion of steels, some familiarity with their nomenclature is desirable. A numerical index, sponsored by the Society of Automotive Engineers (SAE) and the American Iron and Steel Institute (AISI), is used to identify the chemical compositions of the structural steels. In this system, a four-numeral series is used to designate the plain carbon and alloy steels; five numerals are used to designate certain types of alloy steels. The first two digits indicate the type of steel, the second digit also generally (but not always) gives the approximate amount of the major alloying element, and the last two (or three) digits are intended to indicate the approximate middle of the carbon range. However, a deviation from the rule of indicating the carbon range is sometimes necessary. Small quantities of certain elements are present in alloy steels that are not specified as required. These elements are considered as incidental and may be present to the maximum amounts as follows: copper, 0.35 percent; nickel, 0.25 percent; chromium, 0.20 percent; molybdenum, 0.06 percent. The list of standard steels is altered from time to time to accommodate steels of proven merit and to provide for changes in the metallurgical and engineering requirements of industry. [Figure 5-1] Metal stock is manufactured in several forms and shapes, including sheets, bars, rods, tubing, extrusions, forgings, and castings. Sheet metal is made in a number of sizes and thicknesses. Specifications designate thicknesses in thousandths of an inch. Bars and rods are supplied in a variety of shapes, such as round, square, rectangular, hexagonal, and octagonal. Tubing can be obtained in round, oval, rectangular, or streamlined

Series Designation

Types

100xx

Nonsulphurized carbon steels

11xx

Resulphurised carbon steels (free machining)

12xx

Rephosphorized and resulphurised carbon steels (free machining)

13xx

Manganese 1.75%

*23xx

Nickel 3.50%

*25xx

Nickel 5.00%

31xx

Nickel 1.25%, chromium 0.65%

33xx

Nickel 3.50%, chromium 1.55%

40xx

Molybdenum 0.20 or 0.25%

41xx

Chromium 0.50% or 0.95%, molybdenum 0.12 or 0.20%

43xx

Nickel 1.80%, chromium 0.5 or 0.80%, molybdenum 0.25%

44xx

Molybdenum 0.40%

45xx

Molybdenum 0.52%

46xx

Nickel 1.80%, molybdenum 0.25%

47xx

Nickel 1.05% chromium 0.45%, molybdenum 0.20 or 0.35%

48xx

Nickel 3.50%, molybdenum 0.25%

50xx

Chromium 0.25, or 0.40 or 0.50%

50xxx

Carbon 1.00%, chromium 0.50%

51xx

Chromium 0.80, 0.90, 0.95 or 1.00%

51xxx

Carbon 1.00%, chromium 1.05%

52xxx

Carbon 1.00%, chromium 1.45%

61xx

Chromium 0.60, 0.80, 0.95%, vanadium 0.12%, 0.10% min., or 0.15% min.

81xx

Nickel 0.30%, chromium 0.40%, molybdenum 0.12%

86xx

Nickel 0.55%, chromium 0.50%, molybdenum 0.20%

87xx

Nickel 0.55%, chromium 0.05%, molybdenum 0.25%

88xx

Nickel 0.55%, chromium 0.05%, molybdenum 0.35%

92xx

Manganese 0.85%, silicon 2.00%, chromium 0 or 0.35%

93xx

Nickel 3.25%, chromium 1.20%, molybdenum 0.12%

94xx

Nickel 0.45%, chromium 0.40%, molybdenum 0.12%

98xx

Nickel 1.00%, chromium 0.80%, molybdenum 0.25%

*Not included in the current list of standard steels

Figure 5-1. SAE numerical index.

shapes. The size of tubing is generally specified by outside diameter and wall thickness. The sheet metal is usually formed cold in such machines as presses, bending brakes, drawbenches, or rolls. Forgings are shaped or formed by pressing or

hammering heated metal in dies. Castings are produced by pouring molten metal into molds. The casting is finished by machining. Spark testing is a common means of identifying various ferrous metals. In this test the piece of iron or steel is 5-3

held against a revolving grinding stone and the metal is identified by the sparks thrown off. Each ferrous metal has its own peculiar spark characteristics. The spark streams vary from a few tiny shafts to a shower of sparks several feet in length. (Few nonferrous metals give off sparks when touched to a grinding stone. Therefore, these metals cannot be successfully identified by the spark test.) Identification by spark testing is often inexact unless performed by an experienced person, or the test pieces differ greatly in their carbon content and alloying constituents. Wrought iron produces long shafts that are straw colored as they leave the stone and white at the end. Cast iron sparks are red as they leave the stone and turn to a straw color. Low carbon steels give off long, straight shafts having a few white sprigs. As the carbon content of the steel increases, the number of sprigs along each shaft increases and the stream becomes whiter in color. Nickel steel causes the spark stream to contain small white blocks of light within the main burst. Types, Characteristics, and Uses of Alloyed Steels Steel containing carbon in percentages ranging from 0.10 to 0.30 percent is classed as low carbon steel. The equivalent SAE numbers range from 1010 to 1030. Steels of this grade are used for making such items as safety wire, certain nuts, cable bushings, or threaded rod ends. This steel in sheet form is used for secondary structural parts and clamps, and in tubular form for moderately stressed structural parts. Steel containing carbon in percentages ranging from 0.30 to 0.50 percent is classed as medium carbon steel. This steel is especially adaptable for machining or forging, and where surface hardness is desirable. Certain rod ends and light forgings are made from SAE 1035 steel. Steel containing carbon in percentages ranging from 0.50 to 1.05 percent is classed as high carbon steel. The addition of other elements in varying quantities adds to the hardness of this steel. In the fully heat-treated condition it is very hard, will withstand high shear and wear, and will have little deformation. It has limited use in aircraft. SAE 1095 in sheet form is used for making flat springs and in wire form for making coil springs. The various nickel steels are produced by combining nickel with carbon steel. Steels containing from 3 to 3.75 percent nickel are commonly used. Nickel increases the hardness, tensile strength, and elastic limit

5-4

of steel without appreciably decreasing the ductility. It also intensifies the hardening effect of heat treatment. SAE 2330 steel is used extensively for aircraft parts, such as bolts, terminals, keys, clevises, and pins. Chromium steel is high in hardness, strength, and corrosion resistant properties, and is particularly adaptable for heat-treated forgings which require greater toughness and strength than may be obtained in plain carbon steel. It can be used for such articles as the balls and rollers of antifriction bearings. Chrome-nickel or stainless steels are the corrosion resistant metals. The anticorrosive degree of this steel is determined by the surface condition of the metal as well as by the composition, temperature, and concentration of the corrosive agent. The principal alloy of stainless steel is chromium. The corrosion resistant steel most often used in aircraft construction is known as 18-8 steel because of its content of 18 percent chromium and 8 percent nickel. One of the distinctive features of 18-8 steel is that its strength may be increased by cold working. Stainless steel may be rolled, drawn, bent, or formed to any shape. Because these steels expand about 50 percent more than mild steel and conduct heat only about 40 percent as rapidly, they are more difficult to weld. Stainless steel can be used for almost any part of an aircraft. Some of its common applications are in the fabrication of exhaust collectors, stacks and manifolds, structural and machined parts, springs, castings, tie rods, and control cables. The chrome-vanadium steels are made of approximately 18 percent vanadium and about 1 percent chromium. When heat treated, they have strength, toughness, and resistance to wear and fatigue. A special grade of this steel in sheet form can be cold formed into intricate shapes. It can be folded and flattened without signs of breaking or failure. SAE 6150 is used for making springs; chrome-vanadium with high carbon content, SAE 6195, is used for ball and roller bearings. Molybdenum in small percentages is used in combination with chromium to form chrome-molybdenum steel, which has various uses in aircraft. Molybdenum is a strong alloying element. It raises the ultimate strength of steel without affecting ductility or workability. Molybdenum steels are tough and wear resistant, and they harden throughout when heat treated. They are especially adaptable for welding and, for this reason, are used principally for welded structural parts and assemblies. This type steel has practically replaced carbon steel in the fabrication of fuselage tub-

ing, engine mounts, landing gears, and other structural parts. For example, a heat-treated SAE X4130 tube is approximately four times as strong as an SAE 1025 tube of the same weight and size. A series of chrome-molybdenum steel most used in aircraft construction is that series containing 0.25 to 0.55 percent carbon, 0.15 to 0.25 percent molybdenum, and 0.50 to 1.10 percent chromium. These steels, when suitably heat treated, are deep hardening, easily machined, readily welded by either gas or electric methods, and are especially adapted to high temperature service. Inconel is a nickel-chromium-iron alloy closely resembling stainless steel (corrosion resistant steel, CRES) in appearance. Aircraft exhaust systems use both alloys interchangeably. Because the two alloys look very much alike, a distinguishing test is often necessary. One method of identification is to use an electrochemical technique, as described in the following paragraph, to identify the nickel (Ni) content of the alloy. Inconel has a nickel content greater than 50 percent, and the electrochemical test detects nickel. The tensile strength of Inconel is 100,000 psi annealed, and 125,000 psi when hard rolled. It is highly resistant to salt water and is able to withstand temperatures as high as 1,600 °F. Inconel welds readily and has working qualities quite similar to those of corrosion resistant steels. Electrochemical Test Prepare a wiring assembly as shown in Figure 5-2, and prepare the two reagents (ammonium fluoride and dimethylglyoxime solutions) placing them in separate dedicated dropper solution bottles. Before testing, you must thoroughly clean the metal in order for the electrolytic deposit to take place. You may use nonmetallic hand scrubbing pads or 320 to 600 grit “crocus cloth”

Connect the alligator clip of the wiring assembly to the bare metal being tested. Place one drop of a 0.05 percent reagent grade ammonium fluoride solution in deionized water on the center of a 1 inch × 1 inch sheet of filter paper. Lay the moistened filter paper over the bare metal alloy being tested. Firmly press the end of the aluminum rod over the center of the moist paper. Maintain connection for 10 seconds while rocking the aluminum rod on the filter paper. Ensure that the light emitting diode (LED) remains lit (indicating good electrical contact and current flow) during this period. Disconnect the wiring assembly and set it aside. Remove the filter paper and examine it to determine that a light spot appears where the connection was made. Deposit one drop of 1.0 percent solution of reagent grade dimethylglyoxime in ethyl alcohol on the filter paper (same side that was in contact with the test metal). A bright, distinctly pink spot will appear within seconds on the filter paper if the metal being tested is Inconel. A brown spot will appear if the test metal is stainless steel. Some stainless steel alloys may leave a very light pink color. However, the shade and depth of color will be far less than would appear for Inconel. For flat surfaces, the test spot will be circular while for curved surfaces, such as the outside of a tube or pipe, the test spot may appear as a streak. (Refer to Figure 5-3 for sample test results.) This procedure should not be used in the heat affected zone of weldments or on nickel coated surfaces. Nonferrous Aircraft Metals The term “nonferrous” refers to all metals which have elements other than iron as their base or principal constituent. This group includes such metals as alu-

LED

9v battery –

to remove deposits and corrosion products (thermal oxide).

+

Aluminium rod stock

Alligator clip

Figure 5-2. Wiring assembly schematic.

Figure 5-3. Electrochemical test results, Inconel and stainless steel alloys.

5-5

minum, titanium, copper, and magnesium, as well as such alloyed metals as Monel and babbit. Aluminum and Aluminum Alloys Commercially pure aluminum is a white lustrous metal which stands second in the scale of malleability, sixth in ductility, and ranks high in its resistance to corrosion. Aluminum combined with various percentages of other metals forms alloys which are used in aircraft construction. Aluminum alloys in which the principal alloying ingredients are manganese, chromium, or magnesium and silicon show little attack in corrosive environments. Alloys in which substantial percentages of copper are used are more susceptible to corrosive action. The total percentage of alloying elements is seldom more than 6 or 7 percent in the wrought alloys. Aluminum is one of the most widely used metals in modern aircraft construction. It is vital to the aviation industry because of its high strength to weight ratio and its comparative ease of fabrication. The outstanding characteristic of aluminum is its light weight. Aluminum melts at the comparatively low temperature of 1,250 °F. It is nonmagnetic and is an excellent conductor. Commercially pure aluminum has a tensile strength of about 13,000 psi, but its strength may be approximately doubled by rolling or other cold working processes. By alloying with other metals, or by using heat-treating processes, the tensile strength may be raised to as high as 65,000 psi or to within the strength range of structural steel. Aluminum alloys, although strong, are easily worked because they are malleable and ductile. They may be rolled into sheets as thin as 0.0017 inch or drawn into wire 0.004 inch in diameter. Most aluminum alloy sheet stock used in aircraft construction range from 0.016 to 0.096 inch in thickness; however, some of the larger aircraft use sheet stock which may be as thick as 0.356 inch.

are determined by the alloying elements and cannot be changed after the metal is cast. In the other, the alloying elements make it possible to heat treat the casting to produce the desired physical properties. The casting alloys are identified by a letter preceding the alloy number. When a letter precedes a number, it indicates a slight variation in the composition of the original alloy. This variation in composition is simply to impart some desirable quality. In casting alloy 214, for example, the addition of zinc to improve its pouring qualities is indicated by the letter A in front of the number, thus creating the designation A214. When castings have been heat treated, the heat treatment and the composition of the casting is indicated by the letter T, followed by an alloying number. An example of this is the sand casting alloy 355, which has several different compositions and tempers and is designated by 355-T6, 355-T51, or C355-T51. Aluminum alloy castings are produced by one of three basic methods: (1) sand mold, (2) permanent mold, or (3) die cast. In casting aluminum, it must be remembered that in most cases different types of alloys must be used for different types of castings. Sand castings and die castings require different types of alloys than those used in permanent molds. Sand and permanent mold castings are parts produced by pouring molten metal into a previously prepared mold, allowing the metal to solidify or freeze, and then removing the part. If the mold is made of sand, the part is a sand casting; if it is a metallic mold (usually cast iron) the part is a permanent mold casting. Sand and permanent castings are produced by pouring liquid metal into the mold, the metal flowing under the force of gravity alone. The two principal types of sand casting alloys are 112 and 212. Little difference exists between the two metals from a mechanical properties standpoint, since both are adaptable to a wide range of products.

The various types of aluminum may be divided into two general classes: (1) casting alloys (those suitable for casting in sand, permanent mold, or die castings) and (2) wrought alloys (those which may be shaped by rolling, drawing, or forging). Of these two, the wrought alloys are the most widely used in aircraft construction, being used for stringers, bulkheads, skin, rivets, and extruded sections.

The permanent mold process is a later development of the sand casting process, the major difference being in the material from which the molds are made. The advantage of this process is that there are fewer openings (called porosity) than in sand castings. The sand and the binder, which is mixed with the sand to hold it together, give off a certain amount of gas which causes porosity in a sand casting.

Aluminum casting alloys are divided into two basic groups. In one, the physical properties of the alloys

Permanent mold castings are used to obtain higher mechanical properties, better surfaces, or more accu-

5-6

rate dimensions. There are two specific types of permanent mold castings: (1) permanent metal mold with metal cores, and (2) semipermanent types containing sand cores. Because finer grain structure is produced in alloys subjected to the rapid cooling of metal molds, they are far superior to the sand type castings. Alloys 122, A132, and 142 are commonly used in permanent mold castings, the principal uses of which are in internal combustion engines.

at room temperature or by artificial aging at some elevated temperature.

Die castings used in aircraft are usually aluminum or magnesium alloy. If weight is of primary importance, magnesium alloy is used because it is lighter than aluminum alloy. However, aluminum alloy is frequently used because it is stronger than most magnesium alloys.

The first digit of a designation identifies the alloy type. The second digit indicates specific alloy modifications. Should the second number be zero, it would indicate no special control over individual impurities. Digits 1 through 9, however, when assigned consecutively as needed for the second number in this group, indicate the number of controls over individual impurities in the metal.

A die casting is produced by forcing molten metal under pressure into a metallic die and allowing it to solidify; then the die is opened and the part removed. The basic difference between permanent mold casting and die casting is that in the permanent mold process the metal flows into the die under gravity. In the die casting operation, the metal is forced under great pressure. Die castings are used where relatively large production of a given part is involved. Remember, any shape which can be forged can be cast. Wrought aluminum and wrought aluminum alloys are divided into two general classes: non-heat-treatable alloys and heat-treatable alloys. Non-heat-treatable alloys are those in which the mechanical properties are determined by the amount of cold work introduced after the final annealing operation. The mechanical properties obtained by cold working are destroyed by any subsequent heating and cannot be restored except by additional cold working, which is not always possible. The “full hard” temper is produced by the maximum amount of cold work that is commercially practicable. Metal in the “as fabricated” condition is produced from the ingot without any subsequent controlled amount of cold working or thermal treatment. There is, consequently, a variable amount of strain hardening, depending upon the thickness of the section. For heat-treatable aluminum alloys, the mechanical properties are obtained by heat treating to a suitable temperature, holding at that temperature long enough to allow the alloying constituent to enter into solid solution, and then quenching to hold the constituent in solution. The metal is left in a supersaturated, unstable state and is then age hardened either by natural aging

Wrought Aluminum Wrought aluminum and wrought aluminum alloys are designated by a four digit index system. The system is broken into three distinct groups: 1xxx group, 2xxx through 8xxx group, and 9xxx group (which is currently unused).

The last two digits of the 1xxx group are used to indicate the hundredths of 1 percent above the original 99 percent designated by the first digit. Thus, if the last two digits were 30, the alloy would contain 99 percent plus 0.30 percent of pure aluminum, or a total of 99.30 percent pure aluminum. Examples of alloys in this group are: • 1100 — 99.00 percent pure aluminum with one control over individual impurities. • 1130 — 99.30 percent pure aluminum with one control over individual impurities. • 1275 — 99.75 percent pure aluminum with two controls over individual impurities. In the 2xxx through 8xxx groups, the first digit indicates the major alloying element used in the formation of the alloy as follows: • 2xxx — copper • 3xxx — manganese • 4xxx — silicon • 5xxx — magnesium • 6xxx — magnesium and silicon • 7xxx — zinc • 8xxx — other elements In the 2xxx through 8xxx alloy groups, the second digit in the alloy designation indicates alloy modifications. If the second digit is zero, it indicates the original alloy, while digits 1 through 9 indicate alloy modifications. The last two of the four digits in the designation identify the different alloys in the group. [Figure 5-4] 5-7

Percentage of alloying elements (aluminum and normal impurities constitute remainder)

Alloy Copper

Silicon

Manganese

Magnesium

Zinc

Nickel

Chromium

Lead

Bismuth

1100

—

3003

—

—

—

—

—

—

—

—

—

—

1.2

—

—

—

—

—

—

2011

5.5

—

—

—

—

—

—

0.5

0.5

2014

4.4

0.8

0.8

0.4

—

—

—

—

—

2017

4.0

—

0.5

0.5

—

—

—

—

—

2117

2.5

—

—

0.3

—

—

—

—

—

2018

4.0

—

—

0.5

—

2.0

—

—

—

2024

4.5

—

0.6

1.5

—

—

—

—

—

2025

4.5

0.8

0.8

—

—

—

—

—

—

4032

0.9

12.5

—

1.0

—

0.9

—

—

—

6151

—

1.0

—

0.6

—

—

0.25

—

—

5052

—

—

—

2.5

—

—

0.25

—

—

6053

—

0.7

—

1.3

—

—

0.25

—

—

6061

0.25

0.6

—

1.0

—

—

0.25

—

—

7075

1.6

—

—

2.5

5.6

—

0.3

—

—

Figure 5-4. Nominal composition of wrought aluminum alloys.

Effect of Alloying Element 1000 series. 99 percent aluminum or higher, excellent corrosion resistance, high thermal and electrical conductivity, low mechanical properties, excellent workability. Iron and silicon are major impurities. 2000 series. Copper is the principal alloying element. Solution heat treatment, optimum properties equal to mild steel, poor corrosion resistance unclad. It is usually clad with 6000 or high purity alloy. Its best known alloy is 2024. 3000 series. Manganese is the principal alloying element of this group which is generally non-heat treatable. The percentage of manganese which will be alloy effective is 1.5 percent. The most popular is 3003, which is of moderate strength and has good working characteristics. 4000 series. Silicon is the principal alloying element of this group, and lowers melting temperature. Its primary use is in welding and brazing. When used in welding heat-treatable alloys, this group will respond to a limited amount of heat treatment. 5000 series. Magnesium is the principal alloying element. It has good welding and corrosion resistant characteristics. High temperatures (over 150 °F) or excessive cold working will increase susceptibility to corrosion.

5-8

6000 series. Silicon and magnesium form magnesium silicide which makes alloys heat treatable. It is of medium strength, good forming qualities, and has corrosion resistant characteristics. 7000 series. Zinc is the principal alloying element. The most popular alloy of the series is 6061. When coupled with magnesium, it results in heat-treatable alloys of very high strength. It usually has copper and chromium added. The principal alloy of this group is 7075. Hardness Identification Where used, the temper designation follows the alloy designation and is separated from it by a dash: i.e., 7075-T6, 2024-T4, and so forth. The temper designation consists of a letter indicating the basic temper which may be more specifically defined by the addition of one or more digits. These designations are as follows: • F — as fabricated • O — annealed, recrystallized (wrought products only) • H — strain hardened • H1 (plus one or more digits) — strain hardened only • H2 (plus one or more digits) — strain hardened and partially annealed • H3 (plus one or more digits) — strain hardened and stabilized

The digit following the designations H1, H2, and H3 indicates the degree of strain hardening, number 8 representing the ultimate tensile strength equal to that achieved by a cold reduction of approximately 75 percent following a full anneal, 0 representing the annealed state. Magnesium and Magnesium Alloys Magnesium, the world’s lightest structural metal, is a silvery white material weighing only two-thirds as much as aluminum. Magnesium does not possess sufficient strength in its pure state for structural uses, but when alloyed with zinc, aluminum, and manganese it produces an alloy having the highest strength to weight ratio of any of the commonly used metals. Magnesium is probably more widely distributed in nature than any other metal. It can be obtained from such ores as dolomite and magnesite, and from sea water, underground brines, and waste solutions of potash. With about 10 million pounds of magnesium in 1 cubic mile of sea water, there is no danger of a dwindling supply. Some of today’s aircraft require in excess of one-half ton of this metal for use in hundreds of vital spots. Some wing panels are fabricated entirely from magnesium alloys, weigh 18 percent less than standard aluminum panels, and have flown hundreds of satisfactory hours. Among the aircraft parts that have been made from magnesium with a substantial savings in weight are nosewheel doors, flap cover skin, aileron cover skin, oil tanks, floorings, fuselage parts, wingtips, engine nacelles, instrument panels, radio masts, hydraulic fluid tanks, oxygen bottle cases, ducts, and seats. Magnesium alloys possess good casting characteristics. Their properties compare favorably with those of cast aluminum. In forging, hydraulic presses are ordinarily used, although, under certain conditions, forging can be accomplished in mechanical presses or with drop hammers. Magnesium alloys are subject to such treatments as annealing, quenching, solution heat treatment, aging, and stabilizing. Sheet and plate magnesium are annealed at the rolling mill. The solution heat treatment is used to put as much of the alloying ingredients as possible into solid solution, which results in high tensile strength and maximum ductility. Aging is applied to castings following heat treatment where maximum hardness and yield strength are desired.

Magnesium embodies fire hazards of an unpredictable nature. When in large sections, its high thermal conductivity makes it difficult to ignite and prevents it from burning. It will not burn until the melting point of 1,204 °F is reached. However, magnesium dust and fine chips are ignited easily. Precautions must be taken to avoid this if possible. Should a fire occur, it can be extinguished with an extinguishing powder, such as soapstone or graphite. Water or any standard liquid or foam fire extinguisher cause magnesium to burn more rapidly and can cause explosions. Magnesium alloys produced in the United States consist of magnesium alloyed with varying proportions of aluminum, manganese, and zinc. These alloys are designated by a letter of the alphabet, with the number 1 indicating high purity and maximum corrosion resistance. Many of the magnesium alloys manufactured in the United States are produced by the Dow Chemical Company and have been given the trade name of Dowmetal™ alloys. To distinguish between these alloys, each is assigned a letter. Thus, we have Dowmetal J, Dowmetal M, and so forth. Another manufacturer of magnesium alloys is the American Magnesium Corporation, a subsidiary of the Aluminum Company of America. This company uses an identification system similar to that used for aluminum alloys, with the exception that magnesium alloy numbers are preceded with the letters AM. Thus, AM240C is a cast alloy, and AM240C4 is the same alloy in the heat-treated state. AM3S0 is an annealed wrought alloy, and AM3SRT is the same alloy rolled after heat treatment. Titanium and Titanium Alloys Titanium was discovered by an English priest named Gregot. A crude separation of titanium ore was accomplished in 1825. In 1906 a sufficient amount of pure titanium was isolated in metallic form to permit a study. Following this study, in 1932, an extraction process was developed which became the first commercial method for producing titanium. The United States Bureau of Mines began making titanium sponge in 1946, and 4 years later the melting process began. The use of titanium is widespread. It is used in many commercial enterprises and is in constant demand for such items as pumps, screens, and other tools and fixtures where corrosion attack is prevalent. In aircraft construction and repair, titanium is used for fuselage skins, engine shrouds, firewalls, longerons, frames, fittings, air ducts, and fasteners. 5-9

Titanium is used for making compressor disks, spacer rings, compressor blades and vanes, through bolts, turbine housings and liners, and miscellaneous hardware for turbine engines. Titanium, in appearance, is similar to stainless steel. One quick method used to identify titanium is the spark test. Titanium gives off a brilliant white trace ending in a brilliant white burst. Also, identification can be accomplished by moistening the titanium and using it to draw a line on a piece of glass. This will leave a dark line similar in appearance to a pencil mark. Titanium falls between aluminum and stainless steel in terms of elasticity, density, and elevated temperature strength. It has a melting point of from 2,730 °F to 3,155 °F, low thermal conductivity, and a low coefficient of expansion. It is light, strong, and resistant to stress corrosion cracking. Titanium is approximately 60 percent heavier than aluminum and about 50 percent lighter than stainless steel. Because of the high melting point of titanium, high temperature properties are disappointing. The ultimate yield strength of titanium drops rapidly above 800 °F. The absorption of oxygen and nitrogen from the air at temperatures above 1,000 °F makes the metal so brittle on long exposure that it soon becomes worthless. However, titanium does have some merit for short time exposure up to 3,000 °F where strength is not important. Aircraft firewalls demand this requirement. Titanium is nonmagnetic and has an electrical resistance comparable to that of stainless steel. Some of the base alloys of titanium are quite hard. Heat treating and alloying do not develop the hardness of titanium to the high levels of some of the heat-treated alloys of steel. It was only recently that a heat-treatable titanium alloy was developed. Prior to the development of this alloy, heating and rolling was the only method of forming that could be accomplished. However, it is possible to form the new alloy in the soft condition and heat treat it for hardness. Iron, molybdenum, and chromium are used to stabilize titanium and produce alloys that will quench harden and age harden. The addition of these metals also adds ductility. The fatigue resistance of titanium is greater than that of aluminum or steel. Titanium becomes softer as the degree of purity is increased. It is not practical to distinguish between the various grades of commercially pure or unalloyed titanium by chemical analysis; therefore, the grades are determined by mechanical properties. 5-10

Titanium Designations

The A-B-C classification of titanium alloys was established to provide a convenient and simple means of describing all titanium alloys. Titanium and titanium alloys possess three basic types of crystals: A (alpha), B (beta), and C (combined alpha and beta). Their characteristics are: • A (alpha) — all around performance; good weldability; tough and strong both cold and hot, and resistant to oxidation. • B (beta) — bendability; excellent bend ductility; strong both cold and hot, but vulnerable to contamination. • C (combined alpha and beta for compromise performances) — strong when cold and warm, but weak when hot; good bendability; moderate contamination resistance; excellent forgeability. Titanium is manufactured for commercial use in two basic compositions: commercially pure titanium and alloyed titanium. A-55 is an example of a commercially pure titanium. It has a yield strength of 55,000 to 80,000 psi and is a general purpose grade for moderate to severe forming. It is sometimes used for nonstructural aircraft parts and for all types of corrosion resistant applications, such as tubing. Type A-70 titanium is closely related to type A-55 but has a yield strength of 70,000 to 95,000 psi. It is used where higher strength is required, and it is specified for many moderately stressed aircraft parts. For many corrosion applications, it is used interchangeably with type A-55. Both type A-55 and type A-70 are weldable. One of the widely used titanium base alloys is designated as C-110M. It is used for primary structural members and aircraft skin, has 110,000 psi minimum yield strength, and contains 8 percent manganese. Type A-110AT is a titanium alloy which contains 5 percent aluminum and 2.5 percent tin. It also has a high minimum yield strength at elevated temperatures with the excellent welding characteristics inherent in alpha-type titanium alloys. Corrosion Characteristics

The corrosion resistance of titanium deserves special mention. The resistance of the metal to corrosion is caused by the formation of a protective surface film of stable oxide or chemi-absorbed oxygen. Film is often produced by the presence of oxygen and oxidizing agents.

Corrosion of titanium is uniform. There is little evidence of pitting or other serious forms of localized attack. Normally, it is not subject to stress corrosion, corrosion fatigue, intergranular corrosion, or galvanic corrosion. Its corrosion resistance is equal or superior to 18-8 stainless steel. Laboratory tests with acid and saline solutions show titanium polarizes readily. The net effect, in general, is to decrease current flow in galvanic and corrosion cells. Corrosion currents on the surface of titanium and metallic couples are naturally restricted. This partly accounts for good resistance to many chemicals; also, the material may be used with some dissimilar metals with no harmful galvanic effect on either. Copper and Copper Alloys Copper is one of the most widely distributed metals. It is the only reddish colored metal and is second only to silver in electrical conductivity. Its use as a structural material is limited because of its great weight. However, some of its outstanding characteristics, such as its high electrical and heat conductivity, in many cases overbalance the weight factor. Because it is very malleable and ductile, copper is ideal for making wire. It is corroded by salt water but is not affected by fresh water. The ultimate tensile strength of copper varies greatly. For cast copper, the tensile strength is about 25,000 psi, and when cold rolled or cold drawn its tensile strength increases to a range of 40,000 to 67,000 psi. In aircraft, copper is used primarily in the electrical system for bus bars, bonding, and as lockwire. Beryllium copper is one of the most successful of all the copper base alloys. It is a recently developed alloy containing about 97 percent copper, 2 percent beryllium, and sufficient nickel to increase the percentage of elongation. The most valuable feature of this metal is that the physical properties can be greatly stepped up by heat treatment, the tensile strength rising from 70,000 psi in the annealed state to 200,000 psi in the heat-treated state. The resistance of beryllium copper to fatigue and wear makes it suitable for diaphragms, precision bearings and bushings, ball cages, and spring washers. Brass is a copper alloy containing zinc and small amounts of aluminum, iron, lead, manganese, magnesium, nickel, phosphorous, and tin. Brass with a zinc content of 30 to 35 percent is very ductile, but that containing 45 percent has relatively high strength.

Muntz metal is a brass composed of 60 percent copper and 40 percent zinc. It has excellent corrosion resistant qualities in salt water. Its strength can be increased by heat treatment. As cast, this metal has an ultimate tensile strength of 50,000 psi, and it can be elongated 18 percent. It is used in making bolts and nuts, as well as parts that come in contact with salt water. Red brass, sometimes termed “bronze” because of its tin content, is used in fuel and oil line fittings. This metal has good casting and finishing properties and machines freely. Bronzes are copper alloys containing tin. The true bronzes have up to 25 percent tin, but those with less than 11 percent are most useful, especially for such items as tube fittings in aircraft. Among the copper alloys are the copper aluminum alloys, of which the aluminum bronzes rank very high in aircraft usage. They would find greater usefulness in structures if it were not for their strength to weight ratio as compared with alloy steels. Wrought aluminum bronzes are almost as strong and ductile as medium carbon steel, and they possess a high degree of resistance to corrosion by air, salt water, and chemicals. They are readily forged, hot or cold rolled, and many react to heat treatment. These copper base alloys contain up to 16 percent of aluminum (usually 5 to 11 percent), to which other metals, such as iron, nickel, or manganese, may be added. Aluminum bronzes have good tearing qualities, great strength, hardness, and resistance to both shock and fatigue. Because of these properties, they are used for diaphragms, gears, and pumps. Aluminum bronzes are available in rods, bars, plates, sheets, strips, and forgings. Cast aluminum bronzes, using about 89 percent copper, 9 percent aluminum, and 2 percent of other elements, have high strength combined with ductility, and are resistant to corrosion, shock, and fatigue. Because of these properties, cast aluminum bronze is used in bearings and pump parts. These alloys are useful in areas exposed to salt water and corrosive gases. Manganese bronze is an exceptionally high strength, tough, corrosion resistant copper zinc alloy containing aluminum, manganese, iron and, occasionally, nickel or tin. This metal can be formed, extruded, drawn, or rolled to any desired shape. In rod form, it is generally used for machined parts, for aircraft landing gears and brackets.

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Silicon bronze is a more recent development composed of about 95 percent copper, 3 percent silicon, and 2 percent manganese, zinc, iron, tin, and aluminum. Although not a bronze in the true sense because of its small tin content, silicon bronze has high strength and great corrosion resistance. Monel

Monel, the leading high nickel alloy, combines the properties of high strength and excellent corrosion resistance. This metal consists of 68 percent nickel, 29 percent copper, 0.2 percent iron, 1 percent manganese, and 1.8 percent of other elements. It cannot be hardened by heat treatment. Monel, adaptable to casting and hot or cold working, can be successfully welded. It has working properties similar to those of steel. When forged and annealed, it has a tensile strength of 80,000 psi. This can be increased by cold working to 125,000 psi, sufficient for classification among the tough alloys. Monel has been successfully used for gears and chains to operate retractable landing gears, and for structural parts subject to corrosion. In aircraft, Monel is used for parts demanding both strength and high resistance to corrosion, such as exhaust manifolds and carburetor needle valves and sleeves. K-Monel

K-Monel is a nonferrous alloy containing mainly nickel, copper, and aluminum. It is produced by adding a small amount of aluminum to the Monel formula. It is corrosion resistant and capable of being hardened by heat treatment. K-Monel has been successfully used for gears, and structural members in aircraft which are subjected to corrosive attacks. This alloy is nonmagnetic at all temperatures. K-Monel sheet has been successfully welded by both oxyacetylene and electric arc welding. Nickel and Nickel Alloys There are basically two nickel alloys used in aircraft. They are Monel and Inconel. Monel contains about 68 percent nickel and 29 percent copper, plus small amounts of iron and manganese. Nickel alloys can be welded or easily machined. Some of the nickel Monel, especially the nickel Monels containing small amounts of aluminum, are heat-treatable to similar tensile strengths of steel. Nickel Monel is used in gears and parts that require high strength and toughness, such as exhaust systems that require high strength and corrosion resistance at elevated temperatures. 5-12

Inconel alloys of nickel produce a high strength, high temperature alloy containing approximately 80 percent nickel, 14 percent chromium, and small amounts of iron and other elements. The nickel Inconel alloys are frequently used in turbine engines because of their ability to maintain their strength and corrosion resistance under extremely high temperature conditions. Inconel and stainless steel are similar in appearance and are frequently found in the same areas of the engine. Sometimes it is important to identify the difference between the metal samples. A common test is to apply one drop of cupric chloride and hydrochloric acid solution to the unknown metal and allow it to remain for 2 minutes. At the end of the soak period, a shiny spot indicates the material is nickel Inconel, and a copper colored spot indicates stainless steel. Substitution of Aircraft Metals In selecting substitute metals for the repair and maintenance of aircraft, it is very important to check the appropriate structural repair manual. Aircraft manufacturers design structural members to meet a specific load requirement for a particular aircraft. The methods of repairing these members, apparently similar in construction, will thus vary with different aircraft. Four requirements must be kept in mind when selecting substitute metals. The first and most important of these is maintaining the original strength of the structure. The other three are: (1) maintaining contour or aerodynamic smoothness, (2) maintaining original weight, if possible, or keeping added weight to a minimum, and (3) maintaining the original corrosion resistant properties of the metal. Metalworking Processes There are three methods of metalworking: (1) hot working, (2) cold working, and (3) extruding. The method used will depend on the metal involved and the part required, although in some instances both hot and cold working methods may be used to make a single part. Hot Working Almost all steel is hot worked from the ingot into some form from which it is either hot or cold worked to the finished shape. When an ingot is stripped from its mold, its surface is solid, but the interior is still molten. The ingot is then placed in a soaking pit which retards loss of heat, and the molten interior gradually solidifies. After soaking, the temperature is equalized throughout the ingot, then it is reduced to intermediate size by rolling, making it more readily handled.

The rolled shape is called a bloom when its section dimensions are 6 inches × 6 inches or larger and approximately square. The section is called a billet when it is approximately square and less than 6 inches × 6 inches. Rectangular sections which have a width greater than twice their thickness are called slabs. The slab is the intermediate shape from which sheets are rolled.

the steel in a furnace to a specified temperature and then cooling it in air, oil, water, or a special solution. Temper condition refers to the condition of metal or metal alloys with respect to hardness or toughness. Rolling, hammering, or bending these alloys, or heat treating and aging them, causes them to become tougher and harder. At times these alloys become too hard for forming and have to be re-heat treated or annealed.

Blooms, billets, or slabs are heated above the critical range and rolled into a variety of shapes of uniform cross section. Common rolled shapes are sheet, bar, channel, angle, and I-beam. As discussed later in this chapter, hot rolled material is frequently finished by cold rolling or drawing to obtain accurate finish dimensions and a bright, smooth surface.

Metals are annealed to relieve internal stresses, soften the metal, make it more ductile, and refine the grain structure. Annealing consists of heating the metal to a prescribed temperature, holding it there for a specified length of time, and then cooling the metal back to room temperature. To produce maximum softness, the metal must be cooled very slowly. Some metals must be furnace cooled; others may be cooled in air.

Complicated sections which cannot be rolled, or sections of which only a small quantity is required, are usually forged. Forging of steel is a mechanical working at temperatures above the critical range to shape the metal as desired. Forging is done either by pressing or hammering the heated steel until the desired shape is obtained. Pressing is used when the parts to be forged are large and heavy; this process also replaces hammering where high grade steel is required. Since a press is slow acting, its force is uniformly transmitted to the center of the section, thus affecting the interior grain structure as well as the exterior to give the best possible structure throughout. Hammering can be used only on relatively small pieces. Since hammering transmits its force almost instantly, its effect is limited to a small depth. Thus, it is necessary to use a very heavy hammer or to subject the part to repeated blows to ensure complete working of the section. If the force applied is too weak to reach the center, the finished forged surface will be concave. If the center was properly worked, the surface will be convex or bulged. The advantage of hammering is that the operator has control over both the amount of pressure applied and the finishing temperature, and is able to produce small parts of the highest grade. This type of forging is usually referred to as smith forging. It is used extensively where only a small number of parts are needed. Considerable machining time and material are saved when a part is smith forged to approximately the finished shape. Steel is often harder than necessary and too brittle for most practical uses when put under severe internal strain. To relieve such strain and reduce brittleness, it is tempered after being hardened. This consists of heating

Normalizing applies to iron base metals only. Normalizing consists of heating the part to the proper temperature, holding it at that temperature until it is uniformly heated, and then cooling it in still air. Normalizing is used to relieve stresses in metals. Strength, weight, and reliability are three factors which determine the requirements to be met by any material used in airframe construction and repair. Airframes must be strong and yet as light weight as possible. There are very definite limits to which increases in strength can be accompanied by increases in weight. An airframe so heavy that it could not support a few hundred pounds of additional weight would be of little use. All metals, in addition to having a good strength/weight ratio, must be thoroughly reliable, thus minimizing the possibility of dangerous and unexpected failures. In addition to these general properties, the material selected for a definite application must possess specific qualities suitable for the purpose. The material must possess the strength required by the dimensions, weight, and use. The five basic stresses which metals may be required to withstand are tension, compression, shear, bending, and torsion. The tensile strength of a material is its resistance to a force which tends to pull it apart. Tensile strength is measured in pounds per square inch (psi) and is calculated by dividing the load in pounds required to pull the material apart by its cross-sectional area in square inches. The compression strength of a material is its resistance to a crushing force which is the opposite of tensile 5-13

strength. Compression strength is also measured in psi. When a piece of metal is cut, the material is subjected, as it comes in contact with the cutting edge, to a force known as shear. Shear is the tendency on the part of parallel members to slide in opposite directions. It is like placing a cord or thread between the blades of a pair of scissors (shears). The shear strength is the shear force in psi at which a material fails. It is the load divided by the shear area. Bending can be described as the deflection or curving of a member due to forces acting upon it. The bending strength of material is the resistance it offers to deflecting forces. Torsion is a twisting force. Such action would occur in a member fixed at one end and twisted at the other. The torsional strength of material is its resistance to twisting. The relationship between the strength of a material and its weight per cubic inch, expressed as a ratio, is known as the strength/weight ratio. This ratio forms the basis for comparing the desirability of various materials for use in airframe construction and repair. Neither strength nor weight alone can be used as a means of true comparison. In some applications, such as the skin of monocoque structures, thickness is more important than strength, and, in this instance, the material with the lightest weight for a given thickness or gauge is best. Thickness or bulk is necessary to prevent bucking or damage caused by careless handling. Corrosion is the eating away or pitting of the surface or the internal structure of metals. Because of the thin sections and the safety factors used in aircraft design and construction, it would be dangerous to select a material possessing poor corrosion resistant characteristics. Another significant factor to consider in maintenance and repair is the ability of a material to be formed, bent, or machined to required shapes. The hardening of metals by cold working or forming is termed work hardening. If a piece of metal is formed (shaped or bent) while cold, it is said to be cold worked. Practically all the work an aviation mechanic does on metal is cold work. While this is convenient, it causes the metal to become harder and more brittle. If the metal is cold worked too much, that is, if it is bent back and forth or hammered at the same place too often, it will crack or break. Usually, the more malleable and ductile a metal is, the more cold working it can stand. Any process which involves controlled heating and cooling of metals to develop certain desirable characteristics (such as hardness, softness, ductility, tensile strength, or refined grain structure) is 5-14

called heat treatment or heat treating. With steels the term “heat treating” has a broad meaning and includes such processes as annealing, normalizing, hardening, and tempering. In the heat treatment of aluminum alloys, only two processes are included: (1) the hardening and toughening process, and (2) the softening process. The hardening and toughening process is called heat treating, and the softening process is called annealing. Aircraft metals are subjected to both shock and fatigue (vibrational) stresses. Fatigue occurs in materials which are exposed to frequent reversals of loading or repeatedly applied loads, if the fatigue limit is reached or exceeded. Repeated vibration or bending will ultimately cause a minute crack to occur at the weakest point. As vibration or bending continues, the crack lengthens until the part completely fails. This is termed shock and fatigue failure. Resistance to this condition is known as shock and fatigue resistance. It is essential that materials used for critical parts be resistant to these stresses. Heat treatment is a series of operations involving the heating and cooling of metals in the solid state. Its purpose is to change a mechanical property or combination of mechanical properties so that the metal will be more useful, serviceable, and safe for a definite purpose. By heat treating, a metal can be made harder, stronger, and more resistant to impact. Heat treating can also make a metal softer and more ductile. No one heat treating operation can produce all of these characteristics. In fact, some properties are often improved at the expense of others. In being hardened, for example, a metal may become brittle. The various heat-treating processes are similar in that they all involve the heating and cooling of metals. They differ, however, in the temperatures to which the metal is heated, the rate at which it is cooled, and, of course, in the final result. The most common forms of heat treatment for ferrous metals are hardening, tempering, normalizing, annealing, and casehardening. Most nonferrous metals can be annealed and many of them can be hardened by heat treatment. However, there is only one nonferrous metal, titanium, that can be casehardened, and none can be tempered or normalized. Internal Structure of Metals The results obtained by heat treatment depend to a great extent on the structure of the metal and on the manner in which the structure changes when the metal is heated and cooled. A pure metal cannot be hardened

by heat treatment because there is little change in its structure when heated. On the other hand, most alloys respond to heat treatment since their structures change with heating and cooling. An alloy may be in the form of a solid solution, a mechanical mixture, or a combination of a solid solution and a mechanical mixture. When an alloy is in the form of a solid solution, the elements and compounds which form the alloy are absorbed, one into the other, in much the same way that salt is dissolved in a glass of water, and the constituents cannot be identified even under a microscope. When two or more elements or compounds are mixed but can be identified by microscopic examination, a mechanical mixture is formed. A mechanical mixture can be compared to the mixture of sand and gravel in concrete. The sand and gravel are both visible. Just as the sand and gravel are held together and kept in place by the matrix of cement, the other constituents of an alloy are embedded in the matrix formed by the base metal. An alloy in the form of a mechanical mixture at ordinary temperatures may change to a solid solution when heated. When cooled back to normal temperature, the alloy may return to its original structure. On the other hand, it may remain a solid solution or form a combination of a solid solution and mechanical mixture. An alloy which consists of a combination of solid solution and mechanical mixture at normal temperatures may change to a solid solution when heated. When cooled, the alloy may remain a solid solution, return to its original structure, or form a complex solution. Heat-Treating Equipment Successful heat treating requires close control over all factors affecting the heating and cooling of metals. Such control is possible only when the proper equipment is available and the equipment is selected to fit the particular job. Thus, the furnace must be of the proper size and type and must be so controlled that temperatures are kept within the limits prescribed for each operation. Even the atmosphere within the furnace affects the condition of the part being heat treated. Further, the quenching equipment and the quenching medium must be selected to fit the metal and the heattreating operation. Finally, there must be equipment for handling parts and materials, for cleaning metals, and for straightening parts.

Furnaces and Salt Baths

There are many different types and sizes of furnaces used in heat treatment. As a general rule, furnaces are designed to operate in certain specific temperature ranges and attempted use in other ranges frequently results in work of inferior quality. In addition, using a furnace beyond its rated maximum temperature shortens its life and may necessitate costly and time consuming repairs. Fuel fired furnaces (gas or oil) require air for proper combustion and an air compressor or blower is therefore necessary. These furnaces are usually of the muffler type; that is, the combustion of the fuel takes place outside of and around the chamber in which the work is placed. If an open muffler is used, the furnace should be designed to prevent the direct impingement of flame on the work. In furnaces heated by electricity, the heating elements are generally in the form of wire or ribbon. Good design requires incorporation of additional heating elements at locations where maximum heat loss may be expected. Such furnaces commonly operate at up to a maximum temperature of about 2,000 °F. Furnaces operating at temperatures up to about 2,500 °F usually employ resistor bars of sintered carbides. Temperature Measurement and Control

Temperature in the heat-treating furnace is measured by a thermoelectric instrument known as a pyrometer. This instrument measures the electrical effect of a thermocouple and, hence, the temperature of the metal being treated. A complete pyrometer consists of three parts — a thermocouple, extension leads, and meter. Furnaces intended primarily for tempering may be heated by gas or electricity and are frequently equipped with a fan for circulating the hot air. Salt baths are available for operating at either tempering or hardening temperatures. Depending on the composition of the salt bath, heating can be conducted at temperatures as low as 325 °F to as high as 2,450 °F. Lead baths can be used in the temperature range of 650 °F to 1,700 °F. The rate of heating in lead or salt baths is much faster in furnaces. Heat-treating furnaces differ in size, shape, capacity, construction, operation, and control. They may be circular or rectangular and may rest on pedestals or directly on the floor. There are also pit-type furnaces, which are below the surface of the floor. When metal is

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to be heated in a bath of molten salt or lead, the furnace must contain a pot or crucible for the molten bath. The size and capacity of a heat-treating furnace depends on the intended use. A furnace must be capable of heating rapidly and uniformly, regardless of the desired maximum temperature or the mass of the charge. An oven-type furnace should have a working space (hearth) about twice as long and three times as wide as any part that will be heated in the furnace. Accurate temperature measurement is essential to good heat treating. The usual method is by means of thermocouples: the most common base metal couples are copper-constantan (up to about 700 °F), iron-constantan (up to about 1,400 °F), and chromel-alumel (up to about 2,200 °F). The most common noble metal couples (which can be used up to about 2,800 °F) are platinum coupled with either the alloy 87 percent platinum (13 percent rhodium) or the alloy 90 percent platinum (10 percent rhodium). The temperatures quoted are for continuous operation. The life of thermocouples is affected by the maximum temperature (which may frequently exceed those given above) and by the furnace atmosphere. Iron-constantan is more suited for use in reducing and chromel-alumel in oxidizing atmospheres. Thermocouples are usually encased in metallic or ceramic tubes closed at the hot end to protect them from the furnace gases. A necessary attachment is an instrument, such as a millivoltmeter or potentiometer, for measuring the electromotive force generated by the thermocouple. In the interest of accurate control, place the hot junction of the thermocouple as close to the work as possible. The use of an automatic controller is valuable in controlling the temperature at the desired value. Pyrometers may have meters either of the indicating type or recording type. Indicating pyrometers give direct reading of the furnace temperature. The recording type produces a permanent record of the temperature range throughout the heating operation by means of an inked stylus attached to an arm which traces a line on a sheet of calibrated paper or temperature chart. Pyrometer installations on all modern furnaces provide automatic regulation of the temperature at any desired setting. Instruments of this type are called controlling potentiometer pyrometers. They include a current regulator and an operating mechanism, such as a relay.

5-16

Heating

The object in heating is to transform pearlite (a mixture of alternate strips of ferrite and iron carbide in a single grain) to austenite as the steel is heated through the critical range. Since this transition takes time, a relatively slow rate of heating must be used. Ordinarily, the cold steel is inserted when the temperature in the furnace is from 300 °F to 500 °F below the hardening temperature. In this way, too rapid heating through the critical range is prevented. If temperature measuring equipment is not available, it becomes necessary to estimate temperatures by some other means. An inexpensive, yet fairly accurate method involves the use of commercial crayons, pellets, or paints that melt at various temperatures within the range of 125 °F to 1,600 °F. The least accurate method of temperature estimation is by observation of the color of the hot hearth of the furnace or of the work. The heat colors observed are affected by many factors, such as the conditions of artificial or natural light, the character of the scale on the work, and so forth. Steel begins to appear dull red at about 1,000 °F, and as the temperature increases, the color changes gradually through various shades of red to orange, to yellow, and finally to white. A rough approximation of the correspondence between color and temperature is indicated in Figure 5-5. It is also possible to secure some idea of the temperature of a piece of carbon or low alloy steel, in the low temperature range used for tempering, from the color of the thin oxide film that forms on the cleaned surface of the steel when heated in this range. The approximate temperature/color relationship is indicated on the lower portion of the scale in Figure 5-5. It is often necessary or desirable to protect steel or cast iron from surface oxidation (scaling) and loss of carbon from the surface layers (decarburization). Commercial furnaces, therefore, are generally equipped with some means of atmosphere control. This usually is in the form of a burner for burning controlled amounts of gas and air and directing the products of combustion into the furnace muffle. Water vapor, a product of this combustion, is detrimental and many furnaces are equipped with a means for eliminating it. For furnaces not equipped with atmosphere control, a variety of external atmosphere generators are available. The gas so generated is piped into the furnace and one generator may supply several furnaces. If no method of atmosphere control is available, some degree of protection may be secured by covering the work with cast iron borings or chips.

°Fahrenheit 2,700

°Centigrade 1,500

Vacuum furnaces also are used for annealing steels, especially when a bright nonoxidized surface is a prime consideration.

2,600 1,400

2,500 2,400

1,300

Color of hot body

2,300 2,200

1,200 White

2,100

2,000

1,100

1,900 1,800

Yellow 1,000 Orange

1,700 900 1,600

1,500

Light cherry 800 Full cherry

1,400 1,300

700 Dark cherry

1,200 1,100

600 Dark red

1,000 900

500

800 400 700 600

Temper colors 300

500 400

Blue Purple Brown

200

Straw

300 200

Since the work in salt or lead baths is surrounded by the liquid heating medium, the problem of preventing scaling or decarburization is simplified.

100

100 0

Figure 5-5. Temperature chart indicating conversion of Centigrade to Fahrenheit or visa versa, color temperature scale for hardening temperature range, and tempering temperature range.

Soaking The temperature of the furnace must be held constant during the soaking period, since it is during this period that rearrangement of the internal structure of the steel takes place. Soaking temperatures for various types of steel are specified in ranges varying as much as 100 °F. [Figure 5-6] Small parts are soaked in the lower part of the specified range and heavy parts in the upper part of the specified range. The length of the soaking period depends upon the type of steel and the size of the part. Naturally, heavier parts require longer soaking to ensure equal heating throughout. As a general rule, a soaking period of 30 minutes to 1 hour is sufficient for the average heat-treating operation. Cooling The rate of cooling through the critical range determines the form that the steel will retain. Various rates of cooling are used to produce the desired results. Still air is a slow cooling medium, but is much faster than furnace cooling. Liquids are the fastest cooling media and are therefore used in hardening steels. There are three commonly used quenching liquids — brine, water, and oil. Brine is the strongest quenching medium, water is next, and oil is the least. Generally, an oil quench is used for alloy steels, and brine or water for carbon steels. Quenching Media Quenching solutions act only through their ability to cool the steel. They have no beneficial chemical action on the quenched steel and in themselves impart no unusual properties. Most requirements for quenching media are met satisfactorily by water or aqueous solutions of inorganic salts, such as table salt or caustic soda, or by some type of oil. The rate of cooling is relatively rapid during quenching in brine, somewhat less rapid in water, and slow in oil. Brine usually is made of a 5 to 10 percent solution of salt (sodium chloride) in water. In addition to its greater cooling speed, brine has the ability to “throw” the scale from steel during quenching. The cooling ability of both water and brine, particularly water, is considerably affected by their temperature. Both 5-17

Tempering (drawing) temperature for tensile strength (psi)

Quenching medium (n)

Temperatures

100,000 (°F)

125,000 (°F)

150,000 (°F)

180,000 (°F)

200,000 (°F)

Water

—

—

—

—

—

1,575 –1,675

Water

—

—

—

—

—

1,575 –1,650

1,575 –1,675

Water

(a)

—

—

—

—

1,575 –1,650

1,575 –1,625

1,525 –1,600

Water

875

—

—

—

—

1045

1,550 –1,600

1,550 –1,600

1,475 –1,550

Oil or water

1,150

—

—

(n)

—

1095

1,475 –1,550

1,450 –1,500

1,425 –1,500

Oil

(b)

—

1,100

850

750

2330

1,475 –1,525

1,425 –1,475

1,450 –1,500

Oil or water

1,100

950

800

—

—

3135

1,600 –1,650

1,500 –1,550

1,475 –1,525

Oil

1,250

1,050

900

750

650

3140

1,600 –1,650

1,500 –1,550

1,475 –1,525

Oil

1,325

1,075

925

775

700

4037

1,600

1,525 –1,575

1,525 –1,575

Oil or water

1,225

1,100

975

—

—

4130 (x4130)

1,600 –1,700

1,525 –1,575

1,525 –1,625

Oil (c)

(d)

1,050

900

700

575

4140

1,600 –1,650

1,525 –1,575

1,525 –1,575

Oil

1,350

1,100

1,025

825

675

4150

1,550 –1,600

1,475 –1,525

1,550 –1,550

Oil

—

1,275

1,175

1,050

950

4340 (x4340)

1,550 –1,625

1,525 –1,575

1,475 –1,550

Oil

—

1,200

1,050

950

850

4640

1,675 –1,700

1,525 –1,575

1,500 –1,550

Oil

—

1,200

1,050

750

625

6135

1,600 –1,700

1,550 –1,600

1,575 –1,625

Oil

1,300

1,075

925

800

750

6150

1,600 –1,650

1,525 –1,575

1,550 –1,625

Oil

(d)(e)

1,200

1,000

900

800

6195

1,600 –1,650

1,525 –1,575

1,500 –1,550

Oil

(f )

—

—

—

—

NE8620

—

—

1,525 –1,575

Oil

—

1,000

—

—

—

NE8630

1,650

1,525 –1,575

1,525 –1,575

Oil

—

1,125

975

775

675

NE8735

1,650

1,525 –1,575

1,525 –1,575

Oil

—

1,175

1,025

875

775

NE8740

1,625

1,500 –1,550

1,500 –1,550

Oil

—

1,200

1,075

925

850

30905

—

(g)(h)

(i)

—

—

—

—

—

—

51210

1,525 –1,575

1,525 –1,575

1,775 –1,825 (j)

Oil

1,200

1,100

(k)

750

—

51335

—

1,525 –1,575

1,775 –1,850

Oil

—

—

—

—

—

52100

1,625 –1,700

1,400 –1,450

1,525 –1,550

Oil

(f )

—

—

—

—

Corrosion resisting (16-2)(1)

—

—

—

—

(m)

—

—

—

—

Silicon Chromium (for springs)

—

—

1,700 –1,725

Oil

—

—

—

—

—

Steel No.

Normalizing air cool (°F)

Annealing (°F)

Hardening (°F)

1020

1,650 –1,750

1,600 –1,700

1,575 –1,675

1022 (x1020)

1,650 –1,750

1,600 –1,700

1025

1,600 –1,700

1035

Figure 5-6. Heat treatment procedures for steels.

5-18

Notes: (a) Draw at 1,150°F for tensile strength of 70,000 psi. (b) For spring temper draw at 800–900 °F. Rockwell hardness C-40–45. (c) Bars or forgings may be quenched in water from 1,500–1,600 °F. (d) Air cooling from the normalizing temperature will produce a tensile strength of approximately 90,000 psi. (e) For spring temper draw at 850–950 °F. Rockwell hardness C-40–45. (f ) Draw at 350–450 °F to remove quenching strains. Rockwell hardness C-60–65. (g) Anneal at 1,600–1,700 °F to remove residual stresses due to welding or cold work. May be applied only to steel containing titanium or columbium. (h) Anneal at 1,900–2,100 °F to produce maximum softness and corrosion resistance. Cool in air or quench in water. (i) Harden by cold work only.

(j) Lower side of range for sheet 0.06 inch and under. Middle of range for sheet and wire 0.125 inch. Upper side of range for forgings. (k) Not recommended for intermediate tensile strengths because of low impact. (l) AN-QQ-S-770 — It is recommended that, prior to tempering, corrosion-resisting (16 Cr-2 Ni) steel be quenched in oil from a temperature of 1,875–1,900 °F, after a soaking period of 30 minutes at this temperature. To obtain a tensile strength at 115,000 psi, the tempering temperature should be approximately 525 °F. A holding time at these temperatures of about 2 hours is recommended. Tempering temperatures between 700 °F and 1,100 °F will not be approved. (m) Draw at approximately 800 °F and cool in air for Rockwell hardness of C-50. (n) Water used for quenching shall be within the temperature range of 80–150 °F.

Figure 5-6. Heat-treatment procedures for steels. (continued)

should be kept cold — well below 60 °F. If the volume of steel being quenched tends to raise the temperature of the bath appreciably, add ice or use some means of refrigeration to cool the quenching bath. There are many specially prepared quenching oils on the market; their cooling rates do not vary widely. A straight mineral oil with a Saybolt viscosity of about 100 at 100 °F is generally used. Unlike brine and water, the oils have the greatest cooling velocity at a slightly elevated temperature — about 100–140 °F — because of their decreased viscosity at these temperatures. When steel is quenched, the liquid in immediate contact with the hot surface vaporizes; this vapor reduces the rate of heat abstraction markedly. Vigorous agitation of the steel or the use of a pressure spray quench is necessary to dislodge these vapor films and thus permit the desired rate of cooling. The tendency of steel to warp and crack during the quenching process is difficult to overcome because certain parts of the article cool more rapidly than others. The following recommendations will greatly reduce the warping tendency. 1. Never throw a part into the quenching bath. By permitting it to lie on the bottom of the bath, it is apt to cool faster on the top side than on the bottom side, thus causing it to warp or crack. 2. Agitate the part slightly to destroy the coating of vapor that could prevent it from cooling evenly and rapidly. This allows the bath to dissipate its heat to the atmosphere. 3. Immerse irregular shaped parts so that the heavy end enters the bath first.

Quenching Equipment The quenching tank should be of the proper size to handle the material being quenched. Use circulating pumps and coolers to maintain approximately constant temperatures when doing a large amount of quenching. To avoid building up a high concentration of salt in the quenching tank, make provisions for adding fresh water to the quench tank used for molten salt baths. Tank location in reference to the heat-treating furnace is very important. Situate the tank to permit rapid transfer of the part from the furnace to the quenching medium. A delay of more than a few seconds will, in many instances, prove detrimental to the effectiveness of the heat treatment. When heat treating material of thin section, employ guard sheets to retard the loss of heat during transfer to the quench tank. Provide a rinse tank to remove all salt from the material after quenching if the salt is not adequately removed in the quenching tank. Heat Treatment of Ferrous Metals The first important consideration in the heat treatment of a steel part is to know its chemical composition. This, in turn, determines its upper critical point. When the upper critical point is known, the next consideration is the rate of heating and cooling to be used. Carrying out these operations involves the use of uniform heating furnaces, proper temperature controls, and suitable quenching mediums. Behavior of Steel During Heating and Cooling Changing the internal structure of a ferrous metal is accomplished by heating to a temperature above its upper critical point, holding it at that temperature for a time sufficient to permit certain internal changes to 5-19

occur, and then cooling to atmospheric temperature under predetermined, controlled conditions. At ordinary temperatures, the carbon in steel exists in the form of particles of iron carbide scattered throughout an iron matrix known as “ferrite.” The number, size, and distribution of these particles determine the hardness of the steel. At elevated temperatures, the carbon is dissolved in the iron matrix in the form of a solid solution called “austenite,” and the carbide particles appear only after the steel has been cooled. If the cooling is slow, the carbide particles are relatively coarse and few. In this condition, the steel is soft. If the cooling is rapid, as by quenching in oil or water, the carbon precipitates as a cloud of very fine carbide particles, and the steel is hard. The fact that the carbide particles can be dissolved in austenite is the basis of the heat treatment of steel. The temperatures at which this transformation takes place are called the critical points and vary with the composition of the steel. The percentage of carbon in the steel has the greatest influence on the critical points of heat treatment. Hardening Pure iron, wrought iron, and extremely low carbon steels cannot be appreciably hardened by heat treatment, since they contain no hardening element. Cast iron can be hardened, but its heat treatment is limited. When cast iron is cooled rapidly, it forms white iron, which is hard and brittle. When cooled slowly, it forms gray iron, which is soft but brittle under impact. In plain carbon steel, the maximum hardness depends almost entirely on the carbon content of the steel. As the carbon content increases, the ability of the steel to be hardened increases. However, this increase in the ability to harden with an increase in carbon content continues only to a certain point. In practice, that point is 0.85 percent carbon content. When the carbon content is increased beyond 0.85 percent, there is no increase in wear resistance. For most steels, the hardening treatment consists of heating the steel to a temperature just above the upper critical point, soaking or holding for the required length of time, and then cooling it rapidly by plunging the hot steel into oil, water, or brine. Although most steels must be cooled rapidly for hardening, a few may be cooled in still air. Hardening increases the hardness and strength of the steel but makes it less ductile. When hardening carbon steel, it must be cooled to below 1,000 °F in less than 1 second. Should the time required for the temperature to drop to 1,000 °F exceed 5-20

1 second, the austenite begins to transform into fine pearlite. This pearlite varies in hardness, but is much harder than the pearlite formed by annealing and much softer than the martensite desired. After the 1,000 °F temperature is reached, the rapid cooling must continue if the final structure is to be all martensite. When alloys are added to steel, the time limit for the temperature drop to 1,000 °F increases above the 1 second limit for carbon steels. Therefore, a slower quenching medium will produce hardness in alloy steels. Because of the high internal stresses in the “as quenched” condition, steel must be tempered just before it becomes cold. The part should be removed from the quenching bath at a temperature of approximately 200 °F, since the temperature range from 200 °F down to room temperature is the cracking range. Hardening temperatures and quenching mediums for the various types of steel are listed in Figure 5-6. Hardening Precautions A variety of different shapes and sizes of tongs for handling hot steels is necessary. It should be remembered that cooling of the area contacted by the tongs is retarded and that such areas may not harden, particularly if the steel being treated is very shallow hardening. Small parts may be wired together or quenched in baskets made of wire mesh. Special quenching jigs and fixtures are frequently used to hold steels during quenching in a manner to restrain distortion. When selective hardening is desired, portions of the steel may be protected by covering with alundum cement or some other insulating material. Selective hardening may be accomplished also by the use of water or oil jets designed to direct quenching medium on the areas to be hardened. This also is accomplished by the induction and flame hardening procedures previously described, particularly on large production jobs. Shallow hardening steels, such as plain carbon and certain varieties of alloy steels, have such a high critical cooling rate that they must be quenched in brine or water to effect hardening. In general, intricately shaped sections should not be made of shallow hardening steels because of the tendency of these steels to warp and crack during hardening. Such items should be made of deeper hardening steels capable of being hardened by quenching in oil or air.

Tempering Tempering reduces the brittleness imparted by hardening and produces definite physical properties within the steel. Tempering always follows, never precedes, the hardening operation. In addition to reducing brittleness, tempering softens the steel. Tempering is always conducted at temperatures below the low critical point of the steel. In this respect, tempering differs from annealing, normalizing, or hardening, all of which require temperatures above the upper critical point. When hardened steel is reheated, tempering begins at 212 °F and continues as the temperature increases toward the low critical point. By selecting a definite tempering temperature, the resulting hardness and strength can be predetermined. Approximate temperatures for various tensile strengths are listed in Figure 5-6. The minimum time at the tempering temperature should be 1 hour. If the part is over 1 inch in thickness, increase the time by 1 hour for each additional inch of thickness. Tempered steels used in aircraft work have from 125,000 to 200,000 psi ultimate tensile strength. Generally, the rate of cooling from the tempering temperature has no effect on the resulting structure; therefore, the steel is usually cooled in still air after being removed from the furnace. Annealing Annealing of steel produces a fine grained, soft, ductile metal without internal stresses or strains. In the annealed state, steel has its lowest strength. In general, annealing is the opposite of hardening. Annealing of steel is accomplished by heating the metal to just above the upper critical point, soaking at that temperature, and cooling very slowly in the furnace. (Refer to Figure 5-6 for recommended temperatures.) Soaking time is approximately 1 hour per inch of thickness of the material. To produce maximum softness in steel, the metal must be cooled very slowly. Slow cooling is obtained by shutting off the heat and allowing the furnace and metal to cool together to 900 °F or lower, then removing the metal from the furnace and cooling in still air. Another method is to bury the heated steel in ashes, sand, or other substance that does not conduct heat readily. Normalizing The normalizing of steel removes the internal stresses set up by heat treating, welding, casting, forming, or machining. Stress, if not controlled, will lead to failure.

Because of the better physical properties, aircraft steels are often used in the normalized state, but seldom, if ever, in the annealed state. One of the most important uses of normalizing in aircraft work is in welded parts. Welding causes strains to be set up in the adjacent material. In addition, the weld itself is a cast structure as opposed to the wrought structure of the rest of the material. These two types of structures have different grain sizes, and to refine the grain as well as to relieve the internal stresses, all welded parts should be normalized after fabrication. Normalizing is accomplished by heating the steel above the upper critical point and cooling in still air. The more rapid quenching obtained by air cooling, as compared to furnace cooling, results in a harder and stronger material than that obtained by annealing. Recommended normalizing temperatures for the various types of aircraft steels are listed in Figure 5-6. Casehardening Casehardening produces a hard wear-resistant surface or case over a strong, tough core. Casehardening is ideal for parts which require a wear-resistant surface and, at the same time, must be tough enough internally to withstand the applied loads. The steels best suited to casehardening are the low carbon and low alloy steels. If high carbon steel is casehardened, the hardness penetrates the core and causes brittleness. In casehardening, the surface of the metal is changed chemically by introducing a high carbide or nitride content. The core is unaffected chemically. When heat treated, the surface responds to hardening while the core toughens. The common forms of casehardening are carburizing, cyaniding, and nitriding. Since cyaniding is not used in aircraft work, only carburizing and nitriding are discussed in this section. Carburizing

Carburizing is a casehardening process in which carbon is added to the surface of low carbon steel. Thus, a carburized steel has a high carbon surface and a low carbon interior. When the carburized steel is heat treated, the case is hardened while the core remains soft and tough. A common method of carburizing is called “pack carburizing.” When carburizing is to be done by this method, the steel parts are packed in a container with charcoal or some other material rich in carbon. The container is then sealed with fire clay, placed in a furnace, heated to approximately 1,700 °F, and soaked at 5-21

that temperature for several hours. As the temperature increases, carbon monoxide gas forms inside the container and, being unable to escape, combines with the gamma iron in the surface of the steel. The depth to which the carbon penetrates depends on the length of the soaking period. For example, when carbon steel is soaked for 8 hours, the carbon penetrates to a depth of about 0.062 inch.

hours are frequently required to produce the desired thickness of case.

In another method of carburizing, called “gas carburizing,” a material rich in carbon is introduced into the furnace atmosphere. The carburizing atmosphere is produced by the use of various gases or by the burning of oil, wood, or other materials. When the steel parts are heated in this atmosphere, carbon monoxide combines with the gamma iron to produce practically the same results as those described under the pack carburizing process.

Aluminum Alloys In the wrought form, commercially pure aluminum is known as 1100. It has a high degree of resistance to corrosion and is easily formed into intricate shapes. It is relatively low in strength and does not have the properties required for structural aircraft parts. High strengths are generally obtained by the process of alloying. The resulting alloys are less easily formed and, with some exceptions, have lower resistance to corrosion than 1100 aluminum.

A third method of carburizing is that of “liquid carburizing.” In this method, the steel is placed in a molten salt bath that contains the chemicals required to produce a case comparable with one resulting from pack or gas carburizing. Alloy steels with low carbon content as well as low carbon steels may be carburized by any of the three processes. However, some alloys, such as nickel, tend to retard the absorption of carbon. As a result, the time required to produce a given thickness of case varies with the composition of the metal. Nitriding Nitriding is unlike other casehardening processes in that, before nitriding, the part is heat treated to produce definite physical properties. Thus, parts are hardened and tempered before being nitrided. Most steels can be nitrided, but special alloys are required for best results. These special alloys contain aluminum as one of the alloying elements and are called “nitralloys.” In nitriding, the part is placed in a special nitriding furnace and heated to a temperature of approximately 1,000 °F. With the part at this temperature, ammonia gas is circulated within the specially constructed furnace chamber. The high temperature cracks the ammonia gas into nitrogen and hydrogen. The ammonia which does not break down is caught in a water trap below the regions of the other two gases. The nitrogen reacts with the iron to form nitride. The iron nitride is dispersed in minute particles at the surface and works inward. The depth of penetration depends on the length of the treatment. In nitriding, soaking periods as long as 72

5-22

Nitriding can be accomplished with a minimum of distortion, because of the low temperature at which parts are casehardened and because no quenching is required after exposure to the ammonia gas. Heat Treatment of Nonferrous Metals

Alloying is not the only method of increasing the strength of aluminum. Like other materials, aluminum becomes stronger and harder as it is rolled, formed, or otherwise cold worked. Since the hardness depends on the amount of cold working done, 1100 and some wrought aluminum alloys are available in several strain hardened tempers. The soft or annealed condition is designated O. If the material is strain hardened, it is said to be in the H condition. The most widely used alloys in aircraft construction are hardened by heat treatment rather than by cold work. These alloys are designated by a somewhat different set of symbols: T4 and W indicate solution heat treated and quenched but not aged, and T6 indicates an alloy in the heat treated hardened condition. • W — Solution heat treated, unstable temper • T — Treated to produce stable tempers other than F, O, or H • T2 — Annealed (cast products only) • T3 — Solution heat treated and then cold worked • T4 — Solution heat treated • T5 — Artificially aged only • T6 — Solution heat treated and then artificially aged • T7 — Solution heat treated and then stabilized • T8 — Solution heat treated, cold worked, and then artificially aged

• T9 — Solution heat treated, artificially aged, and then cold worked

of freshly quenched (-W) material and material that is in the -T3 or -T4 temper.

• T10 — Artificially aged and then cold worked

The hardening of an aluminum alloy by heat treatment consists of four distinct steps:

Additional digits may be added to T1 through T10 to indicate a variation in treatment which significantly alters the characteristics of the product. Aluminum alloy sheets are marked with the specification number on approximately every square foot of material. If for any reason this identification is not on the material, it is possible to separate the heattreatable alloys from the non-heat-treatable alloys by immersing a sample of the material in a 10 percent solution of caustic soda (sodium hydroxide). The heat-treatable alloys will turn black due to the copper content, whereas the others will remain bright. In the case of clad material, the surface will remain bright, but there will be a dark area in the middle when viewed from the edge. Alclad Aluminum The terms “Alclad and Pureclad” are used to designate sheets that consist of an aluminum alloy core coated with a layer of pure aluminum to a depth of approximately 5 1⁄ 2 percent on each side. The pure aluminum coating affords a dual protection for the core, preventing contact with any corrosive agents, and protecting the core electrolytically by preventing any attack caused by scratching or from other abrasions. There are two types of heat treatments applicable to aluminum alloys. One is called solution heat treatment, and the other is known as precipitation heat treatment. Some alloys, such as 2017 and 2024, develop their full properties as a result of solution heat treatment followed by about 4 days of aging at room temperature. Other alloys, such as 2014 and 7075, require both heat treatments. The alloys that require precipitation heat treatment (artificial aging) to develop their full strength also age to a limited extent at room temperature; the rate and amount of strengthening depends upon the alloy. Some reach their maximum natural or room temperature aging strength in a few days, and are designated as -T4 or -T3 temper. Others continue to age appreciably over a long period of time. Because of this natural aging, the -W designation is specified only when the period of aging is indicated, for example, 7075-W (1⁄ 2 hour). Thus, there is considerable difference in the mechanical and physical properties

1. Heating to a predetermined temperature. 2. Soaking at temperature for a specified length of time. 3. Rapidly quenching to a relatively low temperature. 4. Aging or precipitation hardening either spontaneously at room temperature, or as a result of a low temperature thermal treatment. The first three steps above are known as solution heat treatment, although it has become common practice to use the shorter term, “heat treatment.” Room temperature hardening is known as natural aging, while hardening done at moderate temperatures is called artificial aging, or precipitation heat treatment. Solution Heat Treatment Temperature

The temperatures used for solution heat treating vary with different alloys and range from 825 °F to 980 °F. As a rule, they must be controlled within a very narrow range (±10 °F) to obtain specified properties. If the temperature is too low, maximum strength will not be obtained. When excessive temperatures are used, there is danger of melting the low melting constituents of some alloys with consequent lowering of the physical properties of the alloy. Even if melting does not occur, the use of higher than recommended temperatures promotes discoloration and increases quenching strains. Time at Temperature

The time at temperature, referred to as soaking time, is measured from the time the coldest metal reaches the minimum limit of the desired temperature range. The soaking time varies, depending upon the alloy and thickness, from 10 minutes for thin sheets to approximately 12 hours for heavy forgings. For the heavy sections, the nominal soaking time is approximately 1 hour for each inch of cross-sectional thickness. [Figure 5-7] Choose the minimum soaking time necessary to develop the required physical properties. The effect of an abbreviated soaking time is obvious. An excessive soaking period aggravates high temperature oxidation. With clad material, prolonged heating results in exces5-23

Thickness (inch)

Time (minutes)

Up to .032

30

.032 to 1⁄

30

1⁄

8

to 1⁄ 4

40

Over 1⁄ 4

60

8

Note: Soaking time starts when the metal (or the molten bath) reaches a temperature within the range specified above.

Figure 5-7. Typical soaking times for heat treatment.

sive diffusion of copper and other soluble constituents into the protective cladding and may defeat the purpose of cladding. Quenching

After the soluble constituents are in solid solution, the material is quenched to prevent or retard immediate reprecipitation. Three distinct quenching methods are employed. The one to be used in any particular instance depends upon the part, the alloy, and the properties desired. Cold Water Quenching

Parts produced from sheet, extrusions, tubing, small forgings, and similar type material are generally quenched in a cold water bath. The temperature of the water before quenching should not exceed 85 °F. Using a sufficient quantity of water keeps the temperature rise under 20 °F. Such a drastic quench ensures maximum resistance to corrosion. This is particularly important when working with such alloys as 2017, 2024, and 7075. This is the reason a drastic quench is preferred, even though a slower quench may produce the required mechanical properties. Hot Water Quenching Large forgings and heavy sections can be quenched in hot or boiling water. This type of quench minimizes distortion and alleviates cracking which may be produced by the unequal temperatures obtained during the quench. The use of a hot water quench is permitted with these parts because the temperature of the quench water does not critically affect the resistance to corrosion of the forging alloys. In addition, the resistance to corrosion of heavy sections is not as critical a factor as for thin sections. Spray Quenching High velocity water sprays are useful for parts formed from clad sheet and for large sections of almost all alloys. This type of quench also minimizes distortion 5-24

and alleviates quench cracking. However, many specifications forbid the use of spray quenching for bare 2017 and 2024 sheet materials because of the effect on their resistance to corrosion. Lag Between Soaking and Quenching The time interval between the removal of the material from the furnace and quenching is critical for some alloys and should be held to a minimum. When solution heat treating 2017 or 2024 sheet material, the elapsed time must not exceed 10 seconds. The allowable time for heavy sections may be slightly greater. Allowing the metal to cool slightly before quenching promotes reprecipitation from the solid solution. The precipitation occurs along grain boundaries and in certain slip planes causing poorer formability. In the case of 2017, 2024, and 7075 alloys, their resistance to intergranular corrosion is adversely affected. Reheat Treatment

The treatment of material which has been previously heat treated is considered a reheat treatment. The unclad heat-treatable alloys can be solution heat treated repeatedly without harmful effects. The number of solution heat treatments allowed for clad sheet is limited due to increased diffusion of core and cladding with each reheating. Existing specifications allow one to three reheat treatments of clad sheet depending upon cladding thickness. Straightening After Solution Heat Treatment Some warping occurs during solution heat treatment, producing kinks, buckles, waves, and twists. These imperfections are generally removed by straightening and flattening operations. Where the straightening operations produce an appreciable increase in the tensile and yield strengths and a slight decrease in the percent of elongation, the material is designated -T3 temper. When the above values are not materially affected, the material is designated -T4 temper. Precipitation Heat Treating As previously stated, the aluminum alloys are in a comparatively soft state immediately after quenching from a solution heat-treating temperature. To obtain their maximum strengths, they must be either naturally aged or precipitation hardened. During this hardening and strengthening operation, precipitation of the soluble constituents from the supersaturated solid solution takes place. As precipitation

progresses, the strength of the material increases, often by a series of peaks, until a maximum is reached. Further aging (overaging) causes the strength to steadily decline until a somewhat stable condition is obtained. The submicroscopic particles that are precipitated provide the keys or locks within the grain structure and between the grains to resist internal slippage and distortion when a load of any type is applied. In this manner, the strength and hardness of the alloy are increased.

maximum strength generally being obtained in 24 hours. Alloys 2017 and 2024 are natural aging alloys. The alloys that require precipitation thermal treatment to develop their full strength are artificially aged alloys. However, these alloys also age a limited amount at room temperature, the rate and extent of the strengthening depending upon the alloys.

Precipitation hardening produces a great increase in the strength and hardness of the material with corresponding decreases in the ductile properties. The process used to obtain the desired increase in strength is therefore known as aging, or precipitation hardening.

Many of the artificially aged alloys reach their maximum natural or room temperature aging strengths after a few days. These can be stocked for fabrication in the -T4 or -T3 temper. High zinc content alloys such as 7075 continue to age appreciably over a long period of time, their mechanical property changes being sufficient to reduce their formability.

The strengthening of the heat-treatable alloys by aging is not due merely to the presence of a precipitate. The strength is due to both the uniform distribution of a finely dispersed submicroscopic precipitate and its effects upon the crystal structure of the alloy.

The advantage of -W temper formability can be utilized, however, in the same manner as with natural aging alloys; that is, by fabricating shortly after solution heat treatment, or retaining formability by the use of refrigeration.

The aging practices used depend upon many properties other than strength. As a rule, the artificially aged alloys are slightly overaged to increase their resistance to corrosion. This is especially true with the artificially aged high copper content alloys that are susceptible to intergranular corrosion when inadequately aged.

Refrigeration retards the rate of natural aging. At 32 °F, the beginning of the aging process is delayed for several hours, while dry ice (−50 °F to −100 °F) retards aging for an extended period of time.

The heat-treatable aluminum alloys are subdivided into two classes: those that obtain their full strength at room temperature and those that require artificial aging. The alloys that obtain their full strength after 4 or 5 days at room temperature are known as natural aging alloys. Precipitation from the supersaturated solid solution starts soon after quenching, with 90 percent of the

Precipitation Practices

The temperatures used for precipitation hardening depend upon the alloy and the properties desired, ranging from 250 °F to 375 °F. They should be controlled within a very narrow range (±5 °F) to obtain best results. [Figure 5-8] The time at temperature is dependent upon the temperature used, the properties desired, and the alloy. It ranges from 8 to 96 hours. Increasing the aging tem-

Solution heat treatment

Precipitation heat treatment

Alloy

Temperature (°F)

Quench

Temperature designation

2017

930–950

Cold water

T4

T

2117

930–950

Cold water

T4

T

2024

910–930

Cold water

T4

6053

960–980

Water

T4

Temperature (°F)

7075

960–980

870

Water

Water

T4

Temperature designation

T 445–455

1–2 hours or 8 hours

T5

T6

345–355

18 hours or 8 hours

250

24 hours

T6

345–355 6061

Time of aging

315–325

T6

T6

Figure 5-8. Conditions for heat treatment of aluminum alloys.

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perature decreases the soaking period necessary for proper aging. However, a closer control of both time and temperature is necessary when using the higher temperatures. After receiving the thermal precipitation treatment, the material should be air cooled to room temperature. Water quenching, while not necessary, produces no ill effects. Furnace cooling has a tendency to produce overaging. Annealing of Aluminum Alloys

The annealing procedure for aluminum alloys consists of heating the alloys to an elevated temperature, holding or soaking them at this temperature for a length of time depending upon the mass of the metal, and then cooling in still air. Annealing leaves the metal in the best condition for cold working. However, when prolonged forming operations are involved, the metal will take on a condition known as “mechanical hardness” and will resist further working. It may be necessary to anneal a part several times during the forming process to avoid cracking. Aluminum alloys should not be used in the annealed state for parts or fittings. Clad parts should be heated as quickly and carefully as possible, since long exposure to heat tends to cause some of the constituents of the core to diffuse into the cladding. This reduces the corrosion resistance of the cladding. Heat Treatment of Aluminum Alloy Rivets Aluminum alloy rivets are furnished in the following compositions: Alloys 1100, 5056, 2117, 2017, and 2024. Alloy 1100 rivets are used in the “as fabricated” condition for riveting aluminum alloy sheets where a low strength rivet is suitable. Alloy 5056 rivets are used in the “as fabricated” condition for riveting magnesium alloy sheets. Alloy 2117 rivets have moderately high strength and are suitable for riveting aluminum alloy sheets. These rivets receive only one heat treatment, which is performed by the manufacturer, and are anodized after being heat treated. They require no further heat treatment before they are used. Alloy 2117 rivets retain their characteristics indefinitely after heat treatment and can be driven anytime. Rivets made of this alloy are the most widely used in aircraft construction. Alloy 2017 and 2024 rivets are high strength rivets suitable for use with aluminum alloy structures. They are purchased from the manufacturer in the heat-treated condition. Since the aging characteristics of these 5-26

alloys at room temperatures are such that the rivets are unfit for driving, they must be reheat treated just before they are to be used. Alloy 2017 rivets become too hard for driving in approximately 1 hour after quenching. Alloy 2024 rivets become hardened in 10 minutes after quenching. Both of these alloys may be reheat treated as often as required; however, they must be anodized before the first reheat treatment to prevent intergranular oxidation of the material. If these rivets are stored in a refrigerator at a temperature lower than 32 °F immediately after quenching, they will remain soft enough to be usable for several days. Rivets requiring heat treatment are heated either in tubular containers in a salt bath, or in small screen wire baskets in an air furnace. The heat treatment of alloy 2017 rivets consists of subjecting the rivets to a temperature between 930 °F to 950 °F for approximately 30 minutes, and immediately quenching in cold water. These rivets reach maximum strength in about 9 days after being driven. Alloy 2024 rivets should be heated to a temperature of 910 °F to 930 °F and immediately quenched in cold water. These rivets develop a greater shear strength than 2017 rivets and are used in locations where extra strength is required. Alloy 2024 rivets develop their maximum shear strength in 1 day after being driven. The 2017 rivet should be driven within approximately 1 hour and the 2024 rivet within 10 to 20 minutes after heat treating or removal from refrigeration. If not used within these times, the rivets should be re-heat treated before being refrigerated. Heat Treatment of Magnesium Alloys Magnesium alloy castings respond readily to heat treatment, and about 95 percent of the magnesium used in aircraft construction is in the cast form. The heat treatment of magnesium alloy castings is similar to the heat treatment of aluminum alloys in that there are two types of heat treatment: (1) solution heat treatment and (2) precipitation (aging) heat treatment. Magnesium, however, develops a negligible change in its properties when allowed to age naturally at room temperatures. Solution Heat Treatment

Magnesium alloy castings are solution heat treated to improve tensile strength, ductility, and shock resistance. This heat-treatment condition is indicated by using the symbol -T4 following the alloy designation. Solution heat treatment plus artificial aging is designated -T6. Artificial aging is necessary to develop the full properties of the metal.

Solution heat-treatment temperatures for magnesium alloy castings range from 730 °F to 780 °F, the exact range depending upon the type of alloy. The temperature range for each type of alloy is listed in Specification MIL-H-6857. The upper limit of each range listed in the specification is the maximum temperature to which the alloy may be heated without danger of melting the metal. The soaking time ranges from 10 to 18 hours, the exact time depending upon the type of alloy as well as the thickness of the part. Soaking periods longer than 18 hours may be necessary for castings over 2 inches in thickness. NEVER heat magnesium alloys in a salt bath as this may result in an explosion. A serious potential fire hazard exists in the heat treatment of magnesium alloys. If through oversight or malfunctioning of equipment, the maximum temperatures are exceeded, the casting may ignite and burn freely. For this reason, the furnace used should be equipped with a safety cutoff that will turn off the power to the heating elements and blowers if the regular control equipment malfunctions or fails. Some magnesium alloys require a protective atmosphere of sulfur dioxide gas during solution heat treatment. This aids in preventing the start of a fire even if the temperature limits are slightly exceeded. Air quenching is used after solution heat treatment of magnesium alloys since there appears to be no advantage in liquid cooling. Precipitation Heat Treatment

After solution treatment, magnesium alloys may be given an aging treatment to increase hardness and yield strength. Generally, the aging treatments are used merely to relieve stress and stabilize the alloys in order to prevent dimensional changes later, especially during or after machining. Both yield strength and hardness are improved somewhat by this treatment at the expense of a slight amount of ductility. The corrosion resistance is also improved, making it closer to the “as cast” alloy. Precipitation heat treatment temperatures are considerably lower than solution heat-treatment temperatures and range from 325 °F to 500 °F. Soaking time ranges from 4 to 18 hours. Heat Treatment of Titanium Titanium is heat treated for the following purposes: • Relief of stresses set up during cold forming or machining.

• Annealing after hot working or cold working, or to provide maximum ductility for subsequent cold working. • Thermal hardening to improve strength. Stress Relieving

Stress relieving is generally used to remove stress concentrations resulting from forming of titanium sheet. It is performed at temperatures ranging from 650 °F to 1,000 °F. The time at temperature varies from a few minutes for a very thin sheet to an hour or more for heavier sections. A typical stress relieving treatment is 900 °F for 30 minutes, followed by an air cool. The discoloration or scale which forms on the surface of the metal during stress relieving is easily removed by pickling in acid solutions. The recommended solution contains 10 to 20 percent nitric acid and 1 to 3 percent hydrofluoric acid. The solution should be at room temperature or slightly above. Full Annealing

The annealing of titanium and titanium alloys provides toughness, ductility at room temperature, dimensional and structural stability at elevated temperatures, and improved machinability. The full anneal is usually called for as preparation for further working. It is performed at 1,200–1,650 °F. The time at temperature varies from 16 minutes to several hours, depending on the thickness of the material and the amount of cold work to be performed. The usual treatment for the commonly used alloys is 1,300 °F for 1 hour, followed by an air cool. A full anneal generally results in sufficient scale formation to require the use of caustic descaling, such as sodium hydride salt bath. Thermal Hardening

Unalloyed titanium cannot be heat treated, but the alloys commonly used in aircraft construction can be strengthened by thermal treatment, usually at some sacrifice in ductility. For best results, a water quench from 1,450 °F, followed by reheating to 900 °F for 8 hours is recommended. Casehardening

The chemical activity of titanium and its rapid absorption of oxygen, nitrogen, and carbon at relatively low temperatures make casehardening advantageous for special applications. Nitriding, carburizing, or carbonitriding can be used to produce a wear-resistant case of 0.0001 to 0.0002 inch in depth.

5-27

Hardness Testing Hardness testing is a method of determining the results of heat treatment as well as the state of a metal prior to heat treatment. Since hardness values can be tied in with tensile strength values and, in part, with wear resistance, hardness tests are a valuable check of heattreat control and of material properties. Practically all hardness testing equipment now uses the resistance to penetration as a measure of hardness. Included among the better known hardness testers are the Brinell and Rockwell, both of which are described and illustrated in this section. Also included is a popular portable-type hardness tester currently in use. Brinell Tester

The Brinell hardness tester [Figure 5-9] uses a hardened spherical ball, which is forced into the surface of the metal. This ball is 10 millimeters (0.3937 inch) in diameter. A pressure of 3,000 kilograms is used for fer-

rous metals and 500 kilograms for nonferrous metals. The pressure must be maintained at least 10 seconds for ferrous metals and at least 30 seconds for nonferrous metals. The load is applied by hydraulic pressure. The hydraulic pressure is built up by a hand pump or an electric motor, depending on the model of tester. A pressure gauge indicates the amount of pressure. There is a release mechanism for relieving the pressure after the test has been made, and a calibrated microscope is provided for measuring the diameter of the impression in millimeters. The machine has various shaped anvils for supporting the specimen and an elevating screw for bringing the specimen in contact with the ball penetrator. These are attachments for special tests. To determine the Brinell hardness number for a metal, measure the diameter of the impression, using the calibrated microscope furnished with the tester. Then convert the measurement into the Brinell hardness number on the conversion table furnished with the tester. Rockwell Tester

The Rockwell hardness tester [Figure 5-10] measures the resistance to penetration, as does the Brinell tester. Instead of measuring the diameter of the impres-

Figure 5-9. Brinell hardness tester.

5-28

Figure 5-10. Rockwell hardness tester.

sion, the Rockwell tester measures the depth, and the hardness is indicated directly on a dial attached to the machine. The dial numbers in the outer circle are black, and the inner numbers are red. Rockwell hardness numbers are based on the difference between the depth of penetration at major and minor loads. The greater this difference, the lower the hardness number and the softer the material. Two types of penetrators are used with the Rockwell tester: a diamond cone and a hardened steel ball. The load which forces the penetrator into the metal is called the major load and is measured in kilograms. The results of each penetrator and load combination are reported on separate scales, designated by letters. The penetrator, the major load, and the scale vary with the kind of metal being tested. For hardened steels, the diamond penetrator is used; the major load is 150 kilograms; and the hardness is read on the “C” scale. When this reading is recorded, the letter “C” must precede the number indicated by the pointer. The C-scale setup is used for testing metals ranging in hardness from C-20 to the hardest steel (usually about C-70). If the metal is softer than C-20, the B-scale setup is used. With this setup, the 1⁄16 -inch ball is used as a penetrator; the major load is 100 kilograms; and the hardness is read on the B-scale. In addition to the “C” and “B” scales, there are other setups for special testing. The scales, penetrators, major loads, and dial numbers to be read are listed in Figure 5-11. The Rockwell tester is equipped with a weight pan, and two weights are supplied with the machine. One weight is marked in red. The other weight is marked in black. With no weight in the weight pan, the machine applies a major load of 60 kilograms. If the scale setup calls for a 100 kilogram load, the red weight is placed in the pan. For a 150 kilogram load, the black weight is added to the red weight. The black weight is always used with the red weight; it is never used alone. Practically all testing is done with either the B-scale setup or the C-scale setup. For these scales, the colors may be used as a guide in selecting the weight (or weights) and in reading the dial. For the B-scale test, use the red weight and read the red numbers. For a Cscale test, add the black weight to the red weight and read the black numbers. In setting up the Rockwell machine, use the diamond penetrator for testing materials known to be hard. If the hardness is unknown, try the diamond, since the

Scale Symbol

Penetrator

Major Load (kg)

Dial Color/Number

Diamond

60

Black

16-inch ball

100

Red

Diamond

150

Black

Diamond

100

Black

A B

1⁄

C D E

1⁄

8-inch ball

100

Red

F

1⁄

16-inch ball

60

Red

G

1⁄

16-inch ball

150

Red

H

1⁄

8-inch ball

60

Red

K

1⁄

8-inch ball

150

Red

Figure 5-11. Standard Rockwell hardness scales.

steel ball may be deformed if used for testing hard materials. If the metal tests below C-22, then change to the steel ball. Use the steel ball for all soft materials, those testing less than B-100. Should an overlap occur at the top of the B-scale and the bottom of the C-scale, use the C-scale setup. Before the major load is applied, securely lock the test specimen in place to prevent slipping and to seat the anvil and penetrator properly. To do this, apply a load of 10 kilograms before the lever is tripped. This preliminary load is called the minor load. The minor load is 10 kilograms regardless of the scale setup. The metal to be tested in the Rockwell tester must be ground smooth on two opposite sides and be free of scratches and foreign matter. The surface should be perpendicular to the axis of penetration, and the two opposite ground surfaces should be parallel. If the specimen is tapered, the amount of error will depend on the taper. A curved surface will also cause a slight error in the hardness test. The amount of error depends on the curvature; i.e., the smaller the radius of curvature, the greater the error. To eliminate such error, a small flat should be ground on the curved surface if possible. Clad aluminum alloy sheets cannot be tested directly with any accuracy with a Rockwell hardness tester. If the hardness value of the base metal is desired, the pure aluminum coating must be removed from the area to be checked prior to testing. Barcol Tester

The Barcol tester [Figure 5-12] is a portable unit designed for testing aluminum alloys, copper, brass, or other relatively soft materials. It should not be used on 5-29

Alloy and Temper

Barcol Number

1100-O

35

3003-O

42

3003-H14

56

2024-O

60

5052-O

62

5052-H34

75

6061-T

78

2024-T

85

Figure 5-13. Typical Barcol readings for aluminum alloy.

Figure 5-12. Barcol portable hardness tester.

aircraft steels. Approximate range of the tester is 25 to 100 Brinell. The unit can be used in any position and in any space that will allow for the operator’s hand. It is of great value in the hardness testing of assembled or installed parts, especially to check for proper heat treatment. The hardness is indicated on a dial conveniently divided into 100 graduations. The design of the Barcol tester is such that operating experience is not necessary. It is only necessary to exert a light pressure against the instrument to drive the spring loaded indenter into the material to be tested. The hardness reading is instantly indicated on the dial. Several typical readings for aluminum alloys are listed in Figure 5-13. Note that the harder the material is, the higher the Barcol number will be. To prevent damage to the point, avoid sliding or scraping when it is in contact with the material being tested. If the point should become damaged, it must be replaced with a new one. Do not attempt to grind the point. Each tester is supplied with a test disk for checking the condition of the point. To check the point, press the instrument down on the test disk. When the downward pressure brings the end of the lower plunger guide against the surface of the disk, the indicator reading should be within the range shown on the test disk. 5-30

Forging Forging is the process of forming a product by hammering or pressing. When the material is forged below the recrystallization temperature, it is called cold forged. When worked above the recrystallization temperature, it is referred to as hot forged. Drop forging is a hammering process that uses a hot ingot that is placed between a pair of formed dies in a machine called a drop hammer and a weight of several tons is dropped on the upper die. This results in the hot metal being forced to take the form of the dies. Because the process is very rapid, the grain structure of the metal is altered, resulting in a significant increase in the strength of the finished part. Casting Casting is formed by melting the metal and pouring it into a mold of the desired shape. Since plastic deformation of the metal does not occur, no alteration of the grain shape or orientation is possible. The gain size of the metal can be controlled by the cooling rate, the alloys of the metal, and the thermal treatment. Castings are normally lower in strength and are more brittle than a wrought product of the same material. For intricate shapes or items with internal passages, such as turbine blades, casting may be the most economical process. Except for engine parts, most metal components found on an aircraft are wrought instead of cast. All metal products start in the form of casting. Wrought metals are converted from cast ingots by plastic deformation. For high strength aluminum alloys, an 80 to 90 percent reduction (dimensional change in thickness) of the material is required to obtain the high mechanical properties of a fully wrought structure. Both iron and aluminum alloys are cast for aircraft uses. Cast iron contains 6 to 8 percent carbon and silicon.

Cast iron is a hard unmalleable pig iron made by casting or pouring into a mold. Cast aluminum alloy has been heated to its molten state and poured into a mold to give it the desired shape. Extruding The extrusion process involves the forcing of metal through an opening in a die, thus causing the metal to take the shape of the die opening. The shape of the die will be the cross section of an angle, channel, tube, or some other shape. Some metals such as lead, tin, and aluminum may be extruded cold; however, most metals are heated before extrusion. The main advantage of the extrusion process is its flexibility. For example, because of its workability, aluminum can be economically extruded to more intricate shapes and larger sizes than is practical with other metals. Extruded shapes are produced in very simple as well as extremely complex sections. In this process a cylinder of aluminum, for instance, is heated to 750 – 850 °F and is then forced through the opening of a die by a hydraulic ram. The opening is the shape desired for the cross section of the finished extrusion. Many structural parts, such as channels, angles, T‑sections, and Z-sections, are formed by the extrusion process. Aluminum is the most extruded metal used in aircraft. Aluminum is extruded at a temperature of 700–900 °F (371– 482 °C) and requires pressure of up to 80,000 psi (552 MPa). After extrusion, the product frequently will be subjected to both thermal and mechanical processes to obtain the desired properties. Extrusion processes are limited to the more ductile materials. Cold Working/Hardening Cold working applies to mechanical working performed at temperatures below the critical range. It results in a strain hardening of the metal. In fact, the metal often becomes so hard that it is difficult to continue the forming process without softening the metal by annealing. Since the errors attending shrinkage are eliminated in cold working, a much more compact and better metal is obtained. The strength and hardness, as well as the elastic limit, are increased; but the ductility decreases. Since this makes the metal more brittle, it must be heated from time to time during certain operations to remove the undesirable effects of the working. While there are several cold working processes, the two with which the aviation mechanic will be principally

concerned are cold rolling and cold drawing. These processes give the metals desirable qualities which cannot be obtained by hot working. Cold rolling usually refers to the working of metal at room temperature. In this operation, the materials that have been rolled to approximate sizes are pickled to remove the scale, after which they are passed through chilled finishing rolls. This gives a smooth surface and also brings the pieces to accurate dimensions. The principal forms of cold rolled stocks are sheets, bars, and rods. Cold drawing is used in making seamless tubing, wire, streamlined tie rods, and other forms of stock. Wire is made from hot rolled rods of various diameters. These rods are pickled in acid to remove scale, dipped in lime water, and then dried in a steam room where they remain until ready for drawing. The lime coating adhering to the metal serves as a lubricant during the drawing operation. The size of the rod used for drawing depends upon the diameter wanted in the finished wire. To reduce the rod to the desired size, it is drawn cold through a die. One end of the rod is filed or hammered to a point and slipped through the die opening. Here it is gripped by the jaws of the drawing block and pulled through the die. This series of operations is done by a mechanism known as a drawbench. To reduce the rod gradually to the desired size, it is necessary to draw the wire through successively smaller dies. Because each of these drawings reduces the ductility of the wire, it must be annealed from time to time before further drawings can be accomplished. Although cold working reduces the ductility, it increases the tensile strength of the wire. In making seamless steel aircraft tubing, the tubing is cold drawn through a ring shaped die with a mandrel or metal bar inside the tubing to support it while the drawing operations are being performed. This forces the metal to flow between the die and the mandrel and affords a means of controlling the wall thickness and the inside and outside diameters.

Nonmetallic Aircraft Materials The use of magnesium, plastic, fabric, and wood in aircraft construction has nearly disappeared since the mid-1950s. Aluminum has also greatly diminished in use, from 80 percent of airframes in 1950 to about 15 percent aluminum and aluminum alloys today for airframe construction. Replacing those materials are 5-31

nonmetallic aircraft materials, such as reinforced plastics and advanced composites.

edging is simpler, and crazing and scratches are less detrimental.

Wood The earliest aircraft were constructed of wood and cloth. Today, except for restorations and some homebuilt aircraft, very little wood is used in aircraft construction.

Individual sheets of plastic are covered with a heavy masking paper to which a pressure sensitive adhesive has been added. This paper helps to prevent accidental scratching during storage and handling. Be careful to avoid scratches and gouges which may be caused by sliding sheets against one another or across rough or dirty tables.

Plastics Plastics are used in many applications throughout modern aircraft. These applications range from structural components of thermosetting plastics reinforced with fiberglass to decorative trim of thermoplastic materials to windows. Transparent Plastics Transparent plastic materials used in aircraft canopies, windshields, windows and other similar transparent enclosures may be divided into two major classes or groups. These plastics are classified according to their reaction to heat. The two classes are: thermoplastic and thermosetting. Thermoplastic materials will soften when heated and harden when cooled. These materials can be heated until soft, and then formed into the desired shape. When cooled, they will retain this shape. The same piece of plastic can be reheated and reshaped any number of times without changing the chemical composition of the materials. Thermosetting plastics harden upon heating, and reheating has no softening effect. These plastics cannot be reshaped once being fully cured by the application of heat. In addition to the above classes, transparent plastics are manufactured in two forms: monolithic (solid) and laminated. Laminated transparent plastics are made from transparent plastic face sheets bonded by an inner layer material, usually polyvinyl butyryl. Because of its shatter resistant qualities, laminated plastic is superior to solid plastics and is used in many pressurized aircraft. Most of the transparent sheet used in aviation is manufactured in accordance with various military specifications. A new development in transparent plastics is stretched acrylic. Stretched acrylic is a type of plastic which, before being shaped, is pulled in both directions to rearrange its molecular structure. Stretched acrylic panels have a greater resistance to impact and are less subject to shatter; its chemical resistance is greater,

5-32

If possible, store sheets in bins which are tilted at approximately 10° from vertical. If they must be stored horizontally, piles should not be over 18 inches high, and small sheets should be stacked on the larger ones to avoid unsupported overhang. Store in a cool, dry place away from solvent fumes, heating coils, radiators, and steam pipes. The temperature in the storage room should not exceed 120 °F. While direct sunlight does not harm acrylic plastic, it will cause drying and hardening of the masking adhesive, making removal of the paper difficult. If the paper will not roll off easily, place the sheet in an oven at 250 °F for 1 minute, maximum. The heat will soften the masking adhesive for easy removal of the paper. If an oven is not available, remove hardened masking paper by softening the adhesive with aliphatic naphtha. Rub the masking paper with a cloth saturated with naphtha. This will soften the adhesive and free the paper from the plastic. Sheets so treated must be washed immediately with clean water, taking care not to scratch the surfaces. Note: Aliphatic naphtha is not to be confused with aromatic naphtha and other dry cleaning solvents which have harmful effects on plastic. However, aliphatic naphtha is flammable and all precautions regarding the use of flammable liquids must be observed. Composite Materials In the 1940s, the aircraft industry began to develop synthetic fibers to enhance aircraft design. Since that time, composite materials have been used more and more. When composites are mentioned, most people think of only fiberglass, or maybe graphite or aramids (Kevlar). Composites began in aviation, but now are being embraced by many other industries, including auto racing, sporting goods, and boating, as well as defense industry uses. A “composite” material is defined as a mixture of different materials or things. This definition is so general that it could refer to metal alloys made from several

different metals to enhance the strength, ductility, conductivity or whatever characteristics are desired. Likewise, the composition of composite materials is a combination of reinforcement, such as a fiber, whisker, or particle, surrounded and held in place by a resin, forming a structure. Separately, the reinforcement and the resin are very different from their combined state. Even in their combined state, they can still be individually identified and mechanically separated. One composite, concrete, is composed of cement (resin) and gravel or reinforcement rods for the reinforcement to create the concrete. Advantages/Disadvantages of Composites Some of the many advantages for using composite materials are: • High strength to weight ratio • Fiber-to-fiber transfer of stress allowed by chemical bonding • Modulus (stiffness to density ratio) 3.5 to 5 times that of steel or aluminum • Longer life than metals • Higher corrosion resistance • Tensile strength 4 to 6 times that of steel or aluminum • Greater design flexibility • Bonded construction eliminates joints and fasteners • Easily repairable The disadvantages of composites include: • Inspection methods difficult to conduct, especially delamination detection (Advancements in technology will eventually correct this problem.) • Lack of long term design database, relatively new technology methods • Cost • Very expensive processing equipment • Lack of standardized system of methodology • Great variety of materials, processes, and techniques • General lack of repair knowledge and expertise • Products often toxic and hazardous • Lack of standardized methodology for construction and repairs The increased strength and the ability to design for the performance needs of the product makes composites

much superior to the traditional materials used in today’s aircraft. As more and more composites are used, the costs, design, inspection ease, and information about strength to weight advantages will help composites become the material of choice for aircraft construction. Composite Safety Composite products can be very harmful to the skin, eyes, and lungs. In the long or short term, people can become sensitized to the materials with serious irritation and health issues. Personal protection is often uncomfortable, hot, and difficult to wear; however, a little discomfort while working with the composite materials can prevent serious health issues or even death. Respirator particle protection is very important to protecting the lungs from permanent damage from tiny glass bubbles and fiber pieces. At a minimum, a dust mask approved for fiberglass is a necessity. The best protection is a respirator with dust filters. The proper fit of a respirator or dust mask is very important because if the air around the seal is breathed, the mask cannot protect the wearer’s lungs. When working with resins, it is important to use vapor protection. Charcoal filters in a respirator will remove the vapors for a period of time. If you can smell the resin vapors after placing the mask back on after a break, replace the filters immediately. Sometimes, charcoal filters last less than 4 hours. Store the respirator in a sealed bag when not in use. If working with toxic materials for an extended period of time, a supplied air mask and hood are recommended. Avoid skin contact with the fibers and other particles by wearing long pants and long sleeves along with gloves or barrier creams. The eyes must be protected using leak-proof goggles (no vent holes) when working with resins or solvents because chemical damage to the eyes is usually irreversible. Fiber Reinforced Materials The purpose of reinforcement in reinforced plastics is to provide most of the strength. The three main forms of fiber reinforcements are particles, whiskers, and fibers. A particle is a square piece of material. Glass bubbles (Q-cell) are hollow glass spheres, and since their dimensions are equal on all axes, they are called a particle. A whisker is a piece of material that is longer than it is wide. Whiskers are usually single crystals. They are very strong and used to reinforce ceramics and metals.

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Fibers are single filaments that are much longer than they are wide. Fibers can be made of almost any material, and are not crystalline like whiskers. Fibers are the base for most composites. Fibers are smaller than the finest human hair and are normally woven into cloth-like materials. Laminated Structures Composites can be made with or without an inner core of material. Laminated structure with a core center is called a sandwich structure. Laminate construction is strong and stiff, but heavy. The sandwich laminate is equal in strength, and its weight is much less; less weight is very important to aerospace products. The core of a laminate can be made from nearly anything. The decision is normally based on use, strength, and fabricating methods to be used. Various types of cores for laminated structures include rigid foam, wood, metal, or the aerospace preference of honeycomb made from paper, Nomex, carbon, fiberglass or metal. Figure 5-14 shows a typical sandwich structure. It is very important to follow proper techniques to construct or repair laminated structures to ensure the strength is not compromised. A sandwich assembly is made by taking a high-density laminate or solid face and backplate and sandwiching a core in the middle. The selection of materials for the face and backplate are decided by the design engineer, depending on the intended application of the part. It is important to follow manufacturers’ maintenance manual specific instructions regarding testing and repair procedures as they apply to a particular aircraft.

Reinforced Plastic Reinforced plastic is a thermosetting material used in the manufacture of radomes, antenna covers, and wingtips, and as insulation for various pieces of electrical equipment and fuel cells. It has excellent dielectric characteristics which make it ideal for radomes; however, its high strength-to-weight ratio, resistance to mildew, rust, and rot, and ease of fabrication make it equally suited for other parts of the aircraft. Reinforced plastic components of aircraft are formed of either solid laminates or sandwich-type laminates. Resins used to impregnate glass cloths are of the contact pressure type (requiring little or no pressure during cure). These resins are supplied as a liquid which can vary in viscosity from a waterlike consistency to a thick syrup. Cure or polymerization is effected by the use of a catalyst, usually benzoyl peroxide. Solid laminates are constructed of three or more layers of resin impregnated cloths “wet laminated” together to form a solid sheet facing or molded shape. Sandwich-type laminates are constructed of two or more solid sheet facings or a molded shape enclosing a fiberglass honeycomb or foam-type core. Honeycomb cores are made of glass cloths impregnated with a polyester or a combination of nylon and phenolic resins. The specific density and cell size of honeycomb cores varies over a considerable latitude. Honeycomb cores are normally fabricated in blocks that are later cut to the desired thickness on a bandsaw. Foam-type cores are formulated from combinations of alkyd resins and metatoluene di-isocyanate. Sandwichtype fiberglass components filled with foam-type cores

Face sheet

Honeycomb Adhesive

Face sheet

Figure 5-14. Sandwich structure.

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Fabricated sandwich panel

are manufactured to exceedingly close tolerances on overall thickness of the molded facing and core material. To achieve this accuracy, the resin is poured into a close tolerance, molded shape. The resin formulation immediately foams up to fill the void in the molded shape and forms a bond between the facing and the core. Rubber Rubber is used to prevent the entrance of dirt, water, or air, and to prevent the loss of fluids, gases, or air. It is also used to absorb vibration, reduce noise, and cushion impact loads. The term “rubber” is as all inclusive as the term “metal.” It is used to include not only natural rubber, but all synthetic and silicone rubbers. Natural Rubber Natural rubber has better processing and physical properties than synthetic or silicone rubber. These properties include: flexibility, elasticity, tensile strength, tear strength, and low heat buildup due to flexing (hysteresis). Natural rubber is a general purpose product; however, its suitability for aircraft use is somewhat limited because of its inferior resistance to most influences that cause deterioration. Although it provides an excellent seal for many applications, it swells and often softens in all aircraft fuels and in many solvents (naphthas, and so forth). Natural rubber deteriorates more rapidly than synthetic rubber. It is used as a sealing material for water/methanol systems. Synthetic Rubber Synthetic rubber is available in several types, each of which is compounded of different materials to give the desired properties. The most widely used are the butyls, Bunas, and neoprene. Butyl is a hydrocarbon rubber with superior resistance to gas permeation. It is also resistant to deterioration; however, its comparative physical properties are significantly less than those of natural rubber. Butyl will resist oxygen, vegetable oils, animal fats, alkalies, ozone, and weathering. Like natural rubber, butyl will swell in petroleum or coal tar solvents. It has a low water absorption rate and good resistance to heat and low temperature. Depending on the grade, it is suitable for use in temperatures ranging from −65 °F to 300 °F. Butyl is used with phosphate ester hydraulic fluids (Skydrol), silicone fluids, gases, ketones, and acetones.

Buna-S rubber resembles natural rubber both in processing and performance characteristics. Buna-S is as water resistant as natural rubber, but has somewhat better aging characteristics. It has good resistance to heat, but only in the absence of severe flexing. Generally, Buna-S has poor resistance to gasoline, oil, concentrated acids, and solvents. Buna-S is normally used for tires and tubes as a substitute for natural rubber. Buna-N is outstanding in its resistance to hydrocarbons and other solvents; however, it has poor resilience in solvents at low temperature. Buna-N compounds have good resistance to temperatures up to 300 °F, and may be procured for low temperature applications down to −75 °F. Buna-N has fair tear, sunlight, and ozone resistance. It has good abrasion resistance and good breakaway properties when used in contact with metal. When used as a seal on a hydraulic piston, it will not stick to the cylinder wall. Buna-N is used for oil and gasoline hose, tank linings, gaskets, and seals. Neoprene can take more punishment than natural rubber and has better low temperature characteristics. It possesses exceptional resistance to ozone, sunlight, heat, and aging. Neoprene looks and feels like rubber. Neoprene, however, is less like rubber in some of its characteristics than butyl or Buna. The physical characteristics of neoprene, such as tensile strength and elongation, are not equal to natural rubber but do have a definite similarity. Its tear resistance as well as its abrasion resistance is slightly less than that of natural rubber. Although its distortion recovery is complete, it is not as rapid as natural rubber. Neoprene has superior resistance to oil. Although it is good material for use in nonaromatic gasoline systems, it has poor resistance to aromatic gasolines. Neoprene is used primarily for weather seals, window channels, bumper pads, oil resistant hose, and carburetor diaphragms. It is also recommended for use with Freons™ and silicate ester lubricants. Thiokol, known also as polysulfide rubber, has the highest resistance to deterioration but ranks the lowest in physical properties. Thiokol, in general, is not seriously affected by petroleum, hydrocarbons, esters, alcohols, gasoline, or water. Thiokols are ranked low in such physical properties as compression set, tensile strength, elasticity, and tear abrasion resistance. Thiokol is used for oil hose, tank linings for aromatic aviation gasolines, gaskets, and seals. Silicone rubbers are a group of plastic rubber materials made from silicon, oxygen, hydrogen, and carbon. The silicons have excellent heat stability and very low tem5-35

perature flexibility. They are suitable for gaskets, seals, or other applications where elevated temperatures up to 600 °F are prevalent. Silicone rubbers are also resistant to temperatures down to −150 °F. Throughout this temperature range, silicone rubber remains extremely flexible and useful with no hardness or gumminess. Although this material has good resistance to oils, it reacts unfavorably to both aromatic and nonaromatic gasolines. Silastic, one of the best known silicones, is used to insulate electrical and electronic equipment. Because of its dielectric properties over a wide range of temperatures, it remains flexible and free from crazing and cracking. Silastic is also used for gaskets and seals in certain oil systems.

Shock Absorber Cord Shock absorber cord is made from natural rubber strands encased in a braided cover of woven cotton cords treated to resist oxidation and wear. Great tension and elongation are obtained by weaving the jacket upon the bundle of rubber strands while they are stretched about three times their original length. There are two types of elastic shock absorbing cord. Type I is a straight cord, and type II is a continuous ring, known as a “bungee.” The advantages of the type II cord are that it is easily and quickly replaced and does not need to be secured by stretching and whipping. Shock cord is available in standard diameters from 1⁄4 inch to 13⁄16 inch. Three colored threads are braided into the outer cover for the entire length of the cord. Two of these threads are of the same color and represent the year of manufacture; the third thread, a different color, represents the quarter of the year in which the cord was made. The code covers a 5-year period and then repeats itself. This makes it easy to figure forward or backward from the years shown in Figure 5-15.

Seals Seals are used to prevent fluid from passing a certain point, as well as to keep air and dirt out of the system in which they are used. The increased use of hydraulics and pneumatics in aircraft systems has created a need for packings and gaskets of varying characteristics and design to meet the many variations of operating speeds and temperatures to which they are subjected. No one style or type of seal is satisfactory for all installations. Some of the reasons for this are: (1) pressure at which the system operates, (2) the type fluid used in the sys5-36

Year

Threads

Color

2000

2

black

2001

2

green

2002

2

red

2003

2

blue

2004

2

yellow

2005

2

black

2006

2

green

2007

2

red

2008

2

blue

2009

2

yellow

2010

2

black

Quarter Marking Quarter

Threads

Color

Jan., Feb., Mar.

1

red

Apr., May, June

1

blue

July, Aug., Sept.

1

green

Oct., Nov., Dec.

1

yellow

Figure 5-15. Shock absorber cord color coding.

tem, (3) the metal finish and the clearance between adjacent parts, and (4) the type motion (rotary or reciprocating), if any. Seals are divided into three main classes: (1) packings, (2) gaskets, and (3) wipers. Packings Packings are made of synthetic or natural rubber. They are generally used as “running seals,” that is, in units that contain moving parts, such as actuating cylinders, pumps, selector valves, and so forth. Packings are made in the form of O-rings, V-rings, and U-rings, each designed for a specific purpose. [Figure 5-16] O-Ring Packings O-ring packings are used to prevent both internal and external leakage. This type of packing ring seals effectively in both directions and is the type most commonly used. In installations subject to pressures above 1,500 psi, backup rings are used with O-rings to prevent extrusion. When an O-ring packing is subjected to pressure from both sides, as in actuating cylinders, two backup rings must be used (one on either side of the O-ring). When an O-ring is subject to pressure on only one side, a

standard for MIL-H-5606 systems in which the temperature may vary from −65 °F to +275 °F.

U-ring

O-ring

V-ring

U-cup

Male

Female V-ring adapters

Figure 5-16. Packing rings.

single backup ring is generally used. In this case, the backup ring is always placed on the side of the O-ring away from the pressure. The materials from which O-rings are manufactured have been compounded for various operating conditions, temperatures, and fluids. An O-ring designed specifically for use as a static (stationary) seal probably will not do the job when installed on a moving part, such as a hydraulic piston. Most O-rings are similar in appearance and texture, but their characteristics may differ widely. An O-ring is useless if it is not compatible with the system fluid and operating temperature. Advances in aircraft design have necessitated new O-ring compositions to meet changed operating conditions. Hydraulic O-rings were originally established under AN specification numbers (6227, 6230, and 6290) for use in MIL-H-5606 fluid at temperatures ranging from −65 °F to +160 °F. When new designs raised operating temperatures to a possible 275 °F, more compounds were developed and perfected. Recently, a compound was developed that offered improved low temperature performance without sacrificing high temperature performance, rendering the other series obsolete. This superior material was adopted in the MS28775 series. This series is now the

Manufacturers provide color coding on some O-rings, but this is not a reliable or complete means of identification. The color coding system does not identify sizes, but only system fluid or vapor compatibility and in some cases the manufacturer. Color codes on O-rings that are compatible with MIL-H-5606 fluid will always contain blue, but may also contain red or other colors. Packings and gaskets suitable for use with Skydrol fluid will always be coded with a green stripe, but may also have a blue, grey, red, green, or yellow dot as a part of the color code. Color codes on O-rings that are compatible with hydrocarbon fluid will always contain red, but will never contain blue. A colored stripe around the circumference indicates that the O-ring is a boss gasket seal. The color of the stripe indicates fluid compatibility: red for fuel, blue for hydraulic fluid. The coding on some rings is not permanent. On others it may be omitted due to manufacturing difficulties or interference with operation. Furthermore, the color coding system provides no means to establish the age of the O-ring or its temperature limitations. Because of the difficulties with color coding, O-rings are available in individual hermetically sealed envelopes, labeled with all pertinent data. When selecting an O-ring for installation, the basic part number on the sealed envelope provides the most reliable compound identification. Although an O-ring may appear perfect at first glance, slight surface flaws may exist. These flaws are often capable of preventing satisfactory O-ring performance under the variable operating pressures of aircraft systems; therefore, O-rings should be rejected for flaws that will affect their performance. Such flaws are difficult to detect, and one aircraft manufacturer recommends using a 4 power magnifying glass with adequate lighting to inspect each ring before it is installed. By rolling the ring on an inspection cone or dowel, the inner diameter surface can also be checked for small cracks, particles of foreign material, or other irregularities that will cause leakage or shorten the life of the O-ring. The slight stretching of the ring when it is rolled inside out will help to reveal some defects not otherwise visible. Backup Rings Backup rings (MS28782) made of Teflon™ do not deteriorate with age, are unaffected by any system 5-37

fluid or vapor, and can tolerate temperature extremes in excess of those encountered in high pressure hydraulic systems. Their dash numbers indicate not only their size but also relate directly to the dash number of the O-ring for which they are dimensionally suited. They are procurable under a number of basic part numbers, but they are interchangeable; that is, any Teflon™ backup ring may be used to replace any other Teflon™ backup ring if it is of proper overall dimension to support the applicable O-ring. Backup rings are not color coded or otherwise marked and must be identified from package labels.

Gaskets Gaskets are used as static (stationary) seals between two flat surfaces. Some of the more common gasket materials are asbestos, copper, cork, and rubber. Asbestos sheeting is used wherever a heat resistant gasket is needed. It is used extensively for exhaust system gaskets. Most asbestos exhaust gaskets have a thin sheet of copper edging to prolong their life.

The inspection of backup rings should include a check to ensure that surfaces are free from irregularities, that the edges are clean cut and sharp, and that scarf cuts are parallel. When checking Teflon™ spiral backup rings, make sure that the coils do not separate more than 1⁄4 inch when unrestrained.

Cork gaskets can be used as an oil seal between the engine crankcase and accessories, and where a gasket is required that is capable of occupying an uneven or varying space caused by a rough surface or expansion and contraction.

V-Ring Packings V-ring packings (AN6225) are one-way seals and are always installed with the open end of the “V” facing the pressure. V-ring packings must have a male and female adapter to hold them in the proper position after installation. It is also necessary to torque the seal retainer to the value specified by the manufacturer of the component being serviced, or the seal may not give satisfactory service. An installation using V-rings is shown in Figure 5-17. U-Ring Packings U-ring packings (AN6226) and U-cup packings are used in brake assemblies and brake master cylinders. The U-ring and U-cup will seal pressure in only one direction; therefore, the lip of the packings must face toward the pressure. U-ring packings are primarily low pressure packings to be used with pressures of less than 1,000 psi.

Male V-ring adapter

Female V-ring adapter

Adjustment nuts

V-ring packing

Figure 5-17. V-ring installation.

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A solid copper washer is used for spark plug gaskets where it is essential to have a noncompressible, yet semisoft gasket.

Rubber sheeting can be used where there is a need for a compressible gasket. It should not be used in any place where it may come in contact with gasoline or oil because the rubber will deteriorate very rapidly when exposed to these substances. Gaskets are used in fluid systems around the end caps of actuating cylinders, valves, and other units. The gasket generally used for this purpose is in the shape of an O-ring, similar to O-ring packings. Wipers Wipers are used to clean and lubricate the exposed portions of piston shafts. They prevent dirt from entering the system and help protect the piston shaft against scoring. Wipers may be either metallic or felt. They are sometimes used together, a felt wiper installed behind a metallic wiper. Sealing Compounds Certain areas of all aircraft are sealed to withstand pressurization by air, to prevent leakage of fuel, to prevent passage of fumes, or to prevent corrosion by sealing against the weather. Most sealants consist of two or more ingredients properly proportioned and compounded to obtain the best results. Some materials are ready for use as packaged, but others will require mixing before application. One Part Sealants One part sealants are prepared by the manufacturer and are ready for application as packaged. However, the consistency of some of these compounds may be altered to satisfy a particular method of application. If

thinning is desired, use the thinner recommended by the sealant manufacturer. Two Part Sealants Two part sealants are compounds requiring separate packaging to prevent cure prior to application and are identified as the base sealing compound and the accelerator. Any alteration of the prescribed ratios will reduce the quality of the material. Generally, two-part sealants are mixed by combining equal portions (by weight) of base compound and accelerator. All sealant material should be carefully weighed in accordance with the sealant manufacturer’s recommendations. Sealant material is usually weighed with a balance scale equipped with weights specially prepared for various quantities of sealant and accelerator. Before weighing the sealant materials, thoroughly stir both the base sealant compound and the accelerator. Do not use accelerator which is dried out, lumpy, or flaky. Preweighed sealant kits do not require weighing of the sealant and accelerator before mixing when the entire quantity is to be mixed. After determining the proper amount of base sealant compound and accelerator, add the accelerator to the base sealant compound. Immediately after adding the accelerator, thoroughly mix the two parts by stirring or folding, depending on the consistency of the material. Carefully mix the material to prevent entrapment of air in the mixture. Overly rapid or prolonged stirring will build up heat in the mixture and shorten the normal application time (working life) of the mixed sealant. To ensure a well mixed compound, test by smearing a small portion on a clean, flat metal or glass surface. If flecks or lumps are found, continue mixing. If the flecks or lumps cannot be eliminated, reject the batch. The working life of mixed sealant is from 1⁄2 hour to 4 hours (depending upon the class of sealant); therefore, apply mixed sealant as soon as possible or place in refrigerated storage. Figure 5-18 presents general information concerning various sealants. The curing rate of mixed sealants varies with changes in temperature and humidity. Curing of sealants will be extremely slow if the temperature is below 60 °F. A temperature of 77 °F with 50 percent relative humidity is the ideal condition for curing most sealants. Curing may be accelerated by increasing the temperature, but the temperature should never be allowed to exceed 120 °F at any time in the curing cycle. Heat

may be applied by using infrared lamps or heated air. If heated air is used, it must be properly filtered to remove moisture and dirt. Heat should not be applied to any faying surface sealant installation until all work is completed. All faying surface applications must have all attachments, permanent or temporary, completed within the application limitations of the sealant. Sealant must be cured to a tack-free condition before applying brush top coatings. (Tack-free consistency is the point at which a sheet of cellophane pressed onto the sealant will no longer adhere.)

Aircraft Hardware Aircraft hardware is the term used to describe the various types of fasteners and miscellaneous small items used in the manufacture and repair of aircraft. The importance of aircraft hardware is often overlooked because of its small size; however, the safe and efficient operation of any aircraft is greatly dependent upon the correct selection and use of aircraft hardware. An aircraft, even though made of the best materials and strongest parts, would be of doubtful value unless those parts were firmly held together. Several methods are used to hold metal parts together; they include riveting, bolting, brazing, and welding. The process used must produce a union that will be as strong as the parts that are joined. Identification Most items of aircraft hardware are identified by their specification number or trade name. Threaded fasteners and rivets are usually identified by AN (Air Force-Navy), NAS (National Aircraft Standard), or MS (Military Standard) numbers. Quick-release fasteners are usually identified by factory trade names and size designations. Threaded Fasteners Various types of fastening devices allow quick dismantling or replacement of aircraft parts that must be taken apart and put back together at frequent intervals. Riveting or welding these parts each time they are serviced would soon weaken or ruin the joint. Furthermore, some joints require greater tensile strength and stiffness than rivets can provide. Bolts and screws are two types of fastening devices which give the required security of attachment and rigidity. Generally, bolts are used where great strength is required, and screws are used where strength is not the deciding factor. Bolts

5-39

Application Life (Work)

Storage (Shelf) Life After Mixing

Storage (Shelf) Life Unmixed

Temperature Range

Application and Limitations

EC-807

12 parts of EC-807 to 100 parts of EC-801

2– 4 hours

5 days at −20 °F after flash freeze at −65 °F

6 months

−65 °F to 200 °F

Faying surfaces, fillet seals, and packing gaps

EC-800 (red)

None

Use as is

8 –12 hours

Not applicable

6–9 months

−65 °F to 200 °F

Coating rivet

EC-612 P (pink) MIL-P-20628

None

Use as is

Indefinite non-drying

Not applicable

6–9 months

−40 °F to 200 °F

Packing voids up to 1⁄ 4 inch

PR-1302HT-A

10 parts of PR-1302HT-A to 100 parts of PR-1302HT

2– 4 hours

5 days at −20 °F after flash freeze at −65 °F

6 months

−65 °F to 200 °F

Sealing access door gaskets

PR-727A

12 parts of PR-727A to 100 parts of PR-727

11⁄ 2 hours minimum

5 days at −20 °F after flash freeze at −65 °F

6 months

−65 °F to 200 °F

Potting electrical connections and bulkhead seals

Not applicable

6–9 months

−60 °F to 850 °F

Sealing hot air ducts passing through bulkheads

Not applicable

Indefinite in airtight containers

−65 °F to 250 °F

Top coating

Accelerator (Catalyst)

Mixing Ratio by Weight

EC-801(black) MIL-S-7502A Class B-2

Sealant Base

PR-1302HT (red) MIL-S-8784 PR-727 potting compound MIL-S-8516B

HT-3 (greygreen)

None

Use as is

Solvent release, sets up in 2–4 hours

EC-776 (clear amber) MIL-S-4383B

None

Use as is

8–12 hours

Figure 5-18. General sealant information.

and screws are similar in many ways. They are both used for fastening or holding, and each has a head on one end and screw threads on the other. Regardless of these similarities, there are several distinct differences between the two types of fasteners. The threaded end of a bolt is always blunt while that of a screw may be either blunt or pointed. The threaded end of a bolt usually has a nut screwed onto it to complete the assembly. The threaded end of a screw may fit into a female receptacle, or it may fit directly into the material being secured. A bolt has a fairly short threaded section and a comparatively long grip length or unthreaded portion; whereas a screw has a longer threaded section and may have no clearly defined grip length. A bolt assembly is generally tightened by turning the nut on the bolt; the head of the bolt 5-40

may or may not be designed for turning. A screw is always tightened by turning its head. When it becomes necessary to replace aircraft fasteners, a duplicate of the original fastener should be used if at all possible. If duplicate fasteners are not available, extreme care and caution must be used in selecting substitutes. Classification of Threads Aircraft bolts, screws, and nuts are threaded in the NC (American National Coarse) thread series, the NF (American National Fine) thread series, the UNC (American Standard Unified Coarse) thread series, or the UNF (American Standard Unified Fine) thread series. There is one difference between the American National series and the American Standard Unified

series that should be pointed out. In the 1-inch diameter size, the NF thread specifies14 threads per inch (1-14 NF), while the UNF thread specifies 12 threads per inch (1-12 UNF). Both types of threads are designated by the number of times the incline (threads) rotates around a 1-inch length of a given diameter bolt or screw. For example, a 4-28 thread indicates that a 1⁄4 ‑inch (4⁄16 inch) diameter bolt has 28 threads in 1 inch of its threaded length.

Alloy steel bolts smaller than No. 10-32 and aluminum alloy bolts smaller than 1⁄4 inch in diameter are not used in primary structures. Aluminum alloy bolts and nuts are not used where they will be repeatedly removed for purposes of maintenance and inspection. Aluminum alloy nuts may be used with cadmium-plated steel bolts loaded in shear on land airplanes, but are not used on seaplanes due to the increased possibility of dissimilar metal corrosion.

Threads are also designated by Class of fit. The Class of a thread indicates the tolerance allowed in manufacturing. Class 1 is a loose fit, Class 2 is a free fit, Class 3 is a medium fit, and Class 4 is a close fit. Aircraft bolts are almost always manufactured in the Class 3, medium fit.

The AN-73 drilled head bolt is similar to the standard hex bolt, but has a deeper head which is drilled to receive wire for safetying. The AN-3 and the AN-73 series bolts are interchangeable, for all practical purposes, from the standpoint of tension and shear strengths.

A Class 4 fit requires a wrench to turn the nut onto a bolt, whereas a Class 1 fit can easily be turned with the fingers. Generally, aircraft screws are manufactured with a Class 2 thread fit for ease of assembly. Bolts and nuts are also produced with right-hand and left-hand threads. A right-hand thread tightens when turned clockwise; a left-hand thread tightens when turned counterclockwise. Aircraft Bolts Aircraft bolts are fabricated from cadmium- or zincplated corrosion resistant steel, unplated corrosion resistant steel, or anodized aluminum alloys. Most bolts used in aircraft structures are either general purpose, AN bolts, or NAS internal wrenching or close tolerance bolts, or MS bolts. In certain cases, aircraft manufacturers make bolts of different dimensions or greater strength than the standard types. Such bolts are made for a particular application, and it is of extreme importance to use like bolts in replacement. Special bolts are usually identified by the letter “S” stamped on the head. AN bolts come in three head styles — hex head, clevis, and eyebolt. [Figure 5-19] NAS bolts are available in hex head, internal wrenching, and countersunk head styles. MS bolts come in hex head and internal wrenching styles. General Purpose Bolts The hex head aircraft bolt (AN-3 through AN-20) is an all-purpose structural bolt used for general applications involving tension or shear loads where a light drive fit is permissible (0.006-inch clearance for a 5⁄8-inch hole, and other sizes in proportion).

Close Tolerance Bolts This type of bolt is machined more accurately than the general purpose bolt. Close tolerance bolts may be hex headed (AN-173 through AN-186) or have a 100° countersunk head (NAS-80 through NAS-86). They are used in applications where a tight drive fit is required. (The bolt will move into position only when struck with a 12- to 14-ounce hammer.) Internal Wrenching Bolts

These bolts, (MS-20004 through MS-20024 or NAS495) are fabricated from high-strength steel and are suitable for use in both tension and shear applications. When they are used in steel parts, the bolt hole must be slightly countersunk to seat the large corner radius of the shank at the head. In Dural material, a special heat-treated washer must be used to provide an adequate bearing surface for the head. The head of the internal wrenching bolt is recessed to allow the insertion of an internal wrench when installing or removing the bolt. Special high-strength nuts are used on these bolts. Replace an internal wrenching bolt with another internal wrenching bolt. Standard AN hex head bolts and washers cannot be substituted for them as they do not have the required strength. Identification and Coding Bolts are manufactured in many shapes and varieties. A clear-cut method of classification is difficult. Bolts can be identified by the shape of the head, method of securing, material used in fabrication, or the expected usage. AN-type aircraft bolts can be identified by the code markings on the bolt heads. The markings generally denote the bolt manufacturer, the material of which the bolt is made, and whether the bolt is a standard AN-type or a special purpose bolt. AN standard steel

5-41

Standard head bolt

Drilled hex head bolt

Countersunk head bolt

Internal hex head bolt

Eyebolt AIR

AS

ES

A

E

Clevis bolt

S O I AT C

O

AN standard steel bolt

AN standard steel bolt

AN

AN

4

C

AN standard steel bolt

AN standard steel bolt

4037

S

R N

M

AN standard steel bolt

CO

AN standard steel bolt (corrosion resistant)

O PER

AN standard steel bolt

SPEC

AN standard steel bolt

AN standard steel bolt

Special bolt

63-59131 5

Special bolt

Drilled head bolt

Special bolt

NAS close tolerance bolt

Aluminium alloy (2024) bolt

Orange-dyed magnetically inspected

Clevis bolt

Reworked bolt

Low strength material bolt

W F

O

Magnetically inspected

Figure 5-19. Aircraft bolt identification.

bolts are marked with either a raised dash or asterisk; corrosion resistant steel is indicated by a single raised dash; and AN aluminum alloy bolts are marked with two raised dashes. Additional information, such as bolt diameter, bolt length, and grip length may be obtained from the bolt part number.

indicate corrosion resistant steel, and the absence of the letters would indicate cadmium plated steel. The “5” indicates the length in eighths of an inch (5⁄8), and the “A” indicates that the shank is undrilled. If the letter “H” preceded the “5” in addition to the “A” following it, the head would be drilled for safetying.

For example, in the bolt part number AN3DD5A, the “AN” designates that it is an Air Force-Navy Standard bolt, the “3” indicates the diameter in sixteenths of an inch ( 3⁄16), the “DD” indicates the material is 2024 aluminum alloy. The letter “C” in place of the “DD” would

Close tolerance NAS bolts are marked with either a raised or recessed triangle. The material markings for NAS bolts are the same as for AN bolts, except that they may be either raised or recessed. Bolts inspected magnetically (Magnaflux) or by fluorescent means

5-42

(Zyglo) are identified by means of colored lacquer, or a head marking of a distinctive type. Special-Purpose Bolts Bolts designed for a particular application or use are classified as special-purpose bolts. Clevis bolts, eyebolts, Jo-bolts, and lockbolts are special-purpose bolts. Clevis Bolts

The head of a clevis bolt is round and is either slotted to receive a common screwdriver or recessed to receive a crosspoint screwdriver. This type of bolt is used only where shear loads occur and never in tension. It is often inserted as a mechanical pin in a control system. Eyebolt

This type of special purpose bolt is used where external tension loads are to be applied. The eyebolt is designed for the attachment of such devices as the fork of a turnbuckle, a clevis, or a cable shackle. The threaded end may or may not be drilled for safetying. Jo-Bolt

Jo-bolt is a trade name for an internally threaded threepiece rivet. The Jo-bolt consists of three parts — a threaded steel alloy bolt, a threaded steel nut, and an expandable stainless steel sleeve. The parts are factory preassembled. As the Jo-bolt is installed, the bolt is turned while the nut is held. This causes the sleeve to expand over the end of the nut, forming the blind head and clamping against the work. When driving is complete, a portion of the bolt breaks off. The high shear and tensile strength of the Jo-bolt makes it suitable for use in cases of high stresses where some of the other blind fasteners would not be practical. Jobolts are often a part of the permanent structure of late model aircraft. They are used in areas which are not often subjected to replacement or servicing. (Because it is a three-part fastener, it should not be used where any part, in becoming loose, could be drawn into the engine air intake.) Other advantages of using Jo-bolts are their excellent resistance to vibration, weight saving, and fast installation by one person. Presently, Jo-bolts are available in four diameters: The 200 series, approximately 3⁄16 inch in diameter; the 260 series, approximately 1⁄4 inch in diameter; the 312 series, approximately 5⁄16 inch in diameter; and the 375 series, approximately 3⁄8 inch in diameter. Jo-bolts are available in three head styles which are: F (flush), P (hex head), and FA (flush millable).

Lockbolts

Lockbolts are used to attach two materials permanently. They are lightweight and are equal in strength to standard bolts. Lockbolts are manufactured by several companies and conform to Military Standards. Military Standards specify the size of a lockbolt’s head in relation to the shank diameter, plus the alloy used in its construction. The only drawback to lockbolt installations is that they are not easily removable compared to nuts and bolts. The lockbolt combines the features of a high-strength bolt and rivet, but it has advantages over both. The lockbolt is generally used in wing splice fittings, landing gear fittings, fuel cell fittings, longerons, beams, skin splice plates, and other major structural attachments. It is more easily and quickly installed than the conventional rivet or bolt and eliminates the use of lockwashers, cotter pins, and special nuts. Like the rivet, the lockbolt requires a pneumatic hammer or “pull gun” for installation; when installed, it is rigidly and permanently locked in place. Three types of lockbolts are commonly used: the pull type, the stump type, and the blind type. [Figure 5-20] Pull type. Pull-type lockbolts are used mainly in aircraft primary and secondary structures. They are installed

Pull type

Stump type

Blind type

Figure 5-20. Lockbolt types.

5-43

very rapidly and have approximately one-half the weight of equivalent AN steel bolts and nuts. A special pneumatic “pull gun” is required to install this type of lockbolt. Installation can be accomplished by one person since bucking is not required. Stump type. Stump-type lockbolts, although they do not have the extended stem with pull grooves, are companion fasteners to pull-type lockbolts. They are used primarily where clearance will not permit installation of the pull-type lockbolt. A standard pneumatic riveting hammer (with a hammer set attached for swaging the collar into the pin locking grooves) and a bucking bar are tools necessary for the installation of stump-type lockbolts. Blind type. Blind-type lockbolts come as complete units or assemblies. They have exceptional strength and sheet pull-together characteristics. Blind lockbolts are used where only one side of the work is accessible and, generally, where it is difficult to drive a conventional rivet. This type of lockbolt is installed in the same manner as the pull-type lockbolt. Common features. Common features of the three types of lockbolts are the annular locking grooves on the pin and the locking collar which is swaged into the pin’s lock grooves to lock the pin in tension. The pins of the pull- and blind-type lockbolts are extended for pull installation. The extension is provided with pulling grooves and a tension breakoff groove. Composition. The pins of pull- and stump-type lockbolts are made of heat-treated alloy steel or high strength aluminum alloy. Companion collars are made of aluminum alloy or mild steel. The blind lockbolt consists of a heat-treated alloy steel pin, blind sleeve and filler sleeve, mild steel collar, and carbon steel washer. Substitution. Alloy steel lockbolts may be used to replace steel high-shear rivets, solid steel rivets, or AN bolts of the same diameter and head type. Aluminum alloy lockbolts may be used to replace solid aluminum alloy rivets of the same diameter and head type. Steel and aluminum alloy lockbolts may also be used to replace steel and 2024T aluminum alloy bolts, respectively, of the same diameter. Blind lockbolts may be used to replace solid aluminum alloy rivets, stainless steel rivets, or all blind rivets of the same diameter. Numbering system. The numbering systems for the various types of lockbolts are explained by the breakouts in Figure 5-23.

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Grip Range

Grip No.

Min.

Max.

1

.031

.094

2

.094

3

Grip Range

Grip No.

Min.

Max.

17

1.031

1.094

.156

18

1.094

1.156

.156

.219

19

1.156

1.219

4

.219

.281

20

1.219

1.281

5

.281

.344

21

1.281

1.344

6

.344

.406

22

1.344

1.406

7

.406

.469

23

1.406

1.469

8

.469

.531

24

1.469

1.531

9

.531

.594

25

1.531

1.594

10

.594

.656

26

1.594

1.656

11

.656

.718

27

1.656

1.718

12

.718

.781

28

1.718

1.781

13

.781

.843

29

1.781

1.843

14

.843

.906

30

1.843

1.906

15

.906

.968

31

1.906

1.968

16

.968

1.031

32

1.968

2.031

33

2.031

2.094

Figure 5-21. Pull- and stump-type lockbolt grip ranges.

Grip Range. To determine the bolt grip range required for any application, measure the thickness of the material with a hook scale inserted through the hole. Once this measurement is determined, select the correct grip range by referring to the charts provided by the rivet manufacturer. Examples of grip range charts are shown in Figures 5-21 and 5-24. When installed, the lockbolt collar should be swaged substantially throughout the complete length of the collar. The tolerance of the broken end of the pin relative to the top of the collar must be within the dimensions given in Figure 5-22. Pin diameter

Tolerance Below Above

3⁄ 16

0.079

to

0.032

1⁄ 4

0.079

to

0.050

5⁄ 16

0.079

to

0.050

3⁄ 8

0.079

to

0.060

Figure 5-22. Pin tolerance ranges.

Pull-type lockbolt

Lockbolt collar

ALPP H T 8 8

LC C C

ALPP

LC

Head type ACT509 = close tolerance AN-509 C-sink head ALPP = pan head ALPB = brazier head ALP509 = standard AN-509 C-sink head ALP426 = standard AN-426 C-sink head

H

Class fit H = hole filling (interference fit) N = non-hole filling (clearance fit)

T

Pin Materials E = 75S-T6 aluminum alloy T = heat-treated alloy steel

8

Body diameter in 32nds of an inch

8

Grip length in 16ths of an inch

Lockbolt collar

C

Material C = 24ST aluminum alloy (green color). Use with heat-treated alloy lockbolts only. F = 61ST aluminum alloy (plain color). Use with 75ST aluminum alloy lockbolts only. R = mild steel (cadmium plated). Use with heat-treated alloy steel lockbolts for high temperature applications only.

C

Diameter of a pin in 32nds of an inch

Stump-type lockbolt ALSF E 8 8 ALSF

Blind-type lockbolt BL 8 4 BL

Blind Lockbolt

8

Diameter in 32nds of an inch

4

Grip length in 16ths of an inch, ± 1⁄ 32 inch

Head type ASCT509 = close tolerance AN-509 C-sink head ALSF = flathead type. ALS509 = standard AN-509 C-sink head ALS426 = standard AN-426 C-sink head

E

Pin materials E = 75S-T6 aluminum alloy T = heat-treated alloy steel

8

Body diameter in 32nds of an inch

8

Grip length in 16ths of an inch

Figure 5-23. Lockbolt numbering system.

When removal of a lockbolt becomes necessary, remove the collar by splitting it axially with a sharp, cold chisel. Be careful not to break out or deform the hole. The use of a backup bar on the opposite side of the collar being split is recommended. The pin may then be driven out with a drift punch. Aircraft Nuts Aircraft nuts are made in a variety of shapes and sizes. They are made of cadmium plated carbon steel, stainless steel, or anodized 2024T aluminum alloy, and may be obtained with either right- or left-hand threads. No identifying marking or lettering appears on nuts. They can be identified only by the characteristic metallic luster or color of the aluminum, brass, or the insert when the nut is of the self-locking type. They can be further identified by their construction. Aircraft nuts can be divided into two general groups: non-self-locking and self-locking nuts. Non-self-locking nuts are those that must be safetied by external locking devices, such as cotter pins, safety wire, or locknuts. Self-locking nuts contain the locking feature as an integral part. Non-Self-Locking Nuts

Most of the familiar types of nuts, including the plain nut, the castle nut, the castellated shear nut, the plain

1⁄ -inch Diameter 4

Grip No. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25

Grip Range Min. .031 .094 .156 .219 .281 .344 .406 .469 .531 .594 .656 .718 .781 .843 .906 .968 1.031 1.094 1.156 1.219 1.281 1.344 1.406 1.469 1.531

Max. .094 .156 .219 .281 .344 .406 .469 .531 .594 .656 .718 .781 .843 .906 .968 1.031 1.094 1.156 1.219 1.281 1.343 1.406 1.469 1.531 1.594

5⁄ -inch Diameter 16

Grip No. 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

Grip Range Min. .094 .156 .219 .281 .344 .406 .469 .531 .594 .656 .718 .781 .843 .906 .968 1.031 1.094 1.156 1.219 1.281 1.343 1.406 1.460

Max. .156 .219 .281 .344 .406 .469 .531 .594 .656 .718 .781 .843 .906 .968 1.031 1.094 1.156 1.219 1.281 1.343 1.406 1.469 1.531

Figure 5-24. Blind-type lockbolt grip ranges.

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AN310

AN340

AN320

AN345

AN315

AN316

AN335

AN350

Figure 5-25. Non-self-locking nuts.

hex nut, the light hex nut, and the plain checknut are the non-self-locking type. [Figure 5-25] The castle nut, AN310, is used with drilled shank AN hex head bolts, clevis bolts, eyebolts, drilled head bolts, or studs. It is fairly rugged and can withstand large tensional loads. Slots (called castellations) in the nut are designed to accommodate a cotter pin or lockwire for safety. The castellated shear nut, AN320, is designed for use with devices (such as drilled clevis bolts and threaded taper pins) which are normally subjected to shearing stress only. Like the castle nut, it is castellated for safetying. Note, however, that the nut is not as deep or as strong as the castle nut; also that the castellations are not as deep as those in the castle nut. The plain hex nut, AN315 and AN335 (fine and coarse thread), is of rugged construction. This makes it suitable for carrying large tensional loads. However, since it requires an auxiliary locking device, such as a checknut or lockwasher, its use on aircraft structures is somewhat limited. The light hex nut, AN340 and AN345 (fine and coarse thread), is a much lighter nut than the plain hex nut and must be locked by an auxiliary device. It is used for miscellaneous light tension requirements.

5-46

The plain checknut, AN316, is employed as a locking device for plain nuts, set screws, threaded rod ends, and other devices. The wing nut, AN350, is intended for use where the desired tightness can be obtained with the fingers and where the assembly is frequently removed. Self-Locking Nuts

As their name implies, self-locking nuts need no auxiliary means of safetying but have a safetying feature included as an integral part of their construction. Many types of self-locking nuts have been designed and their use has become quite widespread. Common applications are: (1) attachment of antifriction bearings and control pulleys; (2) attachment of accessories, anchor nuts around inspection holes and small tank installation openings; and (3) attachment of rocker box covers and exhaust stacks. Self-locking nuts are acceptable for use on certificated aircraft subject to the restrictions of the manufacturer. Self-locking nuts are used on aircraft to provide tight connections which will not shake loose under severe vibration. Do not use self-locking nuts at joints which subject either the nut or bolt to rotation. They may be used with antifriction bearings and control pulleys, provided the inner race of the bearing is clamped to the supporting structure by the nut and bolt. Plates

must be attached to the structure in a positive manner to eliminate rotation or misalignment when tightening the bolts or screws. The two general types of self-locking nuts currently in use are the all-metal type and the fiber lock type. For the sake of simplicity, only three typical kinds of self-locking nuts are considered in this handbook: the Boots self-locking and the stainless steel self-locking nuts, representing the all-metal types; and the elastic stop nut, representing the fiber insert type. Boots Self-Locking Nut The Boots self-locking nut is of one piece, all-metal construction, designed to hold tight in spite of severe vibration. Note in Figure 5-26 that it has two sections and is essentially two nuts in one, a locking nut and a load-carrying nut. The two sections are connected with a spring which is an integral part of the nut. The spring keeps the locking and load-carrying sections such a distance apart that the two sets of threads are out of phase; that is, so spaced that a bolt which has been screwed through the load carrying section must push the locking section outward against the force of the spring to engage the threads of the locking section properly. Thus, the spring, through the medium of the locking section, exerts a constant locking force on the bolt in the same direction as a force that would tighten the nut. In this nut, the load-carrying section has the thread strength of a standard nut of comparable size, while the locking section presses against the threads of the bolt and locks the nut firmly in position. Only a wrench applied to the nut will loosen it. The nut can be removed and reused without impairing its efficiency. Boots self-locking nuts are made with three different spring styles and in various shapes and sizes. The wing type, which is the most common, ranges in size for No. 6 up to 1⁄4 inch, the Rol-top ranges from 1⁄4 inch to 1⁄6 inch, and the bellows type ranges in size from No. 8 up to 3⁄8 inch. Wing-type nuts are made of anodized aluminum alloy, cadmium-plated carbon steel, or stainless steel. The Rol-top nut is cadmium-plated steel, and the bellows type is made of aluminum alloy only. Stainless Steel Self-Locking Nut The stainless steel self-locking nut may be spun on and off with the fingers, as its locking action takes place only when the nut is seated against a solid surface and tightened. The nut consists of two parts: a case with a beveled locking shoulder and key, and a threaded insert with a locking shoulder and slotted keyway. Until the

Boots aircraft nut

Flexloc nut

Elastic anchor nut

Fiber lock nut

Elastic stop nut

Figure 5-26. Self-locking nuts.

nut is tightened, it spins on the bolt easily because the threaded insert is the proper size for the bolt. However, when the nut is seated against a solid surface and tightened, the locking shoulder of the insert is pulled downward and wedged against the locking shoulder of the case. This action compresses the threaded insert and causes it to clench the bolt tightly. The cross-sectional view in Figure 5-27 shows how the key of the case fits into the slotted keyway of the insert so that when the case is turned, the threaded insert is turned with it. Note that the slot is wider than the key. This permits the slot to be narrowed and the insert to be compressed when the nut is tightened. Elastic Stop Nut The elastic stop nut is a standard nut with the height increased to accommodate a fiber locking collar. This

Nut case

Locking shoulder Untightened nut

Threaded nut core

Keyway Tightened nut

Figure 5-27. Stainless steel self-locking nut.

5-47

fiber collar is very tough and durable and is unaffected by immersion in hot or cold water or ordinary solvents, such as ether, carbon tetrachloride, oils, and gasoline. It will not damage bolt threads or plating. As shown in Figure 5-28, the fiber locking collar is not threaded and its inside diameter is smaller than the largest diameter of the threaded portion or the outside diameter of a corresponding bolt. When the nut is screwed onto a bolt, it acts as an ordinary nut until the bolt reaches the fiber collar. When the bolt is screwed into the fiber collar, however, friction (or drag) causes the fiber to be pushed upward. This creates a heavy downward pressure on the load carrying part and automatically throws the load carrying sides of the nut and bolt threads into positive contact. After the bolt has been forced all the way through the fiber collar, the downward pressure remains constant. This pressure locks and holds the nut securely in place even under severe vibration. Nearly all elastic stop nuts are steel or aluminum alloy. However, such nuts are available in practically any kind of metal. Aluminum alloy elastic stop nuts are supplied with an anodized finish. Steel nuts are cadmium plated.

Fiber collar

Nut

Normally, elastic stop nuts can be used many times with complete safety and without detriment to their locking efficiency. When reusing elastic stop nuts, be sure the fiber has not lost its locking friction or become brittle. If a nut can be turned with the fingers, replace it. After the nut has been tightened, make sure the rounded or chamfered end of the bolts, studs, or screws extends at least the full round or chamfer through the nut. Flat end bolts, studs, or screws should extend at least 1⁄32 inch through the nut. Bolts of 5⁄16 -inch diameter and over with cotter pin holes may be used with self-locking nuts, but only if free from burrs around the holes. Bolts with damaged threads and rough ends are not acceptable. Do not tap the fiber locking insert. The self-locking action of the elastic stop nut is the result of having the bolt threads impress themselves into the untapped fiber. Do not install elastic stop nuts in places where the temperature is higher than 250 °F, because the effectiveness of the self-locking action is reduced beyond this point. Self-locking nuts may be used on aircraft engines and accessories when their use is specified by the engine manufacturer. Self-locking nut bases are made in a number of forms and materials for riveting and welding to aircraft structure or parts. [Figure 5-29] Certain applications require the installation of self-locking nuts in channels, an arrangement which permits the attachment of many nuts with only a few rivets. These channels are track-like bases with regularly spaced nuts which are either removable or nonremovable. The removable type carries a floating nut, which can be snapped in or out of the channel, thus making possible the easy removal of damaged nuts. Nuts, such as the clinch-type and splinetype, which depend on friction for their anchorage, are not acceptable for use in aircraft structures. Sheet Spring Nuts

Sheet spring nuts, such as speed nuts, are used with standard and sheet metal self-tapping screws in nonstructural locations. They find various uses in supporting line clamps, conduit clamps, electrical equipment, access doors, and the like, and are available in several types. Speed nuts are made from spring steel and are arched prior to tightening. This arched spring lock prevents the screw from working loose. These nuts should be used only where originally used in the fabrication of the aircraft. Figure 5-28. Elastic stop nut.

5-48

Boots aircraft channel assembly

Elastic stopnut channel assembly

Figure 5-29. Self-locking nut bases.

Internal and External Wrenching Nuts

Two commercial types of high strength internal or external wrenching nuts are available; they are the internal and external wrenching elastic stop nut and the Unbrako internal and external wrenching nut. Both are of the self-locking type, are heat treated, and are capable of carrying high strength bolt tension loads. Identification and Coding

Part numbers designate the type of nut. The common types and their respective part numbers are: Plain, AN315 and AN335; castle AN310; plain check, AN316; light hex, AN340 and AN345; and castellated shear, AN320. The patented self-locking types are assigned part numbers ranging from MS20363 through MS20367. The Boots, the Flexloc, the fiber locknut, the elastic stop nut, and the self-locking nut belong to this group. Part number AN350 is assigned to the wing nut. Letters and digits following the part number indicate such items as material, size, threads per inch, and whether the thread is right or left hand. The letter “B” following the part number indicates the nut material to be brass, a “D” indicates 2017-T aluminum alloy, a “DD” indicates 2024-T aluminum alloy, a “C” indicates stainless steel, and a dash in place of a letter indicates cadmium-plated carbon steel. The digit (or two digits) following the dash or the material code letter is the dash number of the nut, and it indicates the size of the shank and threads per inch

of the bolt on which the nut will fit. The dash number corresponds to the first figure appearing in the part number coding of general purpose bolts. A dash and the number 3, for example, indicates that the nut will fit an AN3 bolt (10-32); a dash and the number 4 means it will fit an AN4 bolt (1/4-28); a dash and the number 5, an AN5 bolt (5/16-24); and so on. The code numbers for self-locking nuts end in three or four digit numbers. The last two digits refer to threads per inch, and the one or two preceding digits stand for the nut size in 16ths of an inch. Some other common nuts and their code numbers are: Code Number AN310D5R: AN310 = aircraft castle nut D = 2024-T aluminum alloy 5 = 5⁄16 inch diameter R = right-hand thread (usually 24 threads per inch) Code Number AN320-10: AN320 = aircraft castellated shear nut, cadmium plated carbon steel 10 = 5⁄8 inch diameter, 18 threads per inch (this nut is usually right-hand thread) Code Number AN350B1032: AN350 = aircraft wing nut B = brass 10 = number 10 bolt 32 = threads per inch

5-49

Aircraft Washers Aircraft washers used in airframe repair are either plain, lock, or special type washers.

structures under both the head and the nut of a bolt to prevent crushing the surface.

Plain Washers

Lockwashers, both the AN935 and AN936, are used with machine screws or bolts where the self-locking or castellated-type nut is not appropriate. The spring action of the washer (AN935) provides enough friction to prevent loosening of the nut from vibration. [Figure 5-30]

Plain washers [Figure 5-30], both the AN960 and AN970, are used under hex nuts. They provide a smooth bearing surface and act as a shim in obtaining correct grip length for a bolt and nut assembly. They are used to adjust the position of castellated nuts in respect to drilled cotter pin holes in bolts. Use plain washers under lockwashers to prevent damage to the surface material. Aluminum and aluminum alloy washers may be used under bolt heads or nuts on aluminum alloy or magnesium structures where corrosion caused by dissimilar metals is a factor. When used in this manner, any electric current flow will be between the washer and the steel bolt. However, it is common practice to use a cadmium plated steel washer under a nut bearing directly against a structure as this washer will resist the cutting action of a nut better than an aluminum alloy washer. The AN970 steel washer provides a greater bearing area than the AN960 washer and is used on wooden

Plain AN 960

Ball seat & socket AC9950 & AC955

Taper pin AN975

Special washers

Lockwashers

Lockwashers should never be used under the following conditions: • With fasteners to primary or secondary structures • With fasteners on any part of the aircraft where failure might result in damage or danger to the aircraft or personnel • Where failure would permit the opening of a joint to the airflow • Where the screw is subject to frequent removal • Where the washers are exposed to the airflow • Where the washers are subject to corrosive conditions • Where the washer is against soft material without a plain washer underneath to prevent gouging the surface Shakeproof Lockwashers Shakeproof lockwashers are round washers designed with tabs or lips that are bent upward across the sides of a hex nut or bolt to lock the nut in place. There are O-ring various methods of securing the lockwasher to prevent it from turning, such as an external tab bent downward 90° into a small hole in the face of the unit, or an internal tab which fits a keyed bolt. Shakeproof lockwashers can withstand higher heat than other methods of safetying and can be used under high vibration conditions safely. They should be used only once because the tabs tend to break when bent a second time. Special Washers

Plain AN 935

Star lock washers

Figure 5-30. Various types of washers.

5-50

The ball socket and seat washers, AC950 and AC955, are special washers used where a bolt is installed at an angle to a surface, or where perfect alignment with a surface is required. These washers are used together. [Figure 5-30] The NAS143 and MS20002 washers are used for internal wrenching bolts of the NAS144 through NAS158

series. This washer is either plain or countersunk. The countersunk washer (designated as NAS143C and MS20002C) is used to seat the bolt head shank radius, and the plain washer is used under the nut. Installation of Nuts, Washers, and Bolts Bolt and Hole Sizes

Slight clearances in bolt holes are permissible wherever bolts are used in tension and are not subject to reversal of load. A few of the applications in which clearance of holes may be permitted are in pulley brackets, conduit boxes, lining trim, and miscellaneous supports and brackets. Bolt holes are to be normal to the surface involved to provide full bearing surface for the bolt head and nut, and must not be oversized or elongated. A bolt in such a hole will carry none of its shear load until parts have yielded or deformed enough to allow the bearing surface of the oversized hole to contact the bolt. In this respect, remember that bolts do not become swaged to fill up the holes as do rivets. In cases of oversized or elongated holes in critical members, obtain advice from the aircraft or engine manufacturer before drilling or reaming the hole to take the next larger bolt. Usually, such factors as edge distance, clearance, or load factor must be considered. Oversized or elongated holes in noncritical members can usually be drilled or reamed to the next larger size. Many bolt holes, particularly those in primary connecting elements, have close tolerances. Generally, it is permissible to use the first lettered drill size larger than the normal bolt diameter, except where the AN hexagon bolts are used in light drive fit (reamed) applications and where NAS close tolerance bolts or AN clevis bolts are used. Light drive fits for bolts (specified on the repair drawings as 0.0015 inch maximum clearance between bolt and hole) are required in places where bolts are used in repair, or where they are placed in the original structure. The fit of holes and bolts cannot be defined in terms of shaft and hole diameters; it is defined in terms of the friction between bolt and hole when sliding the bolt into place. A tight drive fit, for example, is one in which a sharp blow of a 12- or 14-ounce hammer is required to move the bolt. A bolt that requires a hard blow and sounds tight is considered to fit too tightly. A light drive fit is one in which a bolt will move when

a hammer handle is held against its head and pressed by the weight of the body. Installation Practices

Examine the markings on the bolt head to determine that each bolt is of the correct material. It is of extreme importance to use like bolts in replacement. In every case, refer to the applicable Maintenance Instructions Manual and Illustrated Parts Breakdown. Be sure that washers are used under both the heads of bolts and nuts unless their omission is specified. A washer guards against mechanical damage to the material being bolted and prevents corrosion of the structural members. An aluminum alloy washer should be used under the head and nut of a steel bolt securing aluminum alloy or magnesium alloy members. Any corrosion that occurs then attacks the washer rather than the members. Steel washers should be used when joining steel members with steel bolts. Whenever possible, place the bolt with the head on top or in the forward position. This positioning tends to prevent the bolt from slipping out if the nut is accidentally lost. Be certain that the bolt grip length is correct. Grip length is the length of the unthreaded portion of the bolt shank. Generally speaking, the grip length should equal the thickness of the material being bolted together. However, bolts of slightly greater grip length may be used if washers are placed under the nut or the bolt head. In the case of plate nuts, add shims under the plate. Safetying of Bolts and Nuts It is very important that all bolts or nuts, except the self-locking type, be safetied after installation. This prevents them from loosening in flight due to vibration. Methods of safetying are discussed later in this chapter. Repair of Damaged Internal Threads

Installation or replacement of bolts is simple when compared to the installation or replacement of studs. Bolt heads and nuts are cut in the open, whereas studs are installed into internal threads in a casting or builtup assembly. Damaged threads on bolts or nuts can be seen and only require replacement of the defective part. If internal threads are damaged, two alternatives are apparent: the part may be replaced or the threads repaired or replaced. Correction of the thread problem is usually cheaper and more convenient. Two

5-51

methods of repairing are by replacement bushings or helicoils. Replacement Bushings Bushings are usually special material (steel or brass spark plug bushings into aluminum cylinder heads). A material that will resist wear is used where removal and replacement is frequent. The external threads on the bushing are usually coarse. The bushing is installed, a thread lock compound may or may not be used, and staked to prevent loosening. Many bushings have lefthand threads external and right-hand threads internal. With this installation, removal of the bolt or stud (righthand threads) tends to tighten the bushing. Bushings for common installations, such as spark plugs, may be up to 0.040 oversize (in increments of 0.005). Original installation and overhaul shop replacements are shrunk fit: a heated cylinder head and a frozen bushing. Helicoils Helicoils are precision formed screw thread coils of 18-8 stainless steel wire having a diamond shaped cross section. [Figure 5-31] They form unified coarse or unified fine thread classes 2-band 3B when assembled into (helicoil) threaded holes. The assembled insert accommodates UNJ (controlled radius root) male threaded members. Each insert has a driving tang with a notch to facilitate removal of the tang after the insert is screwed into a helicoil tapped hole. They are used as screw thread bushings. In addition to being used to restore damaged threads, they are used in the original design of missiles, aircraft engines, and all types of mechanical equipment and accessories to

Drill

Tap

Figure 5-31. Helicoil insert.

protect and strengthen tapped threads in light materials, metals, and plastics, particularly in locations which require frequent assembly and disassembly, and⁄or where a screw locking action is desired. Helicoil installation [Figure 5-32] is a 5 or 6 step operation, depending upon how the last step is classed. Step 1: Determine what threads are damaged. Step 2: (a) New installation of helicoil. Drill out damaged threads to minimum depth specified. (b) Previously installed helicoil. Using proper size extracting tool, place edge of blade in 90° from the edge of the insert. Tap with hammer to seat tool. Turn to left, applying pressure, until insert backs out. Threads are not damaged if insert is properly removed.

Gauge

Figure 5-32. Helicoil installation.

5-52

Install

Step 3: Tap. Use the tap of required nominal thread size. The tapping procedure is the same as standard thread tapping. Tap length must be equal to or exceed the requirement. Step 4: Gauge. Threads may be checked with a helicoil thread gauge.

Basic formula F × L = T F = Applied force L = Lever length between centerline of drive and centerline of applied force (F must be 90 degrees to L) T = Torque

Step 5: Insert assembly. Using proper tool, install insert to a depth that puts end of top coil 1⁄4 to 1⁄2 turn below the top surface of the tapped hole. Step 6: Tang breakoff. Select proper breakoff tool. Tangs should be removed from all drilled through holes. In blind holes, the tangs may be removed when necessary if enough hole depth is provided below the tang of the installed insert.

90° A B

F 90°

These are not to be considered specific instructions on helicoil installation. The manufacturer’s instruction must be followed when making an installation.

750 650 550 450 350 250 150

Helicoils are available for the following threads: unified coarse, unified fine, metric, spark plug, and national taper pipe threads.

L

Fastener Torque

Torque and Torque Wrenches As the speed of an aircraft increases, each structural member becomes more highly stressed. It is therefore extremely important that each member carry no more and no less than the load for which it was designed. To distribute the loads safely throughout a structure, it is necessary that proper torque be applied to all nuts, bolts, studs, and screws. Using the proper torque allows the structure to develop its designed strength and greatly reduces the possibility of failure due to fatigue.

Ratchet type

T

Flexible beam

Torque wrenches. The three most commonly used torque wrenches are the flexible beam, rigid frame, and the ratchet types. [Figure 5-33] When using the flexible beam and the rigid frame torque wrenches, the torque value is read visually on a dial or scale mounted on the handle of the wrench. To use the ratchet type, unlock the grip and adjust the handle to the desired setting on the micrometer type scale, then relock the grip. Install the required socket or adapter to the square drive of the handle. Place the wrench assembly on the nut or bolt and pull the wrench assembly on the nut or bolt and pull in a clockwise direction with a smooth, steady motion. (A fast or jerky motion will result in an improperly torqued unit.) When the applied torque reaches the torque value indicated on

Rigid frame Formula for use with extensions Tw =

Te × A B

A = Lever length of wrench B = Lever length of wrench plus extension Te = Required torque on bolt Tw = Torque reading on wrench dial

Figure 5-33. Common torque wrenches.

5-53

the handle setting, the handle will automatically release or “break” and move freely for a short distance. The release and free travel is easily felt, so there is no doubt about when the torquing process is completed. To assure getting the correct amount of torque on the fasteners, all torque wrenches must be tested at least once a month or more often if necessary. Note: It is not advisable to use a handle length extension on a flexible beam type torque wrench at any time. A handle extension alone has no effect on the reading of the other types. The use of a drive end extension on any type of torque wrench makes the use of the formula mandatory. When applying the formula, force must be applied to the handle of the torque wrench at the point from which the measurements were taken. If this is not done, the torque obtained will be incorrect. Torque Tables. Use the standard torque table as a guide in tightening nuts, studs, bolts, and screws whenever specific torque values are not called out in maintenance procedures. The following rules apply for correct use of the torque table: [Figure 5-34] 1. To obtain values in foot-pounds, divide inchpounds by 12. 2. Do not lubricate nuts or bolts except for corrosionresistant steel parts or where specifically instructed to do so. 3. Always tighten by rotating the nut first if possible. When space considerations make it necessary to tighten by rotating the bolt head, approach the high side of the indicated torque range. Do not exceed the maximum allowable torque value. 4. Maximum torque ranges should be used only when materials and surfaces being joined are of sufficient thickness, area, and strength to resist breaking, warping, or other damage. 5. For corrosion resisting steel nuts, use torque values given for shear type nuts. 6. The use of any type of drive end extension on a torque wrench changes the dial reading required to obtain the actual values indicated in the standard torque range tables. When using a drive end extension, the torque wrench reading must be computed by use of the proper formula, which is included in the handbook accompanying the torque wrench.

5-54

Cotter Pin Hole Line Up When tightening castellated nuts on bolts, the cotter pin holes may not line up with the slots in the nuts for the range of recommended values. Except in cases of highly stressed engine parts, the nut may not be over torque. Remove hardware and realign the holes. The torque loads specified may be used for all unlubricated cadmium-plated steel nuts of the fine or coarse thread series which have approximately equal number of threads and equal face bearing areas. These values do not apply where special torque requirements are specified in the maintenance manual. If the head end, rather than the nut, must be turned in the tightening operation, maximum torque values may be increased by an amount equal to shank friction, provided the latter is first measured by a torque wrench. Aircraft Rivets Sheets of metal must be fastened together to form the aircraft structure, and this is usually done with solid aluminum alloy rivets. A rivet is a metal pin with a formed head on one end when the rivet is manufactured. The shank of the rivet is inserted into a drilled hole, and its shank is then upset (deformed) by a hand or pneumatic tool. The second head, formed either by hand or by pneumatic equipment, is called a “shop head.” The shop head functions in the same manner as a nut on a bolt. In addition to their use for joining aircraft skin sections, rivets are also used for joining spar sections, for holding rib sections in place, for securing fittings to various parts of the aircraft, and for fastening innumerable bracing members and other parts together. The rivet creates a bond that is at least as strong as the material being joined. Two of the major types of rivets used in aircraft are the common solid shank type, which must be driven using a bucking bar, and the special (blind) rivets, which may be installed where it is impossible to use a bucking bar. Aircraft rivets are not hardware store rivets. Rivets purchased at a hardware store should never be used as a substitute for aircraft quality rivets. The rivets may be made from very different materials, the strength of the rivets differs greatly, and their shear strength qualities are very different. The countersunk heads on hardware store rivets are 78°, whereas countersunk aircraft rivets have 100° angle heads for more surface contact to hold it in place.

Torque Values in Inch-Pounds for Tightening Nuts

Bolt, Stud or Screw Size

On standard bolts, studs and screws having a tensile strength of 125,000 to 140,000 psi

On bolts, studs, and screws having a tensile strength of 140,000 to 160,000 psi

On high-strength bolts, studs, and screws having a tensile strength of 160,000 psi and over

Shear type nuts (AN320, AN364 or equivalent)

Tension type nuts and threaded machine parts (AN-310, AN365 or equivalent)

Any nut, except shear type

Any nut, except shear type

8–32

8–36

7–9

12–15

14–17

15–18

10–24

10–32

12–15

20–25

23–30

25–35

25–30

40–50

45–49

50–68

30–40

50–70

60–80

70–90

48–55

80–90

85–117

90–144

60–85

100–140

120–172

140–203

95–110

160–185

173–217

185–248

95–110

160–190

175–271

190–351

140–155

235–255

245–342

255–428

270–300

450–500

475–628

500–756

240–290

400–480

440–636

480–792

290–410

480–690

585–840

690–990

300–420

500–700

600–845

700–990

480–600

800–1000

900–1,220

1,000–1,440

420–540

700–900

800–1,125

900–1,350

660–780

1,100–1,300

1,200–1,730

1,300–2,160

700–950

1,150–1,600

1,380–1,925

1,600–2,250

1,300–1,500

2,300–2,500

2,400–3,500

2,500–4,500

1,300–1,800

2,200–3,000

2,600–3,570

3,000–4,140

1,500–1,800

2,500–3,000

2,750–4,650

3,000–6,300

2,200–3,000

3,700–5,000

4,350–5,920

5,000–6,840

2,200–3,300

3,700–5,500

4,600–7,250

5,500–9,000

3,300–4,000

5,500–6,500

6,000–8,650

6,500–10,800

3,000–4,200

5,000–7,000

6,000–10,250

7,000–13,500

4,000–5,000

6,500–8,000

7,250–11,000

8,000–14,000

5,400–6,600

9,000–11,000

10,000–16,750

11,000–22,500

1⁄ –20 4 1⁄ –28 4 5⁄ –18 16 5⁄ –24 16 3⁄ –16 8 3⁄ –24 8 7⁄ –14 16 7⁄ –20 16 1⁄ –13 2 1⁄ –20 2 9⁄ –12 16 9⁄ –18 16 5⁄

8 –11 5⁄

8 –18

3⁄ –10 4 3⁄ –16 4 7⁄

8 –9 7⁄

8 –14

1"–8 1"–14 11⁄

8 –8

11 ⁄8 –12 11⁄4 –8 11⁄4 –12

Figure 5-34. Standard torque table (inch-pounds).

5-55

Standards and Specifications The FAA requires that the structural strength and integrity of type-certificated aircraft conform to all airworthiness requirements. These requirements apply to performance, structural strength, and integrity as well flight characteristics. To meet these requirements, each aircraft must meet the same standards. To accomplish standardization, all materials and hardware must be manufactured to a standard of quality. Specifications and standards for aircraft hardware are usually identified by the organization that originated them. Some of the common standardizing organizations include: AMS Aeronautical Material Specifications AN

Air Force-Navy

AND

Air Force-Navy Design

AS

Aeronautical Standard

ASA

American Standards Association

ASTM American Society for Testing Materials MS

Military Standard

NAF

Naval Aircraft Factory

NAS

National Aerospace Standard

SAE

Society of Automotive Engineers

When a MS20426-AD4-6 rivet is required, the specifications have already been written for it in the Military Standard (MS) specifications. That information is available to the aircraft manufacturers and to the rivet manufacturers as well as to the mechanic. The specifications designate the material to be used as well as the head type, diameter, and length of the rivet. The use of standardized materials in the production of aircraft makes each aircraft exactly the same as the previous one and makes them less expensive to build. Aircraft rivets are manufactured to much higher standards and specifications than rivets manufactured for general use. When aircraft manufacturers started building all-metal aircraft in the 1930s, different manufacturers had different rivet head designs. Brazier heads, modified brazier heads, button heads, mushroom heads, flatheads, and 78° countersunk heads were used. As aircraft standardized, four rivet head designs almost completely replaced all of the others. Rivets exposed to the airflow over the top of the structure are usually either universal head MS20470 or 100° countersunk head MS20426 rivets. For rivets used in internal structures, the roundhead MS20430 and the flathead MS20442 are generally used.

5-56

Solid Shank Rivets Solid shank rivets are generally used in repair work. They are identified by the kind of material of which they are made, their head type, size of shank, and their temper condition. The designation of the solid shank rivet head type, such as universal head, roundhead, flathead, countersunk head, and brazier head, depends on the cross-sectional shape of the head. [Figure 5-37] The temper designation and strength are indicated by special markings on the head of the rivet. The material used for the majority of aircraft solid shank rivets is aluminum alloy. The strength and temper conditions of aluminum alloy rivets are identified by digits and letters similar to those adopted for the identification of strength and temper conditions of aluminum and aluminum alloy stock. The 1100, 2017‑T, 2024-T, 2117-T, and 5056 rivets are the five grades usually available. The 1100 rivet, which is composed of 99.45 percent pure aluminum, is very soft. It is for riveting the softer aluminum alloys, such as 1100, 3003, and 5052, which are used for nonstructural parts (all parts where strength is not a factor). The riveting of map cases is a good example of where a rivet of 1100 aluminum alloy may be used. The 2117-T rivet, known as the field rivet, is used more than any other for riveting aluminum alloy structures. The field rivet is in wide demand because it is ready for use as received and needs no further heat treating or annealing. It also has a high resistance to corrosion. The 2017-T and 2024-T rivets are used in aluminum alloy structures where more strength is needed than is obtainable with the same size 2217-T rivet. These rivets are known as “ice box rivets,” are annealed, and must be kept refrigerated until they are to be driven. The 2017-T rivet should be driven within approximately 1 hour and the 2024-T rivet within 10 to 20 minutes after removal from refrigeration. The 5056 rivet is used for riveting magnesium alloy structures because of its corrosion-resistant qualities in combination with magnesium. Mild steel rivets are used for riveting steel parts. The corrosion-resistant steel rivets are for riveting corrosion-resistant steels in firewalls, exhaust stack brackets, and similar structures. Monel rivets are used for riveting nickel-steel alloys. They can be substituted for those made of corrosionresistant steel in some cases.

The use of copper rivets in aircraft repair is limited. Copper rivets can be used only on copper alloys or nonmetallic materials such as leather. Metal temper is an important factor in the riveting process, especially with aluminum alloy rivets. Aluminum alloy rivets have the same heat-treating characteristics as aluminum alloy stock. They can be hardened and annealed in the same manner as aluminum. The rivet must be soft, or comparatively soft, before a good head can be formed. The 2017-T and 2024-T rivets are annealed before being driven. They harden with age. The process of heat treating (annealing) rivets is much the same as that for stock. Either an electric air furnace, a salt bath, or a hot oil bath is needed. The heat-treating range, depending on the alloy, is 625 °F to 950 °F. For convenient handling, rivets are heated in a tray or a wire basket. They are quenched in cold water (70 °F) immediately after heat treating. The 2017-T and 2024-T rivets, which are heat-treatable rivets, begin to age harden within a few minutes after being exposed to room temperature. Therefore, they must be used immediately after quenching or else be placed in cold storage. The most commonly used means for holding heat-treatable rivets at low temperature (below 32 °F) is to keep them in a refrigerator. They are referred to as “icebox” rivets. Under this storage condition, they will remain soft enough for driving for periods up to 2 weeks. Any rivets not used within that time should be removed for reheat treating. Icebox rivets attain about one-half their maximum strength in approximately 1 hour after driving and full strength in about 4 days. When 2017-T rivets are exposed to room temperature for 1 hour or longer, they must be subject to reheat treatment. This also applies to 2024-T rivets exposed to room temperature for a period exceeding 10 minutes. Once an icebox rivet has been taken from the refrigerator, it should not be mixed with the rivets still in cold storage. If more rivets are removed from the refrigerator than can be used in 15 minutes, they should be placed in a separate container and stored for reheat treatment. Heat treatment of rivets may be repeated a number of times if done properly. Proper heating times and temperatures are shown in Figure 5-35. Most metals, and therefore aircraft rivet stock, are subject to corrosion. Corrosion may be the result of local climatic conditions or the fabrication process used. It

Heating Time — Air Furnace Rivet Alloy

Time at Temperature

Heat Treating Temperature

2024

1 hour

910 °F–930 °F

2017

1 hour

925 °F–950 °F

Heating Time — Salt Bath Rivet Alloy

Time at Temperature

Heat Treating Temperature

2024

30 minutes

910 °F–930 °F

2017

30 minutes

925 °F–950 °F

Figure 5-35. Rivet heating times and temperatures.

is reduced to a minimum by using metals which are highly resistant to corrosion and possess the correct strength-to-weight ratio. Ferrous metals placed in contact with moist salt air will rust if not properly protected. Nonferrous metals, those without an iron base, do not rust, but a similar process known as corrosion takes place. The salt in moist air (found in the coastal areas) attacks the aluminum alloys. It is a common experience to inspect the rivets of an aircraft which has been operated near salt water and find them badly corroded. If a copper rivet is inserted into an aluminum alloy structure, two dissimilar metals are brought in contact with each other. Remember, all metals possess a small electrical potential. Dissimilar metals in contact with each other in the presence of moisture cause an electrical current to flow between them and chemical byproducts to be formed. Principally, this results in the deterioration of one of the metals. Certain aluminum alloys react to each other and, therefore, must be thought of as dissimilar metals. The commonly used aluminum alloys may be divided into the two groups shown in Figure 5-36. Group A

Group B

1100

2117

3003

2017

5052

2124

6053

7075

Figure 5-36. Aluminum groupings.

5-57

5-58 D DD B

Raised Dot

Raised Double Dash

Raised Cross

2017T-HD

2024T

5056T

Recessed Large and Small Dot

Plain

* New specifications are for design purposes.

Titanium

Brass

Recessed Double Dots

Plain

Monel

Monel (Nickel-Copper Alloy)

Plain

Recessed Dash

Corrosion Resistant Steel

Copper

Recessed Triangle

Carbon Steel

C

M

C

F

D

Raised Dot

2017T

Three Raised Dashes

AD

Recessed Dot

2117T

7075-T73

A

Plain

Head Marking

1100

Material

AN Material Code

X

X

X

X

X

X

AN425 78° Countersunk Head

MS20426

X

X

X

X

X

X

X

AN426 100° Countersunk Head MS20426*

X

X

X

X

X

X

X

AN430 Round Head MS20470*

X

X

X

X

X

X

X

AN456 Brazier Head MS20470*

X

X

X

X

X

X

X

AN470 Universal Head MS20470*

No

No

No

No

No

No

Yes

No

Yes

No

No

Heat Treat Before Use

No

No

X

X

X

X

X

X

X

AN455 Brazier Head MS20470*

X MS20615*

X

X

X

X

X

X

X

AN442 Flat Head MS20470*

No

X

X

X

AN441 Flat Head

X MS20615*

X

X MS20613*

X MS20613*

AN435 Round Head MS20613* MS20615*

Figure 5-37. Rivet identification chart.

X

X

X

X

AN427 100° Countersunk Head MS20427*

95,000

49,000

49,000

23,000

65,000

35,000

27,000

41,000

38,000

34,000

30,000

10,000

Shear Strength psi

90,000

90,000

90,000

136,000

126,000

113,000

100,000

25,000

Bearing Strength psi

Members within either group A or group B can be considered as similar to each other and will not react to others within the same group. A corroding action will take place, however, if any metal of group A comes in contact with a metal in group B in the presence of moisture. Avoid the use of dissimilar metals whenever possible. Their incompatibility is a factor which was considered when the AN Standards were adopted. To comply with AN Standards, the manufacturers must put a protective surface coating on the rivets. This may be zinc chromate, metal spray, or an anodized finish. The protective coating on a rivet is identified by its color. A rivet coated with zinc chromate is yellow, an anodized surface is pearl gray, and the metal sprayed rivet is identified by a silvery gray color. If a situation arises in which a protective coating must be applied on the job, paint the rivet with zinc chromate before it is used and again after it is driven. Identification

Markings on the heads of rivets are used to classify their characteristics. These markings may be either a raised teat, two raised teats, a dimple, a pair of raised dashes, a raised cross, a single triangle, or a raised dash; some other heads have no markings. The different markings indicate the composition of the rivet stock. As explained previously, the rivets have different colors to identify the manufacturers’ protective surface coating. Roundhead rivets are used in the interior of the aircraft, except where clearance is required for adjacent members. The roundhead rivet has a deep, rounded top surface. The head is large enough to strengthen the sheet around the hole and, at the same time, offer resistance to tension. The flathead rivet, like the roundhead rivet, is used on interior structures. It is used where maximum strength is needed and where there isn’t sufficient clearance to use a roundhead rivet. It is seldom, if ever, used on external surfaces. The brazier head rivet has a head of large diameter, which makes it particularly adaptable for riveting thin sheet stock (skin). The brazier head rivet offers only slight resistance to the airflow, and because of this factor, it is frequently used for riveting skin on exterior surfaces, especially on aft sections of the fuselage and empennage. It is used for riveting thin sheets exposed to the slipstream. A modified brazier

head rivet is also manufactured; it is simply a brazier head of reduced diameter. The universal head rivet is a combination of the roundhead, flathead, and brazier head. It is used in aircraft construction and repair in both interior and exterior locations. When replacement is necessary for protruding head rivets — roundhead, flathead, or brazier head — they can be replaced by universal head rivets. The countersunk head rivet is flat topped and beveled toward the shank so that it fits into a countersunk or dimpled hole and is flush with the material’s surface. The angle at which the head slopes may vary from 78° to 120°. The 100° rivet is the most commonly used type. These rivets are used to fasten sheets over which other sheets must fit. They are also used on exterior surfaces of the aircraft because they offer only slight resistance to the slipstream and help to minimize turbulent airflow. The markings on the heads of rivets indicate the material of which they are made and, therefore, their strength. Figure 5-37 identifies the rivet head markings and the materials indicated by them. Although there are three materials indicated by a plain head, it is possible to distinguish their difference by color. The 1100 is aluminum color; the mild steel is a typical steel color; and the copper rivet is a copper color. Any head marking can appear on any head style of the same material. Each type of rivet is identified by a part number so that the user can select the correct rivet for the job. The type of rivet head is identified by AN or MS standard numbers. The numbers selected are in series and each series represents a particular type of head. [Figure 5-37] The most common numbers and the types of heads they represent are: AN426 or MS20426 — countersunk head rivets (100°) AN430 or MS20430 — roundhead rivets AN441 — flathead rivets AN456 — brazier head rivets AN470 or MS20470 — universal head rivets There are also letters and numbers added to a part number. The letters designate alloy content; the num-

5-59

bers designate rivet diameter and length. The letters in common use for alloy designation are: A — aluminum alloy, 1100 or 3003 composition AD — aluminum alloy, 2117-T composition D — aluminum alloy, 2017-T composition DD — aluminum alloy, 2024-T composition B — aluminum alloy, 5056 composition C — copper M — monel The absence of a letter following the AN standard number indicates a rivet manufactured from mild steel. The first number following the material composition letters expresses the diameter of the rivet shank in 32nds of an inch (Examples: 3 indicates 3⁄32 , 5 indicates 5⁄ , and so forth). [Figure 5-38] 32 The last number(s), separated by a dash from the preceding number, expresses the length of the rivet shank in 16ths of an inch (Examples: 3 indicates 3⁄16 , 7 indicates 7⁄16 , 11 indicates 11⁄16 , and so forth). [Figure 5-38] An example of identification marking of a rivet is: AN470AD3-5 — complete part number AN — Air Force-Navy standard number 470 — universal head rivet AD — 2117-T aluminum alloy 3 — 3⁄32 in diameter 5 — 5⁄16 in length

Countersunk angle

Diameter of shank

Figure 5-38. Methods of measuring rivets.

5-60

For use in such places, special rivets have been designed which can be bucked from the front. They are sometimes lighter than solid shank rivets, yet amply strong for their intended use. These rivets are produced by several manufacturers and have unique characteristics that require special installation tools, special installation procedures and special removal procedures. That is why they are called special rivets. Because these rivets are often inserted in locations where one head (usually the shop head) cannot be seen, they are also called blind rivets. Mechanically Expanded Rivets

Two classes of mechanically expanded rivets are discussed here: (1) Nonstructural. (a) Self-plugging (friction lock) rivets. (b) Pull-thru rivets. (2) Mechanical lock, flush fracturing, self plugging rivets. Self-Plugging Rivets (friction lock) The self-plugging (friction lock) blind rivets are manufactured by several companies; the same general basic information about their fabrication, composition, uses, selection, installation, inspection, and removal procedures apply to all of them. Self-plugging (friction lock) rivets are fabricated in two parts: a rivet head with a hollow shank or sleeve, and a stem that extends through the hollow shank. Figure 5-39 illustrates a protruding head and a countersunk head self-plugging rivet produced by one manufacturer.

Length of rivet

Diameter of shank

Blind Rivets There are many places on an aircraft where access to both sides of a riveted structure or structural part is impossible, or where limited space will not permit the use of a bucking bar. Also, in the attachment of many nonstructural parts, such as aircraft interior furnishings, flooring, deicing boots, and the like, the full strength of solid shank rivets is not necessary.

Several events, in their proper sequence, occur when a pulling force is applied to the stem of the rivet: (1) the stem is pulled into the rivet shank; (2) the mandrel portion of the stem forces the rivet shank to expand; and (3) when friction (or pulling action pressure) becomes great enough, it will cause the stem to snap at a breakoff groove on the stem. The plug portion (bottom end of the stem) is retained in the shank of the rivet giving

necessary to install the rivet, it can be used to attach assemblies to plywood or plastics. Factors to consider in the selection of the correct rivet for installation are: (1) installation location, (2) composition of the material being riveted, (3) thickness of the material being riveted, and (4) strength desired. If the rivet is to be installed on an aerodynamically smooth surface, or if clearance for an assembly is needed, countersunk head rivets should be selected. In other areas where clearance or smoothness is not a factor, the protruding head type rivet may be utilized.

Protruding head

Countersunk head

Figure 5-39. Self-plugging (friction lock) rivets.

the rivet much greater shear strength than could be obtained from a hollow rivet.

Material composition of the rivet shank depends upon the type of material being riveted. Aluminum alloy 2117 shank rivets can be used on most aluminum alloys. Aluminum alloy 5056 shank rivets should be used when the material being riveted is magnesium. Steel rivets should always be selected for riveting assemblies fabricated from steel. The thickness of the material being riveted determines the overall length of the shank of the rivet. As a general rule, the shank of the rivet should extend beyond the material thickness approximately 3⁄64 inch to 1⁄8 inch before the stem is pulled. [Figure 5-40]

Self-plugging (friction lock) rivets are fabricated in two common head styles: (1) a protruding head similar to the MS20470 or universal head, and (2) a 100° countersunk head. Other head styles are available from some manufacturers.

Pull-Thru Rivets The pull-thru blind rivets are manufactured by several companies; the same general basic information about their fabrication, composition, uses, selection, installation, inspection, and removal procedures apply to all of them.

The stem of the self-plugging (friction lock) rivet may have a knot or knob on the upper portion, or it may have a serrated portion. [Figure 5-39]

Pull-thru rivets are fabricated in two parts: a rivet head with a hollow shank or sleeve and a stem that extends

Self-plugging (friction lock) rivets are fabricated from several materials. Rivets are available in the following material combinations: stem 2017 aluminum alloy and sleeve 2117 aluminum alloy; stem 2017 aluminum alloy and sleeve 5056 aluminum alloy; and stem steel and sleeve steel. Self-plugging (friction lock) rivets are designed so that installation requires only one person; it is not necessary to have the work accessible from both sides. The pulling strength of the rivet stem is such that a uniform job can always be assured. Because it is not necessary to have access to the opposite side of the work, selfplugging (friction lock) rivets can be used to attach assemblies to hollow tubes, corrugated sheet, hollow boxes, and so forth. Because a hammering force is not

A C B

A = Thickness of material (grip range) B = 3⁄64 – 1⁄8 inch C = Total rivet shank length

Figure 5-40. Determining length of friction lock rivets.

5-61

A C B

Countersunk head

Protruding head

A = Thickness of material (grip range) B = 3⁄64 – 1⁄8 inch C = Total rivet shank length

Figure 5-41. Pull-thru rivets.

Figure 5-42. Determining length of pull-thru rivets.

through the hollow shank. Figure 5-41 illustrates a protruding head and a countersunk head pull-thru rivet.

material thickness approximately 3⁄64 inch to 1⁄8 inch before the stem is pulled. [Figure 5-42]

Several events, in their proper sequence, occur when a pulling force is applied to the stem of the rivet: (1) The stem is pulled through the rivet shank; (2) the mandrel portion of the stem forces the shank to expand forming the blind head and filling the hole.

Each company that manufactures pull-thru rivets has a code number to help users obtain correct rivet for the grip range of a particular installation. In addition, MS numbers are used for identification purposes. Numbers are similar to those shown on the preceding pages.

Pull-thru rivets are fabricated in two common head styles: (1) protruding head similar to the MS20470 or universal head, and (2) a 100° countersunk head. Other head styles are available from some manufacturers.

Self-Plugging Rivets (Mechanical Lock) Self-plugging (mechanical lock) rivets are similar to self-plugging (friction lock) rivets, except for the manner in which the stem is retained in the rivet sleeve. This type of rivet has a positive mechanical locking collar to resist vibrations that cause the friction lock rivets to loosen and possibly fall out. [Figure 5-45] Also, the mechanical locking type rivet stem breaks off flush with the head and usually does not require further stem trimming when properly installed. Self-plugging (mechanical lock) rivets display all the strength characteristics of solid shank rivets and in most cases can be substituted rivet for rivet.

Pull-thru rivets are fabricated from several materials. Following are the most commonly used: 2117-T4 aluminum alloy, 5056 aluminum alloy, Monel. Pull-thru rivets are designed so that installation requires only one person; it is not necessary to have the work accessible from both sides. Factors to consider in the selection of the correct rivet for installation are: (1) installation location, (2) composition of the material being riveted, (3) thickness of the material being riveted, and (4) strength desired. The thickness of the material being riveted determines the overall length of the shank of the rivet. As a general rule, the shank of the rivet should extend beyond the

5-62

Bulbed Cherrylock Rivets The large blind head of this fastener introduced the word “bulb” to blind rivet terminology. In conjunction with the unique residual preload developed by the high stem break load, its proven fatigue strength makes it

(Minimum grip illustrated)

(Minimum grip illustrated)

Figure 5-44. Wiredraw cherrylock rivet. Figure 5-43. Bulbed cherrylock rivet.

the only blind rivet interchangeable structurally with solid rivets. [Figure 5-43] Wiredraw Cherrylock Rivets There is a wide range of sizes, materials, and strength levels from which to select. This fastener is especially suited for sealing applications and joints requiring an excessive amount of sheet takeup. [Figure 5-44]

rivet sleeve; for example, 2017 aluminum alloy rivets for most aluminum alloys and 5056 aluminum rivets for magnesium. Figure 5-46 depicts the sequences of a typical mechanically locked blind rivet. The form and function may vary slightly between blind rivet styles and specifics should be obtained from manufacturers.

Huck Mechanical Locked Rivets Self-plugging (mechanical lock) rivets are fabricated in two sections: a head and shank (including a conical recess and locking collar in the head), and a serrated stem that extends through the shank. Unlike the friction lock rivet, the mechanical lock rivet has a locking collar that forms a positive lock for retention of the stem in the shank of the rivet. This collar is seated in position during the installation of the rivet. Material Self-plugging (mechanical lock) rivets are fabricated with sleeves (rivet shanks) of 2017 and 5056 aluminum alloys, Monel, or stainless steel. The mechanical lock type of self-plugging rivet can be used in the same applications as the friction lock type of rivet. In addition, because of its greater stem retention characteristic, installation in areas subject to considerable vibration is recommended. The same general requirements must be met in the selection of the mechanical lock rivet as for the friction lock rivet. Composition of the material being joined together determines the composition of the

Before installation

After installation

Figure 5-45. Self-plugging (mechanical lock) rivets.

5-63

1

Before pulling begins

2

Clamp-up and hole fill action begin

Sheet gap

3

Clamp-up completed as stem continues to bulb out blind head

Stem is pulled into rivet sleeve and starts to form bulbed blind head.

4

Rivet head firmly seated

Formation of blind head and hole filling are completed. Shear ring now begins to shear from stem cone to allow stem to pull further into rivet. Shear ring

(In minimum grip shear ring may not shear)

5

Shear ring has moved down stem cone until pulling head automatically stops stem break notch flush with top of rivet head. Locking collar is now ready to be inserted

6

Completely installed bulbed cherrylock Pulling head has inserted locking collar, and stem has fractured flush with rivet head

Maximum grip illustrated

Figure 5-46. Cherrylock rivet installation.

Head Styles

Self-plugging mechanical locked blind rivets are available in several head styles depending on the installation requirements. [Figure 5-47] Diameters Shank diameters are measured in 1/32-inch increments and are generally identified by the first dash number: -3 indicates 3⁄32 inch diameter, -4 indicates 1⁄8 inch diameter, and so forth. Both nominal and 1⁄64-inch oversize diameters are available. Grip Length Grip length refers to the maximum total sheet thickness to be riveted and is measured in 1⁄6 of an inch. This is generally identified by the second dash number. Unless otherwise noted, most blind rivets have their grip lengths

5-64

(maximum grip) marked on the rivet head and have a total grip range of 1⁄16 inch. [Figure 5-48] To determine the proper grip rivet to use, measure the material thickness with a grip selection gauge (available from blind rivet manufacturers). The proper use of a grip selector gauge is shown in Figure 5-49. The thickness of the material being riveted determines the overall length of the shank of the rivet. As a general rule, the shank of the rivet should extend beyond the material thickness approximately 3⁄64 inch to 1⁄8 inch before the stem is pulled. [Figure 5-50] Rivet Identification Each company that manufactures self-plugging (friction lock) rivets has a code number to help users obtain the correct rivet for the grip range or material thickness of a particular installation. In addition, MS numbers are used for identification purposes. Figures

100° Countersunk MS 20426 For countersunk applications

Universal MS 20470 For protruding head applications

100° Countersunk NAS 1097 For thin top sheet machine countersunk applications

Unisink A combination countersunk and protruding head for use in very thin top sheets. Stength equal to double-dimpling without the high cost.

156° Countersunk NAS 1097 A large diameter, shallow countersunk head providing wide area for honeycomb applications

Figure 5-47. Cherrylock rivet heads.

Read Max. Grip

1

3

⁄4 "

Figure 5-48. Typical grip length.

⁄16 " Min. Grip

Read 8

6

4

2

Rivet Grip Number to be used: −4

Figure 5-49. Grip gauge use.

5-65

Olympic Screw and Rivet Corporation RV 2 0 0 4 2 RV A C

2

Rivet type 2 = self plugging (friction lock) 5 = holow pull thru

0

Material composition of shank 0 = 2017 aluminium alloy 5 = 5056 aluminium alloy 7 = mild steel

0

Head style 0 = universal head 1 = 100° countersunk

4

Shank diameter in 32nds of an inch: 4 = 1⁄8 inch 6 = 3⁄16 inch 5 = 5⁄32 inch 8 = 1⁄4 inch

2

Grip range in 16ths of an inch

B

A = Thickness of material (grip range) B = 3⁄64 – 1⁄8 inch C = Total rivet shank length

Figure 5-50. Determining rivet length.

5-51 through 5-54 contain examples of part numbers for self-plugging (friction lock) rivets that are representative of each. Special Shear and Bearing Load Fasteners Many special fasteners produce high strength with light weight and can be used in place of conventional AN bolts and nuts. When AN bolts are tightened with the nut, the bolt stretches, narrowing the diameter and then the bolt is no longer tight in the hole. Special fasteners eliminate this loose fit because they are held in place by a collar that is squeezed into position. These fasteners are not under the same tensile loads as a bolt during installation. Special fasteners are also used extensively for light sport aircraft (LSA). Always follow the aircraft manufacturer’s recommendations. Huck Manufacturing Comany 9SP-B A 6 3 9SP-B

A

6

3

Head Style 9SP-B = brazier or universal head 9SP-100 = 100º countersunk head Material composition of shank A = 2017 aluminium alloy B = 5056 aluminium alloy R = mild steel Shank diameter in 32nds of an inch: 4 = 1⁄8 inch 6 = 3⁄16 inch 5 = 5⁄32 inch 8 = 1⁄4 inch

Manufacturer Olympic Screw and Rivet Corporation

Figure 5-52. Olympic Screw and Rivet Corporation codes.

Townsend Company, Cherry Rivet Division CR 163 6 6 CR 163

6

Cherry rivet Series number Designates rivet material, type of rivet, and head style (163 = 2117 aluminium alloy, self-plugging (friction lock) rivet, protruding head) Shank diameter in 32nds of an inch: 4 = 1⁄8 inch 6 = 3⁄16 inch 5 = 5⁄32 inch 8 = 1⁄4 inch

6

Grip range (material thickness): knob stem in 32nds of an inch; serrated stem in 16ths of an inch

Figure 5-53. Townsend Company, Cherry Rivet Division codes.

Military Standard Number MS 20600 B 4 K 2 MS 20600

Military Standard Type of rivet and head style: 20600 = self-plugging (friction lock) protruding head 20600 = self-plugging (friction lock) 100º countersunk head

B

Material composition of sleeve: AD = 2117 aluminium alloy B = 5056 aluminium alloy

4

Shank diameter in 32nds of an inch: 4 = 1⁄ 8 inch 6 = 3⁄16 inch 5 = 5⁄ 32 inch 8 = 1⁄4 inch

K

Type of stem: K = knot head stem W = serrated stem

2

Grip range (material thickness) in 16ths of an inch

Grip range (material thickness) in 16ths of an inch

Figure 5-51. Huck Manufacturing Company codes.

Figure 5-54. Military Standard Numbers.

5-66

Pin Rivets

Pin (Hi-Shear) rivets are classified as special rivets but are not of the blind type. Access to both sides of the material is required to install this type of rivet. Pin rivets have the same shear strength as bolts of equal diameters, are about 40 percent of the weight of a bolt, and require only about one-fifth as much time for installation as a bolt, nut, and washer combination. They are approximately three times as strong as solid shank rivets. Pin rivets are essentially threadless bolts. The pin is headed at one end and is grooved about the circumference at the other. A metal collar is swaged onto the grooved end effecting a firm, tight fit. [Figure 5-55] Pin rivets are fabricated in a variety of materials but should be used only in shear applications. They should never be used where the grip length is less than the shank diameter. Part numbers for pin rivets can be interpreted to give the diameter and grip length of the individual rivets. A typical part number breakdown would be: NAS177-14-17 NAS = National Aircraft Standard 177 = 100° countersunk head rivet OR 178 = flathead rivet 14 = Nominal diameter in 32nds of an inch 17 = Maximum grip length in 16ths of an inch

Stud

Radial Manufacturing head compression Lines of force

Preload

Washer nut

Figure 5-56. Taper-Lok special fasteners.

Taper-Lok

Taper-Loks are the strongest special fasteners used in aircraft construction. The Taper-Lok exerts a force on the walls of the hole because of its tapered shape. The Taper-Lok is designed to completely fill the hole, but unlike the rivet, it fills the hole without deforming the shank. Instead, the washer head nut squeezes the metal with tremendous force against the tapered walls of the hole. This creates radial compression around the shank and vertical compression lines as the metals are squeezed together. The combination of these forces generates strength unequaled by any other fastener. [Figure 5-56] Hi-Tigue

The Hi-Tigue special fastener has a bead that encircles the bottom of its shank. The bead preloads the hole it fills, resulting in increased joint strength. At installation, the bead presses against the sidewall of the hole, exerting radial force that strengthens the surrounding area. Because it is preloaded, the joint is not subjected to the constant cyclic action that normally causes a joint to become cold worked and eventually fail. Hi-Tigue fasteners are made of aluminum, titanium, and stainless steel alloys. The collars are composed of compatible metal alloys and come in two types: sealing and non-sealing. Just like the Hi-Loks, they can be installed using an Allen wrench and a box-end wrench. [Figure 5-57] Captive Fasteners

Collar

Figure 5-55. Pin (Hi-Shear) rivet.

Captive fasteners are used for quick removal of engine nacelles, inspection panels, and areas where fast and easy access is important. A captive fastener has the ability to turn in the body in which it is mounted, but which will not drop out when it is unscrewed from the part it is holding.

5-67

Some of the most commonly used are the Dzus, Camloc, and Airloc. Bead

Dzus Fasteners

The Dzus turnlock fastener consists of a stud, grommet, and receptacle. Figure 5-58 illustrates an installed Dzus fastener and the various parts. The grommet is made of aluminum or aluminum alloy material. It acts as a holding device for the stud. Grommets can be fabricated from 1100 aluminum tubing, if none are available from normal sources.

Figure 5-57. Hi-Tigue special fasteners.

Turnlock Fasteners

Turnlock fasteners are used to secure inspection plates, doors, and other removable panels on aircraft. Turnlock fasteners are also referred to by such terms as quick opening, quick action, and stressed panel fasteners. The most desirable feature of these fasteners is that they permit quick and easy removal of access panels for inspection and servicing purposes. Turnlock fasteners are manufactured and supplied by a number of manufacturers under various trade names.

The spring is made of steel, cadmium plated to prevent corrosion. The spring supplies the force that locks or secures the stud in place when two assemblies are joined. The studs are fabricated from steel and are cadmium plated. They are available in three head styles: wing, flush, and oval. Body diameter, length, and head type may be identified or determined by the markings found on the head of the stud. [Figure 5-59] The diameter is always measured in sixteenths of an inch. Stud length is measured in hundredths of an inch and is the dis-

Stud Detachable part Stud assembly

Grommet

Cut-away view of complete Dzus assembly

Fixed part

Spring and rivets

Spring assembly

Figure 5-58. Dzus fastener.

5-68

DZUS F

61⁄ 2

.50

F = flush head 61⁄ 2 = body diameter in 16ths of an inch .50 = length ( 50⁄100 of an inch)

Figure 5-59. Dzus identification.

tance from the head of the stud to the bottom of the spring hole. Stud assembly

A quarter of a turn of the stud (clockwise) locks the fastener. The fastener may be unlocked only by turning the stud counterclockwise. A Dzus key or a specially ground screwdriver locks or unlocks the fastener. Camloc Fasteners

Camloc fasteners are made in a variety of styles and designs. Included among the most commonly used are the 2600, 2700, 40S51, and 4002 series in the regular line, and the stressed panel fastener in the heavy duty line. The latter is used in stressed panels which carry structural loads.

Grommet Receptacle

The Camloc fastener is used to secure aircraft cowlings and fairings. It consists of three parts: a stud assembly, a grommet, and a receptacle. Two types of receptacles are available: rigid and floating. [Figure 5-60] The stud and grommet are installed in the removable portion; the receptacle is riveted to the structure of the aircraft. The stud and grommet are installed in either a plain, dimpled, countersunk, or counterbored hole, depending upon the location and thickness of the material involved. A quarter turn (clockwise) of the stud locks the fastener. The fastener can be unlocked only by turning the stud counterclockwise. Airloc Fasteners

The Airloc fastener consists of three parts: a stud, a cross pin, and a stud receptacle. [Figure 5-61] The studs are manufactured from steel and casehardened to prevent excessive wear. The stud hole is reamed for a press fit of the cross pin. The total amount of material thickness to be secured with the Airloc fastener must be known before the correct length of stud can be selected for installation. The total thickness of material that each stud will satisfactorily lock together is stamped on the head of the stud in thousandths of an inch (0.040, 0.070, 0.190, and

Figure 5-60. Camloc fastener.

so forth). Studs are manufactured in three head styles: flush, oval, and wing. The cross pin [Figure 5-61] is manufactured from chrome-vanadium steel and heat treated to provide maximum strength, wear, and holding power. It should never be used the second time; once removed from the stud, replace it with a new pin. Receptacles for Airloc fasteners are manufactured in two types: rigid and floating. Sizes are classified by number — No. 2, No. 5, and No. 7. They are also classified by the center-to-center distance between the rivet holes of the receptacle: No. 2 is 3⁄4 inch; No. 5 is 1 inch; and No. 7 is 1 3⁄8 inch. Receptacles are fabricated from 5-69

Commonly used screws are classified in three groups: (1) structural screws, which have the same strength as equal size bolts; (2) machine screws, which include the majority of types used for general repair; and (3) selftapping screws, which are used for attaching lighter parts. A fourth group, drive screws, are not actually screws but nails. They are driven into metal parts with a mallet or hammer and their heads are not slotted or recessed.

Installed fastener

Structural Screws

Structural screws are made of alloy steel, are properly heat treated, and can be used as structural bolts. These screws are found in the NAS204 through NAS235 and AN509 and AN525 series. They have a definite grip and the same shear strength as a bolt of the same size. Shank tolerances are similar to AN hex head bolts, and the threads are National Fine. Structural screws are available with round, brazier, or countersunk heads. The recessed head screws are driven by either a Phillips or a Reed & Prince screwdriver.

Stud receptacles Studs

Cross pin

Figure 5-61. Airloc fastener.

high-carbon, heat-treated steel. An upper wing assures ejection of the stud when unlocked and enables the cross pin to be held in a locked position between the upper wing, cam, stop, and wing detent, regardless of the tension to which the receptacle is subjected. Screws Screws are the most commonly used threaded fastening devices on aircraft. They differ from bolts inasmuch as they are generally made of lower strength materials. They can be installed with a loose fitting thread, and the head shapes are made to engage a screwdriver or wrench. Some screws have a clearly defined grip or unthreaded portion while others are threaded along their entire length. Several types of structural screws differ from the standard structural bolts only in head style. The material in them is the same, and a definite grip length is provided. The AN525 washer head screw and the NAS220 through NAS227 series are such screws.

5-70

The AN509 (100°) flathead screw is used in countersunk holes where a flush surface is necessary. The AN525 washer head structural screw is used where raised heads are not objectionable. The washer head screw provides a large contact area. Machine Screws

Machine screws are usually of the flathead (countersunk), roundhead, or washer head types. These are general purpose screws and are available in low carbon steel, brass, corrosion-resistant steel, and aluminum alloy. Roundhead screws, AN515 and AN520, have either slotted or recessed heads. The AN515 screw has coarse threads, and the AN520 has fine threads. Countersunk machine screws are listed as AN505 and AN510 for 82°, and AN507 for 100°. The AN505 and AN510 correspond to the AN515 and AN520 roundhead in material and usage. The fillister head screw, AN500 through AN503, is a general purpose screw and is used as a capscrew in light mechanisms. This could include attachments of cast aluminum parts such as gearbox cover plates. The AN500 and AN501 screws are available in low carbon steel, corrosion-resistant steel, and brass. The AN500 has coarse threads, while the AN501 has fine threads. They have no clearly defined grip length.

Screws larger than No. 6 have a hole drilled through the head for safetying purposes. The AN502 and AN503 fillister head screws are made of heat-treated alloy steel, have a small grip, and are available in fine and coarse threads. These screws are used as capscrews where great strength is required. The coarse threaded screws are commonly used as capscrews in tapped aluminum alloy and magnesium castings because of the softness of the metal. Self-Tapping Screws

Machine self-tapping screws are listed as AN504 and AN506. The AN504 screw has a roundhead, and the AN506 is 82° countersunk. These screws are used for attaching removable parts, such as nameplates, to castings and parts in which the screw cuts its own threads. AN530 and AN531 self-tapping sheet metal screws, such as the Parker-Kalon Z-type sheet metal screw, are blunt on the end. They are used in the temporary attachment of metal for riveting, and in the permanent assembly of nonstructural assemblies. Self-tapping screws should not be used to replace standard screws, nuts, bolts, or rivets. Drive Screws

Drive screws, AN535, correspond to the Parker-Kalon U-type. They are plain head self-tapping screws used as capscrews for attaching nameplates in castings and for sealing drain holes in corrosion proofing tubular structures. They are not intended to be removed after installation. Identification and Coding for Screws

The coding system used to identify screws is similar to that used for bolts. There are AN and NAS screws. NAS screws are structural screws. Part numbers 510, 515, 550, and so on, catalog screws into classes, such as roundhead, flathead, washer head, and so forth. Letters and digits indicate their material composition, length, and thickness. Examples of AN and NAS code numbers follow. AN501B-416-7 AN = Air Force-Navy standard 501 = fillister head, fine thread B = brass 416 = 4⁄16-inch diameter 7 = 7⁄16-inch length The letter “D” in place of the “B” would indicate that the material is 2017-T aluminum alloy. The letter “C”

would designate corrosion resistant steel. An “A” placed before the material code letter would indicate that the head is drilled for safetying. NAS144DH-22 NAS = National Aircraft Standard 144 = head style; diameter and thread — 1/4-28 bolt, internal wrenching DH = drilled head 22 = screw length in 16ths of an inch — 13⁄8 inches long The basic NAS number identifies the part. The suffix letters and dash numbers separate different sizes, plating material, drilling specifications, and so forth. The dash numbers and suffix letters do not have standard meanings. It is necessary to refer to a specific NAS page in the Standards book for the legend. Riveted and Rivetless Nutplates When access to the back of a screw or bolt installation is impractical, riveted or rivetless nutplates are used to secure the connection of panels. One example in aircraft this technique is especially useful is to secure the floorboards to the stringers and to each other. Nutplates

Nuts that are made to be riveted in place in aircraft are called nutplates. Their purpose is to allow bolts and screws to be inserted without having to hold the nut. They are permanently mounted to enable inspection panels and access doors to be easily removed and installed. When many screws are used on a panel, to make installation easier, normally floating anchor nuts are used. The floating anchor nut fits into a small bracket which is riveted to the aircraft skin. The nut is free to move, which makes it much easier to align it with the screw. For production ease, sometimes ganged anchor nuts are used for inspection panels. Ganged anchor nuts allow the nuts to float in a channel, making alignment with the screw easy. Self-locking nutplates are made under several standards and come in several shapes and sizes. Figure 5-62 shows an MS21078 two-lug nutplate with a nonmetallic insert, and an MS21047 lightweight, all-metal, 450 °F (232 °C) nutplate. Nutplates can also have three riveting points if the added strength is required. Rivnuts

This is the trade name of a hollow, blind rivet made of 6053 aluminum alloy, counterbored and threaded on the inside. Rivnuts can be installed by one person using 5-71

MS21078

MS21051

MS21055

Two-lug anchor nut

One-lug anchor nut

Corner anchor nut

MS21047

MS21059

MS33737

Instrument nut. To reduce magnetic influences in the cockpit, nonmagnetic mounting nuts secure instruments in a control panel.

NAS680A NAS444 or A6195

Right-angle anchor nut

High-temp. two-lug anchor nut

Two-lug floating anchor nut

Anchor type tinnerman nuts are suitable for nonstructural applications.

A1777, A1789, or A1794 Ganged anchor nuts U-type tinnerman nuts provide convenient anchor points for cowlings, fairings, and panels.

Figure 5-62. Various nutplates.

a special tool which heads the rivet on the blind side of the material. The Rivnut is threaded on the mandrel of the heading tool and inserted in the rivet hole. The heading tool is held at right angles to the material, the handle is squeezed, and the mandrel crank is turned clockwise after each stroke. Continue squeezing the handle and turning the mandrel crank of the heading tool until a solid resistance is felt, which indicates that the rivet is set. The Rivnut is used primarily as a nut plate and in the attachment of deicer boots to the leading edges of wings. It may be used as a rivet in secondary structures or for the attachment of accessories such as brackets, fairings, instruments, or soundproofing materials. Rivnuts are manufactured in two head types, each with two ends: the flathead with open or closed end, and the countersunk head with open or closed end. All Rivnuts, except the thin head countersunk type, are available with or without small projections (keys) attached to the head to keep the Rivnut from turning. Keyed Rivnuts are used as a nut plate, while those without keys are used for straight blind riveting repairs where no torque 5-72

loads are imposed. A keyway cutter is needed when installing Rivnuts which have keys. The countersunk style Rivnut is made with two different head angles: the 100° with 0.048 and 0.063 inch head thickness, and the 115° with 0.063 inch head thickness. Each of these head styles is made in three sizes: 6-32, 8-32, and 10-32. These numbers represent the machine screw size of the threads on the inside of the Rivnut. The actual outside diameters of the shanks are 3⁄16 inch for the 6-32 size, 7⁄32 inch for the 8-32 size, and 1⁄4 inch for the 10-32 size. Open end Rivnuts are the most widely used and are recommended in preference to the closed end type wherever possible. However, closed end Rivnuts must be used in pressurized compartments. Rivnuts are manufactured in six grip ranges. The minimum grip length is indicated by a plain head, and the next higher grip length by one radial dash mark on the head. Each succeeding grip range is indicated by an additional radial dash mark until five marks indicate the maximum range.

6-45

6-75

6-100

8-45

8-75

8-100

10-45

10-75

10-100

6B45

6B75

6B100

8B45

8B75

8B100

10B45

10B75

10B100

Notice in Figure 5-63 that some part number codes consist of a “6,” an “8,” or a “10,” a “dash,” and two or three more numbers. In some, the dash is replaced by the letters “K” or “KB.” The first number indicates the machine screw size of the thread, and the last two or three numbers indicate the maximum grip length in thousandths of an inch. A dash between the figures indicates that the Rivnut has an open end and is keyless; a “B” in place of the dash means it has a closed end and is keyless; a “K” means it has an open end and has a key; and a “KB” indicates that it has a closed end and a key. If the last two or three numbers are divisible by five, the Rivnut has a flathead; if they are not divisible by five, the Rivnut has a countersunk head.

6K45

6K75

6K100

An example of a part number code is:

8K45

8K75

8K100

10KB106

10K45

10K75

10K100

10 = Grip length

6KB45

6KB75

6KB100

KB = Closed end and key

8KB45

8KB75

8KB100

106 = Screw and thread size

10KB45

10KB75

10KB100

Flat — 0.32 Head Thickness

100°— 0.48 Head Thickness 6-91

6-121

6-146

8-91

8-121

8-146

10-91

10-121

10-146

6B91

6B121

6B146

8B91

8B121

8B146

10B91

10B121

10B146

100°— 0.63 Head Thickness 6-106

6-136

6-161

8-106

8-136

8-161

10-106

10-136

10-161

6B106

6B136

6B161

8B106

8B136

8B161

10B106

10B136

10B161

6K106

6K136

6K161

8K106

8K136

8K161

10K106

10K136

10K161

6KB106

6KB136

6KB161

8KB106

8KB136

8KB161

10KB106

10KB136

10KB161

Figure 5-63. Rivnut data chart.

Dill Lok-Skrus and Dill Lok-Rivets

Dill “Lok-Skru” and “Lok-Rivet” are trade names for internally threaded rivets. They are used for blind attachment of such accessories as fairings, fillets, access door covers, door and window frames, floor panels, and the like. Lok-Skrus and Lok-Rivets are similar to the Rivnut in appearance and application; however, they come in two parts and require more clearance on the blind side than the Rivnut to accommodate the barrel. [Figure 5-64] The Lok-Rivet and the Lok-Skru are alike in construction, except the Lok-Skru is tapped internally for fastening an accessory by using an attaching screw, whereas the Lok-Rivet is not tapped and can be used only as a rivet. Since both Lok-Skrus and Lok-Rivets

Figure 5-64. Internally threaded rivet (Rivnut).

5-73

are installed in the same manner, the following discussions for the Lok-Skru also applies to the Lok-Rivet. The main parts of a Lok-Skru are the barrel, the head, and an attachment screw. The barrel is made of aluminum alloy and comes in either closed or open ends. The head is either aluminum alloy or steel, and the attachment screw is made of steel. All of the steel parts are cadmium plated, and all of aluminum parts are anodized to resist corrosion. When installed, the barrel screws up over the head and grips the metal on the blind side. The attaching screw is then inserted if needed. There are two head types: the flathead and the countersunk head. The Lok-Skru is tapped for 7-32, 8-32, 10-32, or 10-24 screws, and the diameters vary from 0.230 inch for 6-32 screws, to 0.292 inch for 10-32 screws. Grip ranges vary from 0.010 inch to 0.225 inch. Deutsch Rivets

This rivet is a high strength blind rivet used on late model aircraft. It has a minimum shear strength of 75,000 psi, and can be installed by one person. The Deutsch rivet consists of two parts: the stainless steel sleeve and the hardened steel drive pin. [Figure 5-65] The pin and sleeve are coated with a lubricant and a corrosion inhibitor. The Deutsch rivet is available in diameters of 3⁄16, 1⁄4, or 3⁄8 inch. Grip lengths for this rivet range from 3⁄16 to 1 inch. Some variation is allowed in grip length when installing the rivet; for example, a rivet with a grip length of 3⁄16 inch can be used where the total thickness of materials is between 0.198 and 0.228 inch. When driving a Deutsch rivet, an ordinary hammer or a pneumatic rivet gun and a flathead set are used. The rivet is seated in the previously drilled hole and then the pin is driven into the sleeve. The driving action causes the pin to exert pressure against the sleeve and

forces the sides of the sleeve out. This stretching forms a shop head on the end of the rivet and provides positive fastening. The ridge on the top of the rivet head locks the pin into the rivet as the last few blows are struck. Sealing Nutplates

When securing nutplates in pressurized aircraft and in fuel cells, a sealing nutplate is used instead of the open ended variety previously described. Care must be taken to use exactly the correct length of bolt or screw. If a bolt or screw is too short, there will not be enough threads to hold the device in place. If the bolt or screw is too long, it will penetrate the back side of the nutplate and compromise the seal. Normally, a sealant is also used to ensure complete sealing of the nutplate. Check the manufacturer’s specifications for the acceptable sealant to be used for sealing nutplates. Hole Repair and Hole Repair Hardware Many of the blind fasteners are manufactured in oversized diameters to accommodate slightly enlarged holes resulting from drilling out the original fastener. When using rivets or even bolts, care must be taken to ensure the hole is not elongated or slanted. To reduce the chances of an incorrectly drilled rivet or bolt hole, use a slightly smaller drill bit first, then enlarge to the correct diameter. The last step to prepare the hole for the fastener is to deburr the hole using either a very large drill bit or a special deburring tool. This practice also works well when drilling out a previously attached fastener. If the drill bit does not exactly find the center of the rivet or bolt or screw, the hole can easily be elongated, but when using a smaller drill bit, drill the head only off the fastener, then the ring and stem that is left can be pushed out with a pin punch of the appropriate diameter. If an incorrectly drilled hole is found, the options are to redrill the hole to the next larger diameter for an acceptable fastener or repair the hole using an Acres fastener sleeve. Repair of Damaged Holes with Acres Fastener Sleeves

Figure 5-65. Deutsch rivet.

5-74

Acres fastener sleeves are thin-wall tubular elements with a flared end. The sleeves are installed in holes to accept standard bolts and rivet-type fasteners. The existing fastener holes are drilled 1⁄64 inch oversize for installation of the sleeves. The sleeves are manufactured in 1-inch increments. Along their length, grooves provide a place to break or cut off excess length to match fastener grip range. The grooves also provide a place to hold adhesive or sealing agents when bonding the sleeve into the hole.

Advantages and Limitations The sleeves are used in holes which must be drilled 1⁄64 inch oversize to clean up corrosion or other damage. The oversize hole with the sleeve installed allows the use of the original diameter fastener in the repaired hole. The sleeves can be used in areas of high galvanic corrosion where the corrosion must be confined to a readily replaceable part. Oversizing of holes reduces the net cross-sectional area of a part and should not be done unless absolutely required. Consult the manufacturer of the aircraft, aircraft engine or aircraft component prior to repair of damaged holes with Acres sleeves. Identification The sleeve is identified by a standard code number [Figure 5-66A] which represents the type and style of sleeve, a material code, the fastener shank diameter, surface finish code letter and grip tang for the sleeve. The type and material of the sleeve is represented by the basic code number. The first dash number represents the diameter of the sleeve for the fastener installed and the second dash represents the grip length of the sleeve. The required length of the sleeve is determined on installation and the excess is broken off of the sleeve. A JK5512A-05N-10 is a 100° low profile head sleeve of aluminum alloy. The diameter is for a 5⁄32 -inch fastener with no surface finish and is 5⁄8 inch in length. Hole Preparation Refer to Figure 5-66B for drill number for standard or close fit holes. Inspect hole after drilling to assure all corrosion is removed before installing the sleeve. The hole must also be the correct shape and free from burrs. The countersink must be enlarged to receive the flare of the sleeve so the sleeve is flush with the surrounding surface. Installation After selecting the correct type and diameter sleeve, use the 6501 sleeve breakoff tool for final installation length. Refer to Figure 5-66B for the sleeve breakoff procedure. The sleeve may be installed with or without being bonded in the hole. When bonding the sleeve in a hole, use MIL-S-8802A1⁄2 sealant. Reinstall original size fastener and torque as required. Sleeve Removal Sleeves not bonded in the hole may be removed by either driving them out with a drift pin of the same diameter as the outside diameter of the sleeve or they may be deformed and removed with a pointed tool. Bonded sleeves may be removed by this method, but

care should be used not to damage the structure hole. If this method cannot be used, drill the sleeves out with a drill 0.004 to 0.008 inch smaller than the installation drill size. The remaining portion of the sleeve after drilling can be removed using a pointed tool and applying an adhesive solvent to the sealant. Control Cables and Terminals Cables are the most widely used linkage in primary flight control systems. Cable-type linkage is also used in engine controls, emergency extension systems for the landing gear, and various other systems throughout the aircraft. Cable-type linkage has several advantages over the other types. It is strong and light weight, and its flexibility makes it easy to route through the aircraft. An aircraft cable has a high mechanical efficiency and can be set up without backlash, which is very important for precise control. Cable linkage also has some disadvantages. Tension must be adjusted frequently due to stretching and temperature changes. Aircraft control cables are fabricated from carbon steel or stainless steel. Cable Construction

The basic component of a cable is a wire. The diameter of the wire determines the total diameter of the cable. A number of wires are preformed into a helical or spiral shape and then formed into a strand. These preformed strands are laid around a straight center strand to form a cable. Cable designations are based on the number of strands and the number of wires in each strand. The most common aircraft cables are the 7 × 7 and 7 × 19. The 7 × 7 cable consists of seven strands of seven wires each. Six of these strands are laid around the center strand. [Figure 5-67] This is a cable of medium flexibility and is used for trim tab controls, engine controls, and indicator controls. The 7 × 19 cable is made up of seven strands of 19 wires each. Six of these strands are laid around the center strand. [Figure 5-67] This cable is extra flexible and is used in primary control systems and in other places where operation over pulleys is frequent. Aircraft control cables vary in diameter, ranging from 1⁄ to 3⁄ inch. The diameter is measured as shown in 16 8 Figure 5-67.

5-75

Acres Sleeve

Basic Part Number

Type

100° Tension head plus flange (509 type)

JK5610

Protruding head (shear)

JK5511

100° Low profile head

JK5512

100° Standard profile head (509 type)

JK5516

Protruding head (tension)

JK5517

100° Oversize tension head (1/64 oversize bolt)

JK5533

Part Number Breakdown JK5511 A 04 N 08 L JK5511 A

Basic part number Material Code

1

04

Fastener shank diameter in 32nds

N

Surface finish N = No finish C = Chemical film per MIL-C-554

08 L

Length in sixteenth-inch increments (required installation length by breaking off at proper groove)

Sleeve Part No.

Bolt Size

JK5511( )04( )( ) JK5512( )04( )( ) JK5516( )04( )( ) JK5517( )04( )( )

1/ 8

8

JK5511( )45( )( ) JK5512 JK5516( )45( )( ) JK5517( )45( )( )

#6

8

JK5511( )05( )( ) JK5512( )05( )( ) JK5516( )05( )( ) JK5517( )05( )( )

5/ 32

10

JK5511( )55( )( ) JK5512( )55( )( ) JK5516( )55( )( ) JK5517( )55( )( ) JK5610( )55( )( )

#8

10

JK5511( )06( )( ) JK5512( )06( )( ) JK5516( )06( )( ) JK5517( )06( )( ) JK5610( )06( )( )

#10

12

JK5511( )08( )( ) JK5512( )08( )( ) JK5516( )08( )( ) JK5517( )08( )( ) JK5610( )08( )( )

1/ 4

16

JK5511( )10( )( ) JK5512( )10( )( ) JK5516( )10( )( ) JK5517( )10( )( ) JK5610( )10( )( )

5/ 16

16

JK5511( )12( )( ) JK5512( )12( )( ) JK5516( )12( )( ) JK5517( )12( )( ) JK5610( )12( )( )

3/ 8

16

Material 5052 Aluminium alloy 1/2 hard

A

6061 Aluminium alloy (T6 condition)

B

A286 Stainless steel (passivate)

Sleeve Length

Acres Sleeve for 1/64 Oversize Bolt

“L” at end of part number indicates cetyl alcohol lubricant

Material Code

2

1

C

Sleeve Part No.

Bolt Size

JK5533( )06( )( )

13/ 64

12

JK5533( )08( )( )

17/ 64

16

JK5533( )10( )( )

21/ 64

16

JK5533( )12( )( )

25/ 64

16

2

Sleeve Length

Notes: 1 Acres sleeve, JK5533 1/64 oversize available in A286 steel only 2

Acres sleeve length in sixteenth-inch increments

Figure 5-66A. Acres sleeve identification.

5-76

Hole Preparation for 1⁄64 Oversize Bolt Bolt Size

Drill No.

Drill Dia.

13/

7/

0.2187

17/

9/

0.2812

21/

11/

0.3437

25/

13/

0.4062

64

32

64

32

64

32

64

32

Existing fastener

Acres sleeve

Hole Preparation Standard Fit

Close Fit

Bolt Size Drill No.

Drill Dia.

Drill No.

Drill Dia.

1⁄ 8

9⁄ 64

0.1406

28

0.1405

#6

23

0.1540

24

0.1520

5⁄ 32

11⁄ 64

0.1719

18

0.1695

#8

15

0.1800

16

0.1770

#10

5

0.2055

6

0.2040

17⁄ 64

0.2656

1⁄ 4

14

0.2660

5⁄ 16

21⁄ 64

0.3281

3⁄ 8

25⁄ 64

0.3908

1

Structure

⁄64 Oversize hole

Installation Procedure A. Drill out corrosion or damage to existing hole to 1⁄ 64 oversize. B. Select proper type and length acres sleeve for existing fastener. C. Bond sleeve in structure hole with MIL-S-8802 class A 1⁄ 2 sealant.

Acres sleeve installation

Figure 5-66B. Acres sleeve identification, installation, and breakoff procedure.

1

⁄8 − 3⁄8 Diameter 7 × 19

1

⁄16 − 3⁄32 Diameter 7 × 7

Diameter

7 strands, 19 wires to each strand

Diameter

7 strands, 7 wires to each strand

Figure 5-67. Cable cross sections.

5-77

Cable Fittings

Cables may be equipped with several different types of fittings, such as terminals, thimbles, bushings, and shackles.

AN663 Double shank ball end terminal

Terminal fittings are generally of the swaged type. They are available in the threaded end, fork end, eye end, single shank ball end, and double shank ball end. The threaded end, fork end, and eye end terminals are used to connect the cable to a turnbuckle, bellcrank, or other linkage in the system. The ball end terminals are used for attaching cables to quadrants and special connections where space is limited. Figure 5-68 illustrates the various types of terminal fittings.

AN664 Single shank ball end terminal

AN665 Rod end terminal

The thimble, bushing, and shackle fittings may be used in place of some types of terminal fittings when facilities and supplies are limited and immediate replacement of the cable is necessary.

AN666 Threaded cable terminal

Turnbuckles

A turnbuckle assembly is a mechanical screw device consisting of two threaded terminals and a threaded barrel. [Figure 5-69] AN667 Fork end cable terminal

Turnbuckles are fitted in the cable assembly for the purpose of making minor adjustments in cable length and for adjusting cable tension. One of the terminals has right-hand threads and the other has left-hand threads. The barrel has matching right- and left-hand internal threads. The end of the barrel with the left-hand threads can usually be identified by a groove or knurl around that end of the barrel. When installing a turnbuckle in a control system, it is necessary to screw both of the terminals an equal number of turns into the barrel. It is also essential that all turnbuckle terminals be screwed into the barrel until not more than three threads are exposed on either side of the turnbuckle barrel. After a turnbuckle is properly adjusted, it must be safetied. The methods of safetying turnbuckles are discussed later in this chapter.

AN668 Eye end cable terminal

Figure 5-68. Types of terminal fittings.

Push-Pull Tube Linkage

Push-pull tubes are used as linkage in various types of mechanically operated systems. This type linkage eliminates the problem of varying tension and permits the transfer of either compression or tension stress through a single tube. A push-pull tube assembly consists of a hollow aluminum alloy or steel tube with an adjustable end fitting

Length (threads flush with ends of barrel) Swaged terminal

Barrrel

Figure 5-69. Typical turnbuckle assembly.

5-78

Pin eye

Self-aligning, anti-friction rod end assembly, adjustable Check nut

Rod end, threaded

Tube, steel or aluminium alloy

Clevis, rod end, adjustable

Figure 5-70. Push-pull tube assembly.

and a checknut at either end. [Figure 5-70] The checknuts secure the end fittings after the tube assembly has been adjusted to its correct length. Push-pull tubes are generally made in short lengths to prevent vibration and bending under compression loads. Safetying Methods To ensure fasteners do not separate from their nuts or holding ends, various safetying methods are used in aircraft from heavy aircraft to gliders to recreational aircraft. Safetying is the process of securing all aircraft, bolts, nuts, screws, pins, and other fasteners so that they do not work loose due to vibration. A familiarity with the various methods and means of safetying equipment on an aircraft is necessary in order to perform maintenance and inspection. There are various methods of safetying aircraft parts. The most widely used methods are safety wire, cotter pins, lockwashers, snaprings, and special nuts, such as self-locking nuts, pal nuts, and jamnuts. Some of these nuts and washers have been previously described in this chapter. Pins

The three main types of pins used in aircraft structures are the taper pin, flathead pin, and cotter pin. Pins are used in shear applications and for safetying. Roll pins are finding increasing uses in aircraft construction. Taper Pins Plain and threaded taper pins (AN385 and AN386) are used in joints which carry shear loads and where absence of play is essential. The plain taper pin is drilled and usually safetied with wire. The threaded taper pin is used with a taper pin washer (AN975) and

shear nut (safetied with a cotter pin or safety clip) or self-locking nut. Flathead Pin Commonly called a clevis pin, the flathead pin (MS20392) is used with tie rod terminals and in secondary controls which are not subject to continuous operation. The pin is customarily installed with the head up so that if the cotter pin fails or works out, the pin will remain in place. Cotter Pins The AN380 cadmium plated, low carbon steel cotter pin is used for safetying bolts, screws, nuts, other pins, and in various applications where such safetying is necessary. The AN381 corrosion resistant steel cotter pin is used in locations where nonmagnetic material is required, or in locations where resistance to corrosion is desired. Roll Pins The roll pin is a pressed fit pin with chamfered ends. It is tubular in shape and is slotted the full length of the tube. The pin is inserted with hand tools and is compressed as it is driven into place. Pressure exerted by the roll pin against the hole walls keeps it in place, until deliberately removed with a drift punch or pin punch. Safety Wiring

Safety wiring is the most positive and satisfactory method of safetying capscrews, studs, nuts, bolt heads, and turnbuckle barrels which cannot be safetied by any other practical means. It is a method of wiring together two or more units in such a manner that any tendency of one to loosen is counteracted by the tightening of the wire.

5-79

1

2

6

3

7

4

5

8

Figure 5-71. Safety wiring methods.

Nuts, Bolts, and Screws Nuts, bolts, and screws are safety wired by the single wire or double twist method. The double twist method is the most common method of safety wiring. The single wire method may be used on small screws in a closely spaced closed geometrical pattern, on parts in electrical systems, and in places that are extremely difficult to reach. Safety wiring should always be per conventional methods or as required by the manufacturer, especially for Light Sport Aircraft (LSA). Figure 5-71 is an illustration of various methods which are commonly used in safety wiring nuts, bolts, and screws. Careful study of Figure 5-71 shows that: • Examples 1, 2, and 5 illustrate the proper method of safety wiring bolts, screws, squarehead plugs, and similar parts when wired in pairs. • Example 3 illustrates several components wired in series. • Example 4 illustrates the proper method of wiring castellated nuts and studs. (Note that there is no loop around the nut.) • Examples 6 and 7 illustrate a single threaded component wired to a housing or lug. • Example 8 illustrates several components in a closely spaced closed geometrical pattern, using a single wire method. When drilled head bolts, screws, or other parts are grouped together, they are more conveniently safety 5-80

wired to each other in a series rather than individually. The number of nuts, bolts, or screws that may be safety wired together is dependent on the application. For instance, when safety wiring widely spaced bolts by the double twist method, a group of three should be the maximum number in a series. When safety wiring closely spaced bolts, the number that can be safety wired by a 24-inch length of wire is the maximum in a series. The wire is arranged so that if the bolt or screw begins to loosen, the force applied to the wire is in the tightening direction. Parts being safety wired should be torqued to recommend values and the holes aligned before attempting the safetying operation. Never overtorque or loosen a torqued nut to align safety wire holes. Oil Caps, Drain Cocks, and Valves These units are safety wired as shown in Figure 5-72. In the case of the oil cap, the wire is anchored to an adjacent fillister head screw. This system applies to any other unit which must be safety wired individually. Ordinarily, anchorage lips are conveniently located near these individual parts. When such provision is not made, the safety wire is fastened to some adjacent part of the assembly. Electrical Connectors Under conditions of severe vibration, the coupling nut of a connector may vibrate loose, and with sufficient vibration the connector may come apart. When this

Oil caps

Drain cocks

Note: The safety wire is shown installed for right-hand threads. The safety wire is routed in the opposite direction for left-hand threads. Valves

Figure 5-72. Safety wiring attachment for plug connectors.

occurs, the circuit carried by the cable opens. The proper protective measure to prevent this occurrence is by safety wiring as shown in Figure 5-73. The safety wire should be as short as practicable and must be installed in such a manner that the pull on the wire is in the direction which tightens the nut on the plug.

turnbuckles; however, only two methods will be discussed in this section. These methods are illustrated in Figure 5-74(A) and Figure 5-74(B). The clip locking method is used only on the most modern aircraft. The older type aircraft still use the type turnbuckles that require the wire wrapping method.

Turnbuckles After a turnbuckle has been properly adjusted, it must be safetied. There are several methods of safetying

Double Wrap Method. Of the methods using safety wire for safetying turnbuckles, the double wrap method is preferred, although the single wrap methods described

Bulkhead AN3102 receptacle

AN standard fillister head screw (drilled head)

AN3057 adapter AN3106 plug

Figure 5-73. Safety wiring attachment for plug connectors.

5-81

Straight end Hook shoulder Loop end Hook lip

Hook loop Hook end

Direction of pull for inspection

(A) Clip-locking method

Turnbuckle fork AN161 or AN162

Lock wire

Turnbuckle barrel AN155

4 turns wrap

Cable

4 turns wrap

Turnbuckle eye AN170 Lock wire

Thimble AN100

4 turns wrap

Swaged terminal AN666 or AN669 (B) Wire-wrapping method

Figure 5-74. Safetying turnbuckles: (A) clip-locking method and (B) wire-wrapping method.

are satisfactory. The method of double wrap safetying is shown in Figure 5-74(B). Use two separate lengths of the proper wire as shown in Figure 5-75. Run one end of the wire through the hole in the barrel of the turnbuckle and bend the ends of the wire toward 5-82

opposite ends of the turnbuckle. Then pass the second length of the wire into the hole in the barrel and bend the ends along the barrel on the side opposite the first. Then pass the wires at the end of the turnbuckle in opposite directions through the holes in the turnbuckle

Cable size (in)

Type of Wrap

Diameter of Safety Wire (in)

Material (Annealed Condition)

1⁄ 16

Single

.020

Stainless steel

3⁄ 32

Single

.040

Copper, brass1

1⁄ 8

Single

.040

Stainless steel

1⁄ 8

Double

.040

Copper, brass1

1⁄ 8

Single

.057 min

Copper, brass1

5⁄ and greater 32

Single

.057

Stainless steel

1 Galvanized or tinned steel, or soft iron wires are also acceptable.

Figure 5-75. Turnbuckle safetying guide.

eyes or between the jaws of the turnbuckle fork, as applicable. Bend the laid wires in place before cutting off the wrapped wire. Wrap the remaining length of safety wire at least four turns around the shank and cut it off. Repeat the procedure at the opposite end of the turnbuckle. When a swaged terminal is being safetied, pass the ends of both wires, if possible, through the hole provided in the terminal for this purpose and wrap both ends around the shank as described above.

bend the wire ends toward opposite ends of the turnbuckle. Then pass each wire end through the cable eye or fork, or through the hole in the swaged terminal and wrap each wire end around the shank for at least four turns, cutting off the excess wire. After safetying, no more than three threads of the turnbuckle threaded terminal should be exposed. General Safety Wiring Rules

When using the safety wire method of safetying, the following general rules should be followed:

If the hole is not large enough to allow passage of both wires, pass the wire through the hole and loop it over the free end of the other wire, and then wrap both ends around the shank as described.

1. A pigtail of 1⁄4 to 1⁄2 inch (three to six twists) should be made at the end of the wiring. This pigtail must be bent back or under to prevent it from becoming a snag.

Single Wrap Method. The single wrap safetying methods described in the following paragraphs are acceptable but are not the equal of the double wrap methods.

2. The safety wire must be new upon each application.

Pass a single length of wire through the cable eye or fork, or through the hole in the swaged terminal at either end of the turnbuckle assembly. Spiral each of the wire ends in opposite directions around the first half of the turnbuckle barrel so that the wires cross each other twice. Thread both wire ends through the hole in the middle of the barrel so that the third crossing of the wire ends is in the hole. Again, spiral the two wire ends in opposite directions around the remaining half of the turnbuckle, crossing them twice. Then, pass one wire end through the cable eye or fork, or through the hole in the swaged terminal. In the manner described above, wrap both wire ends around the shank for at least four turns each, cutting off the excess wire. An alternate to the above method is to pass one length of wire through the center hole of the turnbuckle and

3. When castellated nuts are to be secured with safety wire, tighten the nut to the low side of the selected torque range, unless otherwise specified, and if necessary, continue tightening until a slot aligns with the hole. 4. All safety wires must be tight after installation, but not under such tension that normal handling or vibration will break the wire. 5. The wire must be applied so that all pull exerted by the wire tends to tighten the nut. 6. Twists should be tight and even, and the wire between the nuts as taut as possible without overtwisting. 7. The safety wire should always be installed and twisted so that the loop around the head stays down and does not tend to come up over the bolt head, causing a slack loop.

5-83

Cotter Pin Safetying Cotter pin installation is shown in Figure 5-76. Castellated nuts are used with bolts that have been drilled for cotter pins. The cotter pin should fit neatly into the hole, with very little sideplay. The following general rules apply to cotter pin safetying: 1. The prong bent over the bolt end should not extend beyond the bolt diameter. (Cut it off if necessary.) 2. The prong bent down should not rest against the surface of the washer. (Again, cut it off if necessary.) 3. If the optional wraparound method is used, the prongs should not extend outward from the sides of the nut. 4. All prongs should be bent over a reasonable radius. Sharp angled bends invite breakage. Tapping lightly with a mallet is the best method of bending the prongs. Snaprings A snapring is a ring of metal, either round or flat in cross section, which is tempered to have springlike action. This springlike action will hold the snapring firmly seated in a groove. The external types are designed to fit in a groove around the outside of a shaft or cylinder, and may be safety wired. Safety wiring of an external type snapring is shown in Figure 5-77. The internal types fit in a groove inside a cylinder, and are never safetied. A special type of pliers is designed to install each type of snapring. Snaprings can be reused as long as they retain their shape and springlike action.

5-84

Optional

Preferred

Figure 5-76. Cotter pin installations.

Snapring

Single-wire method Shaft

Figure 5-77. External type snapring with safety wire installation.

Corrosion Control Many aircraft structures are made of metal, and the most insidious form of damage to those structures is corrosion. From the moment the metal is manufactured, it must be protected from the deleterious effects of the environment that surrounds it. This protection can be the introduction of certain elements into the base metal, creating a corrosion resistant alloy, or the addition of a surface coating of a chemical conversion coating, metal or paint. While in use, additional moisture barriers, such as viscous lubricants and protectants may be added to the surface. The introduction of airframes built primarily of composite components has not eliminated the need for careful monitoring of aircraft with regard to corrosion. While the airframe itself may not be subject to corrosion, the use of metal components and accessories within the airframe means the aircraft maintenance technician must be on the alert for the evidence of corrosion when inspecting any aircraft. This chapter provides an overview to the problems associated with aircraft corrosion. For more in-depth information on the subject, refer to the latest edition of

FAA Advisory Circular (AC) 43-4A, Corrosion Control for Aircraft. The advisory circular is an extensive handbook, which deals with the sources of corrosion particular to aircraft structures, as well as steps the aircraft maintenance technician can take in the course of maintaining aircraft that have been attacked by corrosion. Metal corrosion is the deterioration of the metal by chemical or electrochemical attack. This type of damage can take place internally as well as on the surface. As in the rotting of wood, this deterioration may change the smooth surface, weaken the interior, or damage or loosen adjacent parts. Water or water vapor containing salt combines with oxygen in the atmosphere to produce the main source of corrosion in aircraft. Aircraft operating in a marine environment, or in areas where the atmosphere contains industrial fumes that are corrosive, are particularly susceptible to corrosive attacks. [Figure 6-1] If left unchecked, corrosion can cause eventual structural failure. The appearance of corrosion varies with the metal. On the surface of aluminum alloys and magnesium, it appears as pitting and etching, and is

Figure 6-1. Seaplane operations.

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often combined with a gray or white powdery deposit. On copper and copper alloys, the corrosion forms a greenish film; on steel, a reddish corrosion byproduct commonly referred to as rust. When the gray, white, green, or reddish deposits are removed, each of the surfaces may appear etched and pitted, depending upon the length of exposure and severity of attack. If these surface pits are not too deep, they may not significantly alter the strength of the metal; however, the pits may become sites for crack development, particularly if the part is highly stressed. Some types of corrosion burrow between the inside of surface coatings and the metal surface, and can spread until the part fails. Types of Corrosion There are two general classifications of corrosion that cover most of the specific forms: direct chemical attack and electrochemical attack. In both types of corrosion, the metal is converted into a metallic compound such as an oxide, hydroxide, or sulfate. The corrosion process always involves two simultaneous changes: The metal that is attacked or oxidized suffers what may be called anodic change, and the corrosive agent is reduced and may be considered as undergoing cathodic change.

Figure 6-2. Direct chemical attack in a battery compartment.

Continuous liquid path (electrolyte) Current flow

Anodic area

Cathodic area

Electron flow Electron conductor metal No contact between electrolyte and anode and cathode

Unbroken paint film

Continuous liquid path (electrolyte)

Anodic area

Cathodic area

Electron conductor metal

Figure 6-3. Electrochemical attack.

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Direct Chemical Attack Direct chemical attack, or pure chemical corrosion, is an attack resulting from a direct exposure of a bare surface to caustic liquid or gaseous agents. Unlike electrochemical attack where the anodic and cathodic changes may be taking place a measurable distance apart, the changes in direct chemical attack are occurring simultaneously at the same point. The most common agents causing direct chemical attack on aircraft are: (1) spilled battery acid or fumes from batteries; (2) residual flux deposits resulting from inadequately cleaned, welded, brazed, or soldered joints; and (3) entrapped caustic cleaning solutions. [Figure 6-2] With the introduction of sealed lead-acid batteries and the use of nickel-cadmium batteries, spilled battery acid is becoming less of a problem. The use of these closed units lessens the hazards of acid spillage and battery fumes. Many types of fluxes used in brazing, soldering, and welding are corrosive, and they chemically attack the metals or alloys with which they are used. Therefore, it is important to remove residual flux from the metal surface immediately after the joining operation. Flux residues are hygroscopic in nature; that is, they absorb moisture, and unless carefully removed, tend to cause severe pitting. Caustic cleaning solutions in concentrated form should be kept tightly capped and as far from aircraft as possible. Some cleaning solutions used in corrosion removal are, in themselves, potentially corrosive agents; therefore, particular attention should be directed toward their complete removal after use on aircraft. Where entrapment of the cleaning solution is likely to occur, use a noncorrosive cleaning agent, even though it is less efficient. Electrochemical Attack An electrochemical attack may be likened chemically to the electrolytic reaction that takes place in electroplating, anodizing, or in a dry cell battery. The reaction in this corrosive attack requires a medium, usually water, which is capable of conducting a tiny current of electricity. When a metal comes in contact with a corrosive agent and is also connected by a liquid or gaseous path through which electrons may flow, corrosion begins as the metal decays by oxidation. [Figure 6-3] During the attack, the quantity of corrosive agent is reduced and, if not renewed or removed, may completely react with the metal, becoming neutralized. Different areas of the same metal surface have varying levels of electrical potential and, if connected

by a conductor, such as salt water, will set up a series of corrosion cells and corrosion will commence. All metals and alloys are electrically active and have a specific electrical potential in a given chemical environment. This potential is commonly referred to as the metal’s “nobility.” [Figure 6-4] The less noble a metal is, the more easily it can be corroded. The metals chosen for use in aircraft structures are a studied compromise with strength, weight, corrosion resistance, workability, and cost balanced against the structure’s needs. The constituents in an alloy also have specific electrical potentials that are generally different from each other. Exposure of the alloy surface to a conductive, corrosive medium causes the more active metal to + Corroded End (anodic, or least noble) Magnesium Magnesium alloy Zinc Aluminum (1100) Cadmium Aluminum 2024-T4 Steel or Iron Cast Iron Chromium-Iron (active) Ni-Resist Cast Iron Type 304 Stainless steel (active) Type 316 Stainless steel (active) Lead-Tin solder Lead Tin Nickel (active) Inconel nickel-chromium alloy (active) Hastelloy Alloy C (active) Brass Copper Bronze Copper-nickel alloy Monel nickel-copper alloy Silver Solder Nickel (passive) Inconel nickel-chromium alloy (passive) Chromium-Iron (passive) Type 304 Stainless steel (passive) Type 316 Stainless steel (passive) Hastelloy Alloy C (passive) Silver Titanium Graphite Gold Platinum

– Protected End (cathodic, or most noble) Figure 6-4. The galvanic series of metals and alloys.

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become anodic and the less active metal to become cathodic, thereby establishing conditions for corrosion. These are called local cells. The greater the difference in electrical potential between the two metals, the greater will be the severity of a corrosive attack, if the proper conditions are allowed to develop. The conditions for these corrosion reactions are the presence of a conductive fluid and metals having a difference in potential. If, by regular cleaning and surface refinishing, the medium is removed and the minute electrical circuit eliminated, corrosion cannot occur. This is the basis for effective corrosion control. The electrochemical attack is responsible for most forms of corrosion on aircraft structure and component parts.

sion will spread under the surface coating and cannot be recognized by either the roughening of the surface or the powdery deposit. Instead, closer inspection will reveal the paint or plating is lifted off the surface in small blisters which result from the pressure of the underlying accumulation of corrosion products. [Figure 6-5] Filiform corrosion gives the appearance of a series of small worms under the paint surface. It is often seen on surfaces that have been improperly chemically treated prior to painting. [Figure 6-6]

There are many forms of corrosion. The form of corrosion depends on the metal involved, its size and shape, its specific function, atmospheric conditions, and the corrosion producing agents present. Those described in this section are the more common forms found on airframe structures.

Dissimilar Metal Corrosion Extensive pitting damage may result from contact between dissimilar metal parts in the presence of a conductor. While surface corrosion may or may not be taking place, a galvanic action, not unlike electroplating, occurs at the points or areas of contact where the insulation between the surfaces has broken down or been omitted. This electrochemical attack can be very serious because in many instances the action is taking place out of sight, and the only way to detect it prior to structural failure is by disassembly and inspection. [Figure 6-7]

Surface Corrosion Surface corrosion appears as a general roughening, etching, or pitting of the surface of a metal, frequently accompanied by a powdery deposit of corrosion products. Surface corrosion may be caused by either direct chemical or electrochemical attack. Sometimes corro-

The contamination of a metal’s surface by mechanical means can also induce dissimilar metal corrosion. The improper use of steel cleaning products, such as steel wool or a steel wire brush on aluminum or magnesium, can force small pieces of steel into the metal being cleaned, which will then further corrode and

Forms of Corrosion

Figure 6-5. Surface corrosion.

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Figure 6-6. Filiform corrosion.

Figure 6-7. Dissimilar metal corrosion.

ruin the adjoining surface. Carefully monitor the use of nonwoven abrasive pads, so that pads used on one type of metal are not used again on a different metal surface. Intergranular Corrosion This type of corrosion is an attack along the grain boundaries of an alloy and commonly results from a lack of uniformity in the alloy structure. Aluminum

alloys and some stainless steels are particularly susceptible to this form of electrochemical attack. [Figure 6-8] The lack of uniformity is caused by changes that occur in the alloy during heating and cooling during the material’s manufacturing process. Intergranular corrosion may exist without visible surface evidence. Very severe intergranular corrosion may sometimes cause the surface of a metal to “exfoliate.” [Figure 6-9] This is a lifting or flaking of the metal at the 6-5

Electrolyte enters through cracks in paint film

Paint film

Cathode

Cladding

Anode

Intergranular corrosion

7075-T6 Aluminium

Steel fastener

Figure 6-8. Intergranular corrosion of 7075-T6 aluminum adjacent to steel fastener.

surface due to delamination of the grain boundaries caused by the pressure of corrosion residual product buildup. This type of corrosion is difficult to detect in its initial stage. Extruded components such as spars can be subject to this type of corrosion. Ultrasonic and eddy current inspection methods are being used with a great deal of success. Stress Corrosion Stress corrosion occurs as the result of the combined effect of sustained tensile stresses and a corrosive environment. Stress corrosion cracking is found in most metal systems; however, it is particularly characteristic of aluminum, copper, certain stainless steels, and high strength alloy steels (over 240,000 psi). It usually occurs along lines of cold working and may

be transgranular or intergranular in nature. Aluminum alloy bellcranks with pressed in bushings, landing gear shock struts with pipe thread type grease fittings, clevis pin joints, shrink fits, and overstressed tubing B-nuts are examples of parts which are susceptible to stress corrosion cracking. Fretting Corrosion Fretting corrosion is a particularly damaging form of corrosive attack that occurs when two mating surfaces, normally at rest with respect to one another, are subject to slight relative motion. It is characterized by pitting of the surfaces and the generation of considerable quantities of finely divided debris. Since the restricted movements of the two surfaces prevent the debris from escaping very easily, an extremely local-

Figure 6-9. Exfoliation.

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ized abrasion occurs. [Figure 6-10] The presence of water vapor greatly increases this type of deterioration. If the contact areas are small and sharp, deep grooves resembling brinell markings or pressure indentations may be worn in the rubbing surface. As a result, this type of corrosion (on bearing surfaces) has also been called false brinelling.

Factors Affecting Corrosion Many factors affect the type, speed, cause, and seriousness of metal corrosion. Some of these factors can be controlled and some cannot. Climate The environmental conditions under which an aircraft is maintained and operated greatly affect corrosion characteristics. In a predominately marine environment (with exposure to sea water and salt air), moisture-laden air is considerably more detrimental to an aircraft than it would be if all operations were conducted in a dry climate. Temperature considerations are important because the speed of electrochemical attack is increased in a hot, moist climate. Foreign Material Among the controllable factors which affect the onset and spread of corrosive attack is foreign material that adheres to the metal surfaces. Such foreign material includes:

• Soil and atmospheric dust. • Oil, grease, and engine exhaust residues. • Salt water and salt moisture condensation. • Spilled battery acids and caustic cleaning solutions. • Welding and brazing flux residues. It is important that aircraft be kept clean. How often and to what extent an aircraft should be cleaned depends on several factors, including geographic location, model of aircraft, and type of operation.

Preventive Maintenance Much has been done to improve the corrosion resistance of aircraft: improvements in materials, surface treatments, insulation, and in particular, modern protective finishes. All of these have been aimed at reducing the overall maintenance effort, as well as improving reliability. In spite of these improvements, corrosion and its control is a very real problem that requires continuous preventive maintenance. During any corrosion control maintenance, consult the Material Safety Data Sheet (MSDS) for information on any chemicals used in the process. Corrosion preventive maintenance includes the following specific functions: 1. Adequate cleaning 2. Thorough periodic lubrication

Figure 6-10. Fretting corrosion.

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3. Detailed inspection for corrosion and failure of protective systems 4. Prompt treatment of corrosion and touchup of damaged paint areas 5. Keeping drain holes free of obstructions 6. Daily draining of fuel cell sumps 7. Daily wipe down of exposed critical areas 8. Sealing of aircraft against water during foul weather and proper ventilation on warm, sunny days 9. Maximum use of protective covers on parked aircraft After any period during which regular corrosion preventive maintenance is interrupted, the amount of maintenance required to repair accumulated corrosion damage and bring the aircraft back up to standard will usually be quite high.

Inspection Inspection for corrosion is a continuing problem and should be handled on a daily basis. Overemphasizing a particular corrosion problem when it is discovered and then forgetting about corrosion until the next crisis is an unsafe, costly, and troublesome practice. Most scheduled maintenance checklists are complete enough to cover all parts of the aircraft or engine, and no part of the aircraft should go uninspected. Use these checklists as a general guide when an area is to be inspected for corrosion. Through experience it will be learned that most aircraft have trouble areas where, despite routine inspection and maintenance, corrosion will set in. In addition to routine maintenance inspections, amphibians or seaplanes should be checked daily and critical areas cleaned or treated, as necessary.

Corrosion Prone Areas Discussed briefly in this section are most of the trouble areas common to all aircraft. However, this coverage is not necessarily complete and may be amplified and expanded to cover the special characteristics of the particular aircraft model involved by referring to the applicable maintenance manual. Exhaust Trail Areas Both jet and reciprocating engine exhaust deposits are very corrosive and give particular trouble where gaps, seams, hinges, and fairings are located downstream from the exhaust pipes or nozzles. [Figure 6-11] 6-8

Figure 6-11. Exhaust nozzle area.

Deposits may be trapped and not reached by normal cleaning methods. Pay special attention to areas around rivet heads and in skin lap joints and other crevices. Remove and inspect fairings and access plates in the exhaust areas. Do not overlook exhaust deposit buildup in remote areas, such as the empennage surfaces. Buildup in these areas will be slower and may not be noticed until corrosive damage has begun. Battery Compartments and Battery Vent Openings Despite improvements in protective paint finishes and in methods of sealing and venting, battery compartments continue to be corrosion prone areas. Fumes from overheated electrolyte are difficult to contain and will spread to adjacent cavities and cause a rapid corrosive attack on all unprotected metal surfaces. Battery vent openings on the aircraft skin should be included in the battery compartment inspection and maintenance procedure. Regular cleaning and neutralization of acid deposits will minimize corrosion from this cause. Bilge Areas These are natural sumps for waste hydraulic fluids, water, dirt, and odds and ends of debris. Residual oil quite often masks small quantities of water that settle to the bottom and set up a hidden chemical cell. Instead of using chemical treatments for the bilge water, current float manufacturers recommend the diligent maintenance of the internal coatings applied to the float’s interior during manufacture. In addition to chemical conversion coatings applied to the surface of the sheet metal and other structural components, and to sealants installed in lap joints during construction, the interior compartments are painted to protect

the bilge areas. When seaplane structures are repaired or restored, this level of corrosion protection must be maintained. Inspection procedures should include particular attention paid to areas located under galleys and lavatories and to human waste disposal openings on the aircraft exteriors. Human waste products and the chemicals used in lavatories are very corrosive to common aircraft metals. Clean these areas frequently and keep the paint touched up. Wheel Well and Landing Gear More than any other area on the aircraft, this area probably receives more punishment due to mud, water, salt, gravel, and other flying debris. Because of the many complicated shapes, assemblies, and fittings, complete area paint film coverage is difficult to attain and maintain. A partially applied preservative tends to mask corrosion rather than prevent it. Due to heat generated by braking action, preservatives cannot be used on some main landing gear wheels. During inspection of this area, pay particular attention to the following trouble spots: 1. Magnesium wheels, especially around bolt heads, lugs, and wheel web areas, particularly for the presence of entrapped water or its effects 2. Exposed rigid tubing, especially at B-nuts and ferrules, under clamps and tubing identification tapes 3. Exposed position indicator switches and other electrical equipment 4. Crevices between stiffeners, ribs, and lower skin surfaces, which are typical water and debris traps

marine operations. It is imperative that incipient corrosion be inhibited and that paint touchup and hard film preservative coatings are maintained intact on seaplane and amphibian engine surfaces at all times. Wing Flap and Spoiler Recesses Dirt and water may collect in flap and spoiler recesses and go unnoticed because they are normally retracted. For this reason, these recesses are potential corrosion problem areas. Inspect these areas with the spoilers and/or flaps in the fully deployed position. External Skin Areas External aircraft surfaces are readily visible and accessible for inspection and maintenance. Even here, certain types of configurations or combinations of materials become troublesome under certain operating conditions and require special attention. Relatively little corrosion trouble is experienced with magnesium skins if the original surface finish and insulation are adequately maintained. Trimming, drilling, and riveting destroy some of the original surface treatment, which is never completely restored by touchup procedures. Any inspection for corrosion should include all magnesium skin surfaces with special attention to edges, areas around fasteners, and cracked, chipped, or missing paint. Piano-type hinges are prime spots for corrosion due to the dissimilar metal contact between the steel pin and aluminum hinge. They are also natural traps for dirt, salt, and moisture. Inspection of hinges should include lubrication and actuation through several cycles to ensure complete lubricant penetration. Use water-displacing lubricants when servicing piano hinges. [Figures 6-12 and 6-13]

Water Entrapment Areas Design specifications require that aircraft have drains installed in all areas where water may collect. Daily inspection of low point drains should be a standard requirement. If this inspection is neglected, the drains may become ineffective because of accumulated debris, grease, or sealants. Engine Frontal Areas and Cooling Air Vents These areas are being constantly abraded with airborne dirt and dust, bits of gravel from runways, and rain erosion, which tends to remove the protective finish. Inspection of these areas should include all sections in the cooling air path, with special attention to places where salt deposits may be built up during

Figure 6-12. Piano hinge.

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Al Alloy extrusions Bare steel hinge pin

Hidden corrosion occurs here. Joint freezes and lugs break off when hinge is actuated.

Figure 6-13. Hinge corrosion points.

Corrosion of metal skins joined by spot welding is the result of the entrance and entrapment of corrosive agents between the layers of metal. This type of corrosion is evidenced by corrosion products appearing at the crevices through which the corrosive agents enter. More advanced corrosive attack causes skin buckling and eventual spot weld fracture. Skin buckling in its early stages may be detected by sighting along spot welded seams or by using a straightedge. The only technique for preventing this condition is to keep potential moisture entry points, including seams and holes created by broken spot welds, filled with a sealant or a suitable preservative compound. Miscellaneous Trouble Areas Helicopter rotor heads and gearboxes, in addition to being constantly exposed to the elements, contain bare steel surfaces, many external working parts, and dissimilar metal contacts. Inspect these areas frequently for evidence of corrosion. The proper maintenance, lubrication, and the use of preservative coatings can prevent corrosion in these areas. All control cables, whether plain carbon steel or corrosion resistant steel, should be inspected to determine their condition at each inspection period. In this process, inspect cables for corrosion by random cleaning of short sections with solvent soaked cloths. If external corrosion is evident, relieve tension and check the cable for internal corrosion. Replace cables that have internal corrosion. Remove light external corrosion with a nonwoven abrasive pad lightly soaked in oil or, alternatively, a steel wire brush. When corrosion products have been removed, recoat the cable with preservative. 6-10

Corrosion Removal In general, any complete corrosion treatment involves the following: (1) cleaning and stripping of the corroded area, (2) removing as much of the corrosion products as practicable, (3) neutralizing any residual materials remaining in pits and crevices, (4) restoring protective surface films, and (5) applying temporary or permanent coatings or paint finishes. The following paragraphs deal with the correction of corrosive attack on aircraft surface and components where deterioration has not progressed to the point requiring rework or structural repair of the part involved. Surface Cleaning and Paint Removal The removal of corrosion necessarily includes removal of surface finishes covering the attacked or suspected area. To assure maximum efficiency of the stripping compound, the area must be cleaned of grease, oil, dirt, or preservatives. This preliminary cleaning operation is also an aid in determining the extent of the spread of the corrosion, since the stripping operation will be held to the minimum consistent with full exposure of the corrosion damage. Extensive corrosion spread on any panel should be corrected by fully treating the entire section. The selection of the type of materials to be used in cleaning will depend on the nature of the matter to be removed. Modern environmental standards encourage the use of water-based, non-toxic cleaning compounds whenever possible. In some locations, local or state laws may require the use of such products, and prohibit the use of solvents that contain volatile

organic compounds (VOCs). Where permitted, dry cleaning solvent (P-D-680) may be used for removing oil, grease, or soft preservative compounds. For heavy-duty removal of thick or dried preservatives, other compounds of the solvent emulsion type are available. The use of a general purpose, water rinsable stripper can be used for most applications. There are other methods for paint removal that have minimal impact upon the aircraft structure, and are considered “environmentally friendly.” Wherever practicable, chemical paint removal from any large area should be accomplished outside (in open air) and preferably in shaded areas. If inside removal is necessary, adequate ventilation must be assured. Synthetic rubber surfaces, including aircraft tires, fabric, and acrylics, must be thoroughly protected against possible contact with paint remover. Care must be exercised in using paint remover. Care must also be exercised in using paint remover around gas or watertight seam sealants, since the stripper will tend to soften and destroy the integrity of these sealants. Mask off any opening that would permit the stripping compound to get into aircraft interiors or critical cavities. Paint stripper is toxic and contains ingredients harmful to both skin and eyes. Therefore, wear rubber gloves, aprons of acid repellent material, and goggletype eyeglasses. The following is a general stripping procedure: 1. Brush the entire area to be stripped with a cover of stripper to a depth of 1⁄32 to 1⁄16 inch. Any paintbrush makes a satisfactory applicator, except that the bristles will be loosened by the effect of paint remover on the binder, and the brush should not be used for other purposes after being exposed to paint remover. 2. Allow the stripper to remain on the surface for a sufficient length of time to wrinkle and lift the paint. This may be from 10 minutes to several hours, depending on both the temperature and humidity, and the condition of the paint coat being removed. Scrub the surface with a bristle brush saturated with paint remover to further loosen finish that may still be adhering to the metal. 3. Reapply the stripper as necessary in areas where the paint remains tightly adhered or where the stripper has dried, and repeat the above process. Only nonmetallic scrapers should be used to assist in removing persistent paint finishes. Nonwoven

abrasive pads intended for paint stripping may also prove to be useful in removing the loosened paint. 4. Remove the loosened paint and residual stripper by washing and scrubbing the surface with water and a broom, brush or fresh nonwoven abrasive pad. If water spray is available, use a low to medium pressure stream of water directly on the area being scrubbed. If steam-cleaning equipment is available and the area is sufficiently large, cleaning may be accomplished using this equipment together with a solution of steam-cleaning compound. On small areas, any method may be used that will assure complete rinsing of the cleaned area. Use care to dispose of the stripped residue in accordance with environmental laws.

Corrosion of Ferrous Metals One of the most familiar types of corrosion is ferrous oxide (rust), generally resulting from atmospheric oxidation of steel surfaces. Some metal oxides protect the underlying base metal, but rust is not a protective coating in any sense of the word. Its presence actually promotes additional attack by attracting moisture from the air and acting as a catalyst for additional corrosion. If complete control of the corrosive attack is to be realized, all rust must be removed from steel surfaces. Rust first appears on bolt heads, hold-down nuts, or other unprotected aircraft hardware. [Figure 6-14] Its presence in these areas is generally not dangerous and has no immediate effect on the structural strength of any major components. The residue from the rust may also contaminate other ferrous components, promoting corrosion of those parts. The rust is indicative of a need for maintenance and of possible corrosive attack in more critical areas. It is also a factor in the general appearance of the equipment. When paint failures occur or mechanical damage exposes highly stressed steel surfaces to the atmosphere, even the smallest amount of rusting is potentially dangerous in these areas and must be removed and controlled. Rust removal from structural components, followed by an inspection and damage assessment, must be done as soon as feasible. [Figure 6-15] Mechanical Removal of Iron Rust The most practicable means of controlling the corrosion of steel is the complete removal of corrosion products by mechanical means and restoring corrosion preventive coatings. Except on highly stressed 6-11

Figure 6-14. Rust.

Figure 6-15. Rust on structural components.

steel surfaces, the use of abrasive papers and compounds, small power buffers and buffing compounds, hand wire brushing, or steel wool are all acceptable cleanup procedures. However, it should be recognized that in any such use of abrasives, residual rust usually remains in the bottom of small pits and other crevices. It is practically impossible to remove all corrosion products by abrasive or polishing methods alone. As a result, once a part cleaned in such a man-

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ner has rusted, it usually corrodes again more easily than it did the first time. The introduction of variations of the nonwoven abrasive pad has also increased the options available for the removal of surface rust. [Figure 6-16] Flap wheels, pads intended for use with rotary or oscillating power tools, and hand-held nonwoven abrasive pads all can be used alone or with light oils to remove corrosion from ferrous components.

cessing, using mild abrasive papers such as rouge or fine grit aluminum oxide, or fine buffing compounds on cloth buffing wheels. Nonwoven abrasive pads can also be used. It is essential that steel surfaces not be overheated during buffing. After careful removal of surface corrosion, reapply protective paint finishes immediately.

Corrosion of Aluminum and Aluminum Alloys

Figure 6-16. Nonwoven abrasive pads.

Chemical Removal of Rust As environmental concerns have been addressed in recent years, interest in noncaustic chemical rust removal has increased. A variety of commercial products, which actively remove the iron oxide without chemically etching the base metal, are available and should be considered for use. Generally speaking, if at all possible, the steel part should be removed from the airframe for treatment, as it can be nearly impossible to remove all residues. The use of any caustic rust removal product will require the isolation of the part from any nonferrous metals during treatment, and will probably require inspection for proper dimensions.

Corrosion on aluminum surfaces is usually quite obvious, since the products of corrosion are white and generally more voluminous than the original base metal. Even in its early stages, aluminum corrosion is evident as general etching, pitting, or roughness of the aluminum surfaces. NOTE: Aluminum alloys commonly form a smooth surface oxidation that is from 0.001 to 0.0025 inch thick. This is not considered detrimental; the coating provides a hard shell barrier to the introduction of corrosive elements. Such oxidation is not to be confused with the severe corrosion discussed in this paragraph. General surface attack of aluminum penetrates relatively slowly, but is speeded up in the presence of dissolved salts. Considerable attack can usually take place before serious loss of structural strength develops.

Chemical Surface Treatment of Steel There are approved methods for converting active rust to phosphates and other protective coatings. Other commercial preparations are effective rust converters where tolerances are not critical and where thorough rinsing and neutralizing of residual acid is possible. These situations are generally not applicable to assembled aircraft, and the use of chemical inhibitors on installed steel parts is not only undesirable but also very dangerous. The danger of entrapment of corrosive solutions and the resulting uncontrolled attack, which could occur when such materials are used under field conditions, outweigh any advantages to be gained from their use.

At least three forms of attack on aluminum alloys are particularly serious: (1) the penetrating pit-type corrosion through the walls of aluminum tubing, (2) stress corrosion cracking of materials under sustained stress, and (3) intergranular corrosion which is characteristic of certain improperly heat-treated aluminum alloys.

Removal of Corrosion from Highly Stressed Steel Parts Any corrosion on the surface of a highly stressed steel part is potentially dangerous, and the careful removal of corrosion products is required. Surface scratches or change in surface structure from overheating can also cause sudden failure of these parts. Corrosion products must be removed by careful pro-

Treatment of Unpainted Aluminum Surfaces Relatively pure aluminum has considerably more corrosion resistance compared with the stronger aluminum alloys. To take advantage of this characteristic, a thin coating of relatively pure aluminum is applied over the base aluminum alloy. The protection obtained is good, and the pure-aluminum clad surface (commonly called “Alclad”) can be maintained in a polished condition. In cleaning such surfaces, how-

In general, corrosion of aluminum can be more effectively treated in place compared to corrosion occurring on other structural materials used in aircraft. Treatment includes the mechanical removal of as much of the corrosion products as practicable, and the inhibition of residual materials by chemical means, followed by the restoration of permanent surface coatings.

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ever, care must be taken to prevent staining and marring of the exposed aluminum and, more important from a protection standpoint, to avoid unnecessary mechanical removal of the protective Alclad layer and the exposure of the more susceptible aluminum alloy base material. A typical aluminum corrosion treatment sequence follows: 1. Remove oil and surface dirt from the aluminum surface using any suitable mild cleaner. Use caution when choosing a cleaner; many commercial consumer products are actually caustic enough to induce corrosion if trapped between aluminum lap joints. Choose a neutral Ph product. 2. Hand polish the corroded areas with fine abrasives or with metal polish. Metal polish intended for use on clad aluminum aircraft surfaces must not be used on anodized aluminum since it is abrasive enough to actually remove the protective anodized film. It effectively removes stains and produces a highly polished, lasting surface on unpainted Alclad. If a surface is particularly difficult to clean, a cleaner and brightener compound for aluminum can be used before polishing to shorten the time and lessen the effort necessary to get a clean surface. 3. Treat any superficial corrosion present, using an inhibitive wipe down material. An alternate treatment is processing with a solution of sodium dichromate and chromium trioxide. Allow these solutions to remain on the corroded area for 5 to 20 minutes, and then remove the excess by rinsing and wiping the surface dry with a clean cloth. 4. Overcoat the polished surfaces with waterproof wax. Aluminum surfaces that are to be subsequently painted can be exposed to more severe cleaning procedures and can also be give