MODULE 07A. MAINTENANCE PRACTICES

European Aviation Safety Agency (EASA) Part-66

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JAR 66 CATEGORY B1 MODULE 7 MAINTENANCE PRACTICES (mechanical)

CONTENTS 1 INTRODUCTION................................................................................1 2 SAFETY PRECAUTIONS..................................................................1 2.1 Fire...................................................................................................1 2.1.1 The ‘Fire Triangle’...............................................................2 2.1.2 Classes of Fire....................................................................2 2.1.3 Fire Extinguishants and their Uses.....................................3 2.2 First Aid...........................................................................................5 2.3 Workshop and Hangar Safety........................................................5 2.3.1 Machinery...........................................................................5 2.3.2 Electricity............................................................................6 2.3.3 Noise..................................................................................7 2.3.4 High-Pressure Gases.........................................................7 2.3.5 Gas Bottle Identification......................................................8 2.3.6 High-Pressure Gas Replenishing........................................8 2.3.7 Oxygen Systems................................................................9 2.3.8 Aviation Oils and Fuels.......................................................10 2.3.9 Chemical and Physiological Hazards..................................11 2.3.10 Lifting and Shoring............................................................11 2.3.11 Slinging.............................................................................12 2.4 Flight-Line Safety...........................................................................13 2.4.1 Towing and Taxying...........................................................14 2.4.2 Parking...............................................................................15 2.4.3 Marshalling.........................................................................16 2.4.4 Fuelling...............................................................................17 2.4.5 Weather Radar...................................................................18

3 WORKSHOP PRACTICES................................................................1 3.1 Care of Tools...................................................................................1 3.2 Control of Tools..............................................................................2 3.3 Calibration of Tools and Equipment..............................................3 3.3.1 General Notes on Calibration.............................................3 3.3.2 Procedures.........................................................................4 3.4 Use of Workshop Materials............................................................6 3.5 Standards of Workmanship...........................................................7

4 TOOLS................................................................................................1 4.1 Common Hand Tools......................................................................1 4.1.1 Engineer’s Rule..................................................................1 4.1.2 Scriber................................................................................2 4.1.3 Key-Seat Rule....................................................................2 4.1.4 Fitter’s Square....................................................................3 4.1.5 Combination Set.................................................................4 4.1.6 Surface Plates and Tables..................................................5 4.1.7 V Blocks.............................................................................5 4.1.8 Surface Gauge (Scribing Block)..........................................6 4.1.9 Dividers..............................................................................7 4.1.10 Callipers...........................................................................7 4.1.11 Hammers..........................................................................8

71811175.doc Issued December 2002

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4.1.12 Punches...........................................................................9 4.1.13 Metal-Cutting Chisels........................................................10 4.1.14 Bench Vice.......................................................................12 4.1.15 Hand Vice.........................................................................13 4.1.16 Hacksaws.........................................................................14 4.1.17 Sheet Metal Shears and Snips.........................................15 4.1.18 Files..................................................................................16 4.1.19 Filing Techniques.............................................................19 4.1.20 Hand Brace (Hand Drill)....................................................21 4.1.21 Twist Drills........................................................................22 4.1.22 Stop, and Press (Dimpling), Countersinking Tools............27 4.1.23 Reamers...........................................................................29 4.1.24 Internal Screw Thread, Cutting Taps................................32 4.1.25 External Screw Thread, Cutting Dies................................34 4.1.26 Screwdrivers.....................................................................36 4.1.27 Pliers................................................................................38 4.1.28 Wire Snips (Nippers).........................................................39 4.1.29 Spanners, Sockets and Wrenches....................................39 4.2 Common Power Tools....................................................................45 4.2.1 Electrically Powered Pillar Drills..........................................45 4.2.2 Electrically Powered Hand Drills.........................................46 4.2.3 Pneumatically Powered Hand Drills....................................46 4.2.4 Pneumatically Powered Riveting Hammers........................48 4.2.5 Pneumatic Miller (Microshaver)..........................................49 4.2.6 Nibblers..............................................................................49 4.2.7 Pneumatic Tool Maintenance.............................................50 4.2.8 Abrasive Wheels.................................................................50 4.3 Precision Measuring Instruments..................................................52 4.3.1 External Micrometers..........................................................52 4.3.2 Internal Micrometers...........................................................56 4.3.3 Micrometer Depth Gauge...................................................57 4.3.4 Vernier Micrometers...........................................................58 4.3.5 Vernier Callipers.................................................................60 4.3.6 Vernier Height Gauge.........................................................61 4.3.7 Vernier Protractor...............................................................62 4.4 Miscellaneous Measuring Tools....................................................63 4.4.1 Gauge Blocks.....................................................................63 4.4.2 Dial Test Indicator (DTI)......................................................64 4.4.3 Feeler Gauges....................................................................64 4.4.4 Screw Pitch and Radius Gauges........................................65 4.4.5 Go/No-Go Gauges..............................................................65 4.4.6 Straight Edges....................................................................65 4.5 Lubrication Methods and Equipment............................................66 4.5.1 Lubrication Methods...........................................................66 4.5.2 Lubrication Equipment........................................................69

5 ENGINEERING DRAWING, DIAGRAMS AND STANDARDS.........1 5.1 Types of Drawing............................................................................1 5.2 Methods of Drawing Solid Objects................................................2 5.2.1 Pictorial Projections............................................................3 5.2.2 Orthographic Projections....................................................4

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5.2.3 Sectional Views..................................................................5 5.2.4 Exploded Views..................................................................6 5.2.5 Drawing Lines, Symbols and Abbreviations........................7 5.2.6 Conventional Representations............................................9 5.2.7 General and Geometric Tolerances....................................9 5.3 Drawing Identification System.......................................................10 5.3.1 Title Block...........................................................................11 5.3.2 Drawing Number.................................................................11 5.3.3 Handed Parts......................................................................11 5.3.4 Sheet Numbers...................................................................11 5.3.5 Drawing Changes...............................................................11 5.3.6 Part Referencing.................................................................12 5.3.7 Validation of Modification/Repair Drawings.........................12 5.3.8 Summary of Recommended Drawing Information..............13 5.4 Auxiliary Diagrams and Charts......................................................14 5.4.1 Electical Wiring Diagrams...................................................14 5.4.2 Component Location Diagrams..........................................15 5.4.3 Schematic Diagrams..........................................................16 5.4.4 Block Diagrams..................................................................17 5.4.5 Logic Flowcharts.................................................................17 5.5 Microfilm, Microfiche and Computerised Presentations..............19 5.5.1 Microfilm.............................................................................19 5.5.2 Microfiche...........................................................................19 5.5.3 Computer CD-ROM............................................................20 5.5.4 Supplementary Information.................................................20 5.6 Aeronautical Standards..................................................................21 5.6.1 Air Transport Association Specification No. 100.................21 5.6.2 International Organisation for Standardisation (ISO)..........24 5.6.3 British Standards (BS).......................................................24 5.6.4 Military Standard (MS).......................................................24 5.6.5 Air Force and Navy (AN)....................................................24 5.6.6 National Aerospace Standard (NAS).................................24

6 FITS AND CLEARANCES.................................................................1 6.1 Dimensions......................................................................................1 6.1.1 Allowances.........................................................................1 6.1.2 Tolerances..........................................................................2 6.2 Drilling Sizes for Holes...................................................................3 6.3 Classes of Fits.................................................................................3 6.3.1 Newall System....................................................................4 6.3.2 British Standards System...................................................5 6.4 Schedule of Fits and Clearances...................................................5 6.4.1 Limits for Wear...................................................................6 6.4.2 Limits for Ovality.................................................................6 6.4.3 Limits for Bow.....................................................................7 6.4.4 Limits for Twist....................................................................8

7 RIVETING...........................................................................................1 7.1 Types of Solid Rivet........................................................................1 7.1.1 Rivet Materials....................................................................2 7.1.2 Basic Rivet Location Terminology.......................................2


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7.2 Types of Riveted Joints..................................................................4 7.3 Closing Solid Rivets.......................................................................4 7.4 Closing Hollow Rivets...................................................................5 7.4.1 Tucker-pop.........................................................................5 7.4.2 Chobert...............................................................................5 7.4.3 Avdel..................................................................................6 7.4.4 Cherry Max.........................................................................7 7.4.5 Hi-Lok.................................................................................8 7.4.6 Rivnuts...............................................................................8 7.5 Inspection of Riveted Joints..........................................................9 7.6 Rivet Removal Procedure...............................................................10

8 PIPES AND HOSES...........................................................................1 8.1 Pipe Bending...................................................................................1 8.1.1 Simple Bending Jigs...........................................................2 8.1.2 Hand Pipe-Bending Machines............................................2 8.2 Pipe Flaring.....................................................................................3 8.2.1 Flaring Tool........................................................................3 8.2.2 Standard Flared Pipe Couplings.........................................4 8.2.3 Flareless Couplings............................................................5 8.3 Inspection and Testing of Pipes and Hoses.................................6 8.3.1 Bore Testing of Pipes.........................................................7 8.3.2 Hydraulic Pressure Testing of Pipes...................................7 8.3.3 Pneumatic and Oxygen Pressure Testing of Pipes.............7 8.3.4 Cleaning After Test.............................................................7 8.3.5 Testing Flexible Hoses.......................................................8 8.4 Installation and Clamping of Pipes................................................8 8.4.1 Pipe Supports.....................................................................8 8.5 Connection of Pipes.......................................................................10 8.6 Maintenance of Pipes and Hoses..................................................10 8.7 Pipe Identification Tape..................................................................11

9 SPRINGS............................................................................................1 9.1 Inspection and Testing of Springs................................................1

10 BEARINGS.......................................................................................1 10.1 Cleaning and Inspection of Bearings..........................................1 10.2 Inspection of Bearings.................................................................2 10.2.1 Normal Fatigue.................................................................2 10.2.2 Excessive Loads...............................................................2 10.2.3 Installation and Misalignment...........................................3 10.2.4 Loose Fit...........................................................................3 10.2.5 Brinelling...........................................................................3 10.2.6 Overheating and Lubrication Failure.................................4 10.2.7 Contamination and Corrosion...........................................5 10.3 Safety Precautions........................................................................5 10.4 Storage..........................................................................................5

11 TRANSMISSIONS............................................................................1 11.1 Gears..............................................................................................1 11.2 Belts and Pulleys..........................................................................1

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11.3 Chains and Sprockets..................................................................2 11.4 Screw Jacks..................................................................................3 11.5 Levers............................................................................................4 11.5.1 Push-Pull Rod Systems....................................................5

12 CONTROL CABLES........................................................................1 12.1 Swaging of End Fittings...............................................................1 12.2 Inspection and Testing of Control Cables..................................1 12.2.1 Cable Wear.......................................................................1 12.2.2 Bowden and Teleflex Cable Systems...............................3 12.3 Inspection of Control Cable Pulleys............................................4

13 SHEET METAL WORK....................................................................1 13.1 Marking Out...................................................................................2 13.2 Forming of Sheet Metal Parts.......................................................3 13.2.1 Cutting..............................................................................3 13.2.2 Bending and Calculation of Bend Allowance....................4 13.3 Inspection of Sheet Metal Work...................................................8

14 WELDING, SOLDERING AND BONDING......................................1 14.1 Welding..........................................................................................1 14.2 Methods of Welding......................................................................1 14.2.1 Oxy-Acetylene Flame.......................................................1 14.2.2 Manual Metal Arc..............................................................2 14.2.3 Metal Arc Gas-Shielded (MAGS)......................................2 14.2.4 Tungsten Arc Gas-Shielded (TAGS) ................................2 14.2.5 Flash Butt Welding...........................................................3 14.2.6 Spot Welding....................................................................3 14.2.7 Seam Welding..................................................................3 14.3 Inspection and Testing of Welds.................................................3 14.4 Soldering.......................................................................................4 14.5 Methods of Soldering...................................................................4 14.5.1 Hard Soldering (Brazing and Silver Soldering).................4 14.5.2 Soft Soldering...................................................................5 14.5.3 Using Indirectly Heated (Electric) Soldering Irons.............6 14.5.4 Active and Passive Fluxes................................................8 14.5.5 Flux Removal....................................................................10 14.6 Inspection and Testing of Soldered Joints.................................10 14.7 Bonding.........................................................................................10 14.8 Methods of Bonding.....................................................................11 14.8.1 Thermoplastic Adhesives..................................................11 14.8.2 Thermosetting Adhesives.................................................12 14.9 Inspection and Testing of Bonded Joints...................................12

15 AIRCRAFT MASS AND BALANCE................................................1 15.1 Definitions.....................................................................................1 15.2 Mass and Balance.........................................................................2 15.2.1 Mass and Balance Documentation...................................3 15.3 Frequency of Weighing................................................................4 15.3.1 Fleet Mass and CG Position.............................................4


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15.4 Weighing Requirements...............................................................4 15.5 Centre of Gravity Limits (CG Envelope)......................................5 15.6 Records.........................................................................................5 15.7 Calculation of Mass and CG of any System................................5 15.8 Principles of Weight and Balance of Aircraft..............................7 15.9 Calculation of Mass and CG of Aircraft.......................................7 15.10 Aircraft Weighing Methods.........................................................8 15.10.1 Preparation for Weighing................................................9 15.10.2 Weighing on Aircraft Jacks.............................................9 15.10.3 Calculation of Aircraft’s CG.............................................10 15.10.4 CG as Percentage Standard Mean Chord (SMC)...........12 15.11 Changes in Basic Weight...........................................................12 15.11.1 Examples of Alterations to Dry Operating Mass..............13 15.12 Loading of Aircraft (Typical Aircraft Load Sheet).....................15

16 AIRCRAFT HANDLING AND STORAGE.......................................1 16.1 Moving Methods ...........................................................................2 16.1.1 Moving by Hand and Steering Arm...................................2 16.1.2 Using a Bridle and Steering Arm.......................................2 16.1.3 Using a Purpose-Made Towing Arm.................................3 16.1.4 Precautions when Towing Aircraft....................................3 16.1.5 Taxiing Aircraft..................................................................4 16.2 Aircraft Jacking.............................................................................5 16.2.1 Special Considerations.....................................................5 16.2.2 Aircraft Jacks....................................................................6 16.2.3 Jack Maintenance and General Notes..............................7 16.2.4 Jacking Precautions.........................................................8 16.2.5 Jacking Procedures..........................................................8 16.2.6 Trestles.............................................................................9 16.2.7 Lowering Aircraft off Jacks................................................10 16.3 Slinging .........................................................................................10 16.3.1 Lifting Tackle....................................................................11 16.4 Parking and Mooring Aircraft.......................................................12 16.4.1 Parking.............................................................................12 16.4.2 Mooring (Picketing)...........................................................13 16.4.3 Typical Small Aircraft Procedures.....................................14 16.4.4 Large Aircraft Procedures.................................................14 16.4.5 Chocking of Aircraft..........................................................15 16.5 Aircraft Storage.............................................................................16 16.6 Aircraft Fuelling Procedures........................................................20 16.6.1 Fuelling Safety Precautions .............................................20 16.6.2 Refuelling.........................................................................21 16.6.3 Checking Fuel Contents...................................................21 16.6.4 Defuelling.........................................................................22 16.7 Ground De-Icing/Anti-Icing of Aircraft.........................................23 16.7.1 Ice Types..........................................................................23 16.7.2 Definitions.........................................................................25 16.7.3 De-Icing and Anti-Icing Methods.......................................25 16.7.4 Chemical De-Icing............................................................26

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16.7.5 Treatment of Frost Deposits.............................................26 16.7.6 Removal of Ice and Snow Deposits..................................27 16.7.7 Hold Over Times...............................................................29 16.7.8 Inspection after De-Icing/Anti-Icing Procedures................30 16.8 Ground Electrical Supplies..........................................................31 16.9 Ground Hydraulic Supplies..........................................................33 16.9.1 Safety, Health and Servicing Precautions.........................33 16.9.2 Rig Maintenance...............................................................34 16.10 Ground Pneumatic Supplies......................................................34 16.11 Effects of Environmental Conditions on Handling...................35 16.11.1 Cold and Wet..................................................................35 16.11.2 Snow and Ice..................................................................36 16.11.3 High Winds.....................................................................36 16.11.4 High Temperature...........................................................37

17 PREVENTATIVE MAINTENANCE TECHNIQUES..........................1 17.1 Types of Defects...........................................................................1 17.1.1 External Damage..............................................................2 17.1.2 Inlets and Exhausts..........................................................3 17.1.3 Liquid Systems.................................................................3 17.1.4 Gaseous Systems............................................................4 17.1.5 Dimensions.......................................................................5 17.1.6 Tyres................................................................................5 17.1.7 Wheels.............................................................................6 17.1.8 Brakes..............................................................................6 17.1.9 Landing Gear Locks.........................................................7 17.1.10 Indicators........................................................................7 17.1.11 External Probes..............................................................8 17.1.12 Handles and Latches......................................................8 17.1.13 Panels and Doors...........................................................8 17.1.14 Emergency System Indication........................................9 17.1.15 Lifed Items......................................................................9 17.1.16 Light Bulbs......................................................................9 17.1.17 Permitted Defects...........................................................9 17.2 Locations of Corrosion in Aircraft...............................................10 17.2.1 Exhaust Areas..................................................................10 17.2.2 Engine Intakes and Cooling Air Vents..............................10 17.2.3 Landing Gear....................................................................10 17.2.4 Bilge and Water Entrapment Areas..................................11 17.2.5 Recesses in Flaps and Hinges.........................................11 17.2.6 Magnesium Alloy Skins.....................................................11 17.2.7 Aluminium Alloy Skins......................................................11 17.2.8 Spot-Welded Skins and Sandwich Constructions.............12 17.2.9 Electrical Equipment.........................................................12 17.2.10 Control Cables................................................................12 17.3 Corrosion Removal, Assessment and Reprotection..................13 17.3.1 Cleaning and Paint Removal............................................13 17.3.2 Ferrous Metals..................................................................14 17.3.3 Aluminium and Aluminium Alloys......................................14 17.3.4 Alclad................................................................................15 17.3.5 Magnesium Alloys............................................................16


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17.3.6 Acid Spillage.....................................................................16 17.3.7 Alkali Spillage...................................................................16 17.3.8 Mercury Spillage...............................................................17 17.4 Permanent Anti-Corrosion Treatments.......................................18 17.4.1 Electro-Plating..................................................................18 17.4.2 Sprayed Metal Coatings...................................................18 17.4.3 Cladding...........................................................................18 17.4.4 Surface Conversion Coatings...........................................19 17.5 Non-Destructive Testing/Inspection (NDT/NDI) Techniques.....20 17.5.1 Visual/Assisted Visual Inspections....................................21 17.5.2 Remote Viewing Instruments............................................22 17.5.3 Penetrant Flaw Detection (PFD).......................................25 17.5.4 Ultrasonic Flaw Detection (UFD)......................................34 17.5.5 Eddy Current Flaw Detection (ECFD)...............................40 17.5.6 Magnetic Particle Flaw Detection (MPFD)........................47 17.5.7 Radiographic Flaw Detection (RFD).................................53 17.5.8 Miscellaneous Radiation Techniques ..............................59 17.6 Disassembly and Re-assembly Techniques...............................60 17.6.1 Complete Airframes..........................................................60 17.6.2 Replacement of Major Components/Modules...................61 17.6.3 Replacement of Minor Components/Modules...................62 17.6.4 Disassembly and Re-assembly of Major Components......62 17.6.5 Disassembly and Re-assembly of Minor Components......62 17.6.6 Basic Disassembly and Re-assembly Techniques............63 17.6.7 Small Part and Component Identification..........................64 17.6.8 Discarding of Parts...........................................................65 17.6.9 Freeing Seized Components............................................65 17.6.10 Use of Correct Tools.......................................................65 17.6.11 ‘Murphy’s Law’................................................................66

18 ABNORMAL EVENTS.....................................................................1 18.1 Types of abnormal occurrences..................................................1 18.2 Types of damage...........................................................................1 18.3 Lightning Strikes ..........................................................................2 18.3.1 Effects of a Lightning Strike..............................................2 18.3.2 Inspection.........................................................................2 18.4 Example of a Post Lightning Strike Procedure ..........................3 18.4.1 Basic Protection................................................................3 18.4.2 Strike Areas......................................................................4 18.4.3 Signs of Damage..............................................................5 18.4.4 External Components at Risk...........................................5 18.4.5 Electrical Components at Risk..........................................6 18.4.6 Examination of External Surface.......................................6 18.4.7 Functional Tests...............................................................7 18.4.8 Examination of Internal Components................................8 18.4.9 Return the Aircraft to Service............................................9 18.5 High Intensity Radiated Fields (HIRF) Penetration.....................9 18.5.1 Specific Testing – HIRF....................................................10 18.5.2 Protection against HIRF Interference................................11 18.6 Heavy Landings............................................................................12

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18.6.1 Example of Post Heavy Landing Inspection......................12 18.7 Flight through Severe Turbulence...............................................14

19 MAINTENANCE PROCEDURES.....................................................1 19.1 Maintenance Planning..................................................................1 19.2 Modification Procedures..............................................................2 19.2.1 Major Modifications...........................................................2 19.2.2 Minor Modifications...........................................................2 19.3 Stores Procedures........................................................................3 19.4 Certification and Release Procedures.........................................3 19.4.1 Interface with Aircraft Operation.......................................4 19.5 Maintenance Inspection/ Quality Control and Assurance.......5 19.6 Additional Maintenance Procedures...........................................6 19.7 Control of Life-Limited Components...........................................6


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INTRODUCTION

Most accidents are, in the main, caused by human carelessness and accidents in the work place are among the main causes of death and disability. They are, additionally, the cause of a great loss of man-hours and, thus, cost companies (and individuals) large amounts of money. All personnel should be aware, not only of the potential for accidents and injury, wherever they work, but also of the legislation and information that is available in an attempt to prevent accidents actually happening. While it is incumbent upon companies (in accordance with The Management of Health and Safety at Work Regulations 1992), to ensure that all personnel receive adequate training in Health and Safety matters, this Module contains a reminder of some of the general safety precautions which are necessary, when working in the aerospace industry. The Module continues with further topics, which are concerned with the practices recommended for the safe and efficient maintenance of aircraft and aerospace components.


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SAFETY PRECAUTIONS

Aircraft, by their very nature and design, make for a dangerous working environment. The danger is further increased by the wide variety of machines, tools and materials required to support and maintain aircraft. Personal safety starts with being appropriately dressed for the work being undertaken, combined with the correct use of eye and ear protection whenever necessary. Technicians should only operate equipment with which they are familiar and which they can operate safely. Hand tools should be kept in good working order. Good ‘housekeeping’ in workshops, hangars, and on flight line ramps is essential to safe and efficient maintenance. Pedestrian and fire lanes should be clearly marked and NEVER obstructed. They should always be used to keep non-technical personnel clear from the work area. Any spillage of oils, greases and fuels should be immediately covered with absorbent material and cleaned up, to prevent fire or injury. Spillage should be prevented, from running into floor drains. It is very important, that all personnel know the location of the fixed points where fire fighting equipment and First Aid treatment are available. They must also be aware of the types of emergency that can occur in the workplace (whether in the workshop, hangar or on the ramp), and of the procedures to be followed in any emergency. 2.1 Fire WARNING: ALWAYS ENSURE THAT CORRECT FIRE PRECAUTIONS ARE OBEYED AND THAT ESCAPE ROUTES ARE NOT OBSTRUCTED. LETHAL FUMES AND SMOKE CAN BE PRODUCED BY CERTAIN MATERIALS AND THEY CAN BURN VERY RAPIDLY. Personnel, engaged in the maintenance, overhaul and repair of aircraft, should be fully conversant with the precautions required to prevent outbreaks of any fire. They should be qualified in the operation of any fire protection equipment that is provided, and should know the action to be taken in the event of discovering a fire.


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2.1.1 The ‘Fire Triangle’ Fire results from the chemical reaction that occurs when oxygen combines rapidly with fuel to produce heat, (and light). Three essentials of this process form the ‘Fire Triangle’ (refer to Fig.1).

The ‘Fire Triangle’ Fig. 1 As can be seen, a fire requires three components to burn, and the removal of any one of these components will extinguish the fire. The requirements of the three components, forming the ‘Fire Triangle’, are: • • •

Fuel: a combustible material, which may be a solid, liquid or gas Oxygen: in sufficient volume to support the process of combustion Heat: of sufficient intensity to raise the temperature of the fuel to its ignition (or kindling) point.

2.1.2 Classes of Fire There are, generally, four classes of fires, each determined by the type of material that is being burned. In alphabetical, order the classes of fire are: • • • •

Class A: often known as solid fires, which occur in materials such as paper, wood, textiles and general rubbish. Class B: often described as liquid fires, and include fires in materials such as internal combustion engine fuels, alcohol, oils, greases and oil-based paints. Class C: include fires involving flammable gases and electrical fires (which can occur in fuse boxes, switches, appliances, motors and generators). Class D: refer to fires of high intensity, which may occur in such metals as magnesium, potassium, sodium, titanium and zirconium. The greatest hazard in these materials, is when they are either in liquid (molten) form, or in finely divided forms such as dust, chippings, turnings or shavings.

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2.1.3 Fire Extinguishants and their Uses The methods of extinguishing fires have led to the development of several types of extinguishants to cater for different types of fire. These methods include: • • •

Cooling the fuel Excluding the oxygen Separating the fuel from the oxygen

The materials, used as general ‘domestic and commercial’ extinguishants, differ from those used in aircraft Fire Protection systems and, while the aircraft systems are discussed in other Modules of this course, consideration is given here only to the extinguishants and extinguishers which conform to the EN3 Standard fire extinguisher code. The materials used in these extinguishers are: • • • •

Water (Water/Gas) Aqueous Film-Forming Foam (AFFF) Carbon Dioxide (CO2) Dry Powder

Applying the incorrect extinguishant to a fire can do more harm than good and may, actually, be dangerous. It is, therefore, important that extinguishers are well marked for quick identification in an emergency. It is also vital that all personnel are aware of the markings, which appear on extinguishers, so that the correct one is chosen to deal with a specific fire. Table 2 shows how the EN3 Standard fire extinguisher code has replaced the older Standard, whereby the extinguisher containers were colour-coded all over to signify their contents. The EN3 Standard has the bodies of every fire extinguisher coloured red all over, with an identifying band of colour, separated by white lines, identifying the extinguishant contained in the extinguisher. Table 2 FIRE EXTINGUISHER IDENTIFICATION AND USES EN3 Standard Extinguishers (All-red Container) Extinguishant Band Colour Types of Fire Water Red Solids only, but NOT Electrical NOR (Water/Gas) Flammable Liquids Aqueous Film-Forming Foam Cream Oil, Fats, Paint, Petrol, and Solids, (AFFF) but NOT safe on Electrical fires Carbon Dioxide Black Gases, Electrical, Flammable Liquids (CO2) and Solids but NOT Burning Metals Dry Powder Blue Burning Metals, Flammable Liquids, and Electrical (<1000 V, >1 m) fires


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From Table 2 it can be seen that Water or Water/Gas extinguishers are ONLY to be used on fires involving burning solids (Class A fires). Water could also cause liquid fires to spread and, obviously, using water on electrical equipment could have lethal results, so these extinguishers must NOT be used on Class B NOR on Class C fires. Water should, also, NOT be used on burning metal (Class D) fires, as the oxygen, in the water, will cause the fires to burn more fiercely and its use could lead to violent explosions. Aqueous Film-Forming Foam (AFFF) is best suited for Class B fires, due to its smothering and cooling action and to the fact that its finer particles will not cause the fire to spread. AFFF extinguishers can also be used on Class A fires (though its cooling action is not as effective as the water extinguishers), but, because Foam does contain water, AFFF extinguishers are considered to be NOT safe on electrical fires where high voltages are encountered. Carbon Dioxide (CO2) is the ‘universal’ fire extinguisher and, being non-corrosive, non-conductive, and leaving no residue, it is suitable for almost all types of fire. CO2 extinguishers must NOT, however, be used on Class D fires, as the extinguishant reduces the temperature very quickly, which (in a similar way to the use of water extinguishers) could cause serious explosions. Due to the fact that CO2 gas tends to dissipate quickly, the extinguisher is provided with a horn device, which helps to concentrate the CO2 at the site of the fire. This horn must NOT be held with bare hands, as the intense cold of the released CO2 will freeze the skin to the horn, resulting in severe injury to the hands. A rubber, insulated coating is provided on the discharge tube and the CO2 must be directed towards the fire by grasping and manipulating the insulated tube. Dry Powder is another extinguishant which is suitable for most classes of fire, and, in particular, those involving burning metals (aircraft wheel brake fires). It is, however, limited in its use on electrical fires, as the powder particles are capable of conducting high voltages (in excess of 1000 V) and, possibly, lesser voltages if they are used at distances of less than 1 metre from electrical fires. Dry Powder (in a similar way to Foam), leaves a messy residue after its use, which could present a problem to electrical contacts and circuitry. Note: It is possible that the older Standard ‘Halon’ fire extinguishants (in greencoloured containers) may be found at many indoor locations. Unfortunately, while Halons (Halogenated Hydrocarbons) are extremely effective as extinguishants of virtually every class of fire, it is felt that they contribute to the depletion of the ozone layer surrounding Earth and, so, they are being phased out of use. Buckets of dry sand may also be placed at the FIRE POINT in workshops (and especially in hangars) as an additional aid to fire fighting.

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2.2 First Aid It has been previously discussed that, when working indoors, whether it is in an office, a workshop or a hangar, there will be fixed points where fire-fighting equipment is available. Similarly, there will be First Aid points where emergency kits, eye washing equipment and call bells are installed and there will be trained First Aid personnel to assist in the treatment of injuries. It is the responsibility of every person at work to know: • • • •

The location of the First Aid Points The methods of calling for help The locations of alarm bells, and the siting of appropriate telephones which may be used to summon help in an emergency The identity of the trained First Aid personnel in their vicinity

In the event of an injury (however slight), it is important that the injured person, or the attending First Aider, should complete an entry in the Accident Book, which is usually kept near the First Aid Point. 2.3 Workshop and Hangar Safety When working in a workshop or in any hangar, there are a number of safety precautions that must be followed, if injury (or death) is to be avoided. 2.3.1 Machinery A machine can be defined as an ‘apparatus for applying power, having fixed and moving parts, each having a definite function’. In particular, machines embrace: • •

Operational Parts - performing the principal output function (Chucks or Bits) Non-Operational Parts - conveying power or motion (Motor Drives).

The wide range of machinery, available in workshops and hangars, precludes giving specific rules and regulations for each machine. The basic drilling, grinding and milling types of machine, all require the use of eye protection, attachment of guards, secure holding of work and, most importantly, correct training before being operated. Possible accidents from machinery, in general, include personnel: • • • • •

Coming into contact with the machinery Being trapped between machinery and material Being struck by machinery or being entangled in its motion Being struck by ejected parts or material Receiving electric shocks from the machinery


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2.3.2 Electricity The human body conducts electricity. Furthermore, electrical current, passing through the body, disrupts the nervous system and causes burns at the entry and exit points. The current, used in domestic 220-240 volt, 50Hz ac electricity, is particularly dangerous because it affects nerves in such a way that a person, holding a current-carrying conductor, is unable to release it. Table 2 shows some typical harmful values and effects of both ac and dc electricity supplies. Table 2 HARMFUL VALUES OF ELECTRICITY Voltage/Current Possible Outcome 50V ac or 100V dc May give rise to dangerous shocks 1 mA Harmless tingle 1 – 12 mA Painful, but can be released 12 – 20 mA Very painful, cannot be released 20 – 50 mA Paralysis of respiration > 50 mA Heart stoppage Since water also conducts electricity, great care must be taken to avoid handling electrical equipment of all kinds when standing on a wet surface or when wearing wet shoes. The water provides a path to earth and heightens the possibility of electric shock. To ensure that equipment is safe, the minimum requirement is through the use of three-core cable (which includes an earth lead) and, possibly, a safety cut-out device. In conjunction, more often than not, with ignorance or carelessness, electrical hazards generally arise due to one or more of the following factors: • • • • •

Inadequate or non-existent earthing Worn or damaged wiring, insulation, plugs, sockets and other installations Bad wiring systems and the misuse of good systems Incorrect use of fuses Inadequate inspection and maintenance of power tools and equipment

All electrical equipment must be regularly checked and tested for correct operation and electrical safety. To show that this has been done, a dated label should be attached, showing when the equipment was last tested and when the next inspection is due. Any new item of equipment must have a test label attached. The presence of a test label does not, however, absolve the user from checking the equipment for any external signs of damage, such as a frayed power cord (or missing safety devices) before use. In the event of a person witnessing another person receiving an electric shock, the basic actions, to be followed by the witness, are: Page 6


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Shout for help and ensure there is no danger of also becoming a victim Switch off the electrical current or remove the victim from the supply by means of insulated material If the victim has ceased breathing, initiate resuscitation Call for professional medical help If the victim is suffering from burns, exclude air from wounds Treat for shock by keeping the victim warm

The approved methods of artificial resuscitation must, by law, be displayed on wall charts in workplaces. 2.3.3 Noise Workshops, hangars and flight lines can be very noisy places of work, so it is essential that ear defenders, or some other protection such as ear plugs, are used at all times that noise is perceived to be a risk. Loss of hearing, leading to deafness, can be the result of operating in a noisy environment without adequate ear protection. Ear protection is optional where noise levels are less than 85 dB, but is mandatory when greater than 90 dB. 2.3.4 High-Pressure Gases Compressed gases are frequently used in the maintenance and servicing of aircraft. The use of compressed gases requires a special set of safety measures. The following rules apply for the use of compressed gases: • • • • • • • • • •

Cylinders of compressed gas must be handled in the same way as any highenergy (and therefore potentially explosive) sources Eye protection must always be worn when handling compressed gases Never use a cylinder that cannot be positively identified When storing or moving a cylinder, have the cap securely in place to protect the valve stem When large cylinders are moved, ensure that they are securely attached to the correct trolley or vehicle Use the appropriate regulator on each gas cylinder Never direct high-pressure gases at a person Do not use compressed gas or compressed air to blow away dust and dirt, as the resulting flying particles are dangerous Release compressed gas slowly. The rapid release of a compressed gas will cause an unsecured gas hose to whip about and even build up a static charge, which could ignite a combustible gas Keep gas cylinders clean. Oil or grease on an oxygen cylinder can cause spontaneous combustion and explosions


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2.3.5 Gas Bottle Identification High-pressure gas cylinders contain various types of gas, the most common used on commercial aircraft being nitrogen and oxygen. To ensure correct identification of these containers, they are colour coded and the name of the gas is stencilled on the side. In the UK, gas containers use BS 381C as the standard to determine the correct colour and shade for each gas type. Nitrogen bottles are painted grey on the body with a black neck, whilst oxygen bottles are black with a white neck. Be aware that bottles of US manufacture use an alternative system, the main difference being oxygen bottles are painted green all over. 2.3.6 High-Pressure Gas Replenishing When replenishing aircraft services such as tyres and hydraulic accumulators with high-pressure gas, care must be taken to ensure that only the required pressure enters the container. When full, a gas storage bottle can hold as much as 200 bar (3000 psi) whilst an aircraft tyre pressure may only require 7 bar (100 psi). To safely control the gas, two pressure regulating valves are used, one at the storage bottle end and one at the delivery end of the system. If one valve fails the other will prevent the receiving vessel from taking the full bottle pressure with the consequence of an explosion. For added safety the gas bottle valve opening key should be left in the valve whilst decanting operations are completed. If problems occur then the highpressure bottle can be quickly isolated before the situation becomes dangerous. The transfer of high-pressure gases from a large storage bottle to the aircraft component is often called decanting and must be done at a very slow rate. If the gas is decanted rapidly the temperature of the receiving component will increase in accordance with the gas laws. Again using the same gas laws the temperature of the gas in the container will drop to ambient, and the pressure in that vessel will reduce. The component pressure will now be incorrect and require the decanting process to be repeated.

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In workshops, compressed air is, sometimes, produced by a compressor (which is housed in a remote building), and fed, via galleries, to work stations. Care must be taken to ensure that no damage occurs to the piping whilst in use. If a concentrated stream of compressed air is blown across a cut in a person’s skin, then air can enter the blood stream and cause injury or death. For this reason, air-dusting guns are restricted to about 2000 kPa (30 psi). Aircraft tyres can require very high pressures and must be inflated inside a strong cage. This cage would protect the personnel working on the wheels in the event of a tyre or wheel bursting. 2.3.7 Oxygen Systems Modern aircraft fly at altitudes where life support systems are needed. Even though most of these aircraft are pressurised, emergency oxygen must be carried in the event that the pressurisation system fails. Smaller aircraft can carry oxygen in cylinders whilst the larger, civil aircraft have individual oxygen generator units. These units are stowed in the overhead cargo bins, above the passenger seats, and are known as the passenger service units or PSUs. A PSU produces oxygen, by means of a chemical reaction, and is activated when its mask (which drops from the overhead bin in an emergency) is pulled by a passenger. Note: When PSUs reach their life expiry and have to be returned to their manufacturer, it is vital that all precautions are followed to make the units ‘safe’ for transit. PSUs get very hot when working and have caused the destruction, due to fire, of an aircraft, which was carrying these units as cargo. The main oxygen systems are serviced from trolleys or vehicles that carry a number of high-pressure bottles of oxygen, which can be at 140 bar (2000 psi) or more. Some trolleys may also have a bottle of nitrogen, to allow the replenishment of hydraulic accumulators and landing gears. The two types of bottles must be separated, in order to prevent the accidental mixing of the gases. It is extremely important that oxygen cylinders be treated with special care, because, in addition to having all the dangers inherent with all other highpressure gases, oxygen always possesses the risk of combustion and explosion. As previously stated, oxygen must never be allowed to come into contact with petroleum products such as oil and grease, since oxygen causes these materials to ignite spontaneously and to burn. Furthermore, an oil-soaked rag, or tools that are oily or greasy (or badly oil-stained overalls), must never be used when installing an adapter or a regulator on an oxygen cylinder.


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Due to the risk of fire and explosion, replenishing trolleys must never be parked close to hydraulic oil replenishing rigs, or in any area where petroleum products are likely to come into contact with the oxygen servicing equipment. Similarly only specially approved thread lubricants can be used when assembling oxygen components. 2.3.8 Aviation Oils and Fuels Aviation oils, generally, are a fairly low-risk material when compared with the more volatile, higher distillates of petroleum such as the aviation fuels - petrol (gasoline) and paraffin (kerosene). Most lubricating oils are flammable, if enough heat is generated but, when the materials are kept away from excessive heat sources, they are (comparatively) quite benign. Synthetic lubricating oils, methanol and some hydraulic oils may be harmful or even toxic if their vapours are inhaled. Also, if they come into contact with the skin or eyes, they can cause injury or blindness. Particular note should be taken of any warnings of dangers to health that may be contained in the relevant maintenance manuals and the recommended procedures for the handling of these liquids should always be observed. Oils and fuels also have an adverse effect on paintwork, adhesives and sealants and, thus, may inhibit corrosion-prevention schemes. Care should, therefore, be taken not to spill any of these liquids but, if a spillage should occur, it must be cleaned up immediately. Note: Sweeping up gasoline spillage with a dry broom can cause a build up of static electricity, with the accompanying risk of explosion. With gasoline and kerosene there is a much greater chance of fire, so more thorough precautions are required. These start with the basic rules, such as not wearing footwear with nails or studs (to prevent sparks), not carrying matches or cigarette lighters and ensuring that ALL replenishing equipment is fully serviceable. Note: All references to refuelling, normally, also include the action of de-fuelling and both are considered under the common term of fuelling. During any fuelling operation, in a workshop, a hangar or on the flight line, the relevant fire extinguishers must be in place.

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2.3.9 Chemical and Physiological Hazards Many chemical compounds, both liquid and solid, are used in aircraft maintenance and these may need specific precautions. Any precautions can be found in the relevant maintenance manuals and in the Control of Substances Hazardous to Health (COSHH) leaflets applicable to those materials. The range of adhesives used for repair and sealing during the maintenance of aircraft is vast. A large number of these produce vapours which, generally, can be dangerous in any enclosed space, both from the results of inhalation of narcotic fumes and from the fire risk associated with those which give off volatile, flammable, vapours. Surface finishes present another area where the various types of material used (etchants, celluloses, acrylics, enamels, polyurethanes etc.), dictate specific precautions. The solvents used, before the actual painting and afterwards, need safety precautions with regards to ventilation, reaction with other materials and, most importantly, their possible corrosive, toxic, irritant and addictive effects on personnel. Some materials have a mildly radioactive property, although they emit little ionising radiation in normal circumstances. These materials are sometimes referred to as ‘heavy metals’ and can be found in balance-weights as well as in smoke detectors, luminescent ‘EXIT’ signs and instruments. This radiation differs from that used for non-destructive testing (NDT) procedures, where high levels of radiation are employed, by specially trained personnel, and which, therefore, require many safety precautions to avoid personal injury. The safety precautions for NDT procedures will be found within the manuals applicable to their employment. 2.3.10 Lifting and Shoring Aircraft must often be raised from the hangar floor for weighing, maintenance or repair. There are several methods of doing this, however, and the maintenance manuals must be followed, during whichever method is used. It is often necessary to lift only one wheel from the floor, to change a wheel or to service a wheel assembly or brake unit. For this type of jacking, some manufacturers have made provision on the undercarriage leg for the placement of a short hydraulic jack. When using this method, never place the jack under the brake housing or in any location that is not approved by the manufacturer.


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When jacking an aircraft asymmetrically, there is usually some movement by the other legs. Care must be shown, when jacking a single leg, that the aircraft is raised strictly in accordance with the maintenance manual. Other places where a larger jack may be connected to the airframe might be: • • • •

Under the wings, at the main spar position Under the nose Under the tail assembly On the side of the front fuselage (in place of the nose jack)

The location and operation of ALL jacks must be carried out both with great care and with the correct number of personnel, who must be well briefed. Most of the larger jacks have a screw-type, safety locking collar, to prevent the jack collapsing in the event of a sudden leak. The jack operator must ensure that these safety collars are gradually screwed down, as the aircraft is being raised, so that they are very close to the jack body at all times. As an additional precaution, especially if the aircraft is to be worked on for an extended period, trestles or ‘steadies’ can be installed under the wings and fuselage to augment the jacks and also to provide an additional means of shoring (supporting) the aircraft. 2.3.11 Slinging It can be necessary, on occasions, to lift either the major components of an aircraft, such as wing or tail assemblies or the complete aircraft (refer to Fig. 2). For example, when recovering an aircraft from an ‘overrun’, it may be easier, and safer, to lift the entire aircraft and place it onto a hard standing, than to try and pull it out of soft ground, using a tug or similar vehicle. When lifting either major components or an entire aircraft, the slings must be produced or approved by the manufacturer of the aircraft. The manufacturer’s slings ensure that the centre of gravity of the component, is always directly beneath the lifting hook of the sling.

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Lifting an Aircraft with Slings Fig. 2 2.4

Flight-Line Safety

Many sources of accidents on the flight line are involved with propellers and rotor blades. They are difficult to see when they are turning, and personnel (despite being familiar with the hazards of propellers and rotors), sometimes become distracted and forget about the danger. The main difference between these, and other flight-line accidents, is that they are almost always fatal. Most blades have high-visibility markings, to ensure that they can be seen when they are turning. These markings vary from a yellow blade tip marking, to black and white alternate stripes along the full blade length. To reduce the risk of propeller and rotor blade strikes, it is best to follow strict rules as to the correct way to approach and leave the vicinity of an aircraft or helicopter whilst it is under power. For example (and allowing for the fact that there are specific rules laid down for each aircraft), installing and removing chocks should normally be done from the wing-tip direction. Boarding and leaving a helicopter should always be done from the side. When dealing with running jet engines there are similar dangers. These come not only from the noise risk, which can result in deafness, but also from the risk of intake suction, which has resulted in ramp personnel being sucked into the engine and being killed. At the rear of the aircraft, there is the risk of jet blast, which, at maximum thrust is quite capable of overturning a vehicle if it passes too close behind the aircraft. (refer to Fig. 3). Piston-powered aircraft (depending on their size) will have similar danger areas.


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MODULE 7 MAINTENANCE PRACTICES (mechanical)

engineering Distance (metres)

0

10

20

30

40

Typical Aircraft Danger Areas Fig. 3 2.4.1 Towing and Taxying If an aircraft requires moving and no pilot is available, then a tug and towing arm must be used. This task will require a qualified tug driver, a supervisor, a ‘brake man’ and other personnel to keep a lookout. A qualified pilot always does the taxiing of larger aircraft, although engineers sometimes taxi light aircraft. Each aircraft and its operator will have laid down rules regarding the way in which each aircraft will be towed. These rules will include the number of people needed, the type of tug, the radio calls if the aircraft is on the manoeuvring area, the maximum towing speed and many other details. These must always be followed if accidents are to be avoided. Aircraft, when moving, either under power or whilst being towed, are sources of numerous risk areas. An airliner can be over 60 metres long and have a wing span greater than 60 metres. This means that when manoeuvring in restricted spaces, there is always the risk of part of the aircraft striking another object, due to a phenomenon known as ‘Swept Wing Growth’ (refer to Fig. 4). It must be borne in mind that, when turning, the wing tips and tail of a large aircraft can move considerable distances in the opposite direction to that of the nose. This is why, whenever an aircraft is approaching its parking spot, there must be personnel available to watch out for any potential conflicts. Driving in the vicinity of a parked aircraft must always be done with care, especially if the driver is alone or visibility from the cab of the vehicle is limited.

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Tail Sweep Area

Wing Tip Sweep Area

Aircraft Turning To Left

Path of Tail

Path of Wing Tip

Swept Wing Growth Fig. 4 2.4.2 Parking When an aircraft has to be parked for a period of time, especially overnight and in inclement weather conditions, there are a number of precautions that must be observed: • • • •

A chock must be placed at the front and rear of a number of wheels, depending on the aircraft type The engine intakes and exhausts may need to be covered with special blanks The control surfaces may have to be locked in place with integral control or gust locks or, if these are not installed, external locks may be attached to all of the surfaces that could be damaged in high winds Other devices required could include blanks for the pitot tubes and static vents.


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engineering 2.4.3 Marshalling

When marshalling an aircraft, it is essential that personnel are fully conversant with all the marshalling signals (refer to Fig. 5). It is also useful to know extra details such as: • • •

The need for additional, ‘lookout’ men on the wing tips or tail The correct place to stand to enable the aircraft’s crew to have sight of the marshaller The point at which the aircraft is required to stop.

Come Ahead

Stop

Right Turn

Left Turn

Remove Chocks

Insert Chocks

Emergency Stop

All Clear (OK)

Slow Down

Some Basic Marshalling Signals for Fixed-Wing Aircraft Fig. 5

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2.4.4 Fuelling While the topic of fuelling is covered more fully in Module 11 and in the relevant Chapter (28) of the Maintenance Manual, brief consideration of some of the general safety precautions is given here. The first, obvious precaution, is the identification of the type of fuel in the fuel tanker (or bowser), ensuring it is of the type and grade required for the aircraft. There have been many times when petrol-powered aircraft have been filled with turbine fuel and, on occasions, the reverse has occurred. The type and grade of fuel should always be stencilled or painted, adjacent to the fuelling point, but it is wise if a responsible person is consulted before starting fuelling. This is because there may be a requirement for some special fuel, or simply that the aircraft is only to be part-filled, due to a weight limitation. The fuel tanker must be parked as far as possible from the aircraft, limited by the hose length, and parallel or facing away from it. This reduces the risk of fire passing from the aircraft to the tanker or vice versa, and also allows a clear path for the tanker to vacate the area quickly, should the need arise. The fuel tanker, the fuelling hose, the aircraft and the ground must all be electrically bonded together, to allow the static electricity (generated during the fuel flow) to run to earth. A safety zone of 6m (20 ft) should be established from the filling and venting points of the aircraft and attendant fuelling equipment. This area should be free from naked lights, smoking and the operation of electrical switches of any kind. There can also be a risk from the operation of radio and radar equipment, so these should also be switched off before fuelling commences. Also, during the fuelling of aircraft, Auxiliary Power Units (APU) and Ground Power Units, (GPU), must be made safe, by checking that their exhausts and intakes are clear of any fuel vapours, and that GPU’s, are located as far as practical from the fuelling point(s). NO switching of power from APU’s or GPU’s will be made during fuelling procedures. There are many precautions involved when defuelling, due to the tanks being left empty of fuel, leaving potentially explosive vapours in its place. ALL necessary safety precautions must be followed during aircraft fuelling procedures.


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2.4.5 Weather Radar The heating and radiation effects of weather radar can be hazardous to life. Personnel should remain a safe distance from the radar if it is in operation. There are published figures and charts in the maintenance manual of each aircraft, showing the safe distances for personnel, depending on the power of the radar in use. As an example, the aerial in the nose of the aircraft should have an unobstructed ‘view’ of something like 30 metres, with the aerial tilted upwards. There should also be a barrier erected about 3 metres or so from the nose of the aircraft, to prevent personnel getting too close. Finally, there should be no fuelling operations in progress during the testing of weather radar.

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WORKSHOP PRACTICES

Despite the enormous advances in the mechanisation and computerisation of the engineering industry in general, there remains the requirement for a high degree of hand skills on the part of technicians who are engaged in the day-to-day maintenance of aircraft and their associated components. While the majority of aerospace components are manufactured under stringent standards, in factory (and laboratory) conditions, it is necessary to remove many items of equipment for cleaning, inspection, overhaul and, if needed, repair before they are, subsequently, re-installed in their appointed locations. These actions may entail the use of many specialist tools and materials, which are used while following written procedures, while it is quite possible that some, comparatively simple, repairs may call upon such basic hand skills as the cutting, filing, drilling, riveting and painting of metals or other materials. No matter whether there are specialist or basic skills required, all will demand a certain quality of the work practices (and of the work-force) involved. 3.1 Care of Tools Engineers are responsible for the maintenance of their personal tools, whilst other personnel, in designated Tool Stores, must care for all the different, specialist tools for which they have the responsibility. It is also the responsibility of engineers to ensure that any tools, or other items of equipment they use, are not left in an aircraft or associated components. The care required for different tools can vary. Ordinary hand tools may merely require racking or locating within sturdy tool boxes, with careful, daily, maintenance restricted to little more than a visual check. Precision instruments however, require great care both in storage and in use. They may need to be kept in special, soft-lined, boxes within other storage facilities. Prior to use they should have a ‘zero’ check or calibration. Some tools require that they have a light coating of machine oil, to prevent the onset of corrosion. Each tool (whether it be a hammer or a micrometer), will require some special care, to ensure its optimum performance for, without this care, even the most expensive tools very quickly become second rate and useless.


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MAINTENANCE PRACTICES (mechanical)

3.2 Control of Tools Control of tools is important to good engineering practices and is also vital to flight safety. A variety of systems can be used to control tools but, whichever system is used, it must allow a 100% check of the tools in use before it can be considered as acceptable. One form of control is the ‘shadow board’ and ‘tool tag’ system, (refer to Fig. 6). Each tool is positioned over its silhouette, on the tool board. Technicians are issued with identification tokens (numbered ‘tags’) which are exchanged for the tool and, usually, a tag is hung above the silhouette, to be reclaimed, in exchange for the tool, when it is returned to the board. The shadow board/tool tag system works equally well when the tools are held within a designated Tool Store arrangement.

6

Shadow Board and Tool Tag Fig. 1 In workshops and bays it is normal for a toolkit to be held by the department in addition to its engineers holding personal sets of tools. The tools held by the department are often referred to as ‘special tools’, meaning that they are only for maintenance work on the items being serviced in that workshop. A wheel bay, for example, may have sets of special spanners, levers, seal applicators and pre-set torque wrenches, which are used primarily for the servicing of particular types of aircraft wheels. This dedicated tool kit makes tool control much simpler and safer, with the tools all being clearly marked as belonging to that specific bay.

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No matter where tools are being used, it is the responsibility of each technician to keep track of ALL of the tools used during a particular task. The most important check of all is the final, ‘End of Work’ tool check, when all tools must be collected and checked off against personal inventories, ensuring all borrowed tools (from the Tool Store for example), are returned and any personal tool tags collected. 3.3 Calibration of Tools and Equipment Requirements within the relevant airworthiness codes, applicable to the United Kingdom Civil Aviation Industry, such as the British Civil Aviation Requirements (BCARs), Joint Aviation Requirements (JARs), and Air Operators Certificates, prescribe that, where necessary, tools, equipment and, in particular, test equipment are all calibrated to acceptable standards. This topic provides an overall picture of the types of requirements and tests required in establishing and maintaining an effective calibration system. It takes into account factors such as the degree of accuracy required, frequency of use and the reliability of the equipment. The key factor is the need to establish confidence in the accuracy of the equipment when it is required for use. The required calibration frequency for any particular piece of test equipment is that which will ensure it is in compliance with the standards applicable to its intended use. In all cases, standards used are attributed upon the need for ultimate traceability to one of the following: • •

The standard specified by the equipment manufacturer/design organisation The appropriate National/International Standards.

3.3.1 General Notes on Calibration The appropriate standards are used to achieve consistency between measurements made in different locations, possibly using alternate measuring techniques. The calibration of test equipment is best achieved by the operation of a methodical system of control. This system should be traceable by an unbroken chain of comparisons, through measurement standards of successively better accuracy up to the appropriate standard. Where recommendations for calibration standards are not published, or where they are not specified, calibration should be carried out, in the UK, in accordance with British Standard EN 30012-1: Quality Assurance Requirements for Measuring Equipment. As an alternative to operating an internal Measurement and Calibration System, an Approved Organisation or an Approved/Licensed Engineer may enter into a sub-contracting arrangement to use an Appliance Calibration Service. This arrangement does not absolve the contractors of the service from maintaining standards as if they were carrying out the work themselves. 71811175.doc Issued December 2002

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In all instances, it is the responsibility of the user to be satisfied that the unbroken traceability chain is in place. External organisations, which supply an external Calibration Service, should be those holding accreditation of the National Accreditation of Measurement and Sampling, (NAMAS). 3.3.2 Procedures The definition of appliances requiring calibration are those items which are necessary to perform measurements or tests of an aircraft, a system or a component, to defined limits, as specified in the technical documentation of the Type Certificate holder. Procedures, controlling regular inspection, servicing and, where appropriate, calibration of such items, are to indicate to the users that the item is within any inspection time limit. These ‘Next Inspection’ labels must clearly state when, and, if necessary, where the next calibration is due. There should be a programme that plans the periodic inspection, service or calibration within the defined time limit, which ensures that the item remains in calibration. It is common sense to stagger the calibration of items, so that the largest number are available for use at all times. It is also important, that a register of all items requiring calibration is held, so that cross-checking can be simply carried out. Where a small number of particular items are held, then contract loan of equipment is permitted. The intervals at which calibration is required, can vary with the nature of the equipment, the conditions under which it is used and the consequences of incorrect results. The frequency will be in accordance with the manufacturer or supplier’s instructions, unless the organisation can show that a different interval is warranted in a particular case. This would normally require a system of continuous analysis of calibration results to be established, to support the variation to the recommended calibration intervals. Any appliance, the serviceability of which is in doubt, should be removed from service and clearly labelled accordingly. The appliance must not be returned to service unless the reason for its unserviceability has been eliminated and its continued calibration re-validated. Action must be taken, if an item of equipment is found, during re-calibration, to have a significant error. This must include rechecking of measurements made prior to finding the fault. The scope of the records maintained, are dependent upon the standards required and the nature of the equipment. The record system can also provide a valuable reference in case of dispute or warranty claims. These records can also indicate ‘drift’ and can help in reassessing calibration intervals.

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Calibration records or certificates should, as a minimum, contain the following information for each appliance calibrated: • • • • • • • • •

Identification of equipment Limits of permissible error Standard used Authority under which the document was issued Results obtained Any limitation of use of equipment Uncertainty of measurement Date when each calibration was conducted Assigned calibration interval.

Where calibration services are provided by outside organisations, it is acceptable that the accuracy of the equipment is attested by a release document in the name of the Calibration Company. Any measurement is affected, to some degree, by the environment in which it is made. The equipment will need to be calibrated, transported and stored under conditions compatible with the type of equipment, to ensure its accuracy is not impaired. To provide valid and repeatable test results, the facilities used for calibration must have a controlled environment. It is necessary to control the temperature, humidity, vibration, dust, cleanliness, electromagnetic interference, lighting and other factors that may affect the standard of the results. If any of these requirements cannot be met, then compensation corrections must be applied to the calibration standard to ensure continued accuracy. A measurement Checking Standard can be applied, at the work place, to check the accuracy of an appliance and to ensure its continued correct functioning. The Checking Standard will be robust and its accuracy will not match that of a full calibration check, but it will give confidence between checks that the equipment is functioning correctly. The company Quality System has the responsibility of ensuring the continued accuracy, not only of the items of equipment, but also of the actual testing facilities.


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3.4 Use of Workshop Materials Many of the wide variety of materials, used in workshops, require some form of control in their handling. This control can involve: •

Safety: relating to such topics as the toxicity, corrosiveness or other health risks associated with the use of certain materials

•

Management: referring to the storage, use and correct handling of all materials whether they are solid, liquid, or, in some instances, gaseous

•

Economy: involving such matters as to the using of the correct dosage or proportions when mixing compounds, using only as much material as required for a specific task and to the keeping in stock of only sufficient materials and thus avoiding ‘lifed’ items reaching their expiry dates before being used.

Abrasive papers, solder and brazing materials, wire wool, tyre powder, oil spill powder and so on, all require control of issue and use, though they may not, normally, require stringent safety precautions. A huge range of liquids can be used in the workshop situation, some of which are harmless and some of which are extremely toxic. It is vital that the work-force make themselves aware of the risks involved when dealing with ANY materials, and especially when working within enclosed areas. Some materials are flammable and must, therefore, be stored outdoors. These include oils, greases, some adhesives, sealing and glazing compounds in addition to many paints, enamels and epoxy surface finishes, which are stored in metal cabinets and, usually, located (in the Northern hemisphere) on the North side of a workshop or hangar. This ensures that the cabinet remains in the shade of the building and does not get exposed to the sun’s hot rays during the day. It is also important that only the minimum amount of these materials is taken indoors for the work which is being done. When handling materials that give off fumes, it may be necessary to have the area well ventilated and/or have the operator wearing a mask or some form of remote breathing apparatus. The finished work may also give off fumes for some time afterwards, so care must be taken to keep it ventilated if necessary. Obviously all liquids must only be used for the purpose for which they are designed and never mixed together, unless the two materials are designed to be mixed, such as with two part epoxy adhesives and sealants. Many liquids used in workshops and in the hangar have (as mentioned earlier) a fixed ‘life’. This date is printed on the container and must be checked before use, because many materials are unsafe if used beyond their expiry date.

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The disposal of liquids is a critical operation, and must only be carried out in accordance with company (and, often, national or international) regulations. Liquids must never be disposed of by pouring them into spare or unidentified containers and they must not be allowed to enter the ‘domestic’ drains systems. The working with, and the use of, high pressure gas containers and oxygen systems, was adequately discussed in the Safety Precautions topic. 3.5 Standards of Workmanship Whilst the standards of workmanship, during the hand-working of metals and other materials, is controlled by the craftsperson, once machinery is used in the manufacturing process, then the standards of finish and workmanship depend upon the allowances set by the designer and on the type of machinery being used. With hand tools, there are standards of finish, but these depend upon the skill of the craftsperson and, again, on the tools being used. For example, when filing metal, different grades of files are used, to obtain a comparatively smooth surface finish while other methods, such as abrasive papers, pastes and polishes, are then used, to provide the final finish. When sawing, the same procedures apply in that blades with finer teeth will give a better finish to the sawn edges, which may then be further smoothed, using an appropriate selection of files. When drilling a hole, the conventional twist drill will only produce a finish of a certain standard. If a finer finish, to the inside of the hole, is required, then a reamer would be used, to smooth the material inside the hole, so that, if a tight fitting pin is to be fitted through the hole, there will be better surface contact. There are a variety of machines that can generate a smooth surface on a piece of metal, the selection between them being decided by the quality of finish. A lathe can produce an exceptionally smooth surface on a bar or some other rotated shape. If a large area is required to have a smooth finish, then perhaps, after initial casting or forging, the choice may be of employing either a grinding machine or a milling machine, to .provide the desired result. In summary, the quality of the finished article is dependent both on the skill of the craftsperson and the equipment available to complete the task. It does not matter whether the tools in use are files and emery cloth or an expensive milling machine; the standard of workmanship of the craftsperson can make a great deal of difference to the finished article.


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TOOLS

Servicing of an aircraft, requires the dismantling, cleaning, examination, adjustment and re-assembly of the parts in accordance with the maintenance schedule. Further aspects of the work may require the manufacture of simple components from metal or other materials, the drilling and tapping of holes, removal of burrs and other operations. A reasonable degree of skill, in the use of hand tools is, therefore, to be expected from all trades-persons. This skill can only be obtained by practice, but it may be stated, that the more efficient the tool, then the better will be the finished work. 4.1 Common Hand Tools The best results are always obtained by using the correct tool for the task. Care and maintenance of all tools is very important, since damaged or inefficient tools can lead to injury of the user or damage to the components. A range of common hand tools is considered in this part of the course. 4.1.1 Engineer’s Rule An engineer’s rule (refer to Fig. 1) is made from high-carbon steel and is graduated in Imperial and Metric units. Rules are classified by the length and width of their graduated portion, must be kept free from rust and should not be subjected to rough usage. The most common engineer’s rule has a length of 300mm (1ft) but rules can be obtained in lengths of up to 1,800mm (6ft). The increment graduation marks are etched into the rule surface providing a grooved recess. These grooves enable dividers to be set to a greater accuracy, as the divider points can be felt to ‘drop in’ to the recess. Metric Scale 1 2 3 4 5 6 7 8 9 10 11 1 2 3 4

Edge View

24 25 26 27 28 29 30 10 11 12 Imperial Scale

Grooves Engineer’s Rule Fig. 1 71811175.doc Issued December 2002

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4.1.2 Scriber A scriber (refer to Fig. 2) is used for marking lines on the surface of metals. Scribers are made from high-carbon steel and are classified by their length. One end of the scriber is usually bent at right angles to enable lines to be scribed in difficult places such as through a hole. All scribed lines, on soft materials, must only be cutting (boundary) lines, and none must be left on the surface of the metal on completion, as they can cause cracks. Other lines, including bend lines and lines for the position of rivets must be marked with a sharp pencil. Scriber points must be kept sharp and fine by careful ‘stoning’, with an oil stone, rather than an abrasive wheel (grindstone). Using a wheel is likely to generate too much heat, which will result in the temper being drawn from the steel and the point of the scriber becoming soft and useless. When not in use (and as with other tools with sharp points), placing pieces of cork, plastic or similar material over their points will protect them. 4.1.3 Key-Seat Rule Key-seat rules are used for marking-off lines, parallel to the axis, on the surface of tubes or round bars (refer to Fig. 2). Sometimes referred to as ‘Box Squares’, key-seat rules are usually graduated and are classified by their length.

Key Seat Rule

Round Bar

Scriber

Scriber and Key Seat Rule Fig. 2

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4.1.4 Fitter’s Square The fitter’s square is used for setting out lines at right angles to an edge or surface, and for checking right angular work for ‘truth’. Squares are made, to very fine limits, of high-carbon steel and are classified by the length of the blade. The blade and the stock have their opposing edges ground truly parallel with the two limbs set at exactly 90° to each other. To preserve its accuracy it is essential that it is handled carefully at all times and, when not in use, kept in a protective case or box. When testing a square for accuracy, it may be checked for truth against an accurately machined right angular test piece such as a ‘V’ block or master square. If this is not possible, a test may be carried out (refer to Fig. 3) as follows: • •

Place the stock against the true edge of a flat surface and scribing a line on the surface, using the outside edge of the blade Turn the square over and check the outside edge of the blade against the previously scribed line.

If the square is accurate, the blade edge and the scribed line will be in line. In a similar manner, the inside edge of the blade can be tested.

True Edge

Error Testing a Square Fig. 3


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4.1.5 Combination Set The Combination Set (refer to Fig. 4), consists of a graduated steel rule, which has a machined groove running along the centre of its entire length. The rule can be slid into three different ‘heads’ and secured, by a locking screw device, so that the combination of rule and ’head’ will enable certain tasks to be accomplished. The Centre Head is used, with the rule, to locate the centre line of bars or round tubes. The Square Head has one working surface at 90° and another at 45° to the locked rule. This allows the tool to be used, either in a similar manner to the Fitter’s Square (to check the squareness of work), or it may be used for the marking out of mitre joints and bevels. A spirit level and scriber are, sometimes, accommodated in the base of the Square Head, to permit a check to be done on the horizontal or vertical accuracy of workpieces. The Protractor Head also has a spirit level, which rotates with the head, and allows the head to be used, singly, as a clinometer or, in conjunction with the rule, it may be used to mark out and check angles on workpieces. Scriber Spirit Level Centre Head

Square Head

Groove

Protractor Head

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4.1.6 Surface Plates and Tables Surface plates (and surface tables which are larger), while not actually being classed as marking out or measuring tools, are simply blocks of grey cast iron with finely machined faces which can be used as a standard of flatness. They may also be used to provide a true surface, from which marking out, measuring and testing can be done. Surface plates are usually mounted on a bench and, normally, only have three supports, or feet, to ensure steadiness, if the surface of the bench were to be slightly uneven. Surface tables are free standing, on the workshop floor, and their sheer weight provides the required steadiness. The standard of the surface finish varies. The better grades are scraped and the cheaper ones are merely planed. The accuracy of a planed table depends upon the accuracy of the machine producing it. Surfaces of grade ‘A’ standard would only be used in Standards Rooms, grade ‘B’ surfaces are for inspection work while grade ‘C’ surface plates and tables would be found in typical workshops. Surface plates and tables can be used to test for flatness, providing the standards required are not too high. The surface of the plate is lightly smeared with a mixture of engineer’s blue and a few drops of oil. The piece to be tested has to be rubbed lightly on to the surface plate and any high spots will show up as blue spots on the test piece. These spots will be filed or scraped until the whole surface shows blue. After use, a light film of oil should be applied to the working surface of the surface plates and tables. They should, then, be protected with a wooden cover, to prevent the onset of corrosion. 4.1.7 V Blocks V Blocks are accurately machined, six-sided, rectangular blocks (generally made of cast iron), which may be used, on surface plates and tables, to hold a round bar, which can then be marked in a variety of ways, to give centres and lines parallel to its side. V blocks are classified by the maximum diameter of the work, which they can hold. All opposite sides of the blocks are parallel and all adjacent faces are square to each other. A 90° groove (in the shape of a V) is machined in two (longer) opposite faces, but the grooves are cut at different depths, to cater for bars of different diameters.


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The V-cut grooves have a small, square-cut, clearance groove in the bottom of the V. This ensures that any oil, or dirt runs off the sides of the V and does not clog the bottom of the V, causing an imperfect seating of any bar which were to be placed in the blocks. V blocks are made in (identified) matching pairs, which must always be used together, so that a block of one pair should not be used with one of another pair. Some V blocks also have grooves machined along the other two longer, parallel, sides, to locate specially designed clamps, which may be used to securely hold work while it is being accurately marked out or drilled. 4.1.8 Surface Gauge (Scribing Block) A Surface Gauge, or Scribing Block (refer to Fig. 5), is another marking out tool, used, on a surface plate or table, in conjunction with a scriber (and, occasionally, with V blocks), for the marking of lines, which are parallel to a true surface. The scriber is clamped to a spindle, which can be accurately pivoted, by means of a fine adjustment screw, on the heavy base. The base, which is generally made from cast iron (or hardened steel) is machined to be as flat as the surface plate on which it slides, but it is also grooved (in a similar manner to the V block) so that it can be used on round stock when required. Two friction-fit pins, in the base, may be pushed down, to assist in drawing lines parallel to a true edge.

V Blocks

Scribing Block

Surface Plate

Scribing Block with V Blocks and Surface Plate Fig. 5

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4.1.9 Dividers Dividers are used to set out distances and to scribe arcs and circles. The legs are made of high-carbon steel, the spring made of spring steel and the adjusting mechanism of mild steel. Dividers are classified by the length of their legs. The points should be kept sharp and of equal length by stoning only the outside of the legs. If grinding is used to sharpen the points, it must be done very carefully, as the temper of the points can be drawn, leaving them soft. The points of dividers should be protected, when not in use, in a similar manner to those of scribers and such tools. 4.1.10 Callipers Callipers (refer to Fig. 6) are a type of measuring device, typically used to measure diameters and distances or for comparing sizes. The three basic types of calliper are: • • •

Outside Callipers: Used to measure the outside diameter of an object and have legs that point inwards Inside Callipers: Used to measure the inside of a hole and have legs that point outwards Odd-Leg Callipers (Hermaphrodite or ‘Jenny’ Callipers): This tool is really half callipers and half dividers. It may be used for scribing arcs on metal surfaces from an edge, for scribing lines parallel to an edge or surface, (provided accuracy is not of great importance), and for finding the centre of a round bar.

Outside

Inside

Callipers Fig. 6


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4.1.11 Hammers Hammers (refer to Fig. 7) are classified by their weight and type of head. Steel heads are forged and manufactured from high-carbon steel. Most shafts are made from straight-grained Ash or Hickory and are secured to the head by wedging.

Ball Pein

Straight Pein

Cross Pein Hide/Copper Face

Hide Face

Rubber Head

Plastic Face

Types of Hammers Fig. 7

As can be seen from Fig. 7, the main types of engineering hammers are the: • •

Ball Pein: The flat surface is used for most general-purpose work whilst the ball pein is used primarily for riveting-type operations Straight Pein: Used for general work, the narrow, straight pein being particularly suitable for use where access to the work is limited

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Cross Pein: As for the straight pein, but the axis of the pein is at 90° to that of the shaft Hide/Copper Face: The rawhide facing enables heavy blows to be delivered without damaging the surface of the work, while the copper face may be used for heavier types of work than hide faced hammers Rubber Head and Plastic Face: More modern versions of the Hide Face hammer. Can often have one of each type of face on each end of the head Claw Hammer (not shown): More commonly used for woodworking. The face is used for hammering nails whilst the claw is used for removing nails Body Hammer (not shown): Little used in aircraft work, as they are primarily used to remove dents and blemishes from sheet metal. They are also known as planishing hammers.

The weight of hammer required can be found with experience. Before use, it must be ensured that the head is secure on the shaft. The shaft should be gripped close to the end opposite the head, as proper control is not possible if it is held close to the head. 4.1.12 Punches Although punches are not ‘pounding tools’, they do allow the force from a hammer blow to be concentrated in the immediate area of the punch tip. This in turn means that the pressure at the end of the punch is increased compared to a hammer blow without a punch. Over a period of time, the hammered shank end of a punch, tends to deform into the shape of a mushroom. To reduce the chance of a metal chip flying off and causing injury, during punching operations, the deformation should be removed and the shank end returned to its original shape by the use of a bench grinder. Eye or face protection should always be used when using punches of any type. The types of punches, more commonly found in an engineer’s toolkit, include: • • • •

Centre Punches Pin Punches Hollow Punches Drifts

The first three punches are, usually, constructed from hexagonal (or knurled, round) rods of tempered, cast steel with a length of approximately 127 mm (5 in), a gripping diameter of approximately 3.175 mm (0.125 in) and a smaller, driving end of the appropriate size.


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Centre Punches are relatively sharp-pointed tools, used to make an indentation in metal. The indentation aids in locating the centre of a hole and for starting a drill bit when drilling the hole. The points may be ground at angles between 60° to 90°, depending on the hardness of the metal on which the punch is being used. The softer the metal, then the larger will be the angle of the punch’s point. When using a centre punch, it must be struck hard enough to give an indentation large enough for a drill bit to start, but not so hard as to distort the metal. Another form of Centre Punch is the ‘Dot’ or ‘Prick’ Punch (also ‘Pricker’), which has a finer point and is used to make indentations along a drawn line when the line is, otherwise, difficult to see. The indentations may also be used, when sawing down to a line, as ‘witness’ points, to show that the cutting is accurate. Centre punches should not be used to drive out pins or rivets from their holes. Pin Punches, as their name implies, are the tools to be used for the removal of pins and rivets from their respective holes. The driving end of a Pin Punch is cut flat, and its diameter ground to match that of the pin or rivet which is being driven from its hole. Pin Punches may be found with parallel or tapered driving ends. Hollow Punches are used to punch out bolt (or stud) holes in soft, thin sheets, such as shimming or gasket materials, which are difficult to cut with drills. The material being cut, should be supported by a wooden block, to avoid damaging the cutting end of the Hollow Punch. Drifts may be fashioned from aluminium alloy, copper or steel bars (or tubes), and are used for driving out bearings, bushes or shafts from their respective cages or housings. Only steel drifts should be used on bearings, due to the possibility of small metal chips, from the softer metals, breaking off and fouling the bearing assemblies. 4.1.13 Metal-Cutting Chisels Metal-cutting chisels (also called Cold Chisels) are used in conjunction with steel hammers. Chisels are forged, usually using short lengths of hexagonal-sectioned, high-carbon steel bars, with the cutting edge hardened and tempered. To prevent flying particles when hammering, the striking end is not hardened and is, therefore, comparatively softer. Periodically, the burr, that forms at the striking end of the chisel, should (in a similar manner to punches), be removed by filing or grinding. Alternatively, the chisels may be made of nickel-alloy steel, specially heat-treated, to produce a long-lasting cutting edge.

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Chisels are classified by their shape, overall length, cross-section of shank and width of cut. There are four principal shapes of chisels (refer to Fig. 8), in general use. They are the: • • • •

Flat Cross-Cut Diamond-Point Half-Round.

Flat chisels are used for general chipping work, such as parting sheet metal or cutting flat surfaces, preparatory to filing. The cutting edge is formed slightly convex. Cross-Cut (or Cape) chisels are used to cut narrow, flat-bottomed, grooves, such as keyways in shafts or where it is not practical to use a flat chisel. These chisels are also used to remove the heads of round-headed rivets during repairs. Diamond-Point chisels are particularly useful for cutting in corners, cutting small oil grooves and for rectifying an incorrect start when drilling. Half-Round (and may, also, be called Round) chisels are general-purpose, grooving chisels, which are suitable for cutting half-round, bottomed, grooves. They are also suitable for rectifying an incorrect start when drilling.

Flat

Cross-Cut

Diamond-Point

Half-Round

Chisel Types Fig. 8 When selecting a chisel for a specific task, consideration must be given both to the nature of the work and to the material that is to be cut. The nature of the work governs the choice of shape, whilst the angle formed by the cutting edge is influenced by the hardness of the metal.


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In general, it may be assumed that the softer the metal the more acute should be the cutting angle. Table 5 shows some suggested cutting angles for use on typical metals, found in aircraft engineering workshops. Table 1 SUGGESTED CHISEL CUTTING ANGLES Hard Steels 70° Mild Steels 60° Soft Metals 40° High-carbon, steel chisels, should be sharpened by grinding on an abrasive wheel, but nickel-alloy, steel chisels are sharpened by filing. The cutting edge of the chisel must be kept cool, during grinding, by frequent immersion in water, which will prevent the temper being drawn from the metal. 4.1.14 Bench Vice The bench vice (refer to Fig. 9) is used to firmly grip the material or item upon which work is being done in a workshop. The body of the vice is provided with detachable steel jaws. The screw is made with a square or with a buttress thread. Most types of bench vice have a quick-release mechanism, operated by a small lever. The jaws can then be slid either open or closed until the correct position is reached. The lever disengages the half nut from the thread to permit the sliding action and it is driven back into engagement by a strong spring. Bench vices are classified by the length of their jaws. The height of the top of the vice above the ground is important, and should ideally, be level with the technician’s elbow when standing adjacent to the vice. With the vice at the correct height, work will be less tiring and correct control of the tools, such as files and saws, will be achieved. The vice must be secured firmly to the bench (with occasional checks of the holding-down nuts), and the screw should be kept clean and lubricated. The jaws must not be over-tightened as the mechanism may be damaged or the workpiece become distorted. To protect soft materials from the hardened, serrated, vice jaws, aluminium ‘vice clamps’ (or clams) can be positioned over the jaws. Other, special holding devices, such as ‘V’ blocks (made out of wood to protect tubular items) can be manufactured locally.

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Bench Vice Fig. 9 4.1.15 Hand Vice A hand vice (refer to Fig. 10) is classified by its overall length and can be used when splicing cables or holding small objects that are to be shaped or drilled. The body and screw are made of mild steel, with a wing nut provided for the operation of the hand vice. Small vice clamps can also be used with these vices when working with soft material.

Hand Vice Fig. 10


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4.1.16 Hacksaws The hacksaw, is the most widely-used, metal-cutting, hand saw. Hacksaws are used for parting off, or for cutting materials approximately to size. They are designed primarily for cutting metal, but may be used on other materials. The saw consists of a mild steel frame, with a suitable handle and a replaceable, serrated blade, which is made from high-carbon or alloy steel. Fine-toothed blades have 24 or 32 teeth per inch and are used for cutting thin material. Coarser blades, with 14 or 18 teeth per inch are for thicker material. A ‘rule of thumb’ is that at least two teeth must be in contact, with the work being cut, at all times (refer to Fig. 11).

Thicker Sections - Less Teeth per Inch

Thinner Sections - More Teeth per Inch Hacksaw Teeth in Contact with Workpiece Fig. 11 The blade mountings must be set in the most convenient position with the teeth facing away from the handle. This allows the blade to cut on the more efficient, forward stroke. Hand pressure should be applied on this forward stroke and relieved on the return stroke. The full length of the blade should be used for each stroke, if at all possible. This action prolongs the life of the blade, lessens the chance of teeth breaking away from the blade and reduces the chance of the saw jamming during use.

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Saw blades are given an alternate ‘set’, during manufacture, which results in the cutting slot (the ‘kerf’) being wider than the blade thickness (refer to Fig. 12). This prevents the blade from jamming, which may result in a bent or broken blade.

Teeth ‘Set’ Fig. 12 4.1.17 Sheet Metal Shears and Snips Shears are another type of cutting tool used on aircraft sheet metal. Long, straight cuts, across a piece of sheet metal, are made on a guillotine, which may also be referred to as ‘squaring shears’. The fabrication of smaller parts requires hand cutting, followed with further trimming to the final dimensions. This can be achieved with different types of shears, known as Tinman’s Shears or Aviation Snips. They can vary in length from 175 mm (7 in) up to 300 mm (12 in) and can be straight or curved cutting. Straight shears (or snips) are primarily for cutting straight or wide radius curves whilst the curved shears are dedicated solely to cutting curves. Curved shears can be found in symmetrical form, which can be used to cut curves in either direction, or they can be asymmetrical and dedicated to cutting curves in one direction only. The handles of asymmetrically curved shears are usually colour-coded (red and green), to indicate the intended cutting direction. ‘Left-cutting’ shears are coloured red while ‘right-cutting’ shears are coloured green). Unlike hacksaws (and files), shears simply part the metal without removing any material. This can, however, cause tiny fractures to occur along the severed lines and so, for this reason, cuts should be made approximately 0.8 mm (0.03 in) from the marking out line and the metal then filed down to the line.


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4.1.18 Files Files are cutting tools for removing metal from a surface and are made of highcarbon steel. The blade is hardened, whilst the tang (to which, a handle must always be attached, for safety reasons, before the file is used), is left in a softer, tougher condition and is, therefore, less brittle. Hand files are classified by their: • • • • •

Length Shape Cross-Section Cut Grade.

The length of a file is measured from the shoulder to the tip of the blade. Files are available, for special work, in lengths from 75 mm (3 in) to 350 mm (14 in). The most common sizes are 150 mm (6 in), 200 mm (8 in) and 250 mm (10 in). Files are available in a variety of shapes (refer to Fig. 13), and the most common shapes are those which are: • • •

Parallel Tapered Bellied. Shoulder

Tip Parallel Length

Tang

Tapered

Bellied

Three Most Common Shapes of Files Fig. 13

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The various shapes and the cross-sections of files allow them to be used on a wide range of tasks. The standard file cross-sections (refer to Fig. 14) are: • • • • •

Hand Round Half-Round Square Three-Square.

Hand

Half-Round

Round

Square

Three-Square

File Cross-Sections Fig. 14 The Hand is the most commonly used section for general filing; and the blade is usually parallel in shape. One edge may be without teeth, to permit safe working against a finished surface. Such a file is called a ‘Hand Safe Edge’ (HSE) file. The Round section is used in association with bellied, parallel and tapered blade shapes, with the bellied being the one most commonly used. These files are suitable for filing small radii. Half-Round files are mostly associated with a bellied-shaped blades. Such files are suitable for use on work of irregular shape or for filing large internal radii. Square files may be bellied, tapered or parallel in shape. They are used for internal work. Three-Square (or Triangular) files are, usually, of the bellied shape. They are particularly useful for filing internal corners.


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The ‘Cut’ of a file refers to the arrangement of the cutting teeth, on the blade of the file. The pattern, in which the teeth are cut, will depend upon the type of material to be filed. The common cuts of files (refer to Fig. 15) are the: • • • •

Single Cut Double Cut Dreadnought Rasp.

Single Cut

Double Cut

Dreadnought

Rasp

File Cuts Fig. 15 The Single Cut file has its teeth cut parallel, in a single direction and (for general engineering), usually, at an angle of approximately 60° to the main axis of the blade. This type of cut is relatively open and the teeth do not clog easily. Sometimes referred to as ‘Floats’, single cut Hand files are, chiefly, used for filing hard metals. Round files and the curved surface of Half-Round files are usually single cut. The Double Cut file also has one set of teeth cut at an angle of 60° to the centre line of the file, with another, crossing set, cut at angle of approximately 75°. This is the most widely used type of file for general purposes. The cut of the Dreadnought’s teeth, make this file especially suitable for heavy cutting on broad, soft metal surfaces. Its use is generally restricted to the larger sizes of flat files. The teeth of the Rasp are ‘cut’ with a punch, while the metal is hot, at the time of manufacture. This type of cut is used for filing very soft materials such as wood and leather. Manufacturers will cut files to cater for a wide range of specialised materials, such as encountered when working with aluminium and other non-ferrous alloys.

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The ‘Grade’ of a file refers to the depth and spacing (number of teeth per inch) of the cutting teeth in a similar manner to the size and spacing of the particles employed on abrasive papers and wheels. The rate of cutting and the finish given to the work is determined, to a large extent, by the grade of the file. While there are several more grades of files, available from manufacturers, the most common grades (or degrees of coarseness) of the single and double cut files, found in a typical aerospace technician’s toolkit, are the: Bastard Second-Cut Smooth. The Bastard is a comparatively coarse grade of file and, though the number of teeth per inch varies with each manufacturer, the Bastard file has approximately 30 teeth per inch. It removes metal fairly quickly and is intended, primarily, for roughing out, but may be used for the entire work, if the finish is not important. The Second-Cut files are finer (40 teeth per inch) and, consequently, give a better surface finish to the work, but are slower cutting. Smooth files (50 to 60 teeth per inch) enable a good finish to be obtained, but such files cut comparatively slowly. They should, therefore, be used for finishing work only. 4.1.19 Filing Techniques Good filing is not just a matter of removing surplus metal. The correct amount of material, at each point on the surface of the workpiece, needs to be removed, so that the dimensions and tolerances, set by the drawing, will be met. Proficiency comes with practice. New files should, if possible, be first used on soft metal. This achieves ‘tempering’ of the cutting teeth and will contribute to a longer life for the file. Before starting work, it must be ensured that the workpiece is secure and correctly placed, as both hands are required for filing tasks. A file must never be used without a handle. The file will not be under full control and the risk of puncturing the wrist or palm is very great. Files must be handled carefully. File blades, being hard, are also brittle and will break if dropped. After use, all files should be returned to their respective racks or bandolier-type holdalls, to prevent them knocking together and being damaged. The length and grade of file, appropriate to the shape (and material) of the workpiece, and to the quality of the desired surface finish, must always be used. 71811175.doc Issued December 2002

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As mentioned previously, the height of the vice is important and platforms may be constructed, to ensure that the elbows of shorter persons are level with the top of the vice. Any platforms, so constructed, should ensure that a correct stance be attained, by the work-person, in front of the vice. It is recommended that the person stand, with feet apart and (depending on whether the person is left- or right-handed), one foot advanced, in a manner similar to a boxer (or a fencer) taking guard. The body weight should be taken on the balls of the feet but, primarily, over the rearmost foot. Again depending on the person, the handle of the file is gripped in the appropriate hand, while the palm of the other hand is placed, flat on the back of the file, near the tip, when the tip of the file is resting on the workpiece. Using a rocking action, the body weight is transferred over the forward foot while pushing the file forward (and, simultaneously, to the left or right) with the gripping hand, and exerting equal downward pressure, on the file, with both hands. The full length of the file should be used for each stroke (which should not be rushed) and, at the completion of the stroke, the action is reversed, excepting that the downward pressure is relieved on the backstroke, as the file does not cut in the rearward direction. Obviously, if attempting to file a flat surface, then it must be ensured that the file is kept level during the filing action and that regular checks are made to verify the accuracy of the dimensions. During work (and particularly so with non-ferrous metals), the teeth of the file gradually become clogged (pinned) with small particles. If these pinnings are ignored they will cause scratches to the surface of the workpiece with subsequent loss of surface finish. To this end, pinnings should be regularly removed by the use of a ‘file card’ (also called a ‘scratch card’) or wire brush. Chalk, rubbed along the face of the file, before starting the finishing work, will assist in minimising pinning. Draw-filing, by grasping the file between the fingers and thumbs of both hands, on either side of a workpiece, and rubbing back and forth on the surface, may be used to rectify any ‘hollows’, which may appear on a filed surface, due to incorrect filing action. It may also be used, in conjuction with chalk, applied as previously described, to assist in creating a finer surface finish.

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4.1.20 Hand Brace (Hand Drill) Whenever it is necessary to cut accurate, circular holes in materials, then, where possible, the material should be securely clamped and the holes cut, using fixed, power-operated drilling machines. These machines are usually found in workshops and bays, bolted down to the floor (or to benches) and will be discussed in later topics. Where it is impractical to do the work with fixed machines, then the drilling is done, using either portable power tools or handoperated drills. Portable, power-operated tools will also be discussed later. The Hand Brace, or, as it is more usually called, the Hand Drill (refer to Fig. 16), is, typically, only used to drill holes of up to 6.5 mm, (¼ in) diameter in thin and comparatively soft materials. The device shown is similar to those most commonly found in the toolkits of aircraft technicians, though the actual design will depend upon each manufacturer. Another hand-operated drill, the Breast Brace, being larger, is designed to hold larger drills than the hand drill and is, normally, used (in workshops etc.) for drilling holes between 6.5 mm and 12 mm (¼ and ½ in). The breast brace has one other advantage over the hand drill, in that two running speeds can be selected, which will more closely match the correct speed, required by the various sized twist drills being employed.

Hand Brace (Hand Drill) Fig. 16


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4.1.21 Twist Drills While the range of tools, designed to create holes in metals and other materials is vast, the Morse-type (named after its inventor, an American engineer) of Twist Drill (refer to Fig. 17) is the one most commonly used in aircraft (and in general) engineering. The shank is the part of the twist drill that is gripped and driven by the chuck of the drilling machine and it is on the shank that the details of the type (grade) and diameter of the drill can usually be found printed or engraved. On drills up to 12.5 mm (½”) diameter, the shank is parallel and placed into the jaws of a self-centring chuck. On drills above 12.5 mm the shank is usually tapered (to a Morse Taper) of 1:20. The tapered shank fits directly into a matching tapered housing in the drilling machine spindle. The tapered shank usually ends in a tang and this arrangement provides a more positive drive, which is necessary to overcome the higher forces when drilling with the larger diameter drills. Land

Point

Body

Flute

Shank

Twist Drill Fig. 17 The helical flute (or fluting), formed in the drill body, provides a rake angle for the cutting edges of the drill. The fluting also allows any lubricant to flow towards the cutting edges and provides a path for the waste metal (‘swarf’), to move clear. The land of the drill actually touches the wall of the hole and steadies the drill during rotation. Immediately behind the land, metal is removed from the body of the drill, to reduce the friction during rotation.

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In order that the drill will cut properly, the point must be ground to the correct shape (refer to Fig. 18). The cutting, angle of 59° (118° inclusive), a clearance angle of 12° and a web angle of 130°, are typical for normal metal cutting, such as aluminium alloys, steels, cast iron and copper. These can be changed to suit the cutting of different materials such as harder metals, softer metals or plastics. Web or Point Angle 115°-135° Inclusive

Cutting Angle 59°

Clearance Angle 12°-15°

Cutting Edges

Typical Twist Drill Point Angles Fig. 18 It is essential that the drill point is central and that the cutting angles of 59° are equal. An offset point or unequal cutting angles will cause an unbalanced rotation that will, in turn, produce an oversized hole. To achieve the desired cutting and clearance angles (and resulting web angles), a drill grinding attachment may be found attached to a grinding wheel in a workshop. Hand grinding/sharpening of drills can be achieved (especially after practice), to an acceptable standard for general work. For the high standard of hole, required to receive rivets, in the pressurised skins of aircraft, it is common practice to discard drills, which have become blunt and to replace them with new drills. There are many different grades of metal, used in the manufacture of twist drills, the most common being: • • •

Carbon Steel High Speed Steel Cobalt Steel.


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Carbon Steel drills, in addition to iron and carbon, contain various amounts of manganese, silicon, sulphur and phosphorus. The letters CS may be found on the shanks of these drills.

High Speed Steel (HSS) drills, contain a comparatively high percentage of tungsten (8%-12%) with a lesser percentage of chromium (5%) and smaller amounts of vanadium and, possibly, molybdenum added to the carbon and iron in this steel. HSS drills retain their hardness at low red heat and can, thus, be used at much higher speeds than carbon steel drills. This results in much less damage to the cutting edges and, although HSS drills are more expensive than CS drills they can, over a period of time, result in a greater economy in the purchase of drills. Cobalt Steel drills, contain up to 12% cobalt, with as much as 20% tungsten, 4% chromium, 1%-2% vanadium and traces of molybdenum combined with 0.8% carbon. These drills are normally used on metals such as stainless steels, titanium and other very hard metals. Being extremely hard, Cobalt Steel drills are also quite brittle. Because of this, the use of these drills can be very dangerous, and, so, strict observance of the recommended cutting speeds is essential. Drill diameter sizes are also usually marked upon the shank of the drill and can be identified by the method used in their sizing. The most common methods of identifying the diameter of twist drills are: • • •

Metric Fractions of an inch The Number/Letter range.

In the Metric range, the smallest, commercially available, drill has a diameter size of 0.35 mm. The full range proceeds in increments of 0.05 mm up to 5.0 mm, and, for larger sizes, in increments of 0.1 mm. The Fractional (Inch) range has a minimum size of 1/64” diameter, proceeding in steps of 1/64” up to 15/8”, and then in steps of 1/32” up to 3” diameter. Table 6 shows an extract from the Number/Letter Range method of sizing drills. This method utilises numbers from 80 to 1 and letters from A to Z. The smallest size being the Number 80 (0.35 mm diameter) drill, and the decreasing number of sizing indicating an increase in the drill diameter.

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The letters supersede the numbers after number 1 (5.80 mm) is reached, with the next largest drill diameter being labelled A (5.85 mm). The letters then move through the alphabet to the drill diameter size Z (10.50 mm), which is the end of the scale. Table 2 EXTRACT FROM THE NUMBER/LETTER RANGE OF DRILL SIZES Number or Standard Number or Standard Number or Letter Metric Size Letter Metric Size Letter 80 0.35 4 5.30 S 79 0.38 3 5.40 T 78 0.40 2 5.60 U 77 0.45 1 5.80 V 76 0.50 A 5.85 W 75 0.52 B 6.05 X 74 0.58 C 6.15 Y 73 0.60 D 6.25 Z

Standard Metric Size 8.85 9.10 9.35 9.55 9.80 10.10 10.30 10.50

The Metric sizes have virtually superseded the numbered and lettered ranges and, usually, a metric size can be found as a replacement for an obsolete size. If the drill is too small to have the size stamped on the shank, then either a drill gauge or a micrometer should be used to establish its size correctly. The use of a suitable lubricant when drilling is very important, not only does the use of lubricant improve the quality of the hole, but it also assists in dissipating the heat produced by drilling. This improves the cutting efficiency and prolongs the life of the drill. Table 3 shows some of the recommended lubricants, which may be employed when drilling metals. Table 3 RECOMMENDED LUBRICANTS FOR METALS Material Lubricant Mild Steel Soluble Oil High-Carbon and Alloy Steels Kerosene or Turpentine Aluminium Alloys Kerosene Cast Iron and Brass Usually no Lubricant Required For a twist drill to cut efficiently it must rotate at the correct speed, in a particular metal, for a given diameter drill. Most hand drills (excepting Breast Braces) are limited to one speed, which is a compromise on the ideal speed for the material and for the drill size. The speed of most static drilling machines can be varied by means of a gearbox or variable drive belt/pulley arrangement.


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When drilling small holes, up to 6.0 mm (¼”) diameter, the depth of the centre punch mark will, usually, accommodate the non-cutting, chisel-like point of the drill, keeping it on centre and guiding the drill until it is established in the metal. When a hole larger than 6.0 mm diameter is to be drilled, the centre punch mark is not large enough to accept the non-cutting point of the drill. In this instance it will be necessary to employ the use of a pilot drill (refer to Fig. 19) to provide a guide for the larger drill. Firstly the centre of the hole is marked out on the metal and care must be taken to accurately centre punch the metal. A small drill (the pilot drill), whose diameter is slightly larger than the non-cutting point of the ‘finished size’ drill, is selected and a pilot hole is drilled in the metal (ensuring that the correct lubricant, for the particular metal, is used). The pilot drill is replaced by the ‘finished size’ drill, which can, then (and again using lubricant), be guided through the pilot hole to complete the hole to the appropriate size.

Using a Pilot Drill Fig. 19

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4.1.22 Stop, and Press (Dimpling), Countersinking Tools Two special tools, used during the riveting process, are the ‘Stop’ countersinking bit and the ‘Press’ countersinking, or (as it is more commonly known), the Dimpling tool. Both of these tools have evolved as a result of the need for flush skins on high-performance aircraft. In order to have the rivet heads flush with the surface, the skin must be prepared by either cutting away a portion of the metal to match the taper of the rivet head, or by indenting (by pressing) the edges of the hole to accept the rivet head. If the top sheet of the metal, being joined, is thicker than the tapered portion of the rivet head, then the material should be ‘cut’ countersunk. Whilst the standard countersink bit (or a twist drill, twice the diameter of the rivet hole) can be used, in a hand or power drill, to form a countersunk hole, the lack of accuracy and consistency means they are only useful for small jobs and certainly they should not be used where pressurised skins are concerned. Where a large number of holes need to be countersunk to a consistent depth, then the Stop Countersink tool should be used (refer to Fig. 20). This tool can be adjusted to cut an exact countersink repeatedly, regardless of the force applied to the drill/tool combination. The pilots can be changed, depending on the size of holes in the material, leaving the remainder of the tool to be used for all jobs unchanged. The stop may be held rigidly, during cutting, to prevent marking the surface.

Stop

Locknut

Fibre Collar Face

Pilot Drill Chuck Fitting

Chip Opening Stop Countersink Tool Fig. 20


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Press countersinking or ‘dimpling’ is done where the aircraft skin is too thin to countersink, and without the attendant risk of enlarging the drilled hole. The edges of the hole are formed, to accommodate the head of the rivet, by using a set of dimpling dies, using either ‘coin dimpling’ or ‘radius dimpling’ methods. Coin dimpling forces the sheets into the lower die, leaving a sharply defined and parallel-sided hole. This process also allows a number of sheets to be ‘stacked’ together at the expense of a complex pair of tools and leaves a neat, clean dimpled hole with smooth sides (refer to Fig. 21). Radius dimpling uses a male die to drive the sheets into a female die. The sides of the formed holes are not as smooth as the coin dimpling method, but this lessprecise operation is quicker and cheaper to achieve. With harder materials, such as magnesium and certain aluminium alloys, a process called hot dimpling is used. This method involves pre-heating the metal, so that it forms more easily and is less likely to crack when shaping takes place.

Punch

Dimpled Skin

Skin

After Rivet has been Formed Die Dimpling Tool Fig. 21

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4.1.23 Reamers Reamers are precision-ground tools, which are used to enlarge drilled holes to accurate dimensions and provide a smooth internal finish to accommodate precision-ground bolts and some special types of rivets. Reamers are manufactured from high-carbon steel or alloy steel and are fluted to provide a series of cutting edges. They are available, either for use by hand, or for using with a suitable drilling machine. Machine reamers can usually be identified by the Morse tapered shank, which is inserted directly into the spindle of a drilling machine. The use of machine reamers is, usually, the prerogative of specialist machinists and not of line- or hangar-based aircraft engineers so that only the hand-operated reamers will be discussed here. Hand reamers are rotated, by means of the hand wrench, which locates on the squared portion of the shank. They must always be rotated only in the cutting direction, even when withdrawing from a hole. The cutting lubricants, used on specific materials, are those which are used for drilling procedures. Reamers are used for removing only small amounts of material, which, typically, for hand reamers, is approximately 0.2 mm-0.3 mm (0.008 in-0.012 in), so holes should be drilled with this fact in mind. Reamers are supplied in protective sleeves, to protect the fine, vulnerable cutting edges, which run along the body of the tool and, to preserve the sharp edges, they should be kept in their sleeves when not in use. The three most common types of hand-operated reamers are the: • • •

Hand Parallel Reamer Hand Expanding Reamer Hand Taper Reamer.

Hand Parallel Reamers (refer to Fig. 22) are fixed-size, parallel-bodied reamers, possessing either straight or spiral flutes. The straight fluted reamer can be considered to be the general-purpose reamer, whilst the spiral fluted reamer is used for reaming holes which have keyways or grooves as the spiral flutes smoothly bridge the edges of the gap in the metal while the reamer rotates.


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Straight Fluted Reamer

Spiral Fluted Reamer

Hand Parallel Reamers Fig. 22 Hand Expanding Reamers (refer to Fig. 23) are used where standard parallel reamers of the required dimension are not available. This type of reamer has separate, replaceable blades that slide, in tapering slots, and which are held in position by a pair of circular nuts. The reamer blades can be adjusted to the required cutting size by slackening one nut and tightening the other. The shape of each blade is such that, at any point along the slot, its cutting edge is always parallel to the axis of the reamer. The size range of each expanding reamer is stamped on its shank. The actual size set during adjustment can be checked using either a ring gauge or micrometer/calliper.

Hand Expanding Reamer Fig. 23

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Hand Taper Reamers (refer to Fig. 24), are used to produce a tapered hole for the insertion of a standard taper pin. The two types of tapered reamers are the: •

Metric: This type, and its corresponding taper pins, have a taper of 1:50. Its size is etched, or stamped, on the shank, and refers to its smaller diameter

•

Imperial: The reamers and the taper pins, for which they are used, have a taper of 1:48. The size of a reamer is indicated by numbers (which range from 0 to 10), or by a fractional designation. The size is etched or stamped on the shank and refers to its larger diameter.

Hand Taper Reamer Fig. 24 The difference between the Metric and Imperial tapers is very slight, but it is sufficient to make the taper pins incompatible. When replacement taper pins are required, particularly when both types are available, then great care must be taken to ensure that pins of the correct taper, size and type are installed.


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4.1.24 Internal Screw Thread, Cutting Taps Taps are used for the hand cutting of internal (female) screw threads of the common types, up to a maximum diameter of approximately 25.4 mm (1.0 in). They are short, threaded bars of hardened and tempered steel, which are fluted to give cutting edges and the end of the shank is squared, to facilitate turning with a wrench (refer to Fig. 25). Taps are normally made in sets of three, with the exception of the BA thread tap sets, which have only two taps to a set. A tap set, which all have the same maximum diameter, normally consists of a: • • •

Taper Tap Second Tap Plug Tap

The Taper Tap is used to start the thread cutting process. It is tapered gradually from the point for about two thirds of the threaded length, so that it can enter the pre-drilled hole easily and assist in the correct alignment of the tap (which is very important) before cutting commences. The last third of its length has fully formed threads. The Second (or Intermediate) Tap is used, following the taper tap, to deepen the thread. This tap is tapered for the first two or three threads only and, where it is possible for the tap to pass the whole length through a hole, it is capable of cutting a fully formed thread. The Intermediate is the tap that is not available in BA thread tapping sets. The Plug (or Bottoming) Tap has no taper and its purpose is to finish the threads in deep, through holes or to cut threads to the bottom of ‘blind’ holes. Before the thread can be cut, a hole must be drilled in the workpiece. This hole must be of the correct size and the drill that is selected (the ‘tapping’ drill), must have the same diameter as the minor diameter of the thread needed to be cut. The correct tapping drill size can be obtained from workshop charts and reference books. Unfortunately, because taps are ‘glass hard’ they are also brittle and can, thus, be easily broken if due care is not given to their use. It is imperative that the tap’s location in the drilled hole be constantly confirmed and that its main axis is maintained in proper alignment with the corresponding axis of the hole. Adequate cutting fluid (as used in the drilling procedure) must be applied, and the arms of the wrench should be of an appropriate length (not too long) so that the possibility of the tap wobbling in the hole, or excessive turning force being applied to the tap (and especially to the smaller diameter taps), is minimised. If a tap jams, and snaps off in a hole, its removal can cause serious difficulties. Page 32


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Plug Tap Full Length Threads

Second Tap 2-3 Thread Taper

Full Threads

Taper Tap Gradual Taper

Full Threads

Conventional Tap Set Fig. 25

Following the drilling of the correct sized hole, the tapping procedure involves ensuring that the workpiece is securely held (firmly attached to another component or clamped in a vice) and that the taper tap is attached to the correct sized wrench. Taps, incidentally, may have ‘right’ or ‘left hand’ threads. Applying lubricant as required, the taper tap is inserted into the hole and its correct alignment verified (by use, for example, of an Engineer’s square), before it is rotated clockwise (for a ‘right hand’ thread), slowly and gently, until the initial threads are established. Once the initial threads are established, the tap must not be rotated continuously, otherwise the cuttings will not break off and the tap will, consequently, jam in the hole and, if forced, it will shatter. To this end, the tap, after each full turn, is rotated backwards, approximately ½ to ¾ of a turn, to break the cuttings off. The forward rotation is then continued, with subsequent cutting breaks, until the full thread portion of the tap has cut sufficient full threads in the hole. After the preliminary cut, the process is repeated, using the second tap (if not a BA thread), and, if required, repeated again using the plug tap. The thread, and each end of the hole (where accessible), should be cleaned out if burrs or swarf are present and, with ‘blind’ holes, the swarf must be cleared out of the hole regularly to prevent the tap binding at the bottom of the hole. In the event of a tap breaking in a hole, it may be necessary to resort to specialist procedures (spark erosion for example) for its removal without causing further, and, possibly, expensive damage, to the component or workpiece.


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4.1.25 External Screw Thread, Cutting Dies Dies are used for the hand cutting of external (male) threads on round rods or bars of comparatively small diameters. While there are several designs of dies (depending on the diameter of the thread being cut), consideration is given here only to the ‘split’ circular or button dies (refer to Fig. 26), which are, typically, found in aircraft maintenance workshops and may be used, by aircraft technicians, for the manufacture of studs and similar items. Circular dies consist of an internally threaded (‘right’ or ‘left handed’) disc of hardened and tempered steel, which is fluted to form several cutting edges. Dies also need to be rotated (in a similar manner to the previously mentioned taps), in order to cut threads but, unlike a tap and wrench, a die is rotated by the use of a stock. Die discs, within the smaller diameter ranges have a standard outside diameter, which allows a range of dies, with different internal sizes, to be used with the same, standard, stock. The discs are ‘split’, to allow for a degree of adjustment to the depth of the thread being cut. The manufacturers name, thread type, diameter and number of threads per millimetre (or inch) are marked on the face of the die (Taps, incidentally, are similarly marked on their shanks). Split Die Shoulder

Stock

Outer Securing and Adjusting Screws Centre Adjusting Screw

Circular Die and Stock Fig. 26

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Before external thread cutting is commenced, it is necessary to obtain a suitable length of rod, the diameter of which is equal to the major diameter of the thread to be cut. Care needs to be taken in this matter (and especially where closely sized Imperial and Metric rods are available) because it is possible to create a thread on slightly undersized or oversized rods. The undersized rod would, of course, be a looser fit with the corresponding female threaded item, which would not be acceptable, while the oversize rod may cause damage to the other threaded device by being too tight a fit. The die should be placed in the stock with the tapered threads (if any) away from the shoulder and the split aligned with the centre adjusting screw. It is next necessary to set the die to the maximum diameter, by slightly slackening the outer adjusting screws and gently tightening the centre adjusting screw. This will ensure that the first cut will be shallow. Failure to do this will invariably result in a poor quality thread. A shallow taper, or chamfer, must be ground or filed onto the end of the rod; to assist in the location of the die before cutting commences and the rod should be clamped firmly, and, preferably, vertically in the bench vice with the tapered end uppermost. Once more, adequate lubrication must be used throughout the procedure, again, using the same lubricants as used for the drilling and tapping tasks. Ensuring that the die is set to cut the maximum diameter, as described previously, the die should be placed squarely onto the taper of the rod and, with steady downward hand pressure, the die is carefully rotated (clockwise) to start the cut. It must be ensured that the die remains square to the rod at all times during the cutting, which is continued in a series of small arcs, reversing each time to sever the cuttings, in a similar manner as is done when using the taps. When enough thread has been cut, the die is removed and the thread checked, using a finished nut. If the thread proves to be too tight, then, after backing off the centre adjusting screw and (carefully) turning the outer adjusting screws inwards another cut is made with the die. The procedure is repeated as often as necessary until a satisfactory fit is achieved between the two, mating, threaded items. As the internal tapped thread is NOT adjustable, the internal thread should be cut first. The external thread, which CAN be slightly adjusted, should always be cut last to ensure the desired degree of fit between the respective threads.


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engineering 4.1.26 Screwdrivers

Despite the many shapes and sizes which may be met, it can be stated that screwdrivers may be divided into two main groups, which, basically, are: • •

Blade Screwdrivers Cross-Point Screwdrivers.

Blade (or Common) screwdrivers consist of a high carbon or an alloy steel blade, mounted into a wooden or composite handle. The end of the blade is ground to engage the diagonal slot in the head of the screw. If the blade is of high carbon steel, it will be hardened and tempered. Screwdrivers in this category are classified by type and by the length of blade, which can be from approximately 35 mm (1.5 in) to 300 mm (12 in) long, although special screwdrivers can be obtained with blade lengths of 500 mm (20 in). Some variations may incorporate a reversible ratchet device in the handle while others may also have an Archimedes’ drive (as in a ‘Pump’ screwdriver)). All of these features would form part of the classification of the screwdriver. The correct engagement of the screwdriver blade in the slotted head of the screw or bolt is most important (refer to Fig. 27). The most common faults can be seen in the illustration. The end of the blade should never be ground to a sharp chisel edge and a blade of the correct thickness and width should always be chosen. Screwdrivers of the wrong size can cause serious damage to fasteners, surrounding aircraft structure and to the persons using them.

Blade too Small

Blade Correct

Blade too Large

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Cross-Point Screwdrivers have been designed, by the several manufacturers of the different types of screw heads (refer to Fig. 28). These screw heads allow greater torque to be applied to the fasteners but, due to the variations in design, it is vital that the correct screwdriver be used with each type of screw head as they are not compatible. The accurate fit of cross-point screwdrivers into the recess in the respective screw head is essential if damage is to be prevented.

Reed and Price

Phillips

Posidrive

Triwing

Various Types of Cross-Point Screwdrivers Fig. 28 There is also a wide selection of other specialist screwdrivers, which have been made to allow certain tasks to be accomplished. These can include: • • •

Offset (or Cranked) Screwdrivers: which can reach screws with little clearance above their heads (and which may, also, have a blade at one end and a cross-point at the other) Reversible Tip Screwdrivers: with hexagonal shanks, that allow the shank of the screwdriver to be reversed in the handle to provide a different tip, with a blade at one end and a cross-point at the other end of the hexagonal shank Interchangeable Tip Screwdrivers: which have a selection of socket-like tips, that can be interchanged to suit any particular type of screw head.


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4.1.27 Pliers Pliers are classified by type and overall length and usually made from alloy steel, with an insulated handle. They are designed for gripping, bending or moving small items that cannot be easily handled with the fingers. Some of the many types, that are available for a variety of purposes, include: •

Side-Cutting Pliers: which are the general-purpose type, and are useful for the installation and removal of split pins. They also have a facility for cutting wire

•

Round-Nosed Pliers: which are useful for putting small radius bends into wire in addition to a variety of other tasks

•

Flat-Nosed Pliers: which, because the jaws are much thinner, may be used for many small holding and bending tasks, that are not possible with the sidecutting pliers

•

Needle-Nosed Pliers: which have finely pointed jaws and can be used in electrical and electronic work, that involves holding small components and thin wires. Needle-nosed pliers may, sometimes, have the jaws turned at right angles to the handles, to allow the operator to see the work being held

•

Wire-Locking Pliers: which are used for the specific task of gripping wire, during the wire-locking of components. Due to their integral Archimedes’ screw, they are also able to spin and so twist two wires, so that a neat and tight wire-locking is obtained

•

Circlip Pliers: which may be found in two, basic forms (Internal and External). Both types have pins on the ends of the jaws, which are used to install and remove circlips from around (and from within) components. The mechanisms are designed so that, squeezing the handles together, either results in the jaw pins coming together, (Internal), or spreading apart (External).

There are other groups of gripping tools that could, loosely be called pliers, but they usually go under the names of grips or clamps. These include ‘Mole’-type Grips: which can be locked, holding a component, freeing up the operator’s hand for other work, Pipe Clamps, which can be used for gripping pipe unions, and Slip-Joint (or Water Pump) Pliers that can have several, different gripping ranges, due to their multi-pivot mechanisms.

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4.1.28 Wire Snips (Nippers) Wire Snips (which are also, often, called ‘Nippers’) can be found with straight or diagonal jaws and are both very useful for cutting and stripping wire. They are also useful for removing split pins. Care must be used, when cutting with wire snips, as the cut-off pieces (locking wire and split pin legs in particular) can fly off, causing injury or getting lost within the aircraft structure or engine bays, which could lead to corrosion or to the jamming of vital control systems. 4.1.29 Spanners, Sockets and Wrenches The commonest spanners are those which are used on the standard hexagonal heads of bolts, nuts, screws and similarly shaped fastening devices. Other spanners are often referred to as special or non-standard spanners, and are used on different types of screw thread fastenings. Some of these special spanners have a limited application, whilst others are specifically produced for a particular component, and will only be found in special toolkits applicable to that component. Most spanners are manufactured from case-hardened mild steel, hardened and tempered high-carbon steel or alloy-steel, though there are some which are made from copper alloys, where spark-resistant tools are required. The size of a spanner, is either marked on the jaw face, or on the shank, in the units of the type of thread system being used on the fastening device. The units, shown on a particular spanner, however, relate to different parts of the fastening devices (refer to Fig. 29), so a knowledge of the spanner sizing systems is necessary. The two main sizing systems are those of the: • •

British Standard Institution (BS) and British Association (BA) Imperial system American/Unified (Imperial) and the Metric system.

The British Standard system uses Imperial units (fractions of an inch etc.) and embraces two of the three main thread systems, used in British engineering, one of which is no longer used in aircraft engineering. The sizing, on BS spanners, relates to the nominal diameter of the nut, bolt or stud, upon which the spanner is to be used. For example, a spanner marked as ½ BS indicates that the spanner is used on a ½" diameter bolt (nut, stud etc.), although the actual distance across the jaws of this spanner would be 0.820".


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Earlier BS spanners can be found with two figures stamped adjacent to each other (e.g. 7/16 BSW 1/2 BSF). The BSW figure relates to the Whitworth thread system, which is not used on aircraft, due to its tendency to loosen when subjected to vibration. The BSF refers to the British Standard Fine thread but, to avoid confusion, the older system has now been brought into line with the BS system, so that a ½" BS (BSF) spanner can (for general engineering purposes) also be used on a ½" Whitworth bolt/nut combination. British Association (the third British thread system) sizes, also use Imperial measurements, and, although they are in decimal fractions of an inch, they are represented by a whole number (2BA, 4BA, 6BA and so on) which again relates to the nominal diameter of the fastening device. The American Fine and Unified thread systems, also use Imperial measurements. The sizes, stamped on spanners, refer to the dimensions across the spanner jaws (or across the flats of the hexagon of the fastening device). A spanner marked ½" A/F, would be used on a bolt with an actual diameter of 5/16". Metric spanners are marked with a number also denoting the width (millimetres) across the flats, of the hexagon shaped fastener on which it is used.

BS and BA (Imperial) Dimensions

American/Unified (Imperial) and Metric Dimensions

Spanner Sizing Systems Fig. 29 It is important that the correct procedure is followed to avoid incorrect tools being used to install or remove a nut, bolt, stud or any other fastening device. In some instances the correct tool size may be quoted in the maintenance manual. This must be strictly followed.

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There are so many tool catalogues, crammed with a bewildering range of tools that it is impossible to include so many in these course notes. Some of the more common spanners and wrenches (in addition to the previously-mentioned tools), which are liable to be found in the average toolkit, of an aircraft maintenance technician, include such general tools as: • • • • • •

‘Set’ (Open-Jaw) Spanners Ring Spanners Flare Nut Spanners. Sockets Allen Keys Torque Wrenches.

The Set or Open-Jaw spanners are usually made in double-ended form, to provide two available sizes in one tool. The open jaws are ‘set’ at an angle (usually 15°) to the axis of the shank, which is a useful feature, because (when replacing nuts and bolts in restricted spaces), by turning the spanner over, the nut or bolt can be approached from a different angle. They are not, however, totally satisfactory devices, as the jaws only bear against two of the available six flats of the hexagon. There is always the tendency for the jaws to spring open when force is applied to the spanner. Ring spanners are preferred to set spanners as they give full enclosure of the hexagonal head of the nut or bolt, each corner of which engages snugly within an angle in the aperture of the spanner. This aperture is usually bi-hexagonal, to facilitate the use of the spanner when angular movement is restricted. Ring spanners are usually supplied in double-ended form, to fit nuts and bolts of consecutive sizes. The ends are normally offset but straight (and also cranked) types of ring spanners can be obtained. Flare Nut spanners are designed with a gap in the ring, which allows the spanner to be placed over a pipeline or electrical loom, and then to be moved onto the hexagon of the union nut or plug. Sockets spanners (but, more commonly, simply referred to as sockets) typically, have a six- or twelve-pointed opening, designed to enclose different sized nuts and bolt heads in one end, with a square hole, for the standard ‘T’ bar driver (or an alternative turning device), in the other end. Socket sets are available in a variety of drive sizes. However, in aircraft maintenance, the ¼" square drive and the 3/8" square drive are the most popular. Other sizes available are the ½", ¾" and 1" square drives.


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Sockets are available in both Imperial and Metric sizes (though the drive sizes tend to be quoted in Imperial units) and can be used with several accessories, which greatly facilitate the use of the sockets and extend the range of their application. The socket spanners are usually supplied in complete sets, of incremental sizes to suit various tasks. Deep sockets are used where a bolt extends further through a nut than normal, preventing the use of a standard socket. They are also used to remove spark plugs from piston-type aero engines. The main accessories, supplied with socket sets, can (in addition to the standard T’ bar driver) include: • • • • • •

Ratchet Handles Drive Bars Speed Braces Extension Bars Universal Joints Converter/Adapters.

Ratchet handles allow the turning to continue, even if the space does not allow full rotation of the normal ‘T’ bar driver. Most ratchets are reversible, either by the use of a selector lever or by the square drive being able to be ‘floated’ through the mechanism, to be available on both sides of the ratchet handle. Drive bars are usually produced with long handles and so, should normally be used only to break the ‘stick’ of a tight nut and not for tightening up. These are also known as breaker bars or knuckle bars. Speed braces can have a socket or screwdriver blade ‘snapped’ onto their ends. They are normally used to turn down nuts or screws, which have many threads before they tighten-up. Final tightening is completed using either a ‘T’ bar, a ratchet handle or (more usually) a torque wrench. Extension bars are used where access for a standard drive handle is restricted. Extension bars are made from forged alloy steel and come in a range of nominal lengths from 50 mm (2 in) to 1 m (39 in). Universal joints allow tightening of nuts, bolts and screws where it is not possible to obtain access in a straight line. They function better if the angle they are working through is not too great. Converter/Adapters allow sockets from one type of drive to be used with another type. For example, a 3/8" drive socket with a 1/4" drive ratchet would use a ‘stepup’ or ‘step-down’ adapter. Care must be taken, when using larger drive equipment on smaller sockets, that the nuts or screws are not over tightened. Page 42


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Certain bolts and screws, are sunk (or set) below the surface of a component and are used for locking purposes. These set-bolts and set-screws, have a hexagonal recess in their heads and the tool used for tightening and loosening these bolts and screws is the Allen Key (also called Allen Wrenches). Allen Keys are made from hexagonal-section, steel bar, suitably hardened and tempered and are cranked at 90° to give the desired leverage. Allen keys are supplied in a variety of sizes to locate with the recesses in the various screws and bolts. They are classified (in Metric or Imperial units), by the dimension across the flats of the hexagon bar from which they are made. The holding power, of a threaded fastener is greatly increased, when it is placed under an initial tensile load that is greater than the loads to which the fastener is, normally, subjected. This task is accomplished, by tightening a bolt or nut, to a pre-determined torque or pre-load. If a fastener is under-torqued, there is danger of the joint being subjected to unnecessary loads, leading to premature failure. When a fastener is over-torqued then the threads are over stressed and can fail. A Torque Wrench is a precision tool that governs the amount of force applied to a fastener and allows accurate torque values to be applied consistently. Under controlled conditions, the amount of force required to turn a fastener is directly related to the tensile stress within the fastener. The amount of torque is the product of the turning force multiplied by the distance between the centre of the fastener and the point at which the force is applied (usually the length of the wrench handle). Table 4 shows various units of torque, including Imperial, Metric and SI values.

Imperial pound force foot (lbf.ft) pound force inch (lbf.in)

Table 4 VARIOUS UNITS OF TORQUE Metric kilogram force metre (kgf.m) kilogram force centimetre (kgf.cm)


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SI Newton metre (Nm) centi-Newton metre (cNm)

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There are, again, many different designs of torque wrenches, so consideration is given here only to three basic types of torque wrench. They are the: • • •

Deflecting Beam Torsion Bar Toggle Type.

The Deflecting Beam torque wrench, has a square drive, on one end of an accurately-ground beam, with a handle, mounted on a pivot, at the other end. A pointer is attached to the square drive end of the beam, whilst a scale is attached to the beam near the handle. When a force is applied to the handle, the beam bends and the pointer deflects over the scale. The deflection is directly proportional to the torque applied. The Torsion Bar torque wrench, uses the principle that a bar accurately deflects in torsion, as well as bending, when a force is applied. The square drive is accurately ground and has a rack gear on one end. When the bar is twisted, the rack moves across a pinion gear in a dial indicator, which shows the amount of bar deflection. The dial is calibrated in units of torque. The Toggle type of torque wrench, is pre-set to the desired torque before it is put on a fastener. When this pre-set torque is reached, a sound (a click), is heard and the handle releases a few degrees, indicating that the set torque value has been exerted. Once this release occurs, then all force is removed. Note; When a castellated nut is being torque loaded, it must, first, be torqued to the lowest value of the given torque range. The torque may then be increased until the holes are in line, but before the maximum torque value is reached.

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4.2 Common Power Tools Powered tools have to be treated with respect because they can injure, and in exceptional cases, can cause death if they are incorrectly operated. Before using any powered machine/tool, personnel must make sure that: • • • • • • • • •

They have been properly trained and are currently authorised to use it All protective guards and fences are securely in place No part of the body or clothing can come into contact with moving parts Protective clothing is fastened and neck ties (if worn) tucked in or removed All rings and other jewellery are removed Safety glasses/goggles are worn wherever there is a debris risk Where necessary, the appropriate fire extinguisher is readily to hand A safety mat is available to stand on where electrical machinery is used Machinery is checked for any “Warning” notices indicating it is unsafe for use.

Possibly the most common method of powering tools is through the use of electricity, which is readily available from the ac mains supply and can also be provided from portable, dc batteries. However, because of the fire hazard, associated with the operation of electrically powered tools, and where there is a possibility of flammable vapours being present, pneumatically powered hand tools are provided for aircraft maintenance tasks, such as drilling, cutting, shaping, screw driving, riveting, nut running and setting. As previously mentioned, these pneumatic tools may be operated from a fixed air supply gallery, in a workshop or hangar, or from a mobile air compressor. 4.2.1 Electrically Powered Pillar Drills Electrically powered, Pillar Drills, are used for heavy-duty drilling tasks, where larger drill sizes and rigid holding-down of the workpiece are required. Pillar drills also have an advantage in that they are equipped with a method of altering the speed of rotation (rpm) of the chuck to suit the material being drilled and the size (and type) of the drill being used. This flexibility is needed to enable drills of all sizes to cut efficiently and safely for different types of materials. If the rpm of the machine were constant, then the cutting speed of any drill being used would be dependent upon the diameter of the drill. Small drills would cut slowly and larger drills more rapidly.


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For example, at a constant rpm, a point, on the circumference of a drill with a diameter of 10 mm, will travel twice as far, and cut at a much faster rate, than a similar point on a drill, which has a diameter of 5 mm. At this excessive rpm, the larger drill would become very difficult to control and would, almost certainly, be damaged by over-heating. The speed of rotation of most fixed drilling machines may be changed, either by means of a gearbox or by a system of coned pulleys. The work, being drilled, must be clamped in a manner that will prevent any movement during the drilling operation. Failure to observe this precaution may result in spoilt work, a broken drill and it may also cause serious injuries. Larger pieces of work are clamped directly to the drilling table of the machine, whereas small items are usually held in a machine vice, which has smooth jaws. It is essential to ensure that the point of the drill aligns with the centre punch mark and adequate cutting fluid (where required) is applied before drilling starts. 4.2.2 Electrically Powered Hand Drills These drills are useful in certain locations when work cannot be taken to a fixed, pillar drill and where there is no risk of fire from inflammable materials or vapour. They are quicker than the hand brace and, when used correctly, can be perfectly safe. The smaller electric drills have a 6.5 mm (¼") chuck, whilst other larger drills can be found with chuck sizes up to 13 mm (½") and larger. This size classification simply indicates the largest size of twist drill that the chuck will hold. Battery powered (cordless), drills offer more freedom than ac powered or pneumatically powered drills, but they should not be used in the vicinity of flammable vapours as they are not considered to be ‘spark proof’. 4.2.3 Pneumatically Powered Hand Drills The type of pneumatic drill, used for a specific task, depends very much on the access available. Three typical types of pneumatic hand drills, in common use, are the: • • •

Straight Drills Angled Drills Pistol Grip Drills.

Straight Drills have conventional chucks and keys to accept twist drills with diameters up to 5 mm (13/64”) and have push-button operation. These drills can be used for all conventional drilling operations where direct access is possible. Page 46


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Angled Drills are available for drilling holes in positions where access is not possible with straight types. The most common types of angled drills are the Angled and Offset Head drills (refer to Fig. 30), both of which will accept twist drills with diameters in sizes up to 4.8 mm (3/16”). Each drill size requires its own chuck collet, which is tightened into place with collet spanners. Pistol Grip Drills (refer to Fig. 30), have standard chuck and key arrangements, accept twist drills of diameters up to 8 mm (5/16”) and have a trigger operation. All drills may be found with built-in filters, pre-set compressed air pressurereducing devices and a requirement for lubrication. The air supply is normally via a quick release, male and female coupling (bayonet type), allowing the tool to be moved from place to place, as the work requires.

Typical Angled and Pistol Grip Pneumatic Hand Drill Fig. 30


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4.2.4 Pneumatically Powered Riveting Hammers The two basic types, into which these hammers may be divided, are: • •

Short-Stroke: fast-hitting hammers, which produce light blows Long-Stroke: slower-hitting hammers, which produce heavy blows.

The short-stroke hammers are usually used for 3/32" or 1/8" rivets and their bodies are made from light-weight, aluminium alloy castings. The long-stroke hammers may be of either the slow-hitting, reciprocating type, or may be a one-shot type, that drives the rivet set only one blow at a time, when the trigger is pulled. These hammers are used to drive the larger rivets and are much heavier than the fast-hitting hammers. Different handle styles are provided for both types of hammers (refer to Fig. 31). The Pistol Grip and Swan Neck are the most popular styles, with the Push Button (Straight) type being available for special applications where access is not possible for either of the more popular styles of hammer.

Pistol Grip

Swan Neck

Push Button or Straight

Pneumatic Riveting Hammers Fig. 31

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4.2.5 Pneumatic Miller (Microshaver) Certain hollow rivets leave a mandrel projecting from the work after the closing action. These are removed, leaving a flush surface, by careful use of a Miller or Microshaver (refer Fig. 32). The miller has an adjustable stop, to prevent the cutting tool (which rotates at high speed), from damaging the aircraft skin. Two rubber feet give the tool stability during the cutting operation. When the shank of the rivet is closed into a countersunk hole (where the rivet is installed from inside the aircraft skin), there can be a small amount of metal projecting above the skin line. This excess metal is also removed using a miller.

Unformed Rivet

Formed Rivet

Milled Rivet

Pneumatic Miller Fig. 32 4.2.6 Nibblers Nibbler are tools used for rough cutting small-to-medium sized holes in skins, radio chassis, instrument panels and other light alloy sheets. Whilst a handoperated nibbler can, occasionally, be found in use, the powered nibbler (powered by either electricity or compressed air), is the most common type of tool. The machine operates by using a reciprocating punch to cut a groove out of the metal, in small bites or ‘nibbles’. The holes, that have been ‘nibbled’, have to be filed and cleaned afterwards, to the limit marks of the true hole. One limitation, of the powered nibbler, is that it can become uncontrollable, if it is not held securely by the operator. Care and skill will, thus, be required to take advantage of the benefits of the tool, namely its fast removal of metal when hole cutting is involved.


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4.2.7 Pneumatic Tool Maintenance When used, maintained and stored correctly, air operated tools have a long and trouble-free life. They will not, however, tolerate lack of lubrication, nor the entry of moisture and foreign particles introduced via the air supply. These effects can be reduced by: • • • •

Draining the oil and water traps in the air supply system at least daily and more often if the tools are in prolonged use. Ensuring that both the male and female parts of the air supply couplings are clean before the connection is made. Before using a tool, introducing the specified lubricating oil into the air supply opening, in the correct quantity. Before the tool is stored, after use, repeating the lubricating procedure and operating the tool as slowly as possible, to distribute the lubricant throughout the tool.

4.2.8 Abrasive Wheels The most common types of Abrasive (or Grinding) Wheels, found in workshops are the double-wheeled, bench-mounted machines, with a coarse abrasive wheel, used for rapid metal removal, and a finer grade wheel, used for smoother work. Protective guards are secured around the moving parts, for the protection of the operator, and adjustable rests are provided for the support of tools, during grinding operations. A word of caution is necessary here, because of the terms that are (carelessly) applied to the various abrasives, which may be used in engineering procedures. The two main types of abrasives, used for grinding wheels, are: • •

Aluminium Oxide or Corundum: next in hardness to diamond, the blue variety of which is the sapphire, while the red variety is the ruby Silicon Carbide: formed by the fusing together of silica (or sand) with carbon, in an electric furnace.

Aluminium Oxide (Corundum), abrasive wheels are used for steel and other ferrous metals of high tensile strength. Silicon Carbide (better known under the trade name of ‘Carborundum’), wheels are used, primarily, for hard, brittle metals such as cast iron, but may also be used for grinding aluminium, brass, bronze or copper. Wheels, which are designated for use with steels, must NEVER, under any circumstances, be used for the grinding of any other materials, and in particular, NOT soft materials (light and copper alloys, wood, plastics etc.). Page 50


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These materials are liable to clog the wheel and, if ignited by a spark, will cause the wheel to explode, with devastating consequences. Only authorised personnel are allowed to use abrasive wheels and, before use, it is mandatory to ensure that: • • • • • • • •

The machine is securely attached to the bench or wall The wheels are secure, not chipped and have no excessive bearing play The operator’s clothes are not loose or in danger of fouling the wheel Suitable fire extinguishers are available All guards are correctly attached and secure The tool rests are set at minimum distance, clear of the wheels Protective goggles, in addition to any safety screens, are worn The operator stands on an insulated mat, where provided.

During grinding operations, the item, being ground, should be moved in alternate directions, across the width of the wheel, so that the grinding area of the wheel will remain flat and true and will not become dangerously grooved. An uneven or grooved wheel will require ‘dressing’ (and, possibly, need being trued) by a qualified ‘dresser’, using special fixtures and extra-hard tools. Care must be taken, during grinding, to ensure that tools do not become overheated. Cutting tools (chisels, punches etc.) will have their ‘temper’ drawn from them if they get too hot, so that it is necessary to ensure that the item is kept as cool as possible, by the frequent use of water or, possibly, a directed jet of cooling air. After completion of the grinding task, the machine should be switched ‘off’, but it should not be left until the wheels become stationary, as this takes a little time and (particularly in a noisy workshop), unattended, rotating wheels pose a danger to unsuspecting personnel.


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4.3 Precision Measuring Instruments Precision Measuring Instruments are provided to measure dimensions to a greater accuracy than can be obtained by the use of a simple engineer’s rule. Where the smallest graduation on a rule is usually either 1 mm or, perhaps, 1/64", precision instruments are available which measure to 0.01 mm or to 0.0001”. The precision instruments mentioned here would normally be found either in a workshop environment or in a ‘clean room’, which may be part of a company’s Quality Department. It should also be noted that, whilst very basic forms of the different instruments are described, in order that the principles of operation be understood, the actual precision instruments, found in workshops and ‘clean rooms’ may appear quite different and, in all probability, will possess digital readout facilities. 4.3.1 External Micrometers An External Micrometer (refer to Fig. 33), as the name implies, is used for measuring (or testing the level of accuracy of) the external sizes of objects. Graduated Barrel with Fiducial Line Ratchet Stop

Spindle Anvil

Graduated Thimble Locking Ring

Frame

External Micrometer Fig. 33 The standard (or common) external micrometer consists of an appropriately shaped frame, to one end of which is attached an internally threaded barrel (or sleeve).

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A partially, externally threaded spindle, located in a hollow, tubular thimble, is able to be screwed into (or out of) the sleeve by means of rotating the thimble. The working tip of the spindle has an accurately machined face, to match the equally accurately machined face of the anvil. The anvil is located at the opposite end of the frame and, with the spindle moved sufficiently, the object to be measured is placed between the faces of the anvil and the spindle. The ratchet stop is used to rotate the thimble so that the spindle moves until the object is held between the faces of the spindle and the anvil. To prevent distortion of the frame and to ensure that the reading is constant when taken by different users of the instrument, the ratchet stop ‘slips’ (3 clicks!) when sufficient pressure is applied to the object being measured. The principle of the micrometer is based on the lead of the screw thread. This is the distance the thread moves, either forwards or backwards, during one complete revolution of the thimble. If the lead is known, together with the number of revolutions, then the total distance the screw moves can be calculated. The circumference of the thimble and the length of the barrel are graduated to indicate the measurement of the object that is in contact with the faces of the anvil and the spindle. The barrel also has a datum (fiducial) line, against which the measurements are made, from the bevelled end of the thimble as it uncovers the markings on the fiducial line. The thimble is bevelled so that its graduations are brought close to those on the fiducial line. The bevelling eliminates shadows and also lessens parallax error when reading the measurement. The body of the micrometer usually has a matt finish, which serves to reduce glare and, thus, aids accurate readings. The locking ring (some micrometers have a locking lever) is used to lock the spindle, when the instrument is employed as a fixed (or snap) gauge. The mechanism of the external micrometer is arranged so that the spindle face can only move between 0 - 25 mm (or 0 – 1in) from the anvil face and, thus, the standard micrometer has the capability to measure items which are in this range. For larger items, the size of the frame is simply increased in successive increments of 25 mm (or 1in). For example, the next size of micrometer would be able to measure between 25 mm – 50 mm (1 in – 2 in), the next 50 mm – 75 mm (2 in – 3 in) and so on. While the frames increase in size to accommodate the larger items, the spindle movement (of external micrometers) remains in the range of 0 – 25 mm (0 – 1 in).


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Both Metric and Imperial micrometers (while their units of measurement are different), are operated in a similar manner. The Metric micrometer uses a thread pitch and, thus, a lead, of 0.5 mm (two threads per millimetre), so that the thimble moves over the barrel a distance of 0.5 mm per revolution. The fiducial line, on the barrel, is marked in increments of 0.5 mm and 1 mm, with numerals at intervals of 5 mm (5, 10, 15, etc.) to 25 mm. The thimble has a total of 50 markings, so that one thimble division represents 1/50 of 0.5 mm, or 0.01 mm. When reading a Metric micrometer (refer to Fig. 34) it is, first of all, necessary to decide on the number of divisions, on the fiducial line, which are exposed by the thimble and to note the division on the thimble which also coincides with the fiducial line. The subsequent actions, to arrive at the dimension being measured, are to: • • • •

Note the number of main divisions exposed (as shown at A = 8.00 mm) Note the additional number of sub-divisions (as shown at B = 0.50 mm) Note the number of divisions on the thimble (as shown at C = 0.28 mm) Add all the numbers together to provide the total dimension (8.78 mm).

Thimble (0.01 mm divisions)

Fiducial Line (0.5 mm divisions) Barrel

30 0

5

25

C

B A

A = B = C = Total =

8.00 mm 0.50 mm 0.28 mm 8.78 mm

Metric Micrometer Reading Fig. 34

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Imperial Micrometers measure in decimals of an inch. Their screw threads have forty threads per inch, giving a ‘lead’ of 1/40" (0.025"), which is the length of each sub-division on the fiducial line and represents one revolution of the thimble. The thimble circumference is, now, divided into only 25 equal divisions, making one division read 1/25 of 1/40”, which equals 1/1000” (or 0.001") movement of the spindle. Barrel markings are made at each tenth of an inch (1, 2, 3, 4, etc) with four subdivisions between each main mark. Again, in a similar manner to the Metric micrometer, when taking a dimension, it is necessary to deduce the number of division, on the fiducial line, exposed by the thimble. Next note the mark on the thimble which aligns with the fiducial line and follow similar actions to those employed with the Metric micrometer. If, for example, nineteen divisions, on the barrel of an Imperial micrometer, were exposed, while the eighth mark on the thimble aligned with the fiducial line, then the total dimension would consist of: • • •

Four 1/10” divisions (sixteen 1/40” divisions) on the barrel Three further 1/40” divisions on the barrel (making nineteen in all) Eight 1/1000” divisions on the thimble

In this example the total dimension would be 0.400” + 0.075” + 0.008” = 0.483”. To ensure the integrity of any dimensions it is imperative that the faces of the spindles and anvils of micrometers are kept scrupulously clean. Micrometers should be stored in a protective case, preferably with a sachet of desiccant (or VPI paper) and not used in extremes of temperature (the temperature of a standards room is usually maintained at 20°C). Never store a micrometer with its spindle and anvil in contact. Changes in temperature will cause distortion of the frame, with the obvious consequences. Prior to use, the accuracy of a micrometer should be confirmed by doing a check on the zero setting (with the spindle and anvil faces in contact) and a sample check (using slip gauges or similar, accurate standard test pieces), of measurements within the range of the micrometer. It is possible to do adjustments with special tools, which are provided with micrometers, but any adjustments should normally, only be done by qualified personnel, who will then certify that the micrometer is accurate enough, to be used for aerospace work.


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4.3.2 Internal Micrometers Internal micrometers are used for the precision measuring of internal dimensions, using much the same principles as those used with the external types. While there are many designs for internal micrometers, to suit particular tasks, space (and time) dictates that consideration be given here only to the type more commonly referred to as the ‘Stick’ micrometer (refer to Fig. 35), which is found in both Imperial and Metric versions.

Handle (replaced by a Grub Screw when the Handle is not required)

Collar

2 1 0

Extension Rod

Micrometer Head

Internal ‘Stick’ Micrometer Fig. 35 An Imperial, ‘Stick’ micrometer, consists of a micrometer head, with an overall closed length of only 1½”, a ‘spacing’ collar which has a length of ½" and ten extension rods. The lengths of the rods increase in increments of one inch, with the shortest length being ½” and the longest 9½” (e.g. ½”, 1½”, 2½” etc.). The internal micrometer differs from the external type in that the thimble travel is only half an inch and so, from closed, the micrometer is capable of measuring internal dimensions from 1½” up to 2”. For dimensions greater than 2” it is then necessary to close the micrometer and attach the smallest extension rod (½”), enabling dimensions up to 2½” to be measured. By adding the spacing collar (½”) with the smallest extension rod, measurements up to 3” can be made, then, by removing both collar and rod and using the next rod (length 1½”), it is possible to measure dimensions up to 3½”. With alternate use of extension rod and rod/collar combinations, the Imperial internal micrometer has a measuring range from 1½” to 12”. Page 56


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With Metric internal micrometers, similar principles are used, but the dimensions are, obviously, changed and are not directly related to the measurements used with the Imperial type. The closed body length is 50 mm, thimble range is 10 mm, the collar length is also 10 mm and the seven extension rods are provided in a selection of lengths, which allow measurements (in increments of 20 mm), from 50 mm to 210 mm to be made. 4.3.3 Micrometer Depth Gauge Whilst only used in specialist applications, a micrometer depth gauge is useful when the depth of a groove or recess needs to be measured with precision. The device (refer to Fig. 36) has a standard micrometer head (but the scale, on the barrel, is reversed) mounted onto a precisely ground base. When the spindle of the micrometer is flush with the face of the base, then the depth gauge reads zero and the thimble is at its maximum distance from the base.

0 9 8

To measure the depth of a recess, the base is placed over the groove and the spindle screwed down until it contacts the bottom of the groove. The reading on the micrometer head indicates the groove depth.

Micrometer Depth Gauge Fig. 36


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engineering 4.3.4 Vernier Micrometers

Some micrometers (and other precision measuring instruments), have a ‘Vernier’ scale, which enables the instrument to measure to a greater accuracy. The ‘Vernier’ principle (inventor Pierre Vernier 1580 – 1637)) utilises two accurately graduated scales. The main scale may be fixed, whilst the other (the Vernier scale), moves parallel to the main scale (refer to Fig. 37), or, depending on the instrument (such as with micrometers), it could be the other way round, where the main scale moves while the Vernier scale is stationary.

0

10

Main Scale

mm

0

1

2

3 4

5

6

7

8 9

10

Vernier Scale

Vernier Principle Fig. 37 In the very basic example (refer to Fig. 37) ten divisions on the Vernier scale are made to equal nine divisions on the main scale, so that one Vernier scale division equals one tenth of nine millimetres (0.9 mm). The difference between one mainscale and one Vernier division is, therefore, 0.1 mm. When the Vernier scale is moved (to the right in this instance), so that the first of the smaller Vernier divisions is aligned with the first main-scale division, the zeros will be displaced by exactly one tenth of one millimetre. If this principle is continued until the second division of each scale is coincident, then the zeros will have moved exactly two tenths of a millimetre apart. From this it can be seen that, whichever lines on the main and Vernier scales align, then the zero (or datum) marks will be displaced by the small amount shown on the Vernier scale.

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When the Vernier principle is applied to a micrometer (refer to Fig. 38), the Vernier scale is engraved on the barrel and is, therefore, stationary. The Vernier graduations are scaled, usually, to represent one tenth of those on the thimble, which enables a Vernier micrometer to read dimensions to an accuracy of one tenth of that of a standard micrometer. Consequently the graduations on the Vernier of a Metric micrometer represent 0.001 mm, while those on an Imperial micrometer represent 0.0001”. The example shows a Metric micrometer reading, where the graduation on the thimble scale does not exactly coincide with the datum line on the barrel. The procedure for reading the dimension is to: • • • •

Note the main and sub divisions visible on the barrel (8.5) Note the nearest thimble reading below the datum line (27) Note the Vernier line which aligns with a thimble line (6) Add the readings to provide the total dimension

= 8.500 mm = 0.270 mm = 0.006 mm = 8.776 mm.

A similar procedure would be followed with an Imperial micrometer.

Ten Vernier Scale Marks on Barrel. 0 8 6 4 2 0

35 30

5

25

Thimble Markings

Barrel Markings with Fiducial Line.

Vernier Micrometer Fig. 38 Care must be taken that it is the Vernier number, which is added, and not the value of the main scale (thimble) reading which aligns with the Vernier line. This is a common fault when reading Verniers. It may also be found advantageous, to use a magnifying glass, to assist in the reading of the smaller Vernier scale and in deciding which lines are actually in alignment.


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4.3.5 Vernier Callipers A Vernier Calliper (refer to Fig. 39), is a versatile precision instrument, used to measure both inside and outside dimensions. In many situations, a Vernier calliper is faster to use than a micrometer but, possibly, needs greater skill in manipulation in order to obtain the correct ‘feel’ and to, thus, ensure accurate readings. Callipers, furthermore, have a working range of up to 150 mm (6 in) as opposed to the micrometer’s more limited movement. Jaws for Internal Measurement. Main Scale

Vernier Scale

Position Lock

Jaws for External Measurement Vernier Calliper Fig. 39 The Vernier scales on Imperial instruments are accurate to 0.001 inch, while Metric Verniers have an accuracy of 0.02 mm. With some types of calliper, ‘nibs’ are located at the end of both jaws. The nib size, which is etched on the jaw, must be added to any internal dimensions that have been measured. Two ‘target’ points may also be found on some callipers, one on the beam and one on the sliding jaw. These are used to set spring dividers accurately, when they are being used in a comparator mode. The target points are exactly the same distance apart as the reading on the Vernier and main scale.

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4.3.6 Vernier Height Gauge The Vernier Height Gauge (refer to Fig. 40) is similar in construction to the Vernier calliper, except that an accurately machined, solid base replaces the fixed jaw and the beam is mounted perpendicular to the base, which enables the instrument to be used on a surface plate or table. The measurements, on the beam, are read in the same manner as those on the Vernier Calliper and they, usually, have both metric and Imperial markings on the same face of the beam. This instrument can be used for various purposes, when used in conjunction with other suitable attachments. These can include measuring height, comparing and transferring height dimensions (for marking-off), and also as a depth gauge.

Initial Locking Screw

Fine Adjustment Control

Vernier Scale

Final Locking Screw

Scriber

Precision-Ground Beam and Base

Vernier Height Gauge Fig. 40


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engineering 4.3.7 Vernier Protractor

The Vernier Protractor (refer to Fig. 41) provides greater precision than is possible when using a standard bevel protractor (or the protractor head of a combination set), and enables angles to be measured to an accuracy of five minutes of arc. It consists of a grooved blade, a graduated protractor head and a stock with true edges. The protractor head can be slid along the length of the blade to any required position and locked. The stock rotates about the centre of the protractor and can also be locked in any position. The angles formed by the edges of the stock, relative to the blade, are indicated on the protractor by an index mark ‘0’ on the Vernier scale that is attached to the rim of the stock disc. The protractor scale is graduated in 180° from each end, meeting at 90° at the middle. This enables both acute and obtuse angles to be measured.

Grooved Blade

Vernier Scale under Magnifier Blade Locking Device

Main Scale on Head

Stock

Fine Adjustment Scale Locking Device

Vernier Protractor Fig. 41

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The Vernier scale is formed into twelve equal parts, which are compared to twenty-three protractor main scale divisions (23°), so that one Vernier scale division represents 23/12 = 1° 55’. The difference between two protractor scale divisions (2°), and one Vernier scale division, (1° 55’) is, therefore, 5’ of arc. The Vernier scale has each third division numbered 15, 30, 45 and 60, indicating the number of minutes (which make up one degree). There are two separate scales, reading from the centre ‘0’ to left and right, to match the two protractor scales. The protractor is read from the zero on the protractor scale to the zero on the Vernier scale. This provides the number of whole degrees. The Vernier scale is read in the same direction until the coinciding line is met. The number of the coinciding line, (indicating minutes) must be added to the degrees, read from the protractor scale, to obtain the total value of the angle. 4.4 Miscellaneous Measuring Tools There are a number of specialist measuring tools, which are, usually, only found in selected workshops or in Quality Departments. These tools are normally used in conjunction with surface plates or tables, which are used to give the measuring operation a standard flat surface to base the measuring upon. 4.4.1 Gauge Blocks Gauge Blocks (also known as Slip Gauges), are, simply, precision-ground blocks of metal that are used either alone, or in combination with other blocks, to give extremely accurate measurements. The blocks are made from high-carbon steel or cemented carbide and are hardened, ground and lapped so that: • • •

Opposite faces are flat Opposite faces are parallel Opposite faces are , accurately, the stated distance apart.

The opposite faces are of such a high degree of surface finish, that, when two blocks are wrung (pressed, with a simultaneous slight twist, by hand) together, they will remain firmly attached to each other. This characteristic, of gauge blocks, enable them to be built up, into combinations, which give sizes varying in increments of 0.01 mm (0.0004 in), and whose overall accuracy is of the order of 0.00025 mm (0.00001 in) even with workshop grade blocks. Gauge blocks are supplied in sets of 50, 78 or 105 pieces and protective blocks are provided for use with inspection and workshop grades. The protective blocks should, where possible, be used as the end blocks of all combinations, and the smallest number of gauge blocks should always be used when making up a combination.


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4.4.2 Dial Test Indicator (DTI) The Dial Test Indicator, or DTI, consists of a small dial, to the circumference of which, is connected a spring-loaded plunger. When the plunger is placed on a surface and moved over that surface (or the surface moved beneath the plunger), any variations in the surface condition will be indicated on the dial of the gauge. A DTI (also called a Clock Gauge) is used, not for measuring the actual size of a component, but to indicate small differences in size or for indicating the amount of eccentricity (parallelism, run out) of revolving parts. Its dial is graduated to indicate thousandths of an inch or, in metric values, in increments of 0.01 mm or 0.001 mm, depending on the sensitivity required. The dial has the zero datum at the top (12 o’clock position), with scales of equal value to either side, which enables plus and minus values to be measured. An important feature of the gauge dial is that the dial can be rotated by a ring bezel, enabling it to be readily set to zero. The gauge can thus be used as a comparator, or as an instrument for comparative measurements, as well as for direct measurements. Internally, the plunger has a rack (or straight) gear, which drives a small pinion. The pinion is fixed to a larger gear, which drives a second pinion. This pinion is also fixed to a second, larger gear, which drives a further, third, small pinion to which the pointer of the gauge is attached. The compound gear train magnifies the plunger movement and outputs its displays, via the pointer, onto the dial. A spring keeps the plunger in contact with the surface being tested. The flatness of a surface of a workpiece, can be checked, by attaching a DTI to a scribing block that is standing on a surface plate. The surface being checked is set beneath the DTI plunger and the bezel is zeroed. The workpiece is moved beneath the DTI and variations in flatness are displayed and quantified by the dial reading. A bar may also be checked for bowing by using a DTI, attached to a scribing block, whilst the bar is supported by ‘V’ blocks. 4.4.3 Feeler Gauges Feeler Gauges have a wide application and consist of a series of thin, flexible, steel blades in varying thicknesses (normally from 0.04 mm to 1.00 mm or from 0.0015 in to 0.015 in). The blades are secured in a protective, metal scabbard, by a pin. It is important that those blades not in use should be withdrawn into the scabbard, to prevent accidental distortion, especially of the thinnest blades.

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Feeler gauges are used to measure very small, fixed gaps between faces. The blades are chosen to comply with the measurement given in the maintenance instructions. Sometimes there will only be a need to use a single blade whilst, at other times, a combination of blades may be required to achieve the given dimension. Feeler gauges are classified by the length of the blade. 4.4.4 Screw Pitch and Radius Gauges These gauges are used to check the profiles of threads and radii, by comparison with sets of blades held in a case much like feeler gauges. The thread gauges are available in all thread types and the radius gauges have both an internal and external radius. 4.4.5 Go/No-Go Gauges Where a single dimension has to be repeatedly measured, a comparator-type of gauge is used which checks, simply, whether a component is within a pair of dimensions, usually referred to as maximum and minimum dimensions. These gauges are referred to as ‘GO/NO-GO’ gauges (Fig 42) and, providing the item being checked passes through one jaw of the gauge, (the ‘GO’ dimension), and fails to pass through the other jaw, (the ‘NO-GO’ dimension), it is considered to be satisfactory.

GO G O

NOGO

Plug Type Gauge

N O G O

Calliper or Snap Gauge

Limit (GO/NO-GO) Gauges Fig. 42 4.4.6 Straight Edges Straight Edges can be found in a variety of types, from a precision-ground rule, to heavy-duty, cast iron straight edges, (e.g. the ‘camel back’ straight edge). The lighter straight edges are used to either visually check the flatness of a surface (by holding it up to a light), or to use feeler gauges. Heavy-duty straight edges will, probably, be used, to check other items for straightness, by a similar ‘bluing’ method to that used on surface tables.


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4.5 Lubrication Methods and Equipment Solid surfaces are never perfectly smooth, as the actual rubbing contact (the friction), between two moving surfaces, is concentrated on a number of high spots on the respective surfaces. Any load between the two surfaces tends to wear away these high spots until the area of contact is large enough to support the load without further deformation. During the wearing action fragments of the surface are broken away and high local temperatures are generated. The effects are potentially dangerous, in that the fragments can cause serious damage to the surfaces while excess heat leads to expansion of the materials. The expansion is likely to cause higher frictional forces, leading to further damage (due to more particles breaking from the surface) and can possibly result in eventual ‘seizure’ and stoppage of movement. Seizure could have disastrous results in any moving mechanism. Lubrication is a process for reducing friction and wear, through the introduction of an unguent between two moving surfaces. The materials, commonly used for these purposes, are greases and oils and this topic discusses the various methods and equipment used in the lubrication of aerospace mechanisms. A large number of different greases and oils are in use in aircraft maintenance. All lubricants should be kept clean and covered to keep out foreign objects. The correct quantity of the respective lubricant must, always, be used, as overlubrication may cause all manner of problems, from contamination of electrical and mechanical equipment, to dirt and dust collecting on the lubricant residue. Care must also be taken that lubricants do not remain in prolonged contact with unprotected skin (and particularly eyes) as many of them are severe irritants and present serious hazards to health. Hot oil also constitutes a danger to personnel. 4.5.1 Lubrication Methods Grease is the preferred lubricant, in certain circumstances, for the following reasons: •

• • •

Where conditions are dirty, dusty or wet, grease will provide a sealing medium, which will tend to prevent the entry of foreign matter into moving parts Grease will stay in vertical bearings, whereas oil will drain away Grease, packed into a bearing or housing, will provide sufficient lubrication for prolonged periods of time Grease lubrication systems use much cheaper fittings, and less complicated designs than those required for oil systems.

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Grease, however, has limitations in certain situations, in that grease: • • •

Cannot replace oil when heat needs to be carried away Cannot be used where contamination is likely to occur Cannot be used where debris and contaminants have to be washed away.

Greases (as with oils), are produced to a range of specifications that depend upon the applications for which they are used. Some greases have to be waterresistant, while all have to posses good storage characteristics and be of a specific viscosity. A grease, generally, is required to: • • • •

Remain chemically stable when it is heated, and especially if that grease is to be used in bearings that are running at high temperature Be thin enough to flow into applicators and spaces, whilst having the ability to remain in the desired location Resist the tendency to harden at the low temperatures met at high altitudes Retain the limits, set on its alkalinity or acidity, to prevent the onset of corrosion.

Oils are also required to remain chemically stable and to possess (and retain) a suitable viscosity during their useful life. In general, every oil must: • • • •

Adequately wet the surface needing lubrication Not evaporate too much in service Not produce gum, sludge or carbon Not damage the material with which it normally comes into contact.

In addition to protecting surfaces from corrosion, other properties of oils include the previously mentioned ability to wash away small particles of debris and to remove heat from system components. The heat can be as a result of the friction of motion, or from other sources such as the heat of combustion within a gas turbine or piston-type aero engine. Oil provides lubrication in two distinct phases, which are referred to as: • •

‘Boundary’ lubrication ‘Fluid’ lubrication.

Boundary lubrication occurs in a stationary engine, when the oil tends to drain away from surfaces, leaving only an extremely thin film of oil, clinging to the microscopically ‘rough’ surfaces of the metals. Boundary lubrication will assist in the initial movement of one surface against another but, if it is not renewed, the surfaces will slide until the film disappears and seizure follows.


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Fluid lubrication is a thin, but continuous, film of oil, separating the moving surfaces, and so preventing metal to metal contact. The oil also acts as a cushion between the surfaces. If, however, the oil is driven from the space between the surfaces, possibly due to excess loading, the fluid film is reduced to almost nothing and again, the surfaces will slide until eventual seizure occurs. A typical example of oil fluid lubrication would be as found in a typical gearbox in which the gears are immersed in oil or are flooded with oil from pressure jets. Oils in a similar manner to greases, have specialised purposes which include: • • • •

Normal Lubrication High Pressure (Gearboxes) Extreme High Pressure (Hydraulics) Preservation and Inhibiting of components or systems.

As a general rule, the specification will identify the lubricant that is required for each application. It is not necessary for engineers to know the properties of every lubricant, as component manufacturers specify the lubricants approved for use on their equipment. Generally, lubricating oils do not deteriorate during storage, but low-temperature greases can suffer and must be stored in a cool place. If water gets into either type of lubricant, the result can be very serious. Water contamination can cause: • • • •

Breakdown of normal properties under bearing loads Oil additives rendered ineffective, giving a tendency to ‘sludge’ Failure of normal properties due to oil emulsification Frothing of engine oil, which can cause excessive loss of oil through the system vents.

Many contaminants, such as rust and dust in suspension, may lead to blocking of oil passages or damage to moving parts. In grease, these solids produce a sort of grinding paste, which wears moving parts very quickly. When oils and greases are in storage, a number of precautions will prevent subsequent problems. Good lubricant storage and usage demands that: • • • •

All containers have their lids firmly secured at all times when not in use The majority of lubricants need to be stored in a cool place Different types or groups of lubricants must be kept apart, to prevent a risk of cross-contamination All equipment must be kept totally clean.

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engineering 4.5.2 Lubrication Equipment

Because of the numerous types of lubricants that are available, the equipment used for applying those lubricants is vast and so mention is made here of only some of the more typical equipment, which includes; • • •

Grease Guns Oil Cans Risbridger Replenishing Rigs.

Grease Guns cover a wide range of tools, which are all designed to inject grease, under pressure, into bearings and other components requiring greasing. One common ‘Universal’ model (refer to Fig. 43), - which can also be used to inject oil - is usually supplied with four nozzles for use with different nipples. The four nozzles are the: • • • •

Standard: Tecalamit standard sized hexagonal nipples Miniature: Tecalamit miniature sized nipples Push-on: Tecazerk and similar push-on nipples Hydraulic: Used on hydraulic nipples for priming

Note: If it becomes necessary to change nozzles, it is advisable to prime the new nozzle before its first use. Pumping Handle Filling Point Adapter

Grease Bleed Point Various Hose Lengths (Flexible and Rigid)

Various Nozzles

Universal Grease Gun Fig. 43 The gun consists of a barrel that is closed, at one end with a spring type cap, and, at the other end, by a pump head.


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The barrel houses a piston follower assembly. The pump head, which screws on to the barrel, houses a lever-operated piston and a spring-loaded non-return valve. In addition, installed in the front face of the head, is an adapter for attaching the various nozzles. A pressure- relief valve is also incorporated into the adapter. For oil filling, the cap is removed, to reveal a light chain. The chain is attached to the piston follower assembly, which is pulled out by using the chain. Once the pump is aligned head down, the barrel is filled to approximately 40 mm (1½ in) from the top, then the piston is replaced and the cap secured. For grease filling, the pump head is, firstly, removed. Next the cap is removed and the piston is pulled down to the cap end. Grease is loaded from the ‘head end’ until the barrel is full. The pump head is replaced before the cap is finally secured to the opposite end of the barrel. Note: All guns must be clearly marked with their contents and, most importantly, a check must be made to ensure that the gun is refilled with the same grease or oil as stated on its label. Oil Cans are often used to apply oils, in smaller quantities, to parts and areas which require more exact lubrication than is possible with a general spray. Cans are able to apply oil in droplets, without spillage, just where the oil is wanted. Oil cans must also be clearly labelled to show the type of oil that they contain. Risbridger Replenishing Rigs are a number of differently designed, replenishing rigs, which are normally used to replenish engine and hydraulic systems. The basic principle of the rigs is that, by attaching a pump assembly to normal cans of the correct oil, the pumping action will draw the oil from the can and deliver the oil to the tank/reservoir of the relevant system. One of the commonest rigs is attached to a can of oil by means of integral clamps, which seal the pump and hose assembly to the top of the can. A sharp blow will puncture the top of the can, depositing the collector pipe into the oil and the pump is then ready for action. The hose end will have one of a variety of connectors (such as bayonet and push-on). These will be of the correct type to enable connection to the relevant system requiring replenishment, and so avoid refilling with the incorrect oil. Note: All engineers must be careful not to attach the incorrect can to the replenisher, although most tool stores will employ a system of labels and colourcoding, to minimise the risk of this happening. On completion, it is normal practice to leave the can attached to the pump (even if it is empty), as an additional precaution against attaching the incorrect can at the next replenishment.

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ENGINEERING DRAWING, DIAGRAMS AND STANDARDS

The purpose of an engineering drawing is to record and convey the designer’s requirements to other, interested, people. The drawing must, therefore, include sufficient information to enable production planning, manufacture, assembly, testing, inspection and subsequent maintenance of the particular component or assembly to be achieved in the most cost-effective manner. So that there can be no misrepresentation of drawings, it is essential that the person preparing the drawing and those using the drawing should have a knowledge of the methods of presentation symbols, terms, and abbreviations, used in the preparation of an engineering drawing. This section is not intended as a standard for the production of drawings, but should be regarded as a general guide to drawing procedures and interpretation. The reference for drawing practices, in the United Kingdom, is that produced by the British Standards Institution, (BSI), in their publication BS 308. There are other standards available, which supplement BS 308, such as the Society of British Aerospace Companies’ (SBAC) Technical Specification (TS) 88. Companies, that have design approval from the CAA or the JAA, can modify these standards to suit their own particular drawing requirements. They must, however, publish their preferred standards of drawing, to obtain the approval of their National Aviation Authority (NAA). 5.1 Types of Drawing There are four main types of drawings recommended by the BSI, although there are many other types and sub-types of drawing used at different times. The main drawing types are the: • • • •

Single-part: unique parts or assemblies Collective: parts or assemblies of similar shape, but of different dimensions Combined: complete assemblies, including all individual parts on a single drawing Constructional: assembly drawing with sufficient dimensional and other information to describe the component parts of a construction.

A complete set of drawings for an aircraft, and any documents or specifications referenced on the drawings, represents a complete record of the information required to manufacture and assemble that aircraft. The manner, in which a set of aircraft drawings is arranged, enables any particular component, material, dimension, procedure or operation to be traced.


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Drawings of individual parts contain all the information necessary to enable the parts to be manufactured to design requirements. The material specification, dimensions and tolerances, machining details and surface finish, and any other treatment required, would all be specified on the drawings. Sub-Assembly drawings are issued to convey specific information on the assembly of component parts. When the method of assembly involves welding or a similar process, the drawing will include details of any heat treatment or anticorrosive treatment that may be necessary. Sub-assembly drawings are sometimes issued in connection with spares provisioning and also in instances where assembly would be difficult without special tools, jigs or techniques. Installation drawings are issued to clarify the details of external dimensions and attitudes of components, locations, adjustments, clearances, settings, connections, adapters and locking methods between components and assemblies. A main General Arrangement (GA) drawing of the aircraft and GA drawings of main assemblies and systems are also provided. These drawings usually contain overall profile particulars only, with locations and references of the associated main assembly and installation drawings. They also provide a guide to the identification of drawing groups used by the particular design organisation. Main Assembly drawings may also contain profile particulars only, but will include the information required for the assembly of individual parts of sub-assemblies. The sequence of assembly is given where appropriate but the information contained in single-part or sub-assembly drawings is not repeated. Parts, as such, are referenced but, in the case of sub-assemblies, only the sub-assembly will be referenced and not its individual parts. There are a number of other drawings, which are used to display alternative views of a component, or to show where that component appears in a system, while pictorial diagrams or charts, are used, to show complete or part representations of functional systems such as hydraulic and electrical systems. 5.2 Methods of Drawing Solid Objects Several methods are employed in representing three-dimensional, solid objects on the flat surface of a sheet of paper (or of other materials, used in producing engineering drawings). The two common methods, used to depict components, in drawings, are by: • •

Pictorial Projections Orthographic Projections.

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engineering 5.2.1 Pictorial Projections

Pictorial Projections provide a three-dimensional, single image of the object, as if it were being viewed, in perspective, by eye (in a similar manner to a painting or a photograph). The main types of pictorial projections (refer to Fig. 1) may be considered as the Perspective Projection, Oblique Projection and Isometric Projection. A Parallel Perspective Projection is when one of the principal faces is parallel to the picture plane.

Plan Front

Vanishing Point Side

Parallel Perspective Projection Plan

Plan Front

Side Front

Side

45° or 30°

30°

30° Oblique

Isometric

Pictorial Projections Fig. 1 Whilst perspective and oblique projections are not normally, used in aircraft engineering drawings, they may sometimes, be used in Maintenance or Overhaul manuals, to provide initial images of uncomplicated components or to portray a general view of a constructional assembly. Isometric projections are the types mostly used for sketches and for the majority of images in Maintenance and many other manuals, used in aircraft servicing.


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engineering 5.2.2 Orthographic Projections

Orthographic Projections are the types mainly used in the production of aircraft (and most other) engineering drawings of components and structures. They are drawn as if the viewer is infinitely remote from the object and rays (or projectors) lead out from the object so that the projection lines of opposite sides appear to be parallel. This method of projection provides a two-dimensional view of only one surface of the object. This means it must have multiple views (usually three, but there can be as many as six) of the relevant surfaces (drawn on three mutually perpendicular planes) to provide an accurate depiction of the whole object. There are two conventions, used for orthographic projections (refer to Fig. 2), and they are: • •

The older First Angle Projection The more recent Third Angle Projection.

Side View

Plan View

Front View

Front View

Plan View FIRST ANGLE PROJECTION

Side View

THIRD ANGLE PROJECTION

Orthographic Projections Fig. 2 The internationally recognised symbol, of the truncated cone (frustum), is the key as to whether the First or Third Angle projection is being portrayed on a drawing.

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The First Angle projection is being used when the truncated end of the cone is viewed and the two concentric circles are drawn at the remote end of the cone. In the same way, the surface of interest (of the object) is drawn remote from that surface in First Angle projections. Third Angle projections show the surface of interest drawn adjacent to that surface, in the same manner that the two concentric circles are drawn adjacent to the truncated end of the cone. Note; It is possible, on some drawings, to find the cone reversed (end for end), but the location of the two concentric circles, relative to the truncated end, will always provide the information as to how the drawing is to be read. 5.2.3 Sectional Views When it is necessary to show the internal construction or shape of a part, a sectional view is used. The four main types of sectional views are the: • • • •

Revolved Section Removed Section Complete Section Half Section.

If only the shape of a part needs to be shown, it is drawn with either a revolved or with a removed section (refer to Fig. 3). The symbols, used for sectioning, indicate where the object has been cut or sectioned and also indicate from whichever direction the section is to be viewed. A

A-A

A Revolved Section

Removed Section

Revolved and Removed Sections Fig. 3


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The Revolved Section view is often used to illustrate simple items with no interior parts. Basically, a revolved section shows how a part is sectioned and revolved to illustrate it from a different view. The Removed Section view is also used to illustrate simple objects. However, to do this, the object is cut, by a cutting plane line, and a section is removed to illustrate another angle. Section (or Hatching) lines help to enhance the clarity of the sectioned view and are, conventionally, drawn at 45° to the axis of the section. Dissimilar metals, other materials, or adjacent parts of similar materials, within the section, are indicated by the hatching lines being drawn in different directions or with different spacing between the lines. The usual spacing between hatching lines is, preferably, not less than 4 mm, with the exception of small areas where they are usually not less than 1 mm apart. Assemblies of multiple parts are often shown in Complete or in Half Section views (refer to Fig. 4). The complete section view makes it easier to identify individual parts within an assembly (particularly where the assembly is more complex). Sometimes it helps to be able to see the outside of the item being sectioned, and, if the inside of the component is symmetrical, unnecessary detail can be omitted from the view. Both of these situations are overcome by half-sectioning the component.

Complete Section

Half Section

Complete and Half Sections Fig. 4 5.2.4 Exploded Views Illustrated Parts Catalogues often make use of exploded views, to show every part of an assembly. In this type of drawing, all parts are, typically, in their relative positions and expanded outward. Each part is identified, both by its physical appearance and by its reference number, which is used on the Parts List. An exploded view drawing can be of great assistance, when dismantling and reassembling a complex component.

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5.2.5 Drawing Lines, Symbols and Abbreviations Table 1 shows some of the types of line used in most drawings (as laid down in BS 308). The lines are designed to ensure that interpretation is clear at all times.

Type

Table 1 LINE TYPES (EXTRACT FROM BS 308) Description Width Application (mm) Continuous (Thick) 0.7 Visible outlines and edges. Continuous (Thin)

0.3

Continuous irregular (Thin)

0.3

Short dashes (thin)

0.3

Chain (Thin)

0.3

Chain (thick at ends and changes of direction, thin elsewhere) Chain (Thick)

0.7

Fictitious outlines and edges, dimensions and leader lines, hatching, outlines of adjacent parts and revolved sections. Limits of partial views or sections when the line is not on axis. Hidden outlines and edges. Centre lines and extreme positions of moveable parts. Cutting Planes.

0.3 0.7

Indicates surfaces which have to meet special requirements

The width of the lines, shown in Table 1, relates to the width of the nibs of the pens which are used to complete drawings that are produced in design departments or in drawing offices. These widths are only approximate and will change as soon as the drawing is photocopied. Some drawings may be completed by pencil and then the lines are differentiated by the use of pencils, the leads of which possess varying degrees of hardness, so that the softer leads create a blacker line while the feinter lines are drawn, using a ‘harder’ pencil.


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The lines, used for basic dimensioning (refer to Fig. 5), are depicted, using a standard format, which permits commonality between draughtsman and engineer. Dimensions are usually shown so they can be read when the drawing is held on the bottom or right hand edge. This is done to reduce the number of times a drawing is handled and, thus, potentially increases its working life.

∅ 80

∅ 35

3

∅ 70

50

∅ 30

6 12

∅ 25 Dimensioning Examples Fig. 5 In order to save time and space, when compiling a drawing, abbreviations and symbols, as found in BS 308, can be used. Capital letters are normally used to ensure clarity, but lower case letters may be found when it is deemed appropriate. Machining symbols, for example (refer to Fig. 6), indicate the type of surface finish that a component requires. Type of surface finish Value of surface finish (mm)

Lap 0.08

Lap 0.08 Surface to be machined Typical Machining Symbols Fig. 6

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5.2.6 Conventional Representations Common features, which may appear several times on a drawing, are seldom drawn in full, since this would take up space and time unnecessarily. Table 2 shows how some of these features are illustrated by conventional representations, which are listed in BS 308. Table 2 TWO EXAMPLES OF CONVENTIONAL REPRESENTATIONS Title Convention External screw threads (detail)

Holes in a linear patch

5.2.7 General and Geometric Tolerances A general tolerance is usually given for all dimensions on a drawing. Where general tolerances are inadequate or restrictive, an individual tolerance may be given to a dimension. Tolerances (as discussed in Workshop Practices) may be expressed by quoting the upper and lower limits, the nominal dimension or the limits of tolerance above and below that dimension. Geometric Tolerances are used where it is sometimes necessary to place tolerances both on geometric features and dimensions, in order to control the shape of a part adequately. A recommended system can be found in the BS 308. This is information usually required during manufacture.


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5.3 Drawing Identification System An aircraft engineering drawing (refer to Fig. 7), must certain data, which is used to prove its validity (and legality). All alterations to drawings must be made in accordance with a drawing amendment system, which will ensure amendment to design records. If an alteration is made, a new issue number and date must be allocated to the drawing. To comply with legislation, procedures must be introduced to progressively amend the total definition of the product in terms of its associated list of drawings at specific issues. Each particular variant of a product and its state of modification must be identifiable in relation to the appropriate list of drawings. DRAWING No.

TC002

A

DRAWING ISSUE No.

N/A

APPROVAL DATE OF ISSUE

New Drawing

ALTERATION

WORKBENCH EXERCISE No 1

65.0

15.0

R5

45°

25.0

R5

10.0 60.0 115.0

1

Note: Stamp initials on workpiece before case hardening

TC002-1

ITEM

AIRCRAFT

CHECKED BY.

SCALE

ANY ERRORS SHOULD BE REPORTED TO THE INSTRUCTORS

DO NOT SCALE IF IN DOUBT ASK

DATE

N/A

19-09-01

TREATMENT

Riveting Block

QTY

TOLERANCE

uk

mm

THIRD ANGLE PROJECTION

TITLE

USED ON: N/A

engineering

DRAWING No.

RIVETING BLOCK

HURRICANE WAY NORWICH AIRPORT NORWICH NORFOLK ENGLAND NR6 6HB

TECHNICAL COLLEGE

TC002

SHEET 1 OF 1

Typical Drawing Data Fig. 7 The following details indicate some of the items of information that might be found on a conventional drawing. Page 10

1

DESCRIPTION

± 0.05 mm

N/A N/A

Case Harden

1:1

DIMENSIONS IN

APPROVED BY.

THIS DOCUMENT IS THE PROPERTY OF KLM uk ENGINEERING LIMITED AND MAY NOT BE COPIED OR COMMUNICATED TO A THIRD PARTY OR USED FOR ANY PURPOSE OTHER THAN THAT FOR WHICH IT IS SUPPLIED WITHOUT THE EXPRESS WRITTEN AUTHORITY OF KLM uk ENGINEERING LIMITED.

MATERIAL / SPECIFICATION

DRAWN BY.

STRESS

CAD GENERATED DRAWING - NO MANUAL ALTERATIONS

Mild Steel

PART No.


uk engineering


5.3.1 Title Block The title block is generally pre-printed and contains the essential information required for the identification, administration and interpretation of the drawing. It is recommended that the title block should be at the bottom of the sheet with the drawing number in the lower right hand corner. Adjacent to this drawing number should be the title and issue (alteration) information. For convenience, the drawing number may appear elsewhere on the drawing, usually inverted so it can be read whichever way it is filed. 5.3.2 Drawing Number No two drawings should bear identical drawing numbers and a design office should maintain a register of all drawings issued. The Drawing Number may refer to elements such as the project identity, the group breakdown, and the individual register number. Except for repair drawings, the Drawing Number is also generally the Part Number. 5.3.3 Handed Parts Drawings of handed parts usually have the left-hand, upper, inner or forward part drawn. This item is allocated the odd number, with the opposite hand the consecutive even number. The drawing sheet bears the legend ‘AS DRAWN’ and ‘OPP HAND’ in the item quantity column. Where necessary the handed condition is indicated by a local view or annotation. 5.3.4 Sheet Numbers Where a complete drawing cannot be contained on a single sheet, successive sheets are used. The first sheet is identified as ‘SHEET 1 of X SHEETS’, as applicable and subsequent sheets by the appropriate sheet number. Where a Schedule of Parts (Parts List), applicable to all sheets, is required, it appears on Sheet 1. 5.3.5 Drawing Changes Change to a design drawing, with the exception of minor clerical corrections, is usually accompanied by a new issue number and date. New parts added to the drawing, or ‘drawn on’ parts affected by the change, take a new issue number, and parts, which are not affected, retain the original issue number. In all cases where interchangeability is affected, a new Drawing Number and Part Number are allocated.


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Details of the drawing changes are recorded in the appropriate column on the drawing, or recorded separately on an ‘Alteration Sheet’, which is referenced on the drawing. The issue number may, sometimes, be represented by a letter. Some organisations use alphabetical issues for prototype aircraft drawings and numerical issues for production aircraft drawings; thus all drawings of a prototype aircraft become ‘Issue 1’ when production commences. An alteration to a single part drawing may also result in changes to associated drawings, and it may be necessary to halt manufacture or assembly of the product. The Drawing Office system usually makes provision for the proper recording of drawing changes, by publishing concurrently with the re-issued drawing, an instruction detailing the effects these will have on other drawings, on work-in-progress and on existing stock. As a further safeguard, some organisations publish ‘Drawing Master Reference Lists’, which give details of the current issues of all drawings which are associated with a particular component or assembly. 5.3.6 Part Referencing Every item called up on a drawing is given an item number, which is shown in a ‘balloon’ on the face of a drawing. No other information is given in, or adjacent to, the balloon, with the exception of information necessary for manufacture or assembly, such as ‘equally spaced’ or ‘snap head inside’. A Schedule of Parts is, normally, also included. Materials such as locking wire and shimming, which are available in rolls and sheets, will be detailed by specification number in the ‘Part No’ column and the quantity will be entered as ‘As Required’ or ‘A/R’. 5.3.7 Validation of Modification/Repair Drawings When a modification or a repair is required to be embodied into an aircraft structure or component part, it usually necessitates the use of a working drawing to assist with the work. To ensure the authenticity (and legality) of the drawing, it should bear a ‘Validity’ stamp (using red ink) which is applied by the issuing department. The stamp consists of the authorisation stamp and signature of the issuing person and the date on which the drawing is obtained from the issuing department. In addition the stamp should bear the words VALID ‘TIL: followed by a second date.

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The additional date will be that of the date of the next scheduled revision (usually Quarterly – January, April, July, October or similar) to the relevant manual or document from which the working drawing has been copied. Working drawings must not be used beyond their validation date, but must be returned to the issuing department for checking and re-validation before use. 5.3.8 Summary of Recommended Drawing Information Table 3 provides a fairly comprehensive summary of the recommended basic and additional information, which is likely to be found on typical aircraft engineering drawings. Table 3 RECOMMENDED BASIC AND ADDITIONAL DRAWING INFORMATION Recommended Basic Drawing Information Company Identifier (Name, Logo etc.) Drawing number Copyright clause Descriptive title of part/assembly Date of drawing Units of measurement ‘Issue’ information General tolerances Projection symbol Original scale Sheet number Warning: ‘DO NOT SCALE’ Number of sheets Grid or zoning system ‘Validation’ stamp for working drawings Signature(s) Recommended Additional Drawing Information Material and specification Treatment/hardness Surface texture Finish Screw thread forms Tool references Sheet size Gauge references Print-folding marks Reference to drawing standards Supersedes Equivalent part


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5.4 Auxiliary Diagrams and Charts In addition to the conventional Engineering Drawings, uses are made of other diagrammatic means of presenting information, for specific purposes, during maintenance operations. These auxiliary diagrams can include: • • • • •

Electrical Wiring Diagrams Component Location Diagrams Schematic Diagrams Block Diagrams Logic Flowcharts.

5.4.1 Electical Wiring Diagrams Electrical wiring diagrams are included in most aircraft maintenance manuals and they will specify details such as the size (gauge) of the wires and the types of terminals used for each application. Wiring diagrams, typically, identify each component within a system by its part number, (and sometimes by its serial number), and will include any changes that were made during a production run of an aircraft. There are several types of electrical wiring diagrams. Some diagrams show only one circuit while others show several circuits within a system. More detailed diagrams show the connection of wires at splices and junction boxes in addition to the arrangement of components throughout the aircraft. On modern aircraft, wiring diagrams can vary from a single page diagram (applicable to a light aircraft, for example), to those of a modern jet airliner, which might need to have many dozens of wiring diagrams to give each system and sub-system enough clarity to make them useful. Because some aircraft have very complex electrical and electronic systems, a separate Wiring Manual is often produced. The Wiring Manual can include full wiring diagrams, component location diagrams, and schematic diagrams to provide a system overview (which show all connections to the components etc.) to assist in trouble shooting.

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5.4.2 Component Location Diagrams In a Component Location Diagram (refer to Fig. 8), components, within a specific system, are shown as they actually appear, and not as symbols or as cut-aways. This simplifies understanding of the overall system operation.

Electrical Power Receptacle

Component Location Diagram Fig. 8


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Schematic diagrams can be found in maintenance manuals to represent and give information on aircraft systems such as electrical, hydraulic, pneumatic, lubrication, air-conditioning, and pressurisation, and also to provide details for engine and (where applicable) propeller operation. Coloured diagrams are not used in maintenance manuals, but a system of hatching and shading is normally used to indicate how the system functions. A typical electrical system schematic diagram (refer to Fig. 9), is used to give an overview of the complete aircraft system so that maintenance and diagnosis can be initiated. The diagram may show sources of electrical power and the distribution of that power to a wide range of bus-bars. Each sub-system would have an additional diagram, showing the circuits, in detail, from those bus-bars.

Gen 1

AC Ext

Gen 2

DC Ext

Automatic AC Bus Transfer System AC Bus 1

TRU 1

Battery Charger 1

AC Bus

2

Battery Charger 2

TRU 2

Battery 1

Battery 2

Batt Bus 2

Batt Bus 1

DC Bus Transfer System

Typical Electrical System Schematic Diagram Fig. 9

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5.4.4 Block Diagrams Due to the complexity of electrical and electronic diagrams, a Block Diagram (refer to Fig. 10) may be used to assist in certain tasks, such as trouble shooting, because the purpose of trouble shooting is merely to locate the faulty module, rather than to check it out internally. The ‘blocks’ can represent components, circuit boards, or replaceable modules.

RF Amplifier

Demodulator

AF Amplifier Loudspeaker

Simple Block Diagram Fig. 10 5.4.5 Logic Flowcharts Logic Flowcharts provide another aid to trouble-shooting, by representing the mechanical, electrical or electronic action of a system without expressing the constructional or engineering information. A simple Logic Flowchart (refer to Fig. 11) can be used (by following the arrows through the sequence), to detect faults in an operation and to provide solutions for correcting the faults.


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Start

Obtain correct size drill

No

Is drill size correct? Yes Secure drill in chuck

Adjust drill speed

No

Is drill speed correctly set? Yes Align and secure work in clamp

Drill hole

Finish

Simple Logic Flow Chart Fig. 11

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5.5 Microfilm, Microfiche and Computerised Presentations Due to the increased complexity of modern aircraft, the amount of information needed, within the Maintenance, Spares and Repair manuals, has grown to an enormous extent. For example, the Maintenance manuals, for one modern aircraft alone, consist of twenty volumes, each of which would be more than 76 mm (3 in) thick. To compress even greater amounts of data, other media are utilised, to make the information more easily available to aircraft servicing technicians. These include: • • •

Microfilm Microfiche Computers (CD-ROM).

5.5.1 Microfilm This method entails one publication being reproduced, on a roll of film and contained in a special cartridge case, approximately three inches (76 mm) square. The pages are sequentially copied onto the film and wound upon a drum, within the cartridge case. A microfilm ‘Reader’ (a projector) is used, to wind the film through a ‘gate’ and display a single page of text/drawing upon a screen, which is large enough to enable the text and illustrations to be read and understood. Because of the condensing of the ‘hard copy’ books into a small space; a complete set of maintenance manuals can, thus, be contained in a small number of microfilm cartridges which can be stored close to the Reader. A number of these projectors are provided with a printing facility, that allows the person, reading the film, to print a copy of any sheets which contain information that is required away from the machine. All copies, removed from the microfilm reading room, must be used once only, and not retained for later work. This practise ensures that amendments and updates are not missed. 5.5.2 Microfiche A similar process to microfilm, with the exception that many pages of the manuals are reproduced on one clear sheet of film, measuring approximately 100 mm x 150 mm (4 in x 6 in). Each sheet is capable of storing a large number of pages (over 100) of text/drawings and takes up very little space.


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The Reader is similar to the microfilm Reader except that the film slide is moved about, beneath the viewing lens, until the relevant page appears upon the screen. By simply pressing a button on the machine, a photocopy of the page being viewed can be produced for remote use and, once again, any copies should not be retained for future use. Amendment of both this and the microfilm system is by direct replacement, with local disposal of the unwanted items. 5.5.3 Computer CD-ROM The use of computers, with respect to aircraft maintenance manuals, (and other publications), has the primary advantage of the huge amount of information that can be stored on one Compact Disc (CD). A single computer, located within a maintenance facility, could have all the necessary publications (such as the Maintenance Manual, Illustrated Parts Catalogue and Wiring Diagrams), for the relevant aircraft type, held on one CD. As with the other two systems, there should be the facility to print the necessary information required with, of course, the limitation that the information is only valid ‘on-the-day’, and must not be used for repetitive jobs. Updating of computer-based systems is by the simple replacement of the relevant CD-ROM, although there may be intermediate amendments. 5.5.4 Supplementary Information It is important that only the current issue, of whichever system is in use, is supplied to servicing technicians. This means that the amendment procedures must be carefully monitored (and especially the disposal of the out-dated material). The new amendments come with a ‘Letter of Transmittal’, from the relevant authority, in exactly the same manner as they do with the ‘hard copy’ technical publications. Because of the need to dispose of large amounts of information, whenever even a minor update or amendment is carried out, it is normal to produce Supplementary Information in hard copy form, as an intermediate source of current information. These issues are in addition to either the film/fiche/CD-ROM systems in use and must be not only carefully monitored, but also well publicised. This ensures that the technicians know that the information, contained in the system they are using, could, possibly, contain small items of out-of-date information.

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5.6 Aeronautical Standards A standard is variously defined as: •

•

Something, established for use as a rule, or basis of comparison, in measuring or judging capacity, quantity, content, extent, value or quality, or a level or grade of excellence Any measure of extent, quality or value, established either by law, or by general use, or by consent.

In the normal performance of their duties, technicians can find a wide array of standards, establishing the characteristics of the materials and components that they encounter in their day-to-day work of maintaining and repairing aircraft. 5.6.1 Air Transport Association Specification No. 100 Since 1 June 1956, the Air Transport Association of America (ATA), has used a specification, to establish a standard for the presentation of technical data, by aircraft, engine or component manufacturers, that is required for their respective products. This specification is known as ATA Specification No.100 (ATA 100), and its two Chapters clarify the general requirements of the aircraft industry, with reference to the coverage, preparation and organisation of all technical data. Chapter 2 of the ATA 100 covers policies and standards applicable to specific manuals and it details the names and contents of the various manuals that must be prepared by the manufacturer. Such manuals include the: • • • • • • • • • • • • • •

Aircraft Maintenance Manual Wiring Diagrams Structural Repair Manual Aircraft Illustrated Parts Catalogue Component Maintenance Manual Illustrated Tool and Equipment Manual Service Bulletins Weight and Balance Manual Non-Destructive Testing Manual Power Plant Build-up Manual Aircraft Recovery Manual Fault Reporting and Fault Isolation Manuals Engine Manual Engine Illustrated Parts Catalogue.


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Chapter 1 of the ATA 100 covers policies and standards applicable to all publications and provides a uniform method for arranging technical material, within the relevant publications, in an effort to simplify the technician’s problem in locating instructions and parts. In the ‘Arrangement of Material’ section, in Chapter 1 (1-2) of the ATA 100, the standard details the use of a three-element identifier number. Each element of the identifying number consists of two digits. The first element is designed to provide identification of all topics or systems, within the respective manuals, by reference to specific Chapters. The second element identifies sub-systems (subtopics) as Sections, while the third element identifies associated sub-sub-systems (sub-sub topics) as Subjects. Table 4 illustrates an example of how the ATA 100 numbering system (in this instance using numbers ranging from 27-00-00 to 27-31-14) is used, to identify the material which is covered at particular locations within a typical Maintenance Manual.

First Element, Chapter (system) 27 27

27

27

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Table 4 EXAMPLE OF ATA 100 NUMBERING SYSTEM Second Element, Third Element, Coverage Section Subject (sub-system) (unit) 00 00 Material which is applicable to the system as a whole (in this instance Flight Controls). 31 00 Material which is applicable to the sub-system as a whole (in this instance Elevator and Tab Control System). 31 00 Material which is applicable to the sub-sub-system as a whole. This number (digit) is assigned by the manufacturer. 31 14 Material applicable to a specific unit of the sub-sub-system (Elevator Feel Computer). Both digits are assigned by the manufacturer


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The subject is broken down yet further – into Page Blocks – to provide maintenance personnel with more detailed information on specific topics (or subtopics) which relate to the Subject material. Table 5 shows an example of a Page Block system along with the topics and subtopics, which are allocated to the relevant Page Block numbers.

Table 5 EXAMPLE OF ATA 100 PAGE BLOCK NUMBERING SYSTEM Topic or sub-topic Page Block Description and Operation 1 to 100 Trouble-shooting 101 to 200 Maintenance Practices (if brief) 201 to 300 (Otherwise) Servicing 301 to 400 Removal/Installation 401 to 500 Adjustment/Test 501 to 600 Inspection/Check 601 to 700 Cleaning/Painting 701 to 800 Approved Repairs 801 to 900 Note: The word EFFECTIVITY - which may appear on the left hand side of the bottom of a page – is used to identify the aircraft serial number, or manufacturer’s serial number (MSN), or aircraft model to which a particular Subject topic may refer and those numbers will be shown. If the word ALL appears adjacent to the EFFECTIVITY then the information concerns all types of aircraft (or components), regardless of any serial numbers. Chapter 1 of the ATA 100 also details the policies and standards applicable to all publications with reference to the: • • •

Physical Requirements: Format of media (Paper, Film, Page layout/numbering etc.) and Indexing (List of Effective Pages [LEPs], Table of Contents [TOC], Text, Divider Cards, Sequence, etc.) Issuance and Revision Service Aircraft and Engine Zoning: Access Door, Port, Panel and Area identification.

Many airlines and similar companies also organise their spare parts in stores departments under the relevant ATA specification numbers and, irrespective of the aircraft type, information on similar components will be found in the same Chapter and Section. A complete table of the ATA numbering system, subsystem and titles, allows the technician to establish, precisely, where the information required can be found in the respective manuals.


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5.6.2 International Organisation for Standardisation (ISO) This is an international organisation, which has representatives from each member country, usually consisting of that country’s own standardising organisation. 5.6.3 British Standards (BS) These are controlled by the British Standards Institution (BSI) and are the recognised body for the preparation and promulgation of national standards and codes of practice. The BSI represents the United Kingdom on matters pertaining to ISO. 5.6.4 Military Standard (MS) This standard was developed by the military, and has found its way into all aspects of aviation. The MS (also MIL) standard has, all but, replaced the AN system and, in many cases, former AN parts are now being produced as MS or MIL parts. The suffix usually consists of a letter, which identifies the item (R for rivet, S for steel, C for cable, etc.), and a serial number. These standards apply both to hardware and materials. For example, MIL-C-5424 is a standard 7 x 19, aircraft cable, made of carbon steel. 5.6.5 Air Force and Navy (AN) The AN system is one of the most widely used standards in aircraft hardware. It was developed, together with the MS system, by the US military to ensure quality and uniformity. Items manufactured to this standard are not limited to the military and are found in all classifications of aircraft. 5.6.6 National Aerospace Standard (NAS) Items of hardware used within military aviation, which have been proven satisfactory by the aerospace industry, can be granted a NAS designation.

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FITS AND CLEARANCES

For ease of manufacture and replacement, it is essential that the components of similar mechanisms should be interchangeable. For this reason limits (tolerances and allowances) are imposed on the manufacturing procedures. The limits on dimensions ensure that, if any two mating parts are manufactured to the tolerances and allowances, stated on the drawing, then they will assemble without the need of further hand working or machining to achieve the required standard of fit. Because shafts are much easier than holes to machine small diameters, the main systems of Limits and Fits, for engineering purposes, is usually the ‘hole-based’ system. The holes are created to a certain tolerance and the sizes of the shafts are altered to provide the required class of fit between the two items. 6.1 Dimensions Mass production has long been the basis of the approach to the most economic methods of manufacturing and the complete replacement of a defective item is common practice in the maintenance of aircraft and aerospace components. For this reason, limits are imposed on the manufacturing processes, to ensure that, if any two mating parts are manufactured to the dimensions as stated on the relevant drawings, then the parts will assemble without need of further major adjustments and in the least time possible. The limits are based on the allowances and tolerances imposed on the dimensions of the manufactured parts. These dimensions will be given the accuracy required by the designer of the respective parts. 6.1.1 Allowances An allowance is a difference in dimension that is necessary to give a particular ‘class of fit’ between two parts. If, for example (and using a typical limit system), a shaft were required to locate with a corresponding hole in a component. Then, to assist in the economy of manufacture, either the hole or the shaft is made as accurately as possible to the nominal size and an allowance is applied to the associated item. The term ‘shaft’ also includes bolts and pins. If the shaft is constant and the hole varies in size, then the system used is said to be ‘shaft-based’. If the hole is constant and the shaft varies in size, then the system is ‘hole based’. The hole-based system is the one in more general use.


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The item dimensioned to include the allowance also has high and low limits and, therefore, a tolerance. The correct allowance would be the difference between the high limit of the shaft and the low limit of the hole. 6.1.2 Tolerances The tolerance on a dimension is the variation tolerated and may be considered as a numerical expression of the desired quality of workmanship. It is the difference between the high and low limits of size for that dimension (refer to Fig. 1). Thus, a part that should be exactly 25 mm nominal diameter, will be accepted for a certain purpose if it is within the limits 25.1 mm, (the high limit); and 24.9 mm, (the low limit). The difference between the two (0.2 mm) is the tolerance.

High limit of hole

Low limit of hole

Low limit High limit of shaft of shaft

Shaft/Hole Tolerance Terms Fig. 1

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Table 1 shows that tolerances may be stated in one of three ways, using a hole that has a nominal size of 100 mm diameter and a tolerance of 0.02 mm as an example. Table 1 TYPES OF TOLERANCES Bilateral 100 mm ± 0.01 mm Unilateral 100 mm + 0.02 mm 100 mm – 0.00 mm Limits 100.01 mm 99.99 mm With sheet material, such as patch plates, used in certain repairs, the dimensions quoted in the repair scheme usually have a tolerance in one direction only, the nominal size being the lower limit. In effect the patch plate must never be below the nominal size, although it can be slightly over, in accordance with the repair scheme in the manual. 6.2 Drilling Sizes for Holes The size of hole to be drilled depends upon the purpose of that hole. A hole drilled for a rivet with a specific diameter would differ from those drilled to take a screw thread, or the plain shank of a bolt, of the same diameter. Similarly the size of a hole which is to accommodate a shaft will depend on the size of the shaft and on the manner in which the hole/shaft combination is to be used. Additionally, if the hole is to be reamed, then it must be drilled slightly smaller than its nominal size, to allow for the metal removed by the reamer. Drill sizes (as discussed in the Tools topic) are fixed and can be found on charts that list each standard drill size, together with other columns such as clearance and tapping sizes. These charts may also include equivalent sizes displayed in metric, fractional, letter and in the number/letter system. 6.3 Classes of Fits There are three principal classes of fit, between shafts and holes, and they are the: •

Interference Fit: where the shaft is larger than the hole

•

Transition Fit: where the shaft and hole are approximately the same size

•

Clearance Fit: where the shaft is smaller than the hole.


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Interference Fit

Transition Fit

Clearance Fit

Hole-Based System British Standards System of Fits Fig. 2 6.3.1 Newall System In the early Newall hole-based system of limits, the holes are classified as Class A and Class B fits. Class A holes are manufactured to a closer tolerance than are Class B holes. Table 2 shows how the shafts are classified, using the letters F, P, D, X, Y, and Z.

Class of Fit Interference F

Table 2 NEWALL SYSTEM OF FITS Type of Fit Force

Driving D

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Transition P

Push

Clearance X, Y and Z

Running

Remarks Mechanical pressure is required for assembly and, once assembled, no dismantling is likely to be required. These are a little less tight than Force Fit and one part can be driven into the other. Slight manual effort is required to assemble the parts. Suitable for detachable or locating parts but not for moving parts. Suitable for various types of moving parts. Class Z provides the finest fit


uk engineering


6.3.2 British Standards System The British Standard (BS 4500) hole-based system of fits (refer to Fig. 2), also uses the basic terminology for holes and shafts. The terminologies are similar to those used in the Newall system (and several other systems), the holes and shafts are identified by a more extensive alpha/numeric identifier. It can, however, be seen that, in an Interference Fit, the upper and lower limits of the shaft are greater than the corresponding limits of the hole and, thus, force is necessary to achieve the fit. In the Transition Fit, the differences in the upper and lower limits of both items are negligible so that only light effort is required to insert the shaft into the hole. The upper and lower limits of the shaft, in a Clearance Fit, are always less than those of the hole, so that the shaft moves easily within the hole. 6.4 Schedule of Fits and Clearances Wear occurs at any time that there is motion between two parts. This motion can be intentional, such as when a shaft rotates in a plain (journal) bearing or when a roller moves back and forth over a track. Wear can also be accidental, where two parts, that should be immovable, chafe together. If the parts are intended to move together, then the maintenance documentation will have a Schedule of Fits and Clearances, based on the limit system, issued for each mechanism, used on the aircraft. If the parts are not intended to move together, it will depend upon inspection procedures to discover the problem and repair schemes will be initiated, in an attempt to prevent recurrence. The Schedule of Fits and Clearances contains tables, which specify the limits on wear and other characteristics such as: • • •

Ovality (of a hole or shaft) Bow of a shaft Twist of a shaft.


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6.4.1 Limits for Wear The four dimensions, typically covered in wear tables are: • • • •

Dimension New Permissible Worn Dimension Clearance New Permissible Worn Clearance.

Dimension New relates to the size of the part when new, and will show the relevant tolerances. Permissible Worn Dimension refers to the size to which a part may wear before it must be rejected as unserviceable. Parts, which are not worn beyond this size, can be used again, providing a suitable mating part is chosen to keep the clearance within the permissible figure. This will frequently involve choosing a new part to mate with the worn part. Clearance New is the desired clearance in limit form. Interference fits are quoted as negative clearances. Permissible Worn Clearance refers to the maximum allowable clearance when reassembling the component. 6.4.2 Limits for Ovality This usually occurs as a result of the surface wearing, through friction or linear movement. Ovality and can apply equally to holes and shafts (refer to Fig. 3). Holes may be tested for ovality, using such instruments as Go/No-Go gauges, internal micrometers, or callipers, as were previously discussed in the Tools topic of this course. A shaft may be tested for ovality, by the use of snap gauges, external callipers and micrometers, which were, again, discussed in the Tools topic. It is important to test for ovality of a shaft, before testing it for bow, as the results may be suspect if bow is done first. Bow in a shaft can be determined, in a workshop, by utilising V blocks, a surface gauge and a DTI (in conjunction with a surface table).

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Wear

Wear Hole

Shaft Ovality of a Hole or a Shaft Fig. 3

6.4.3 Limits for Bow When dealing with shafts and tubes, it is vital that not only are the ends square with each other, but that the centreline of the complete shaft or tube is straight. If the centre line of the shaft is not straight, then the item is bowed. When the shaft or tube is rotating, especially at a high speed in a bowed state, there is the risk of vibration, which can lead to mechanical failures, loosening of fasteners and (most critical of all) fatigue. All cylindrical items, both tubular and solid, can be given a limit to the amount of bow permitted. For example a drive shaft, which rotates about 1500 rpm, may have a limit of 0.25 mm (0.01 in) bow over the length of the shaft. This ensures that, within the limits of production, the drive shafts are effectively straight, giving the least possible vibration.


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6.4.4 Limits for Twist Twist is the result of applied torsion on circular or square-sectioned shafts. If the twist disappears, as a result of removing the force, then the shaft will have been loaded below its elastic limit. If the shaft remains twisted, after removal of the load, then it has been loaded above its elastic limit. The action of a shaft (of whatever section), carrying a torque load is to twist in proportion to the torque applied. The result of cyclic loading of shafts is that, at certain times, the shafts have to be checked for permanent twist. If the shaft has a square section, it can be checked for twist on a surface table using a DTI mounted on a surface gauge. Solid or tubular shafts that have to be checked for twist will possibly have witness marks or lines engraved or etched at each end of the shaft. The shafts can be checked, by mounting the shaft in V blocks and, then, locating these marks in the horizontal position. It is possible to measure the amount of twist, to which a shaft is subjected, whilst in operation or rotation, by the use of strain gauges. These emit varying amounts of electric current when under strain, giving an indication (on a calibrated instrument) of the load being applied. The designer of the aircraft or equipment will set all limits, with regards to the distortion of parts and set them down in the relevant manuals. The methods used to measure the distortion will either be standard procedures, such as using a DTI and surface table etc., or will have a special procedure included in the manuals.

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RIVETING

Rivets are a non-detachable form of fastening device, used extensively on aircraft, to secure the items of components built up from sheet metal. They are ideal for forming liquid-tight joints, are cheaper, lighter in weight and are more rapidly fastened than bolts. Rivets, however, have the disadvantage that they are not really suitable for tensile loads. A riveted assembly cannot be readily dismantled. Rivets, basically fall into two classes, which are: • •

Solid rivets Hollow or tubular rivets

Rivets are supplied with one head already formed, the tail being formed by handoperated or machine tools. 7.1 Types of Solid Rivet Solid rivets are available in a variety of shapes and metals. The common types of British rivet (refer to Fig. 1) are the snap head, which is used for general purposes, the mushroom head, where less resistance to the air is essential, and the countersunk head, where a flush finish is required. In the USA the common heads are the universal (similar to the mushroom head) and the countersunk head. Countersunk heads are available in a variety of different head angles, usually 60°, 90°, 100° and 120°, with the most common being the 100°.

Snap Head

Mushroom (or Universal) Head Rivet Types Fig.1


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7.1.1 Rivet Materials Unless otherwise stated, the rivets must be of the same material as the work being riveted. The rivet material may be identified by markings, colour, anticorrosion treatment or magnetic properties. Solid rivet markings are usually situated on the head or tail of the rivet. Tubular rivets are not marked. When in doubt as to the identification of rivets, reference should be made to the packaging label. Solid rivets can be made from a variety of materials with aluminium alloy being the most common. The material and specifications of British and American rivets are not the same. The type of rivet used for repair is dictated by an aircraft’s maintenance manual. Permission from the aircraft manufacturer is required before any changes, to rivet specification, are allowed. 7.1.2 Basic Rivet Location Terminology Basic terminologies (refer to Fig. 2) are employed, to describe the location of rivets relative to each other and to the limiting boundaries of the assembly. The most common terms are the: •

Pitch: which is the distance between two rivets in a row, measured centre to centre, and it should be at least four times the rivet diameter (4D).

•

Spacing: which is the distance between adjacent rows, and it should be between 3D and 4D. A zigzag pattern of spacing is normally used for liquidtight joints.

•

Land: which is the distance between rivet centres and the edge of the metal sheet and it not should be less than 2D.

•

Allowance: which is the amount of rivet shank that protrudes beyond the material before the rivet is formed.

•

Clearance: which is the amount that the rivet hole is larger, than the rivet shank diameter.

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Pitch

Spacing Land

Allowance

Clearance

Terminology of Rivet Locations Fig. 2


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7.2 Types of Riveted Joints The location of the riveting dictates the type of joint (refer to Fig. 3) that is made. An ordinary lap joint is used on lightly loaded members and, to provide a flush surface on one side, the joint may be joggled. Where one flush surface and greater strength is required, the single butt joint is used. The strongest joint is the double strap butt joint.

Lap Joint

Joggled Lap Joint

Single Strap Butt Joint

Double Strap Butt Joint

Types of Riveted Joints Fig. 3 7.3 Closing Solid Rivets This is usually done during a repair or modification to the aircraft structure. The repair scheme or modification leaflet will detail the rivet size, type and spacing to be used which, usually will be the existing layout and materials used in adjacent parts of the structure. The rivet diameter will normally be approximately three times the thickness of a single sheet of the material being joined. The tools required to close (set) a solid rivet are a rivet snap, reaction block (dolly) and pneumatic hammer (rivet gun). If access is restricted, then two people will be required to complete the task, - one holding the work and one riveting. A solid rivet may be closed as follows: • • • •

It should, first, be ensured that the plates are secured by use of gripping pins The hole is prepared and the correct rivets obtained A rivet, of the correct diameter and length, is inserted into the hole The rivet head is supported in the snap and the dolly placed on the tail of the rivet

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Light blows are applied, with a hammer, to the snap whilst the dolly is kept on the rivet’s tail The partially formed rivet is inspected and (if satisfactory) more blows are applied with the hammer until the rivet is formed correctly.

•

7.4

Closing Hollow Rivets

There are a variety of hollow rivets, designed primarily for application where there is access to only one side of the job. These include Tucker-pop, Chobert, Avdel, Cherry and the Hi-Lok family of modern fasteners. 7.4.1 Tucker-pop Hand-operated pop-rivet pliers or Lazy-tongs pliers can be used to close Tuckerpop rivets. A procedure for using the Lazy-tongs type pliers is as follows: • • •

The Lazy tongs are fully extend and the mandrel shank inserted into the chuck The tongs are slightly compressed and the rivet inserted into the hole in the material being riveted The chuck is held firmly and squarely against the material, while the tongs are further compressed, until the head of the mandrel closes the rivet and the stem break off.

The broken off mandrel stems, swarf, rivet heads and shanks, which are discarded during a repair operation, must be removed after completion of the task. The danger of such waste material fouling items such as control cables, cannot be over-emphasised. 7.4.2 Chobert Chobert rivets (refer to Fig. 4) are very similar to Tucker Pop rivets, excepting that the mandrel pulls fully through the rivet and is used many times, (including some which have a magazine facility allowing repetitive operation). To operate the tool: • • • • •

The mandrel is lubricated and threaded with the rivet The rivet is then threaded into the jaws of the tool with the operating handle fully anti-clockwise The rivet and mandrel are pushed fully into the tool jaws The rivet is place into the hole, in the work, and the tool is checked for square. The operating handle is turned clockwise, to pull the mandrel through the rivet, and, when the resistance ceases, the rivet is secured

After closing the Chobert rivet a pin is inserted through the centre of the rivet, to provide additional shear strength and some sealing properties.


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Mandrel

Closing a Chobert Rivet Fig. 4 7.4.3 Avdel Where the strength of a solid rivet and a sealed joint is required, Avdel rivets (refer to Fig. 5) are one of the possible choices. These leave part of the mandrel firmly in place, the top of which can be milled off to leave a flush surface finish. Mandrel Avdel Rivet

Mandrel milled flush with skin Closing Avdel Rivets Fig. 5

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7.4.4 Cherry Max One of the most common riveting systems is the ‘Cherry Max’ system, produced in the USA, by Cherry Textron. The system is one of a range produced by the company, others include Friction-lock and Mechanical-lock Cherry rivets. Cherry Max rivets (refer to Fig. 6) consist of four components, assembled as a single unit. The components consist of a: • • • •

Stem with a break notch shear ring and plug section Locking collar, which locks the stem Fastener sleeve with a locking collar dimple Driving anvil, to ensure correct installation.

Mandrel

Driving Anvil

Tubular Rivet Sleeve Unformed Locking Collar

Bulbed Head Cherry Max Rivets Fig. 6


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One other ‘family’ of fasteners that have taken the place of rivets in certain high load situations is the Hi-Lok (refer to Fig. 7), Hi-Lok Hi-Tigue and Hi-Lite group. These fasteners vary from those mentioned before, not only because they are screwed down to full closure, but they are also installed using ordinary hand tools such as open-ended spanners or Allen wrenches, avoids the need for complex and expensive special closing tools. The collar is tightened onto the pin to close the two sheets of metal, and when it reaches the correct torque the hexagonal drive portion shear off. The collar part of the fastener may have a fibre washer attached to aid sealing properties.

Collar Fibre Collar

Pin

Closing a Hi-Lok Rivet

Fig. 7 7.4.6 Rivnuts Rivnuts are a form of blind rivet, which can be used as an anchor nut, because the internal bore is threaded to receive a bolt or screw. These fasteners can be found with either countersunk or flat heads and are installed with a special tool, fitted with a threaded mandrel. This mandrel is screwed into the rivnut and, when the gun is operated, the pull force on the mandrel expands the shank, leaving the rivnut securely fastened and able to receive a bolt or screw after the tool is removed.

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7.5 Inspection of Riveted Joints Riveted joints must be inspected at all stages of production and operation. This means that the manufacturing stages must be thoroughly inspected to ensure that the finished work meets the required specifications. Whilst in service, rivets must be inspected regularly, to check for a number of faults that might have occurred, such as corrosion, fretting and fatigue. After the rivets have been closed, they should be inspected to ensure that they are tight and fully formed. Rivet heads must not be deformed or cracked and the surrounding area must be free from distortion and undamaged by riveting tools. All aircraft maintenance manuals contain diagrams of formed rivets and their possible faults (refer to Fig. 8). These diagrams show what is acceptable and what is not.

Clinched Head

Cocked Head

Cracked Shop Head

Countersinking too Deep Typical Rivet Faults Fig. 8

Whilst rivets that are clearly not satisfactory must be changed, care must be taken when considering replacing those only slightly below standard. It is possible that more harm could be done replacing them, than leaving them in place.


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If there are any signs of damage to the airframe structure, then a thorough inspection of the whole area must be made. Hidden damage may extend beyond the area of visible deformation, so that any riveted joint that shows an indication of damage should be inspected well beyond the last deformed rivet. Inspection of a rivet head for stretch can be achieved by sliding a feeler gauge under the head or tail. A staining colour of black or grey around a rivet head is an indication that it has stretched. If any doubt exists it may be necessary to drill out the rivet and examine the hole for indications of elongation or tearing. Any stretching will become apparent when the rivets are removed, as the skin will move position. Once the material has settled it may be necessary for the holes to be drilled out oversize, providing this is in accord with the repair publications. 7.6 Rivet Removal Procedure As with all maintenance tasks on aircraft, the procedure for removing solid rivets will be detailed in the AMM. The following procedure explains a basic method of rivet removal: • • • •

The centre of the manufactured rivet head is carefully marked with a centre punch Using a twist drill the same size as the rivet shank diameter, the rivet is drilled to the depth of the head The head is carefully removed, with a flat chisel or is prised out with a pin punch The remaining shank is then punched out with a parallel pin punch of the same diameter as the rivet shank.

An alternate method, occasionally used by some manufacturers, is to drill the tail of the rivet off first and remove the remaining shank from the opposite end. Care needs to be taken, during rivet removal, to ensure that the least possible damage is done to the original hole and its surrounding structure. When removing rivets from bonded assemblies it is essential not to apply shear loads, which are liable to part the bond.

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PIPES AND HOSES

Pipes and hoses can be called upon to carry a wide variety of different fluids within an aircraft, including fuel, hydraulic and engine oils, de-icing fluids, pitot and static air. The pressure within these pipes can vary from ambient to 300 M Pa (300 bar or 4000 psi). All pipes and hoses must be manufactured, installed and connected so that no leaks occur in service, because a leak in a very low-pressure pitot air tube can be just as dangerous as a leak in an extremely high- pressure hydraulic line. Rigid pipelines are, generally, made from stainless steel, Tungum (Trade name for a high-tensile, copper alloy) and aluminium alloy. Replacement pipelines are, usually , supplied by the manufacturer, ready for installation, with the pipe bent to the correct curvature and the pipe ends flared and provided with the appropriate end fittings. In certain circumstances, it may be permissible to manufacture new pipelines from lengths of pipe. A new pipeline will be made, by cutting the basic pipe to the correct length, attaching the correct couplings and expanding the ends by the use of a flaring tool. Requests for the basic pipe material will require details of the: • • • •

Metal specification (DTD, BS, AN etc.) Outside diameter (OD) Gauge of the wall thickness (SWG) Length of pipe required.

Flexible hoses are obtained from the aircraft manufacturer using the aircraft’s Illustrated Parts Catalogue (IPC). It is possible that, in certain circumstances, a replacement hose can be manufactured in a workshop or hose bay. Approval to manufacture the replacement hose must be sought from the aircraft’s manufacturer. 8.1 Pipe Bending To lessen the possibility of the pipe wall kinking when it is being bent, it may be filled with a special alloy, which can be removed after the bending operation. These alloys are known as ’fusible alloys’, some of which melt below 100° C and can, therefore, be melted out by immersion in boiling water. The pipe is oiled first, to prevent the alloy adhering to the tube wall. It is next plugged at one end, pre-heated and then filled with the melted alloy. Once cooled, the pipe can then be bent as required.


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After bending, the pipe should be unloaded, by immersing it in boiling water until all the alloy has run out. The pipe must then be cleaned internally to ensure that any alloy adhering to the walls of the pipe is removed. This is accomplished by using a ‘pull through’ with the pipe immersed in boiling water or by using a steam cleaner. The complete removal of the fusible alloy from the pipe is extremely important as its presence may lead to blockages or corrosion and, in steel tubes, which may be subsequently heat-treated, the presence of any alloy would cause intercrystalline cracking. 8.1.1 Simple Bending Jigs A simple bending jig (refer to Fig. 1) is supplied with a range of rollers and stops and the pipe is bent using the correct combination of components checking the new pipe against either a template or the old pipe.

Simple Bending Jig Fig. 1 8.1.2 Hand Pipe-Bending Machines Hand pipe-bending machines are available for pipe sizes up to 12 mm (< ½ in) and for sizes of 12 mm to 25 mm (½ in to 1 in). A typical hand pipe-bending machine (refer to Fig. 2) would have a matching former and guide for each pipe size in the range, giving a bend radius of approximately four times the pipe diameter. A pressure indicator allows adjustment, so that when bending thin wall pipes, there is no risk of ‘wrinkling’ or ‘flattening’. The roller ensures that the load of bending is transmitted axially to the pipe, via the guide, which ensures that no sliding and so no damage, takes place between the guide and pipe.

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Accurate bends can be made either from a drawing or a template, by following simple instructions when marking out the bend. This is especially true when the bends have to be a specific dimension from the end of the pipe or a series of bends have to be made to produce a complex shaped pipe assembly. Pressure Indicator

Adjustable Stop

Adjustable Screw

Former Roller

Bending Arm

Pipe Guide

Pull this way to Bend Pipe

Hand Pipe-Bending Machine Fig. 2 8.2 Pipe Flaring Flaring can be achieved only when the end of the pipe has been accurately squared off and cleaned out. Once a flare has been formed correctly, it should remain completely fluid tight at all normal pressures. 8.2.1 Flaring Tool Pipe flaring tools, come in a variety of sizes, with a range of pipe sizes that can be flared by each particular tool. A typical flaring tool (refer to Fig. 3), is used to flare tubes in the range 12 mm to 25 mm ((½ in to 1 in). Sets of half-bushes or dies cover the range of tube sizes for each machine. The flaring tool is usually mounted in a hand vice or some other rigid mounting. Once the half-bushes have been installed, the union-nut and collar are placed onto the tube and the tube is then clamped into the bushes, with the tube end flush with the end of the dies or half bushes.


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Latch Fitting Threaded Sleeve Rotation Handle

Pipe Latch Fitting Securing Screw

Expander Cone Rotation Handle Expander Cone Threaded Sleeve

Pipe Flaring Tool Fig. 3

The threaded sleeve is slowly fed into the end of the tube whilst simultaneously turning the expander cone via the rotation handle. This spreads the end of the tube until it contacts the inner face of the bushes. A correctly finished flare should leave prescribed amount of the tube projecting from the collar. The finished flared end with the union nut and collar can be connected to a variety of other end fittings. These can include other pipes, and both internal and external adapters fitted to a number of different components. 8.2.2 Standard Flared Pipe Couplings Various types of standard flared pipe couplings (refer to Fig. 4), are available in aircraft fluid systems. These couplings have different angles and whilst they may look similar, they are not interchangeable. The AGS system uses a 32° flare whilst the AN system uses flares of 74° included angle. Care must be taken to ensure that the correct couplings are fitted when manufacturing these pipes.

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Externally coned adapter

Internally coned adapter

Flared pipe

Union nut

Spherical-ended adapter nipple

Pipe to external cone adapter

Sleeve

Spherical-ended adapter nipple Pipe to internal cone adapter

Pipe to pipe

Standard Flared Pipe Couplings Fig. 4 8.2.3 Flareless Couplings The flaring operation leaves the tube end in a stressed condition which, as the flare takes a large amount of the vibration loadings, can result in fatigue failure. To overcome this situation, the ‘flareless coupling’ was introduced. The flareless coupling, achieves its sealing properties by the deformation of a sleeve, built into the coupling (refer to Fig. 5). The end of this sleeve has a sharp, hooked shape, which is known as a ‘pilot’. It is the action of this sharp hook, cutting into the pipe, which provides the required sealing properties. The individual parts of the coupling are assembled and the nut is simply screwed down on to its union until finger tight, then turned one further turn with a spanner. This action bows the sleeve and causes it to bite into the tube at its forward end. When the nut is slackened, the sleeve remains permanently bowed and attached to the pipe. This pre-setting can be done either with the service union or with a special hardened steel union that is only used for pre-setting.


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Hooked Sleeve

Under-ightened pre-set

Correctly tightened pre-set

Over-tightened pre-set

Flareless Pipe Coupling Pre-set Fig. 5 After pre-setting, the pipe should be inspected to ensure the sleeve is correctly bowed (It is permissible for the sleeve to rotate on the pipe). In service, the nut should be tightened until a distinct resistance is felt, then tightened further, - the amount depending on the tube size and material. Under no circumstances should the nut be tightened further to stop any leaks, this action will permanently damage the tube end and sleeve. 8.3 Inspection and Testing of Pipes and Hoses Before any inspections can be done, it must be ensured that the components are scrupulously clean and that all critical areas are visible if the inspection is done while the component is in its normal, installed location (in situ). Rigid pipes should be inspected for signs of: • • • • •

Chafing Corrosion – both externally and internally where possible Cracking of flared ends where appropriate Deformation and Dents Deterioration in condition of end fittings and their threads.

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Hose assemblies should be inspected for such defects as: • • • • • •

Blistering – Both externally and internally where possible Burn damage or discolouration Chafing, circumferential cracking or crazing of the outer cover Date of manufacture – to ensure that it is within its prescribed life, and that it will remain so until the next inspection Deterioration in condition of end fittings and their threads Flattening, kinking or twisting.

The relevant maintenance manual will state the intervals of inspections and the criteria which must be met before rigid pipes or hose assemblies may be considered fit for further service. 8.3.1 Bore Testing of Pipes Pipes should be tested to ensure that the bore is clear and dimensionally correct after forming. One method of satisfying this requirement is to pass a steel ball, with a diameter of 80% of the internal diameter of the pipe, through the pipe in both directions. When the design or size of the pipe and end fittings, makes this test impractical or when a more searching test is required, the drawing will normally require a flow test to be performed. 8.3.2 Hydraulic Pressure Testing of Pipes Hydraulic pressure testing consists of firstly carrying out a flow test. This means a full bore flow by pumping fluid through the pipe and checking the flow at the open end. If this check is satisfactory, the open end should be suitably blanked. Once the flow test has been carried out, the oil pressure should then be built up to that prescribed on the drawing, usually 1½ times the maximum working pressure. The duration of the test must give the pipe a chance to show any leaks or other problems. 8.3.3 Pneumatic and Oxygen Pressure Testing of Pipes These pipes are usually given an initial hydraulic pressure test, using water as the test medium, followed by a compressed air test that is limited to maximum system pressure. Using high-pressure air during the test is very dangerous and the pipe(s) under test should be placed behind a protective screen and/or submerged in water. 8.3.4 Cleaning After Test


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After a pipe has been tested, it should normally be flushed out using a suitable solvent, dried out using a jet of clean, dry air and blanked off, using the approved blanks. Pipes that will be used in high-pressure air and gaseous or liquid oxygen systems must be scrupulously clean and free from any possible contamination by oil or grease. It is normal to recommend that pipes for use in these systems are flushed with Trichloroethane or some other suitable solvent, blown through with double filtered air and blanked-off, with the approved blanks immediately afterwards. 8.3.5 Testing Flexible Hoses Once the manufactured hose has been checked for satisfactory physical condition, the hose must be flow and pressure tested. The flow test will verify whether the hose inner lining is secure and not acting as a form of non-return valve. This is achieved by passing the fluid through the hose assembly both ways to confirm that there is an equal and free flow. Where a replacement hose has been manufactured in a local hose bay, a bore test may be done, in the same manner as that with rigid pipes, by use of a ball bearing being rolled in both directions through the hose. In this instance, however, the diameter of the ball should be 90% of the internal diameter of the hose’s end fittings. The hose should then be ‘proof-tested’ by capping one end of the hose and applying the test pressure, usually twice the working pressure, to it for between one and five minutes. 8.4 Installation and Clamping of Pipes Prior to installation, the pipe should be checked to establish that it is of the correct type and that there is evidence of prior inspection and testing. This may involve checking the inspector’s stamp and part number. Once the pipe has been checked for signs of damage, dirt or corrosion, and found serviceable, it must then be immediately installed. When transporting lengths of pipe, especially long lengths, great care must be taken not to kink or otherwise damage the pipe prior to installation. Once in position, the pipes should be loosely placed into position in the supporting clamps, and adjusted so that the connections align correctly. The connections can then be tightened up, the clamps fastened and any bonding leads attached. 8.4.1 Pipe Supports

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Multiple pipe supports are often used to save space and these can be made from a variety of materials, such as fibre blocks, aluminium alloy, moulded rubber or nylon. The clamp halves are usually joined together and attached to the aircraft structure by bolts. It is important that the edges of the semi-circular recesses are not sharp and are of the correct size for the pipe in use.


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In some instances, packing will be installed between the pipe and the clamping material. This will usually be to reduce vibration or to insulate the pipe and clamp material, if they are likely to suffer from electrolytic corrosion. Individual pipe clamping is usually achieved using ‘P’ clips. These are light alloy loops with a rubber sleeve, which wrap around the pipe and are held by a single bolt to the aircraft structure. To avoid the risk of fretting occurring between the pipe and various parts of the aircraft, minimum dimensions must be observed between these components, which can be found in the AMM. The CAAIPs list these dimensions as 6 mm (0.25 in) from fixed structure, 18 mm (0.7 in) from control rods and 25 mm (1 in) from control cables, but the AMM must always take precedent. 8.5 Connection of Pipes When connecting pipes with brazed, flared or flareless couplings, there are a number of points to be considered. • • •

Union nuts must be free to rotate and can be slid back from the end of the pipe without fouling. All loose items such as nipples and washers, are of the correct type and correctly located. All pipe ends align correctly without any undue pressure on the pipe. (Pipes should never be forced into position, neither should they ever be pulled-up into position by their union nuts).

8.6 Maintenance of Pipes and Hoses The correct methods of installing pipes and hoses (refer to Fig. 6) must be followed if damage (and possibly disaster) is not to result. Pipes attached to the airframe structure, are often shielded and will not usually be liable to accidental damage. Other pipes may be located in exposed positions, where they may be susceptible to damage or corrosion. Pipes located in wheel bays or attached to an undercarriage leg could easily be damaged by stones and mud or corroded by thrown-up water. Some pipes may be badly sited and may be subject to abuse from carelessly performed and unrelated servicing activities. Chafing can occur in many places, such as clamps and clips, so care must be shown to eliminate or at least reduce the chances of this happening. Cracking of pipes can occur when pulsations are present and/or the pipe has sharp bends. This risk must also be considered when inspecting pipe runs.

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Liquid leaks can be found by the presence of fluid, or at least dampness, on the pipe or clamps. Gaseous leaks must be searched for using one of the proprietary leak-detecting fluids. The relevant AMM will give details on how a particular hose is installed in the aircraft, but, in general, a hose should be at least 3% longer than the maximum distance between end fittings. Consideration should also be given to the orientation of a hose and, once correctly installed, the witness lines, marked on the hose, should be straight. Structure

Hose to tight

Hose twisted and under tension

Hose correct tension Correct and Incorrect Methods of Hose Installation. Fig. 6 8.7 Pipe Identification Tape Once a pipe has been fitted to the aircraft, it should have system identification tape attached to enable engineers to identify which system each pipe belongs to. The tape comes in rolls of about 25 mm wide and uses colours, symbols and letters to differentiate between different pipes. A small length of the tape is wound around the pipe at convenient points.


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SPRINGS

Module 6 (Materials and Hardware) dealt with the various types of springs, the materials from which they are made, their characteristics and their applications within the aerospace industry. Consideration in Module 7 is given only to the inspection and testing of springs. 9.1 Inspection and Testing of Springs Springs will generally require little in the way of maintenance. Those that are in exposed areas can become corroded over time and those in areas of high temperature can, if they become overheated, lose their temper and cease to have the necessary mechanical compliance to satisfy the task for which they were designed. Corrosion, that occurs on static springs, can reduce the loads that the spring can carry, whilst if a spring that carries cyclic loads becomes corroded, then the combination of fatigue and corrosion can result in a serious loss of fatigue strength. Over heating, usually shown as blistering of the surface protection can, in extreme circumstances, show a change of colour of the metal due to the loss of temper. It must be assumed in this event that the spring is not suitable for the designed task. It is important that any exposed springs are carefully inspected for signs of either of the problems of corrosion and overheating. In some instances, springs have to be checked against figures or graphs to prove whether they are in a suitable condition to continue in service. Some checks have to be done out at prescribed intervals whilst others are done on an ‘opportunity basis’, such as when a brake unit a hydraulic actuator is dismantled for overhaul. The most common check, done on coil springs is on its static measurement. The manufacturer will publish the exact dimension of the unloaded spring with some small tolerance, whilst the servicing technician will accurately measure the spring’s length and compare the two dimensions. Providing that the spring is within the published figures, then the spring is considered to be serviceable.


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The other check, usually completed in a workshop environment, is the load/deflection check. A special test rig is used, to load the spring with either a compressive, tensile or a torsional loading and a meter on the rig will display the load versus deflection figures. A series of loads are, subsequently, applied to the spring and the relevant deflections noted. On completion, the figures are compared to a graph, published by the spring manufacturer, to establish the serviceability of the spring. If a spring fails any of these checks it is, simply, replaced by a serviceable item.

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10 BEARINGS When correctly installed and efficiently lubricated, bearings require little but thorough, attention during service, to ensure a long working life. The majority of bearings used in aircraft engineering are of the sealed or shielded type. These bearing are designed to prevent the ingress of foreign matter, which will damage or contaminate the bearing contact surfaces. 10.1 Cleaning and Inspection of Bearings Cleaning Before a bearing can be checked it must be cleaned thoroughly to remove any dirt and the old lubricating fluid. The manual will dictate the cleaning process including use of any solvent, but a typical method is as follows. •

Remove any excess grease with clothes and dry compressed air, whilst ensuring that the rolling elements remain stationary or only rotate slowly.

•

Soak the bearing in an approved solvent, such white spirit, to remove the remaining grease. The solvent may be applied as a forced jet if necessary.

•

Dry the bearing by using clean warm and dry compressed air, again avoiding fast rotation of any rolling elements.

•

Lightly lubricate all bearing surfaces with oil to prevent the onset of corrosion.

Testing Testing a bearing is usually restricted to rotational checks and excessive backlash or free play. Slow rotation of the rolling elements and raceway will highlight any roughness due to damage, corrosion or wear. A serviceable bearing should have a smooth actuation. Free play should be tested in both a radial and axial direction and is normally done by using a DTI. Some wear is usually permitted and will depend on the grade of fit, but any wear that leaves excessive backlash in the system is unsatisfactory. The rate of this wear depends on the speed the bearing is rotated at, with high speed bearings failing quicker than those which rotate slowly or through distances of less than one complete revolution. A bearing that has any indication of a fault should be discarded immediately. Due to their construction internal inspection of shielded bearings will be restricted. Taper bearings can be dismantled and a thorough inspection of the rolling elements and raceway surfaces can be completed.


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Once clean the bearing should be inspected for signs of failure, some of the more common being: 1. 2. 3. 4. 5. 6. 7.

Normal Fatigue Excessive Loading Installation and Misalignment Loose Fitting or Spinning Brinelling Overheating and Lubrication Deficiency Contamination and Corrosion

10.2.1 Normal Fatigue Normal fatigue failure is often shown as a fracture of the running surface, with subsequent removal of small particles of metal and is commonly called spalling. (Refer Fig. 1) It occurs on both rolling elements and raceways, and is always accompanied by an increase in vibration. Moderately spalled areas show the bearing has reached the end of its normal service life. 10.2.2 Excessive Loads Excessive loading of a bearing is usually the same as normal fatigue, but the rolling elements wear path is usually heavier. There is also increased evidence of overheating with a widespread and deeper fatigue or spalled area. This often causes premature bearing failure. (Refer Fig. 1)

Spalled Area Ball Path

Fig. 1 Spalled Areas Page 2


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10.2.3 Installation and Misalignment Installation damage is usually the result of an impact that occurs when a bearing is fitted incorrectly. This may be due to a sharp strike from a drift or pressing the wrong raceway when mounting the bearing. Misalignment damage can be seen on the raceway of the non-rotating ring because the rolling element wear path is not parallel to the raceway edge. Excessive misalignment can cause high temperatures as well as heavy wear of the cage. 10.2.4 Loose Fit A bearing should always be mounted onto a shaft or housing with an interference fit. If the raceway becomes loose then it will rotate on these surfaces and cause fretting. This fretting will remove metal particles, which oxidise and leave a distinctive brown colour. It usually occurs when the bearing outer raceway rotates inside a worn housing. The external surface of the raceway will be scored and discoloured as a result of a loose fitting bearing. (Refer Fig. 2)

Fig. 2 Loose Fit Damage 10.2.5 Brinelling Brinelling marks on a bearing raceway resemble the indentations that result from a Brinell Hardness Test. They are described as being either True Brinell or False Brinell marks. True Brinelling occurs when loads on the bearings raceway exceed the elastic limit of the raceway material. Brinell marks are indentations on the rolling element caused by an excessive static or dynamic loads. The indentations can be seen on the raceways and will increase bearing noise and vibration, which leads to the bearings premature failure. The damage is often caused by dropping the bearing or installing it incorrectly. (Refer Fig. 3 Left) 71811175.doc Issued December 2002

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False Brinelling occurs when there is only small relative motion between the rolling elements and raceways during non-rotation periods. It is characterised by elliptical wear marks in the axial direction at each rolling element position (Refer Fig. 3 Right). If the bearing is not turning then an oil film cannot be formed to prevent raceway wear. False Brinelling marks are normally perpendicular to the line of motion, well defined and maybe surrounded by debris.

Fig. 3 True Brinelling (Left) False Brinelling (Right) 10.2.6 Overheating and Lubrication Failure Excessive heating of a bearing manifests itself as discoloration of the rings, rolling elements and cages from gold to blue. Excessive temperatures will usually be in excess of 400°C. In extreme cases the rolling elements and raceways will deform. A blue/black colour indicates an area close to the heat source and changes to a silver/gold discoloration the further you move away. Failure or lack of lubrication often has similar signs as overheating because good lubrication should cool the material and transfer away any heat produced during rotation. Restricted flow and excessive temperatures can also degrade the chemical composition of the oil, making it ineffective and increase wear rates. The outcome of either overheating or lubrication failure will always result in the eventual failure of the bearing.

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10.2.7 Contamination and Corrosion Contamination is one of the leading causes of premature bearing failure. The symptoms are dents or scratches embedded in the bearing raceway and rolling elements, resulting in bearing vibration and wear. (Refer Fig. 4 Left) The contaminant would be an abrasive substance that gets into the bearing, such as sand, grit or dust. The principal sources are dirty tools, contaminated work areas, dirty hands and foreign matter in the lubricant or cleaning solutions. Corrosion is usually the result of a chemical attack on the bearing material by an incompatible fluid such as moisture. It manifests itself as either black pitting marks or red/brown rust coloured areas on the rolling elements, raceways, or cages. It usually results in increased vibration followed by wear. (Refer Fig. 4 Right).

Fig. 4 Contamination (Left) Corrosion (Right) 10.3 Safety Precautions The cleaning of bearings for inspection normally involves the use of solvents, so the appropriate PPE should be worn. This will include respiratory, eye and skin protection by using breathing masks, goggles and inspection gloves. The moisture from the human hand may contaminate a bearing surface, as easily as the lubricant can cause damage to the skin through dermatitis. 10.4 Storage If a bearing is to be used immediately after inspection, it should be lubricated with correct lubricant and installed. If there is liable to be a delay before installation, then the bearing should be coated in rust-preventing inhibiting oil, wrapped in greaseproof paper, boxed and labelled. The bearing should always be stored horizontally, in a clean dry atmosphere.


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11 TRANSMISSIONS The various types of transmission mechanisms were described in Module 6 (Materials and Hardware). Here, in Module 7, consideration is given to the inspections relative to such transmission mechanisms as: • • • • • •

Gears Belts and Pulleys Chains and Sprockets Screw Jacks Levers Push-Pull Rod Systems.

11.1 Gears Gears can be found in a wide variety of applications, throughout aircraft and engine installations. The most common applications are those used to reduce or increase the rotational speed of an input shaft. Some aircraft engines utilise a reduction gearbox to slow the main shaft speed to a lower figure for either the propeller or the fan. Accessory drive gearboxes, actuators, flap mechanisms and some flying control runs use gears of one form or another. Wear in gears, assuming that the bearings are not worn, will be found on the faces of the gear teeth. This wear shows up as excessive backlash, which can be easily found by simply rotating the input mechanism, such as the main drive shaft, whilst holding the output rigidly. It should be borne in mind that a small amount of backlash is essential for the correct operation of bearings. By rotating the input in both directions, the angular difference, when measured, is the accumulation of wear of all the gear teeth. The maintenance manual should give the total backlash figure for the particular train of gears, to indicate whether the train is serviceable or if further inspection is needed. Because the majority of gears, whether in a train or a single pair of gears, are within some other component, any signs of wear or other problems are usually solved by replacement of the major component. 11.2 Belts and Pulleys It is rare to find belt drives being used on modern aircraft. Due to the risk of slippage once the belt has taken on a slight stretch, there has to be some method, often automatic, to retain the set tension over a long period.


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In some installations, the drive from the high-speed engine to the low-speed propeller is accomplished by the use of a ‘toothed’ belt drive. The teeth on the inside of the belt engage with grooves machined onto the drive (and driven) pulleys. This reduces the chance of slippage. Most piston engines on smaller aircraft have a belt drive to the ac generator or the vacuum pump, similar to that found on many motor cars. The same maintenance applies to most belts, in that the security is checked before each flight. The belts must also be checked, at regular intervals, for signs of wear, by fraying and splitting, in addition to a tension check on the either the belt itself or the tensioning mechanism. The majority of belts (whatever their usage), have a finite life and are also subject to ‘on-condition’ monitoring. 11.3 Chains and Sprockets Chain assemblies should be inspected at the specified frequency, which is laid down in the aircraft maintenance manual. One of the major checks should be that the chain passes smoothly over the sprockets. If there is any roughness or binding, then further checks will be required. Another major check of the chain assembly is that for wear, which involves applying a load to detect whether the chain is worn. The load can be applied to a free length of chain where the amount of deflection can be measured and compared to a limit published in the manuals. An alternative would involve pulling the chain at a pulley and seeing if the chain can be lifted from the sprocket by a significant amount. Failure of either of the preceding checks, followed by a check for chain elongation, would require replacement of the chain. A typical percentage limit of elongation could be 2% when the cleaned and dried chain is loaded with the correct tension. Chains should also be checked for normal faults that can befall most mechanisms; these include damage, corrosion, cleanliness and insufficient lubrication. One other inspection, which could be done on a chain assembly, might be for correct articulation. This check involves the chain being drawn over the plain shank of a screwdriver. Tight joints, found by this method, should be carefully inspected and the chain rejected if there are any doubts as to its serviceability.

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11.4 Screw Jacks There is little maintenance to be carried out on screw jacks, apart from regular greasing of all the exposed threaded portions and checks for backlash between the ‘nut’ and the screw. Some screw jacks are in the form of actuators and some are used to drive flaps and other aerodynamic devices. A screw jack actuator (refer to Fig 1) is a fairly simple design and correct maintenance is vital because it is part of the flying controls. The actuator has a grease nipple fitted, which allows not only the bearings to be kept well lubricated, but the screw will also pick up some grease when the actuator is extended and retracted. In-service checks, other than lubrication will probably only include backlash checks on the actuating shaft. These will probably mean no more than a side-toside hand movement of the rudder trim tab, which ensures that the movement is not excessive.

Thrust Bearings Control Surface Attachment

Control Input

Threaded Screw Shaft Screw Jack Actuator Fig. 1

Another form of screw jack is that used to drive flaps up and down. This form of jack will usually be found with a drive gearbox, transmitting the motive power to the screw and ‘ball nut’, that connects to the flap structure.


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The rotation of the flap motor drives a number of the gearboxes simultaneously, which transmit rotation to their respective screws. The ball nuts are all driven down the screws and these push the flaps to their selected position. Because the screw jacks and their ball nuts are exposed to the elements, it is essential that all checks and lubrication required must be thoroughly done. As previously mentioned, the screw jacks must be thoroughly greased but only after all dirt, sand and other materials have first been completely removed. The nuts will be checked for wear at regular intervals and this check will probably require special tools and measuring jigs. On a day-to-day basis, the backlash on the nut/screw combination can be checked by an up and down movement of the flap trailing edge. 11.5 Levers Levers can be found in numerous places within an aircraft and maintenance of these items can vary, depending on their location and purpose. As a rule, levers will be used to transmit thrust from one medium to another. For example, a push/pull system may drive a lever that operates a service, with an increase or decrease of mechanical advantage or a change of direction. Apart from the bearings of the lever requiring lubrication, (unless they are sealedfor-life bearings), there is little maintenance required, other than physical checks for damage, distortion and cracks.

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11.5.1 Push-Pull Rod Systems The majority of aircraft push-pull systems can be found in both the flying and engine controls (refer to Fig. 2). They may consist of a series of hollow aluminium tubes, which have either fixed or adjustable end fittings. Sometimes, to prevent the tubes vibrating, their length is kept short and idler levers are fitted between each pair of tubes. As an alternative, rollers or bushes can be installed along the length of the push-pull tubes to provide support.

Range of Movement

Engine Fuel Control Unit

Push-Pull Rods Pivot Point Structure

Range of Movement

Support Arm

Push-Pull Rod Mechanisms Fig. 2

The maintenance required for this type of control consists generally of inspection and rigging. As the bearings in both the idler levers if fitted, and the end fittings are normally sealed for life, the only inspections to carry out are for signs of damage and overheating. If the pilot complained of stiff controls, then a check of each bearing assembly would be required, to check which bearing was stiff. Rigging of push pull rods is relatively simple. The rigging pins hold the rods and levers in the datum position and the adjustable ends are altered until all the connecting bolts can be inserted without any force being required.


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12 CONTROL CABLES Control cables, their make up and their associated components were discussed in Module 6. Consideration is given, here in Module 7, to the inspections relative to control cables and components of various types. 12.1 Swaging of End Fittings All cables, used in aircraft controls runs, have some form of end fittings attached to each end of the cables. These end fittings are usually ‘swaged’ onto the cable, meaning that the end fitting is slid over the cable before being squeezed, to reduce its diameter, and cause it to grip the cable very tightly. During production of these cables, the completed end fitting will be carefully checked, using a Go/No-Go gauge, to ensure that the cable has been gripped satisfactorily. The finished cable assembly will also be proof tested to confirm its suitability for use as an aircraft control cable. 12.2 Inspection and Testing of Control Cables Once in service, the cables will be inspected regularly for a variety of possible faults, whilst the swaged end fittings will require minimal inspection. In some installations, red paint is applied at the junction of the end fitting where the cable emerges, leading to a gap showing if the cable has slipped within the swaged end fitting during normal service. Some individual ferrules, fitted to non-critical cables, may be inspected for signs of cracking whilst in service. It is rare for cables to be removed from service to have a scheduled proof load test. If there is any doubt to the possibility of the cables lasting a long time in service, they will be either checked for stretch by measuring their length under load, or they will be given a finite life and replaced when that life is reached. Cable systems have to receive regular inspections due to their being subject to a wide variety of environmental conditions and wear. Their degradation, due to wear, can take the form of wire/strand breakage (which is fairly easy to detect), or may exist as less visible (internal) wear, or as corrosion and distortion. 12.2.1 Cable Wear Critical areas for strand breakage are where the cable passes over pulleys or through fairleads. Examination of cables will normally involve passing a cloth along the length of the cable, which will both clean any dirt from it and detect broken strands if the cloth ‘snags’ on the projecting wires.


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There will be limits, published by the manufacturer, which say how many strands per unit length can be broken. Removed cables can be bent through a gentle radius, which may show up broken internal strands that would not be visible when installed and tensioned. External wear (refer to Fig. 1) will extend along the cable, equal to the distance the cable moves at that location and may occur on one side of the cable or over its entire circumference. The limits of permitted wear will be found in the AMM.

Side View

Side View

Plan View

Plan View

Cable Worn < 50% Diameter

Cable Worn > 50% Diameter

External Cable Wear Fig. 1 Internal wear occurs in similar places in the wire to external wear, around pulleys and fairleads and is much more difficult to detect. Separating the strands, after removing the cable, is the only way to detect internal wear and this only permits limited inspection. Generally any signs of internal wear within a cable will mean its replacement. Broken strands on a cable at a location not adjacent to a pulley or fairlead, could be an indication that the breakage was due to corrosion. The inspection of a cable for internal corrosion should be done off aircraft, and will involve rejection of the cable if corrosion is found.

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The maintenance carried out on cable runs usually involves both regular inspections and preservative measures. With the majority of cables being steelbased, it is vital that cables, passing through high risk areas such as battery bays, toilets and galleys, receive regular rust preventative treatments in addition to visual inspections. Most cables have external corrosion preventative compounds applied in varying amounts, whilst internally they will have been soaked in some form of thin grease or low-temperature oil to resist the formation of the difficult to detect internal corrosion. Normally in dry and desert atmospheres, the application of certain compounds to cables is not permitted. This is because the adhesive properties of these compounds will cause the sand and dust to stick to the cable and, thus, cause extremely high rates of wear. All controls will be monitored, by the flight deck crew, on a day-to-day basis but, during maintenance, more subjective tests must be completed. The tension of the cables will be measured, as will the rigging of the complete runs, to ensure that the controls remain accurate and precise in their operation. Whilst it is not usual to find faults on the cable end fittings, these should all be checked for any signs of damage, corrosion and stressing of the cable at the end fitting. Items checked will include turnbuckles and ball end fittings, to ensure that the cable is operating at the designed angle, tension and over the correct range. 12.2.2 Bowden and Teleflex Cable Systems A typical Bowden cable control might be a brake lever on the control column operating a remote brake control valve. Maintenance of Bowden cable systems is usually restricted to cleaning and lubrication of the inner cable at regular intervals and adjustment of the outer conduit (e.g. if the brakes needed adjustment). The lubrication would keep moisture out of the cable to prevent it freezing at low temperatures. The Teleflex cable system is more complex than the Bowden cable system in that the operating cable, within the conduit, is actually a number of spirally wound cables which surround a core tension cable, giving it support. This allows the cable to transmit a push force as easily as a pull force, doing away with the need for any form of return spring. A typical use of a Teleflex system might be a throttle lever to engine fuel control system connection.


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The Teleflex cable system is a snug fit within the conduit and, because there might be the chance of it becoming seized, due to foreign objects, dirt or freezing, it is vital that the inner cables are regularly removed, cleaned and lubricated with low temperature grease. It is also important that the conduits are thoroughly cleaned using a form of ‘pull-through’, prior to the inner cable being installed. At longer intervals, it might become necessary to inspect the outer conduit for signs of damage or kinking; which can cause the control to become tight or ‘notchy’. 12.3 Inspection of Control Cable Pulleys When inspecting cables for the previously mentioned wear and breakages, the complete cable runs must be examined for incorrect routing, fraying, twisting or wear at fairleads, pulleys and guards. Pulleys must be inspected for wear (refer to Fig. 2), to detect indications of seizure, flat spots, embedded foreign material and excessive tension. Any signs of contact with adjacent structure, pipe-work, wiring and other controls must also be thoroughly investigated.

Excessive Cable Tension

Seized Pulley Bearing

Pulley too Large for Cable

Cable Misplaced or Incorrectly Installed Types of Pulley Wear Fig. 2

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13 SHEET METAL WORK While the majority of metals can be rolled into sheet form, consideration is confined here solely to the working with sheets of the light alloys, which are encountered on aircraft and, in particular, those formed from aluminium alloy ingots. Safe working procedures were covered adequately, in the ‘Workshop and Hangar Safety’ Section of the SAFETY PRECAUTIONS topic, but there are several additional points, which need highlighting, with regard to working with sheets of aluminium alloy. By definition, sheets of aluminium alloy are comparatively thin in cross-section and, as such, they not only pose a health hazard, through cuts, when being handled but they are, also, prone to buckling and creasing if handled carelessly. Large sheets of aluminium alloys are, usually stored upright, on their longest edge and supported, clear of the floor, in a wooden framework so they are protected from damage and corrosion. Care must be taken when removing a large sheet from its storage rack – a task which normally involves at least two persons – and good communication between the carriers is important so that the task is completed in a safe manner and no damage is done to the sheet metal. Some sheets are covered, on one or both surfaces, with a thin protective plastic membrane and, if possible, it may be beneficial to leave at least the underneath protection in place while the marking out is done, to minimise the possibility of the surface sustaining undesirable scratch marks. If no protective membrane is applied to the sheet, then care must be taken over the condition of the surface of the table, or workbench, upon which the sheet is to be laid for the marking out procedures. Other factors, which should be considered (as with all work) concern the requirements to ensure that: • •

Material wastage is kept to a minimum The task is done correctly, first time, so that valuable time, also, is not wasted.

The first point is usually obvious, due to the cost of the materials involved, but the second point quite often gets forgotten, when work is being done, but the actual labour costs far outweigh the material costs on a high percentage of tasks. Repair or modification drawings must be studied very carefully, to ensure there is no doubt about the data and dimensions provided, so that the marking out is correctly done and the approved metal is shaped in exactly the manner that the designer of the drawing intended.


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13.1 Marking Out Having carefully studied all the data and dimensions on the relevant drawing, the technician, after confirming that the correct metal (to the appropriate heattreatment standard) is being used, can proceed with marking out the pattern for the part which is being formed. Firstly the overall dimensions of the part must be computed and, where necessary, a bare outline drawn on the large sheet, so that the metal can be cut and, thus, allow an easier, smaller piece upon which to work. It must be remembered that, the metal should be cut so that any identification markings remain on the larger piece, for future users of the sheet and that scribers must only be used to mark lines which are going to be removed from the surface. Scribed lines penetrate the aluminium cladding of ‘Alclad’ alloys, which can lead not only to subsequent corrosion, but also can create stress raisers and the initiation of cracks in the material. The drawing of the outline is achieved by establishing a datum line or point on the surface of the metal and taking all dimensions from the datum so that errors, due to ‘chaining’ of dimensions, are eliminated. The drawing surface of the metal should be cleaned of any protective oil (or plastic membrane) before marking out commences and the sheet should be laid flat on a clean, firm workbench or table in good lighting conditions. In some instances it may be advantageous to rub chalk on the surface or to apply a thin coat of zinc chromate, to make it easier to distinguish the marking out lines, which (if they are not going to be removed) should be made with a ‘soft’ pencil. Once the outline is completed, the sheet may be (carefully) moved to the squaring shears, or guillotine and the outline cut from the main sheet. The square edge, created by the squaring shears, will make the use of such tools as engineers’ squares, combination sets and Vernier protractors etc. easier, to achieve parallel and appropriately angled lines during completion of the marking out. Note: Before any centre punch marks are made (for the location of the centres of radii or holes) it must be confirmed that they are in the required location. The punch should be only lightly tapped with a hammer (or a suitably adjusted automatic centre punch used), so that the punch marks do not distort the thin metal sheet.

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13.2 Forming of Sheet Metal Parts Once the marking out has been verified as being correct, the forming of the final shape of the sheet metal component can be achieved by the use of appropriate cutting and, if necessary, bending tools. 13.2.1 Cutting While metal-cutting tools were discussed in the earlier topic on TOOLS, mention is made here of the manner in which the relevant tools should be used when working with sheet aluminium alloys. The squaring shears has already been used to produce a convenient size upon which to work and, of course, to provide an accurate straight edge from which to make measurements. Note: The squaring shears must only be used to cut metal of the approved thickness (recommended by its manufacturer) and must never be used on sheets (or strips) of metal thicker than those specified. The alignment of the blade will be distorted and the accuracy of its cut will be degraded if this caution is ignored. When using shears (whether squaring or the hand type), then the cut must be made slightly above the line. This allows for filing down to the line, which will eliminate the possibility of stress raisers being formed at the edges of the metal, due to the shearing action of the various types of shears. Care must be taken when drilling aluminium sheet, due to the danger of cutting enlarged holes in the soft, thin metal and to the tendency to distortion, caused by the application of too great a weight on unsupported aluminium sections. Twist drills must be of the correct type and size, with accurately-ground points, and their passage, through the metal, must be carefully controlled at all times. Off cuts of scrap wood should be placed behind (or underneath) sheet metal parts while drilling is in progress and both the backing piece and the part must be firmly held, to prevent movement during the drilling procedures. Similarly, scrap wood should be used, as backing, when hack-sawing or filing sheet metal and protection must be given, against possible damage, when such components are held in the jaws of vices, by the use of ‘soft’ vice clamps. Obviously fine-toothed hacksaws (32 tpi) and second cut and/or smooth files (used with long, smooth strokes), are the cutting tools, used in the shaping of sheet metal parts. Files, as discussed in the TOOLS topic, must be regularly cleaned, to prevent the build up of pinnings, and the use of file cards and chalk, for this task, has also been, earlier, mentioned.


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13.2.2 Bending and Calculation of Bend Allowance As previously stated, the sheet metal used for aircraft construction and repair, is generally formed from an ingot of aluminum alloy that has been processed through a series of rollers. This process reduces the thickness of the material to a dimension that meets the requirements of the design drawing. As a result of this process, the metal assumes a grain structure, which can easily be detected in a sample of sheet aluminum alloy. When planning any sheet metal work process, the orientation of the metal is to be taken into account so that any bends formed will, where practical and achievable, be made across the grain. Where, however, strength is required along the length of a long, channel section, then, regardless of any bends, the grain should flow along the length of the channel. Great care must be taken, before bending aluminium alloy, to ensure that it is of the correct designation and heat-treatment standard. The subject of the heattreatments of aluminium alloys was discussed fully in Module 6 – Materials and Hardware in the topic on AIRCRAFT MATERIALS – NON-FERROUS. Some alloys must be subjected to either an annealing, or to a solution treatment procedure before (and, again, after) bending but, as this is, usually, beyond the scope of maintenance technicians, mention of it is merely made here to draw attention to its requirement and for the need for vigilance when bending sheets of aluminium alloy. Bending of aluminium alloys is achieved either by the use of: • •

Specially-shaped bending bars: used for small pieces and larger angles and between which, the sheet is clamped, in a vice, while the metal is bent, by hitting with a hide-faced or similarly soft-headed hammer A large, free standing, bending machine (or bending brake): in which the metal sheet is clamped and the bend made, in one movement, by means of a hinged bending leaf.

Caution must also be exercised when forming a bend, using the bending bars and soft-headed hammer method, because too many blows with the hammer will cause work-hardening of the metal, or the metal, in the bend, will become too thin and stretched. Subsequent cracking of the metal will result from these faults For this reason the bending brake is preferred but, in a similar manner to the squaring shears, only the approved thicknesses of metals should be bent in these machines, as any distortion will destroy the accuracy of the bends.

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Square (or sharp) angles, in aluminium alloys, are only formed by adhesive, casting, extrusion or welding methods. Whether it be the bending bars or the bending brake method, which is used to bend aluminium alloy sheet, the bend will always be formed around a radius, as it is not possible to create square angles by bending without cracking the metal. It is recommended that the radii of bends, in aircraft-grade, aluminium alloy sheets, be not less than three times the thickness (3t) of the metal, in sheets thicker than 22 SWG (0.7 mm) and should, preferably, be greater if possible. It is, therefore, usual to create bends, in sheets of 20 SWG (0.9 mm), of not less than 3 mm (3 x 0.9 mm = 2.7 mm) radius and bends in sheets of 18 SWG (1.2 mm), of not less than 4 mm (3 x 1.2 mm = 3.6 mm) radius. As an example, if it were required to form a right angled curve (10 mm radius) in an 18 SWG aluminium alloy sheet, to provide two legs, effectively 76.2 mm (3 in) in length (refer to Fig. 1), it can be seen that the actual length of metal involved is obviously less than 2 x 76.2 mm (6 in). The total length of the metal, required for the curve, is deduced by using the formula: LT = L1 + BA + L2 Where LT = Total Length of Metal Required L1 = X – (r + t) L2 = Y – (r + t) BA = Bend Allowance X and Y = Effective Lengths of Unbent Sections r = Radius of Bend t = Thickness (SWG) of Metal X = 76.2 mm t = 1.2 mm

A L1

r = 10.0 mm

B

L2

Y = 76 .2 m m

Total Length of Metal in a Curve Fig. 1


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The lengths of both L1 and L2 can be found by subtracting the sum of the radius and the metal thickness from the effective lengths of the unbent sections. In this instance, therefore, the lengths are both 76.2 – 11.2 = 65 mm. To calculate the length of metal in the bend (Bend Allowance) it is necessary to consider the fact that, when metal is bent, the metal on the inside of the bend will be compressed while that on the outside of the bend will be subjected to tension or stretching so that the length of metal on the inside and outside of the bend will be different. It may, however, be considered that there is a neutral line (at approximately half thickness) where the compression and tensile forces cancel out. It is this line which is taken, to calculate the length of the arc of the circumference of the circle, which would be described by the radius of the curve. The Bend Allowance is, thus calculated, using the formula: BA = 2π (r + ½t) α 360 Where r = Radius of Bend t = Thickness of Metal α = Angle of Bend Note: Some books use the formula: BA = π (D + t) α 360 Where D = The diameter of the circle However, as curves are normally shown as radii, in engineering drawings, the previously given formula is preferred here. Substituting figures, in the preferred formula, it will be seen that the bend allowance equates to 16.65 mm (0.66 in). When this figure is added to lengths L1 and L2, it can be seen that the total length of metal, required to form the curve, is only 146.65 mm (5.77 in) and not 152.4 mm (6 in). In a simple, single curve, this represents a saving in metal of only 5.75 mm (0.23 in) but, in a multiple bend component, and with larger radii involved, considerable savings of metal can be made by using these formulas to calculate the correct amount of metal required to forms bends in sheet metal. Table 1 shows data relating to bend allowances for forming 90º curves of various radii in both 20 SWG and 18 SWG metals. Using the preferred formula, the student should be able to calculate the bend allowances and enter them in the empty spaces in the table.

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Table 1 BEND ALLOWANCES (BA) FOR A 90° BEND Inside Bend Radius BA - 20 SWG (0.9 mm) 3 mm 4 mm 5 mm Inside Bend Radius BA - 18 SWG (1.2 mm) 4 mm 5 mm 6 mm When the total length of sheet metal, required to form a curve, has been calculated, it will be necessary to draw the development (refer to Fig. 2) of the intended shape, so that the bend lines can be seen. Unbent Section

Bend Allowance

65

16.65

Unbent Section 65

Bend Lines

Development of Shape Fig. 2 An additional line must be drawn on the development drawing before the metal is placed in the bending brake. This line (refer to Fig. 3), is referred to as the sighting line or the brake reference line. It is drawn at a distance, equal to the radius of the curve (in this instance, using the figure of 10 mm from the previous example), parallel to, but away from, the bend line, which is under the clamping nib of the bending brake and towards the bend line which is free of the clamping nib. The sighting line, as the name implies, is then used, to align with the front of the clamping nib and, in this way, allowance is made for the thickness of the metal in the formation of the curve.


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Sighting Line

Bend Lines

Metal to be Bent

Brake Bed

Hinge Point

Bend Leaf Bends Counter-clockwise

Use of Sighting or Brake Reference Line Fig. 3 13.3 Inspection of Sheet Metal Work As far as aircraft maintenance technicians are concerned, the inspection of sheet metal work is confine to visual or assisted visual methods. Personnel who have approval may also perform dye penetrant procedures in the search for cracks in suspect areas. Specially trained and approved NDT personnel may use Eddy Current, Ultrasonic or Radiographic procedures to detect faults in aluminium alloy sheet metal work.

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14 WELDING, SOLDERING AND BONDING Welding, Soldering and Bonding are methods of creating permanent joints between materials and use is made of all three methods in the aerospace industry, primarily at the production stage and to a more limited degree during maintenance and overhaul stages. Within the aircraft industry welding is considered to be a specialist skill and only suitably approved and authorised personnel can undertake welding procedures. Approved welders must satisfy the CAA of their competency, by submitting several ‘test pieces’ of their typical work for testing and they are subjected to similar re-tests every 12 months in order to retain their approvals. Maintenance technicians may, however, be called upon to do some soldering and bonding procedures so, with these facts in mind, only the basic methods of welding will be discussed in this topic, while greater emphasis is placed upon procedures involving soldering and bonding. 14.1 Welding Welding may be defined as the permanent joining, by fusion, of two pieces of material (usually metals), by the progressive melting and subsequent solidification of the materials at the site of the joint. The basic principle, of fusion welding of metals, is the same for all processes, in that the surfaces, or edges, of the metal to be joined, are brought to a molten state and allowed, or caused, to intermix (with or without the addition of a filler metal), so that the parent metal and filler metal (if used) form a homogeneous molten pool which, when cooled, forms the complete weld. 14.2 Methods of Welding Welds require the application of sufficient heat energy to melt the metals involved in the joint and the high temperatures are achieved by various methods. 14.2.1 Oxy-Acetylene Flame The cutting of steel sections and plate material may be done by means of a flame torch, using a mixture of oxygen, with one of the appropriate fuel gases (acetylene, hydrogen, natural gas or propane). For welding, however, only an oxygen and acetylene mixture will provide a sufficiently, high heat input, needed for the welding process. The temperature of the oxy-acetylene flame is approximately 3150°C. The oxy-acetylene method can be used for welding ferrous or non-ferrous metals but, when welding non-ferrous metals, it is necessary that an additional material (a flux) be used, usually with a filler metal, to assist in the fusion process. 71811175.doc Issued December 2002

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The purpose of the flux is to prevent oxidation of the joint site so that the molten metals can fuse together more easily and, thus, eliminate brittleness in the joint. 14.2.2 Manual Metal Arc This welding process uses an electric arc as the heat source. The arc is established between a flux-coated, filler metal rod and the workpiece, which are connected to an electrical power source so that they are the anode and cathode electrodes of the circuit. When the power is switched on, the heat, generated by the resulting arc, melts the flux-coated electrode and the edges of the parent material to form a weld pool. The temperature of the arc is approximately 4000°C to 4500°C. 14.2.3 Metal Arc Gas-Shielded (MAGS) In this semi-automatic welding process the heat source is also an electric arc, but the electrode is a bare wire, which is consumable and is supplied, from a reel, to the welding gun, by a wire feed unit. A shielding gas is employed; in place of a flux material, to protect the weld pool. The type of shielding gas, used, will vary with the application. Some of the gases and gas mixtures used are: • • • • • •

Argon Carbon dioxide Argon/carbon dioxide Argon/oxygen Argon/nitrogen Helium.

Note: This process may also be referred to according to the type of shielding gas (or mixture of gases) which is being used and whether those gases are inert or active. The two types of this process are: • •

Metal Inert Gas (MIG) welding: where the shielding is provided by a shroud of inert gas. Metal Active Gas (MAG) welding: where the shielding is provided by a shroud of active, or non-inert, gas or mixture of gases.

14.2.4 Tungsten Arc Gas-Shielded (TAGS) This process also uses an electric arc as the heat source, but here a tungsten non-consumable electrode is used to form the arc with the workpiece. An inert shielding gas (argon) is required to protect both the weld pool and the tungsten electrode from the oxygen and moisture in the atmosphere. For this reason the process is sometimes called argon arc welding and, also, Tungsten Inert Gas (TIG) welding. A filler rod is usually required to give reinforcement to the weld. Page 2


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14.2.5 Flash Butt Welding The components to be joined are set up as opposite poles in an electric circuit and, when the current is switched on, the components are moved into and out of contact with one another. This action causes an arc to be struck and, when welding temperature is reached, a force is applied to both components, so that their molten surfaces are fused together. 14.2.6 Spot Welding A method used to join comparatively thin sheets of metal, spot welding is a form of resistance welding. The sheets of metal are sandwiched between two, pointed electrodes on which force is exerted as the current is applied. The heat is generated at a local spot where the resistance to the flow of the electricity is at its highest and the metal fuses at these spots. The pointed electrodes are made from copper alloy and are usually water-cooled. 14.2.7 Seam Welding The principle of seam welding is similar to that of spot welding (namely resistance to the flow of electricity). The main difference is that in place of the pointed electrodes, this method uses discs or wheels, which are moved along the length of the weld. The supply of current is intermittent, so causing a spot weld to overlap its neighbour and, thereby, form a continuous seam weld. 14.3 Inspection and Testing of Welds The wide use of welding in industry has resulted in an increasing demand for standards relating to welded constructions in various branches of engineering. These standards generally include requirements for certain welding tests to be conducted, primarily for the qualification of welding procedures and operators. Sophisticated methods of non-destructive testing of welds include the use of Radiographic, Ultrasonic and Magnetic Particle testing procedures, all of which are done by specially trained, and approved, personnel. Specimen welds are also destructively tested, by fracturing or sectioning, to test the integrity of a specific welding procedure. These methods are beyond the scope of unqualified personnel, so that aircraft maintenance technicians are, usually, constrained solely to the visual inspection of welds (following thorough cleaning of the relevant areas). It may, however, be possible that, after suitable training, some technicians can be granted approval to conduct limited Dye Penetrant inspection procedures on certain welds, which will be specified in the appropriate servicing manual.


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14.4 Soldering Soldering differs from welding in that it is done at considerably lower temperatures so that the parent metals do not melt and fuse together. Instead, a fusible and, usually, non-ferrous alloy (with a lower melting point) is applied between the heated metals of the joint, such that the fusible alloy forms a metallic bond with the parent metals and, on cooling, creates a solid joint. The word ‘solder’ does, in fact, come from the same stem as the word ‘solid’ (as does the American term, which is pronounced ‘sodder’, for the same process). 14.5 Methods of Soldering Soldering can be divided into two basic methods, one of which uses higher temperature ranges than the other, but both of which are conducted at temperatures below the melting points of the parent metals of the intended joint. The two basic methods of soldering are: •

Hard Soldering: done at temperatures in excess of 500°C and which include the processes of Brazing and Silver Soldering • Soft Soldering: done at temperatures within the range of 180°C to 330°C, which, consequently, create joints of lower strength (but less expense) than those achieved by the hard soldering methods. Note: The hard soldering processes are, normally, beyond the remit of the aircraft servicing technician, so only brief consideration is given to them here, with more attention being given to the soft soldering method. 14.5.1 Hard Soldering (Brazing and Silver Soldering) Brazing, as the name implies, uses a Copper/Zinc (Brass) alloy, as the filler metal (spelter) between the parent metals of the joint. The degree of alloying will dictate the temperature at which the process is done but the melting point of the brazing alloys can be as high as 880°C. Brazing is a process of joining in which, during, or after heating, the molten filler metal is drawn into, or retained in, the space between closely adjacent surfaces of the parts to be joined, by capillary attraction. In general applications, workshops and small factories, a flame, directed onto the joint area, is the source of heat. However, in the more sophisticated applications, used in industry, heating for hard soldering may be provided by a: • •

Gas, oil or electrically heated, closed furnace High-frequency (HF) induction coil.

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As with welding, it is necessary to employ the use of a flux material to assist the fusion of the filler with the parent metals and to prevent oxidation of the joint. The flux mostly used for brazing processes is borax, which is based on Sodium Borate powder, mixed with water, to a thin paste before being applied, by brush or swab, to the site of the joint. Other fluxes are also available where required. Silver Soldering entails the use of a Copper/Zinc/Silver or Nickel/Silver alloy as the joining metal and (again depending on the alloy employed), can be done at temperatures of between 650°C to 700°C. Brass, copper, monel metal and stainless steel are typical metals on which silver soldering processes can be used. 14.5.2 Soft Soldering Soft Soldering involves the use of a Lead/Tin alloy (with traces of Bismuth and Antimony added when required) as the filler metal, which melts at temperatures between approximately 180°C to 330°C, depending on the composition of the alloy. The lower temperature requirement, of the soft soldering process, allows the use of indirect heat. In earlier times, the heat was provided by the application of an implement with a wooden handle and a smooth, flat, base or ‘bit’ (originally made of iron). The ‘iron’ was directly heated in a flame, then quickly cleaned, before being applied to the solder joint, where the transference of its heat would facilitate the melting of the filler metal. This process possibly needed repeating several times (as the iron tended to lose its heat fairly quickly) before a large task could be completed. It was found that copper is a better heat conductor than iron, is less prone to corrosion and is, therefore, easier to keep clean. Copper, consequently, became the metal most preferred for use as the soldering ‘bit’, though the implement retained its name of the soldering ‘iron’. While needing re-heating less frequently, it remains necessary to regularly reheat the copper bit of the directly heated soldering irons. The advent of electrically heated (and thermostatically controlled) soldering irons has overcome the re-heating problem, associated with directly heated irons, and consideration is given here only to the method of soft soldering with the use of electrically (or indirectly) heated soldering irons. While the method described is the most commonly used in small workshops (or in DIY applications), there are, however, three further methods which are used in industrial applications. Those methods involve: • Applying a naked flame to the joint • Dip soldering • Heating by non-contact techniques. 71811175.doc Issued December 2002

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14.5.3 Using Indirectly Heated (Electric) Soldering Irons Electric soldering irons are available in a variety of sizes and weights with bits shaped to suit the particular application. Typically, the 25 watt, electric soldering iron (refer to Fig. 1), is widely used for making joints in electric circuitry. The heating element contained in the barrel of the iron is supplied directly from the mains electrical supply. Larger, 40 watt (or as large as 125 watt) irons, with proportionately larger bits, may be used when it is required to create overlapping joints (lap joints) of sheet metals (though this is a task, not normally done by aircraft maintenance technicians). Rubber Grip

Protective Shroud

Electric Power Lead

Soldering Iron Body

Copper Bit

Typical 25 Watt Electric Soldering Iron Fig. 1 Before any soldering operation is attempted, the joint surfaces (and the soldering iron) must be properly prepared. It is of paramount importance that the joint surfaces be absolutely free of dirt and grease (and surface oxides), so that the solder will be able to satisfactorily form intermetallic compounds and, thus, bond completely with the parent metals. To ensure this, the approved cleaning methods must be used for the relevant metals (abrasives, etchants de-greasants etc.) and, finally, an appropriate flux is applied to the cleaned surfaces, to prevent oxidation at the joint and to assist in the flow and fusion of the solder. Note: Some solders have a flux included in their hollow core, while others, require the application of a separate flux material. After the surfaces have been carefully prepared, the electric soldering iron can be switched on and allowed to reach its operating temperature. This is, usually, indicated by a small, integral warning lamp but may be deduced by applying a piece of solder to the bit and seeing the solder melt when the temperature is adequate.

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The heated bit of the soldering iron must, next, be ‘tinned’. This is achieved by, firstly, ensuring that the bit is thoroughly cleaned then dipping the bit in flux (if a separate flux is being used) and applying solder to the bit until a thin film of solder completely covers the working area of the soldering bit. It is important that the tinning of the bit is done correctly, otherwise problems will be experienced with the soldering operation. Each surface of the prepared joint must also be carefully tinned (refer to Fig. 2) in a similar manner, so that a thin film of solder covers the total area of the joint surfaces. Direction of Soldering

Solder Stick

Copper Bit

B

C

D

E

F

A

Parent Metal

A B C D E F G

oxide film on parent metal flux solution above oxidised metal surface boiling flux solution removing oxide film bare metal in contact with fused flux liquid solder tin reacting with base metal to form Inter-metallic compound solidifying solder

Tinning the Joint Surface Fig. 2


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Care must be taken, when applying solder to the joint surfaces, to ensure that it is as thin and as smooth as possible and that the heat is maintained, to allow the inter-metallic compound between the parent metal and the layer of solder to form. This compound is an important factor and contributes greatly to the strength of the joint, as it is, actually, stronger than the solder. When the two surfaces of the joint are correctly tinned, they are placed together and the hot iron is applied to an outer surface of the joint. The heat is transmitted through the metal and melts the solder interfaces so that they fuse together and a typical soldered lap joint (refer to Fig. 3) of the metals is completed.

Intermetallic Compound

Solder Parent Metal

Soldered Lap Joint Fig. 3

Note: Even when making electrical connections, using soft solder, a type of lap joint must be made, since an end-to-end joint in wire would be impracticable. 14.5.4 Active and Passive Fluxes Metal surfaces become more reactive to oxygen when they are heated and, as previously discussed, to prevent this oxidation, during the soldering process, a suitable flux is applied to the surfaces being joined. The flux should possess certain characteristics in that it: • • • •

Forms a liquid film over the joint and excludes the gases in the atmosphere Prevents any further oxidation during the heating cycle Assists in dissolving the oxide film on the metal surface and the solder Is displaced from the joint by liquid filler metal.

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Fluxes for soft soldering are often classified into two groups, which are the: • •

Active group: which are corrosive or acid fluxes Passive group: which are non-corrosive fluxes.

The flux can be applied separately, or as a constituent within the solder. Fluxes may take the form of a liquid, paste or solid, and the application, for which they are being used, will govern the type selected. Active (corrosive) fluxes are used where conditions require a rapidly working and highly active flux. The common active fluxes are listed below. WARNING: THESE FLUXES CAN CAUSE BURNS TO FLESH AND CLOTHING. PROTECT THE EYES WITH GOGGLES AND WEAR RUBBER GLOVES AND APRON WHEN USING A CORROSIVE FLUX. •

Zinc Chloride (ZnCl): commonly called ‘killed spirits’. This used on general sheet-metal work and may be obtained commercially under its trade name of ‘Baker’s Soldering Fluid’ • Ammonium Chloride (NH4Cl): commonly called sal ammoniac. This used, in block form, for cleaning the face of the soldering bit before tinning, or in powdered form, with Zinc Chloride, for tinning cast iron • Hydrochloric Acid (HCI): used in the raw state for pickling the surfaces of the metal and rendering them clean. As a flux it is extremely active and is suitable for soldering zinc and galvanised mild steel • Phosphoric Acid: used, primarily, on stainless steels. Note: Flux residues of acid fluxes remain active after soldering and will cause corrosion unless removed by thorough cleansing, - first in a weak solution of caustic soda - and then in water. Passive (Non-Corrosive) fluxes are divided into three types, which are: •

Natural resin: dissolved in suitable organic solvents, it is the closest approximation to a non-corrosive flux and is particularly suitable for use in the electrical industry • Tallow: used by plumbers, for the jointing of lead sheet and pipes. Similar to resin, it is only slightly active when heated to the temperature of the soldering process • Olive Oil: used for soldering pewter items.


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14.5.5 Flux Removal It is essential that all flux residues be removed, since they can present a corrosion hazard. The method of removal will be determined by the type of flux used, but will entail the use of one, or a combination of, the following: • • • •

A solution of caustic soda A solution of sulphuric acid A supply of warm water Physical abrasion.

14.6 Inspection and Testing of Soldered Joints The inspection of soldered joints is done mainly by visual means, though, in some applications, tensile testing is recommended. Electrical contacts, using soft soldering methods, may be tested by gently pulling on the wires to confirm the security of the joint. These joints may also be tested for electrical continuity and resistance, using appropriate instruments. 14.7 Bonding Bonding, by the use of adhesives, is the third method of achieving permanent joints between surfaces, to be considered in this part of the course. Comprehensive coverage of adhesives and sealants is provided in Module 6 (Materials and Hardware), along with details of composite materials, the detection of typical defects and the methods used in their repair, therefore consideration here will be limited merely to a summary of: • • •

Bonding terminologies Methods of bonding The inspection and testing of bonded joints.

Bonding, in the aerospace industry, is employed to form permanent joints between materials ranging from composites, fabrics, metals and metal alloys, to plastics, - all of which are referred to as ‘adherends’. The surface texture of a particular adherend, the type of joint required, and the manner in which loads are applied to the joint will dictate the type of adhesive to be used, and the method to be employed, in effecting the joint. Synthetic resins (and some elastomers) are mainly used as adhesives in the bonding of aircraft structures and associated components and, while most of them are used at the manufacturing stages, some may well be used, by aircraft servicing technicians, during routine maintenance tasks. Page 10


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WARNING: CONTROLLED VENTILATION, PROTECTIVE CLOTHING, AND ANTI-FIRE/EXPLOSION PRACTICES, ARE ABSOLUTELY ESSENTIAL, WHEN WORKING WITH ADHESIVES AND SEALANTS. ALTHOUGH MANY OF THE ADHESIVES IN CURRENT USE ARE SUPPLIED IN FILM FORM, SOME ARE LIQUIDS OR PASTES, FROM WHICH, TOXIC AND FLAMMABLE VAPOURS ARE EMITTED, PRIOR TO CURING. MANY OF THE NECESSARY, SURFACE PREPARATION SOLVENTS, ALSO GIVE OFF TOXIC/FLAMMABLE VAPOURS. The warning is reproduced from Module 6 – (Materials and Hardware), where it also states that the two major groups of adhesives, are: •

•

Flexible adhesives: used where some flexing or slight relative movement, of the joint is required, and where high load-carrying properties are not paramount. These adhesives are, generally, based on flexible plastics or elastomers Structural adhesives: used in applications where high loads must be carried without excessive creep and which are relatively rigid without being excessively hard or brittle. These adhesives are based on resins (commonly of the epoxy or of the polyester types).

Note: Another group of adhesives is the two-polymer type, which has a reasonably even balance of resin and elastomer. This results in a flexible, yet fairly strong, adhesive. 14.8 Methods of Bonding While the two major groups of adhesives are designated as flexible or structural, they are further classified as being of the thermoplastic or of the thermosetting types. Each type’s characteristics will influence the method employed in its use as a bonding agent. 14.8.1 Thermoplastic Adhesives Thermoplastic materials are those which soften on heating and harden when cooled but will, again soften and harden as often as the heat/cool cycle is repeated. Thermoplastic adhesives consist of thermoplastic materials (which may be either acrylic-, cellulose-, epoxy-, rubber- or vinyl-based), in solution with a volatile solvent and which may be applied to the surface of adherends in the form of: • •

Direct application adhesives Contact (or impact) adhesives.


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Direct application adhesives, are spread over the area of both surfaces of the joint before the joint is closed and the solvent continues to evaporate. This method can create problems if the joint area is large, as all of the solvent may not evaporate and a weak joint will result. Contact adhesives are also applied to both surfaces to be joined but, with these adhesives, the solvent is allowed to evaporate until the adhesive feels ‘tacky’, when the surfaces are, then, brought into contact and a complete joint is achieved. 14.8.2 Thermosetting Adhesives Thermosetting materials (thermosets), once set, cannot be reformed by the application of heat and they create permanent heat-resisting bonds. Thermosetting adhesives consist of epoxy- and phenolic-based materials in addition to polyesters, polyurethanes, and silicones. Thermosets require a ‘curing’ process (which is achieved by the application of heat), to cause them to harden. The heat can be obtained by placing the components being joined into an oven or into an autoclave (a pressurised oven). Alternatively, the adhesive in the joint can be heated by the chemical (exothermic) reaction of a hardening agent, which is added to the adhesive, prior to the joint being made. Thermosetting adhesives are the types most widely used in the aerospace industry. 14.9 Inspection and Testing of Bonded Joints The inspection of bonded joints may be done (as discussed in Module 6) visually, usually in good lighting conditions and, possibly, with the aid of magnifying glasses or small microscopes. Delamination and de-bonding of aircraft honeycomb panels and control surfaces may be detected by percussion (ring) testing or coin tapping, while more sophisticated methods, such as ultrasonic and radiographic procedures, may be used by suitably trained and approved personnel. Where repairs are done to composite structures, then samples of the adhesives used are kept for testing, while ‘peel’ tests are done on adhesives which are used to attach de-icing or anti-icing elements to the leading edges of propellers or flying control surfaces.

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15 AIRCRAFT MASS AND BALANCE The main purposes, of monitoring the mass and balance of aircraft, are to maintain safety and to achieve efficiency in flight. The position of loads such as passengers, fuel, cargo and equipment will alter the position of the Centre of Gravity (CG) of the aircraft. Incorrect loading will affect the aircraft rate of climb, manoeuvrability, ceiling, speed and fuel consumption. If the CG were too far forward, it would result in a nose-heavy condition, which could be potentially dangerous on take-off and landing. If the CG is too far aft, the tail-heavy condition will increase the tendency of the aircraft to stall and make landing more difficult. Stability of the aircraft will also be affected with the CG outside the normal operational limits. Provided the CG lies within specified limits, the aircraft should be safe to fly. The unit of measurement for mass and balance are normally dictated by the aircraft manufacturer and can be either Metric or Imperial terms. Specific definitions for mass and balance ensure they are correctly interpreted. 15.1 Definitions •

Datum: The datum is an imaginary vertical plane from which horizontal measurements are taken. The locations of items such as baggage compartments, fuel tanks, seats and engines are relevant to the datum. There is no fixed rule for the location of the datum. The manufacturer will normally specify the nose of the aircraft, but it could be at the front main bulkhead or even forward of the aircraft nose

•

Arm: The horizontal distance from an item or piece of equipment to the datum. The arm's distance is usually measured in inches (or millimetres) and may be preceded by a plus (+) or a minus (-) sign. The plus sign indicates that the distance is aft of the datum and the minus sign indicates distance is forward of the datum

•

Moment: The product of a force multiplied by the distance about which the force acts. In the case of mass and balance, the force is the mass (kg/lb) and the distance is the arm (m/in). Therefore, a mass of 40 kilograms, at 3 metres aft of the datum will have a moment of 40 x 3 = 120 kg/m. It is important to consider whether a value is positive (+ve) or negative (-ve) when moments are calculated and the following conventions are used: Distances horizontal: Weight:


aft of the datum (+), forward of the datum (-). added (+), removed (-).

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•

Centre of Gravity (CG): This is the point about which all of the mass of the aircraft or object is concentrated. An aircraft could be suspended from this point and it would not adopt a nose-down nor a tail-down attitude.

•

Centre of Gravity Balance Limits: For normal operation of the aircraft, the CG should be between the Forward and Aft limits as specified by the manufacturer. If the CG is outside these limits, the aircraft performance will be affected and the aircraft may be unsafe.

•

Dry Operating Mass: The total mass of the aeroplane, ready for a specific type of operation, excluding all usable fuel and traffic load. This mass includes crew and crew baggage, catering and removable passenger service equipment, potable water and lavatory chemicals.

•

Maximum Zero Fuel Mass: The maximum permissible mass of an aircraft with no usable fuel. Fuel contained in certain tanks must be included if this is explicitly mentioned in the aircraft’s Flight Manual limitations.

•

Maximum Structural Take-Off Mass (MTOM): The maximum permissible total aeroplane mass at the start of the take-off run.

•

Maximum Structural Landing Mass: The maximum permissible aeroplane mass upon landing under normal circumstances.

•

Traffic Load: This includes the total mass of passengers, baggage and cargo, including any non-revenue load.

total

15.2 Mass and Balance The document that covers the legal requirements of an aircraft’s mass and balance is ‘JAR-OPS 1 Subpart J’. An aircraft operator must specify in the Operations Manual the principles and methods involved in the loading and mass balance system used. This system must meet the legal requirements of JAROPS, and include all types of intended operations, such as charter, cargo and scheduled flights. The operator has to ensure that, during any phase of operation, the loading, mass and CG of the aeroplane comply with the limitations specified in the approved Flight Manual or the Operations Manual if this is more restrictive. The operator must establish the mass and CG of an aircraft by actual weighing prior to entry into service and at specified intervals thereafter. The accumulated effects of modifications and repair on the mass and balance must be accounted for and documented. If the effect of these changes cannot be established the aircraft must be re-weighed.

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The Dry Operating Mass must be established by weighing or using standard masses. The influence items included in the Dry Operating Mass and their position on the aircraft must also be established, as are other mass items such as the traffic load, fuel load and ballast. Methods for calculating crew and passenger mass values are laid down in JAROPS and include either weighing the individual crew and their baggage or taking standard mass values. Whichever method is used must be acceptable to the relevant Authority. 15.2.1 Mass and Balance Documentation The Mass and Balance documentation used by an operator must include certain basic information, which is listed below. Subject to the approval of the authority, some of this information may be omitted. A. B. C. D. E. F. G. H. I. J. K. L.

Aeroplane registration and type Flight identification number and date Identity of the commander Identity of the person who prepared the document Dry operating mass and the corresponding CG of the aeroplane Mass of the fuel at take-off and the mass of trip fuel Mass of consumables other than fuel Load components that include passengers, baggage, freight and ballast Take-off Mass, Landing Mass and Zero Fuel mass. The load distribution Aeroplane CG positions Limiting mass and CG values

Any last minute changes that occur after the mass and balance documentation has been completed should be brought to the attention of the commander and entered on the mass and balance documentation. The Operations Manual should specify the maximum allowable changes to passenger numbers or hold load. If this is exceeded a new mass and balance documentation should be prepared. Computerised systems are commonly used to generate the mass and balance documentation. These systems can only be used once they have gained approval from the authorities. The integrity of computerised system must be continually verified by the operator, at intervals not exceeding six months. Onboard mass and balance and Datalink systems can also be used, but again if the operator wishes to use these systems as the primary source of mass and balance documentation, he must obtain approval.


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15.3 Frequency of Weighing Aircraft must be weighed before entering service, to determine the individual mass and CG position. This should be done once all manufacturing processes have been completed. The aircraft must also be re-weighed within four years from the date of manufacture, if individual mass is used, or within nine years from the date of manufacture, if fleet masses are used. The mass and CG position of an aircraft must be periodically re-established. The maximum interval between one aircraft weigh and the next, must be defined by the operator, but not exceed the four/nine year limits. In addition the mass and CG position should be re-established either by weighing or calculation when the cumulative changes in the: • •

Dry Operating Mass exceed ± 0.5% CG position exceeds ± 0.5% of the MAC.

An aircraft may be transferred from one JAA operator to another without reweighing provided both have an approved mass control programme. 15.3.1 Fleet Mass and CG Position When an operator has a number of aircraft of the same type and configuration, he may wish to use the average Dry Operating Mass and CG position of this group of aircraft. The use of fleet mass and CG position is controlled by strict rules to ensure that all aircraft in the fleet stay within the specified limits. If one aircraft exceeds these specified limits, it must be removed from the fleet calculations and individual mass restrictions will apply. 15.4 Weighing Requirements Weighing of an aircraft can only by done by the manufacturer or an approved maintenance organisation, and must be done inside a hanger. The aircraft must be clean and complete, with the correct type of equipment fitted in its proper position and the required fluids properly accounted for. The equipment used to weigh an aircraft must be capable of accurately establishing its mass, and used in accordance with the manufacturers instructions. Weighing scales should be zeroed before use and calibrated at least every two years or more often if specified by the either the equipment manufacturer or the approved maintenance organisation.

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15.5 Centre of Gravity Limits (CG Envelope) The certified CG position of an aircraft will have operating margins built into the calculations, and is known as the CG envelope. This allows for any movement of the CG that may be experienced during flight by passengers and crew moving about the cabin, fuel consumption, landing gear and flaps moving position and any possible weighing errors or unaccounted modifications. The operator must show that his procedures account for, the possibility of an extreme CG variation at any time during the operation of the aircraft. 15.6 Records All records of weighing, including the calculations involved, must be available to the authority. The aircraft manufacturer, maintainer or operator must retain weighing records, and when the aircraft is weighed again, the previous records must not be destroyed, but must be retained with the aircraft records. Operators must retain all known weight and CG changes that occur after the aircraft has been weighed. These records are kept for the life of the aircraft. 15.7 Calculation of Mass and CG of any System The position of the CG of any system (refer to Fig. 1) may be found using the following process: •

Total Mass is calculated, by adding the mass of each load (plus the mass of the beam)

•

The moment of each load is calculated, by multiplying the mass by the arm (distance from the reference datum)

•

ALL the moments are added together, to provide the Total Moment

•

Total Moment is divided by the Total Mass to give CG position.


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2.4 m 2.0 m 1.2 m 0.25 m B

A

Datum

Beam Mass 250 kg Mass A 100 kg

Mass B 200 kg

Calculating the Mass and CG of a System Fig. 1 In the example, shown in Fig. 1, the reference datum is to the left of the beam. A mass of 100 kg is 0.25 metres from the datum and another mass of 200 kg is 2 metres from the datum. The mass of the beam is 250 kg and the length of the beam is 2.4 metres. Using the data, in Table 1, the position of the CG can be calculated thus: Centre of Gravity position = Total Moment / Total Mass = 725 / 550 = 1.32 m So the position of the Centre of Gravity is 1.32 metres to the right of the datum.

ITEM Mass 1

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Table 1 CALCULATION OF POSITION OF CG Mass (kg) Arm (m) Moment (Mass x Arm) 100 0.25 25

Mass 2

200

2.0

400

Beam

250

1.2

300

Total

550

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15.8 Principles of Weight and Balance of Aircraft Aircraft mass and balance is concerned with the force produced by the masses on the aircraft acting at a distance from a specific datum point on the aircraft. The sum of all moments about any point can be shown to be equal to the moment of the resultant force about that datum point (refer to Fig. 2). As the entire mass of the aircraft may be considered to be concentrated at the CG, the total moment of the aircraft about the datum is the aircraft mass times the horizontal distance between the CG and the datum.

Arm Limits Arm

Fwd

Aft

Load Load Datum

CG (Average Arm)

Mass and Balance Datum, Arms and C.G. Limits Fig. 2 15.9 Calculation of Mass and CG of Aircraft The mass and CG position of an aircraft is calculated in much the same way as that for any system. The Dry Operating Mass of the aircraft corresponds to the mass of the beam, and is usually found out by weighing the aircraft. The variable and disposable loads or ‘Traffic Loads’, such as fuel, crew, passengers and cargo correspond to the beam loads. Before each flight, the mass and moment of these items should be determined so that the aircraft mass and position of the CG can be determined, prior to flight, to see if they are within the approved limits.


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Aircraft Weighing Methods

Aircraft weighing equipment consist of weighbridge scales, hydrostatic weighing units or electrical/electronic strain gauge type weighing equipment. The capacity of the equipment must be compatible with the load, so that accurate measurements may be obtained. All weighing equipment should be calibrated and zeroed before any weighing commences, with the accuracy of the scale or load cell used depending on its capacity. Scale or Load Cell capacity

Accuracy

< 2000 kg 2000 kg to 20000 kg > 20000 kg

± 1% ± 20 kg ± 0.1%

•

Weighbridge Scales

These consist of a separate weighing platform for each wheel or bogey, the mass at each reaction point being indicated directly on the balance arm or on a dial indicator. Large aircraft may be weighed in a hangar, using portable weighbridge scales, or on weighbridges set permanently into the floor. •

Hydrostatic Weighing Units

These are based upon the principle that fluid pressure in a cylinder, in which a piston is working, depends on the area of the piston and the load applied to it. The units are placed between the lifting jacks and the aircraft jacking points and the weight at each position recorded on a gauge. The gauge may be calibrated directly into weight units or a conversion may be required to obtain the correct units. It is important that the jacks used with these units are vertical and the units correctly positioned, otherwise side loads may be imposed on the units and inaccurate readings obtained •

Electrical or Electronic Weighing Equipment

These type incorporate three or more weighing cells, using metallic resistance elements or strain gauges, the resistance of which varies with change in length, due to elastic strain. These strain gauges are either incorporated into cells between the aircraft and the jacks, or they are used in portable weighbridge platforms placed beneath the aircraft wheels. The output may be measured with a galvanometer, or sent to an instrumentation unit, which adds all of the platform values and digitally displays the aircraft load.

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15.10.1 Preparation for Weighing Before weighing the aircraft should be in a condition that meets the requirements of JAR-OPS. It is recommended that the aircraft be positioned several hours before weighing, so that an even temperature can be assumed and the aircraft is free from moisture. The aircraft should be placed into the ‘Rigging Position’, so that consistent results are obtained and several readings should be taken at each reaction point to obtain reliable average readings. Some light aircraft with tail wheels, have a negative load on the tail when in the rigging position, as a result of the CG being forward of the main wheel centres. In such instances, it may be possible to use a jack at the nose or a spring balance may be anchored to the ground and attached to the tail wheel. The reaction thus obtained will be a negative reaction and its value deducted from the aircraft weight and treated as a minus quantity when calculating the CG position. 15.10.2 Weighing on Aircraft Jacks Jacking should be done in accordance with the Maintenance Manual procedures and suitable jacking adapters should be placed at the jacking points. Weighing units of sufficient capacity should be attached to the jacks and the jacks positioned at each jacking point. Zero indication of each weighing unit should be verified, before the aircraft is raised evenly, until clear of the ground when the aircraft should be levelled. Readings should be made at each weighing point, and to ensure representative readings are obtained, a second reading should be made. The Dry Operating Mass of the aircraft may be deduced by adding all of the readings from each weighing point. With the aircraft weight correctly established, it remains only to calculate the CG.


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15.10.3 Calculation of Aircraft’s CG While the CG of a nose-wheeled aircraft must, obviously, be somewhere near the main wheels, its location may be found, using the previously described methods. Taking, as an example (refer to Fig. 3), a nose-wheeled aircraft, which has been weighed and its Dry Operating ass has been calculated to be 1915 kg. The CG is forward of the main undercarriage and, using the main wheel centre-line position as a reference datum, the CG can be found by means of the formula: AxB÷C Where

A = Mass at the nose wheel B = Distance between the front and rear reactions C = Basic Mass (the sum of all the reactions)

Thus:

A x B = 100 x 6 = 600 kg m and, as C = 1915 kg then, the CG = 600 (kg m) ÷ 1915 (kg) = 0.3133 m or 313.3 mm.

The CG is, therefore, calculated (to 3 significant figures) to be 313 mm forward of the main wheel centre-line. Reference Datum

Nose Wheel Reaction 910 kg 100 kg

Main Wheel Reactions 6m

905 kg

Calculation of Aircraft’s CG (Main Wheel Centre-Line as Reference Datum) Fig. 3 The main wheel centre-line is not, however, always taken as the reference datum and, as another example (refer to Fig. 4), the reference datum could be taken to be somewhere between the nose wheel and the main wheel positions.

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engineering Nose Wheel Reaction

910 kg 100 kg

Main Wheel Reactions

4m 905 kg

2m Reference Datum

Calculation of Aircraft’s CG (Reference Datum Aft of Nosewheel) Fig. 4

In this instance it is necessary to use the formula: CG = Total Moment ÷ Total Mass It is also necessary to remember that Moments, which are AFT of a reference datum are considered to be positive or additive (+ve), while Moments, which are FORWARD of the datum are considered to be negative or subtractive (-ve). Using the data, displayed in Table 2, enables the aircraft’s CG to be calculated. Table 2 CALCULATION OF AIRCRAFT’S CG Position Mass (kg) Arm (m) Moment (kg m) Left Main Wheel 905 +4 3620 Right Main Wheel 910 +4 3640 Nose Wheel 100 -2 -200 TOTALS 1915 +7060 Thus Total Moment (+7060 kg m) ÷ Total Mass (1915 kg) = +3.687 m (to three decimal places). Therefore the CG is located 3.687 metres AFT of the reference datum (which, by observation, remains at 313 millimetres inches forward of the main wheel centreline, as previously calculated). 71811175.doc Issued December 2002

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15.10.4 CG as Percentage Standard Mean Chord (SMC) Since the position of the CG is an aerodynamic consideration, its position is sometimes specified as a percentage of the SMC of the wing, measured AFT from the leading edge (refer to Fig. 5). The percentage SMC may be calculated using the formula: (A - B) ÷ C x 100 Where

A = Distance of the CG from the Reference Datum B = Distance of the SMC leading edge from the Reference Datum C = Length of the SMC.

A – B x 100 C

= 8 – 7 x 100 4.5

=

1 x 100 = 22.2%. 4.5

The CG is, therefore, located, 22.2% aft of the leading edge of the SMC.

B=7m

C = 4.5 m

Reference Datum

A=8m

Calculation of C.G. as a Percentage SMC Fig. 5 15.11

Changes in Basic Weight

When an item of Basic Equipment is added, removed or re-positioned in an aircraft, calculations must be made to determine the effect on both Dry Operating Mass and CG. In the event of modifications, where the Total Mass and Moment, for additional parts is not quoted in the appropriate modification leaflet, the additional parts must be accurately weighed and their moments calculated, relative to the reference datum.

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In order to find the new Dry Operating Mass and Moment of the aircraft, the mass and moment of the equipment added or removed must be considered as follows: • • • •

When equipment has been added, the mass must be added to the original Dry Operating Mass and, if the arm of the new equipment is +ve (aft of the CG reference datum), then the moment must be added to the original moment If the arm is -ve (forward of the CG datum), then the moment must be subtracted When equipment has been removed, the mass must be deducted from the original mass. If the arm is positive the moment must be deducted from the original moment and vice versa The revised CG position is calculated by dividing the revised Total Moment by the revised Dry Operating Mass.

15.11.1 Examples of Alterations to Dry Operating Mass To consolidate the information, discussed in this topic, the following examples are provided, and are typical of an aeroplane with a: • • •

Dry Operating Mass of 7000 kg. CG Reference Datum at Fuselage Station 4000 (i.e. 4000 millimetres aft of Fuselage Station Zero) CG located at Station 4600 (i.e. + 600 millimetres aft of the Reference Datum)

Table 3 shows the relevant data when, in the first example, the components of a Radar System are installed at several locations in the aircraft. The components of the Radar System (their mass and locations) comprise of a: • • •

Transmitter (mass 13 kg.), at Fuselage Station 4700 Controller (mass 2 kg.), at Fuselage Station 1800 Scanner (mass 9 kg), at Fuselage Station 400 Table 3 REVISION TO BASIC MASS AND MOMENT Mass (kg) Arm (m) Original Aircraft 7000 + 0.6 Transmitter 13 + 0.7 Controller 2 - 2.8 Scanner 9 - 3.6 Revised Basic Mass and Moment 7024


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Moment (kg m) + 4200 + 9.1 - 5.6 - 32.4 + 4171.1

JAR 66 CATEGORY B1

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engineering

With the revised Dry Operating Mass and Moment, the revised CG can be calculated using the formula: CG

=

Total Moment ÷ Total Mass

CG

=

4171.1 ÷ 7024

=

0.594 m

The revised Mass and CG Position will, therefore, state: Dry Operating Mass Centre of Gravity

: 7024 kg. : 594 mm aft of the Reference Datum.

Table 4 shows the relevant data when, in the second example, a Heating Unit (mass 66 kg.) is removed from Fuselage Station 1600 and re-installed at Station 6000 in the same aircraft as that in the first example. Table 4 REVISION TO BASIC WEIGHT AND MOMENT Mass (kg) Arm (m) Moment (kg m) Original Aircraft 7000 + 0.6 + 4200 Item Removed - 66 - 2.4 + 158.4 Item Replaced + 66 + 2.0 + 132 Revised Basic Mass and Moment 7000 + 4490.4 With the Dry Operating Mass unchanged, the revised CG position will (to two decimal places), again, be found with the formula: CG

=

Total Moment ÷ Total Mass

CG

=

4490.4 kg. m ÷ 7000 kg.

=

0.641.49 m.

In this instance, the revised Mass and CG Position will state: Dry Operating Mass Centre of Gravity

: :

7000 kg. 641.49 mm aft of the Reference Datum.

Therefore the datum has moved aft by nearly 41.5 mm.

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uk engineering 15.12


Loading of Aircraft (Typical Aircraft Load Sheet)

The Captain (or Commander) of an aircraft, must be satisfied that the load, carried by the aircraft, is of such a weight and is distributed and secured in such a way that it may be safely carried in flight. To ensure this, the Traffic Loads must be added to the Dry Operating Mass of the aircraft and the Total Mass and CG position calculated. Loading of an aircraft must be completed under the supervision of qualified personnel, and ensure that the loading of freight is consistent with the data used to calculate the aircraft mass and balance. Additional structural limitations such as the maximum load per unit area, maximum mass per cargo compartment and the maximum seating limits must also be considered when loading the aircraft. With large passenger carrying and cargo aircraft, the moment of items such as fuel, passengers and cargo are considerable and the calculation of CG can be complicated. In addition to longitudinal CG calculation, it may also be necessary to distribute fuel and cargo in a transverse direction. Most operators utilise specialists who deal with loading calculations, and produce a Load Sheet for each flight. A typical aircraft Load Sheet is reproduced in Table 5.


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Table 5 TYPICAL AIRCRAFT LOAD SHEET Weight (lb) Basic Weight Variable Load Pilot Navigator Engineer Steward Crew Baggage Passenger Seats 50 Business 100 Economy Drinking Water Life-Raft Emergency Transmitter Service Equipment (food etc.) Operating Weight Disposable Load Passengers 1st class (35) Tourist (83) Cargo No 1 hold No 2 hold No 3 hold No 4 hold Zero Fuel Weight Fuel Nos 2 and 4 tanks Nos 1 and 3 Reserve tanks Take Off Weight

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Arm (in)

Moment (lb in/1000) 21000.00

100 000

210

165 165 165 165 100 450 600 250 300 30 200 102 590

100 100 120 300 110 170 280 130 410 120 400 211

16.50 16.50 19.50 49.50 11.00 76.50 168.00 32.50 123.00 3.60 80.00 21 596.60

160 270 100 200 280 350 215 150 200 240 210

924.00 3697.65 50.00 90.00 140.00 140.00 26638.55 1500.00 2000.00 1200.00 31338.55

5 775 13 695 500 450 500 400 123910 10000 10000 5000 148910


CG (SMC) 29.2

30.0

33.3

29.2

uk engineering


16 AIRCRAFT HANDLING AND STORAGE Aircraft need to be moved on the ground, between flights, for a variety of reasons, which can include: • • •

Moving aircraft into, or within hangars for maintenance Re-positioning aircraft for ground running or storm protection Emergency removal of aircraft from a taxi-way.

It is important that the aircraft be moved safely, using the correct equipment, to avoid injury to personnel or damage to aircraft. Small aircraft, generally require little preparation but, with larger aircraft, some or all of the following points may be relevant: Preparation for the reception of the aircraft should be made in advance of its arrival. There should be adequate space available for the aircraft, with consideration given, as appropriate, to clearances for jacking, access for cranes etc. All equipment required for servicing should be available and serviceable. The aircraft should be in a satisfactory condition to move. The brakes should be serviceable and electrical power should be available, if required, for lights and indications in dark or poor light. The route of the proposed move should be free from obstructions, such as servicing platforms, passenger steps, vehicles and any other servicing equipment. Consideration should also be given to sources of F.O.D. along the route. The members of the moving team should be fully conversant with their assigned tasks. The person controlling the move should adequately brief them all, as to their individual responsibilities. This applies equally to the re-positioning a light aircraft in a hangar or to the moving of a giant airliner around a large, international airport. The equipment and method of move should be as stated in the relevant aircraft maintenance manual. All towing limitations should be observed. These should be stated in the maintenance manual under "Ground Handling". Examples of limitations include minimum turning radii and disconnection of nose-wheel steering system on certain aircraft. Clearance from the local Air Traffic Control may be required for the move.

71811175.doc Issued March 2002

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16.1 Moving Methods Normal moving methods of moving aircraft on the ground are by means of: • • •

Hand: by pushing and using a steering arm Tractor: using a bridle and steering arm or with a purpose-made towing arm Taxiing: moving the aircraft, using its own power.

When an aircraft has to be moved from one place to another, either by manhandling, by the use of a tractor (also called a towing ‘tug’) or by taxiing, there are a number of safety precautions which have to be applied every time. 16.1.1 Moving by Hand and Steering Arm This method is generally used for moving light aircraft small distances. Care should be exercised, during the move, to avoid damage to the structure, particularly on aircraft constructed from wood and fabric. On aircraft, which have a nose-wheel, a steering arm is attached to the wheel axle, in order to guide the aircraft, while the moving force is applied to strong parts of the aircraft. It is generally better to push the aircraft backwards, since the leading edges are stronger than the trailing edges. It is also permitted to push at the undercarriage struts and wing support struts. Areas to avoid include: • Flying Control Surfaces • Propellers • Wing and Tail-plane trailing edges. On aircraft with steerable nose wheels, which are connected to the rudder pedals, care should be taken not to exceed the towing limit, which may be marked on the undercarriage leg. On this type of aircraft the rudder controls should not be locked during towing. If the aircraft has a tail skid, in place of a wheel, it is customary to lift the tail clear of the ground, ensuring first that the propeller is positioned horizontally, so that it does not strike the ground. 16.1.2 Using a Bridle and Steering Arm This method is sometimes used, when the aircraft is to be moved over uneven or boggy ground, because, if normal towing procedures were used, they would be likely to cause an unnecessary strain on the nose undercarriage. Using this alternative method, a special bridle (consisting of cables and attaching shackles) is attached to specific points on each main undercarriage leg and a steering arm is attached to the nose undercarriage for directional control.

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The aircraft is normally towed backwards, using a tractor attached to the bridle. It is normal to tow the aircraft backwards as this reduces the stress on the weaker nose undercarriage. If towing points are not available, then ropes may be passed round the legs, as near to the top as possible, taking care not to foul on adjacent pipes or structure. A separate tractor should then be connected to each main undercarriage and steering control achieved by using the steering arm. 16.1.3 Using a Purpose-Made Towing Arm This is the normal method used on large aircraft. The aircraft is normally towed with a suitable tractor (or tug) and using the correct, purpose-made towing arm for the specific aircraft. A person familiar with, and authorised to operate, the aircraft brake system should be seated in the cockpit (or on the flight deck) to apply the brakes in an emergency. The brakes should not normally be applied unless the aircraft is stationary. The relevant maintenance manual will normally specify details of the towing arm and any limitations on the towing procedure. On many aircraft with nose-wheel steering, it is normal practice to disconnect or depressurise the aircraft steering system before towing. 16.1.4 Precautions when Towing Aircraft Towing speed should be kept to a safe level at all times (walking pace is a safe limit). A steering limit is often imposed, so that the radii of turns are kept within specified limits, thus minimising tyre scrubbing and reducing the twisting loads on the undercarriage. It is usual to tow the aircraft forwards in a straight line after executing a turn, in order to relieve stresses built up in the turn. The steering limit is often shown by marks painted on the fixed part of the nose leg, but may, sometimes, be overcome by the disconnection of a pin, joining the torque links. Suitably briefed personnel should be positioned at the wing tips and tail when manoeuvring in or around confined spaces, so that obstructions may be avoided. One person shall be supervising the aircraft movement (NOT the tractor driver) and should be positioned so that all members of the team can be observed. Particular care should be given, when towing swept wing aircraft, to "wing tip growth". This is the tendency of the swept wing to "grow" in a turn and was discussed in ‘Flight-Line Safety’, which is contained in the early topic concerning Safety Precautions.


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Before commencing the towing operation, the brake system should be checked and the brake accumulator charged as necessary. Brake pressure should be carefully monitored during the move. Large, multi-engined aircraft will usually be towed with special-purpose tug and a suitable towing arm that includes a shear pin, designed to shear if a predetermined towing load is exceeded. In an emergency it may be necessary to move an aircraft from the runway if it has one or more deflated tyres. Provided there is one sound tyre on the axle the aircraft may be towed to the maintenance area, but sharp turns must be avoided and towing speed kept to a minimum. If there are no sound tyres on an axle, the aircraft should only be moved the shortest distance in order to clear an active runway and serviceable wheels should be provided before towing. After any tyre failure, the associated wheel and other wheels on the same axle should be inspected for signs of damage. 16.1.5 Taxiing Aircraft When aircraft are to be moved under their own power, whether for ground movements or prior to flight, a fully certified flight crew must be on the flight deck and in command of the aircraft. It is usual for the aircraft to have received a daily inspection before the taxi operation, which ensures items such the oil and fuel levels and brake pressures are sufficient for the task. It will be necessary for a ‘Starter Crew’ to be present before engine starting. This crew should include a supervisor (who will be in visual and/or verbal communication with the aircraft crew), a fireman with a suitable extinguisher and a tractor driver to pull any ground power unit clear after engine starting. Once the aircraft is moving under its own power, the flight crew has responsibility for the safety of the aircraft. The ground team should give assistance to the crew, via the intercom and/or standard marshalling hand signals (refer again to the ‘Flight-Line Safety’ section of the earlier Safety Precautions topic), until the flight crew no longer require their services. When approaching its parking spot, providing it is not using the automatic parking indicating system, found on many parking stands, the pilot may be dependent upon the ground team for clearance indications and stopping cues. Once stopped, the aircraft wheels must be chocked, given ground power, if required, and generally taken control of, by the engineers, prior to its next maintenance procedure.

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engineering 16.2 Aircraft Jacking

Aircraft may need to be jacked for a variety of purposes. These may include component changes, retraction tests, weighing of the aircraft and aircraft rigging checks. Care needs to be taken when jacking, to avoid damage to aircraft or equipment. Jacking points are provided in the wings and fuselage, at strong points, to enable the whole aircraft to be lifted, and there are, usually, other points, at the nose and main undercarriages, to enable individual wheels to be changed (refer to Fig. 1). Some aircraft require a jacking pad to be fitted to each jacking point, while in some, the jacking pads are built into the structure. Special jacking adapters and beams may be available to lift individual axles. In all instances, the Maintenance Manual should be consulted, so that the correct equipment and procedures may be used. Nose Jacking Point (Offset)

Main Jacking Points

Nose Jacking Point (Offset) Typical Jacking Points Fig.1 16.2.1 Special Considerations Because of the position of the jacking points, the C.G. of some aircraft may be well behind, or in front of, the main jacking points. It may be necessary to add ballast forward or rear of the jacking points or to check the fuel load of the aircraft, to bring the centre of gravity within safe limits as specified in the Maintenance Manual. 71811175.doc Issued March 2002

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Each jacking point may have a load limit which, if exceeded, could result in structural damage. To avoid exceeding this limit it may be necessary to install hydraulic or electric load cells. Any special requirements should be listed in the Maintenance Manual. Micro-switches, attached to the undercarriage legs, and operated by the extension of the shock absorbers (weight-on switches), are used to operate various electrical circuits, This operation may not be desirable, so circuits should be isolated, by tripping circuit breakers or removing fuses as necessary. Aircraft should always be as structurally complete as possible before jacking, It is essential that any stressed panels which have been removed are re-installed. Failure to do this may result in distortion or failure of the structure. 16.2.2 Aircraft Jacks Aircraft jacks are intended for raising and lowering loads and should not be used for supporting the loads for long periods. Where a load must remain raised for a long period, it should be supported on blocks or trestles after it has been jacked to the required height. The most common types of aircraft jacks are the pillar, trolley, bipod, tripod and the quadrupod hydraulic jacks. There are several sizes of jacks, with capacities ranging from 4000 kg and greater. The Pillar hydraulic jack consists of a cylinder assembly, a fluid container and a hydraulic pump which, when operated, forces fluid from the container into the cylinder and raises the ram. A release valve is provided which, when opened, causes the fluid in the cylinder to return to the container and the ram to descend. Because of possible hydraulic failure, some jacks are provided with a mechanical locking collar which, when wound down, will prevent the jack from lowering. An air/filler valve, which vents the return side to atmosphere, may also be provided. This should always be open when the jack is operated. Bipod, Tripod and Quadrupod jacks are used, to raise an aircraft for various servicing operations. Their methods of operation and hydraulic mechanisms are similar to the pillar jack. They consist of a hydraulic unit, supported by the relevant number of legs (two, three or four). Because of the problems involved in raising an aircraft and to avoid injury to personnel or damage to the aircraft, care should be taken to use the correct type of jack as stated in the Maintenance Manual. Each jack should be used with the correct adapter head.

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The tripod jack comprises a hydraulic unit with three equally spaced legs. The jack is designed for a vertical lift only and not for a lift involving lateral movement of the jack (such as when raising one side of the aircraft for a wheel change). The resulting side thrust may cause any one of the following: • • • • •

Serious damage to the ram, due to the bending load Distortion of the jack legs Damage to the aircraft, due to the .jack head slipping out of the jacking pad Shearing of the jacking pad fastener Dragging sideways of the serviceable tyre.

To change a single wheel, a pillar jack may be used, while two tripod jacks may be used to raise the complete aircraft (or a bipod jack may be used). The bipod arrangement overcomes the limitations of the tripod jack for an 'arc' lift. On this type of jack, two fixed legs provide the support and a third, trailing leg, follows the lift and steadies the load during the lift. The maximum angle of arc should not be more than 6º. The quadruped jack is used more commonly as it possesses the advantages of both types of jack. Two legs are fixed and two are adjustable. This jack may be used as a bipod jack, by removing the adjustable legs, or as an adjustable, stable jack with one extra leg added. All four legs may be locked solid, by slight adjustment of both adjustable legs. Transportation wheels are often permanently attached to some jacks while they may be provided as detachable units on other jacks. The wheels facilitate easy movement of the jacks that would otherwise need to be dragged around the hangar. Jacks, alternatively, can be dismantled for easier transportation. 16.2.3 Jack Maintenance and General Notes Aircraft jacks should always be positioned correctly and the load raised and lowered gradually. All jacks should be stored in the fully retracted position, kept clean and free from corrosion. Moving parts should be lubricated regularly and the jack should be exercised if it is not used frequently. Jack replenishment is usually through the air valve, up to the level of the bottom of the air valve. Low oil level is indicated by inability to lift to maximum height, whilst over-filling is indicated by leakage of oil when the jack is fully extended.


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16.2.4 Jacking Precautions As a safety precaution, small aircraft should normally be jacked inside a hangar. Larger aircraft may be jacked outside, provided they are positioned nose into wind; the jacking surface is level and strong enough to support the weight, and that any special instructions, stated in the Maintenance Manual, are observed. A maximum wind speed, stated for jacking outside, can also be found within the Maintenance Manual. The aircraft to be jacked should be chocked fore and aft and the brakes positioned to OFF (brakes released). If the brakes are inadvertently left in the ON position (brakes applied) stress could be introduced to the landing gear or to the aircraft structure, due to weight re-distribution as the aircraft is raised. 16.2.5 Jacking Procedures While the following procedures will, generally, ensure safe and satisfactory jacking of most aircraft, precedence must always be given to the procedures and precautions specified in the relevant Maintenance Manual. One person should co-ordinate the operation and one person should control each jacking point. On larger aircraft a levelling station will also need to be monitored and all members of the team may need to be in radio or telephone communication with the co-ordinator. Checks should be made on the aircraft weight, its fuel state, and centre of gravity, to ensure they are within the specified limits as detailed in the Maintenance Manual. The aircraft should be headed into wind (if it is in the open), the main wheels chocked fore and aft, the brakes released and the undercarriage ground locks installed. It is vital that the earth cable be connect to the earth point on the aircraft and it must be ensured that there is adequate clearance above every part of the aircraft and that there is clearance for lifting cranes or other equipment, which may be required. Jacking pads should be attached to the jacking points and adapters provided for the jacks as required. Load cells may also be included if needed. The jacks should be positioned at each jacking point and checks made, to confirm that the jacks are adjusted correctly (i.e. release valve closed, jack body vertical, weight evenly distributed about the legs when the adapters are located centrally in the jacking pads, and the weight of the aircraft is just being taken by the jacks).

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Before jacking commences, the chocks must be removed and then the aircraft should be raised slowly and as evenly as possible. Whilst jacking is in progress, the locking collars should be continually wound down, keeping them close to the body of the jack. When the aircraft is raised to the correct height, the locking collar should be fully tightened down. When jacking is complete, then supports may be placed under the wings and fuselage as indicated in the Maintenance manual. Note: As previously stated, a pillar (bottle) jack and an adapter are often used for raising a single undercarriage for changing a single wheel. Alternatively a trolley jack or stirrup jack may be used. The remaining wheels should be checked to prevent aircraft movement, and it may be specified that a tail support be located when raising a nose undercarriage. The jack should be raised only enough to lift the unserviceable wheel clear of the ground. 16.2.6 Trestles These are provided to support to aircraft structures (main planes, fuselages etc.) and may also be used to support the complete aircraft. Various types are available including plain wooden trestles that are purpose-built and not adjustable. Trestles should only be used at designated strong parts of the structure. It will normally be shown in the Maintenance Manual where they should be positioned. Lines are often painted on the aircraft to show where the trestle beam is positioned The ‘Universal’ trestle is made up from lengths of angle iron, bolts and nuts, and has two jacking heads. By using different lengths of angle iron, trestles of various sizes can be produced. The wooden beam across the jacking heads may be replaced by a wooden former, which is cut to the curvature of the component it supports. Padding is normally attached to the former, to prevent damage to the aircraft finish. The two jacking heads, which are hand-operated screw jacks, enable the beam to be adjusted to suit the angle of the component. Although the trestles have ‘jacking heads’, they should only be used for supporting a load, and not for attempting to raise parts of the aircraft. Damage may be caused to the aircraft if attempts are made to do any more than support the structure. The ‘Tail’ trestle is not suitable for heavy loads and must only be used for supporting a load vertically. Adjustment in height is made by a screw thread. In the same manner as a universal trestle, the beam can be made in the same shape as the contour of the aircraft.


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16.2.7 Lowering Aircraft off Jacks Before lowering the aircraft to the ground, all equipment, trestles, work stands etc. should be moved clear of the aircraft, to prevent collision or contact with the aircraft structure. The wheels should be rotated by hand, to ensure the brakes are off. The jacks should be lowered together, by opening their respective release valves, and the locking collars (if used) unscrewed (but kept close to the jack body), whilst the jacks are lowered. The jacks should be fully lowered after the aircraft is resting on its wheels and the release valves then closed. On no account should the top of the jacks be handled until the jack is clear of the aircraft. It is common for the aircraft shock absorbers to stick and to suddenly collapse, resulting in damage to equipment or serious injury to parts that might be between the aircraft and jack. After the aircraft is lowered and the jacks removed, the jacking pads and adapters should be removed and the chocks placed in position. Any fuses or circuit breakers should be re-set in their correct position. 16.3 Slinging Slings may be required for lifting various parts of an aircraft during maintenance, repair, dismantling and assembly. Sometimes a complete aircraft may need to be lifted for transportation or to clear a runway quickly. The use of the correct equipment for lifting aircraft parts will minimise the risk of damage to the aircraft and personnel. A list of special equipment is usually in the front of the Maintenance Manual. This list will usually include special slings to be used on the aircraft and any other special equipment or tools required. Slings may be of the three-point type, as used for lifting-main planes, while other types, used for lifting engines, fuselages or other large items may be provided with spreader bars or struts. Before removing a main plane, the opposite main plane must be supported with trestles. To attach a sling, some aircraft have special slinging points with threaded holes in the airframe, which are used to accommodate the eye or forkend bolts of the sling. These holes are normally sealed, with removable plugs, when not in use. As an alternative to screw-in devices, some slings are used in conjunction with strong straps that pass under the component to be lifted.

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16.3.1 Lifting Tackle The following is a list of safety precautions that must be used when using lifting tackle: • • • • • • • •

Do not exceed the safe working load of the lifting devices Do not leave a suspended load unattended at any time Do not walk or work under a suspended load Do not tow the hoist at greater than walking pace Do not tow the hoist, other than by hand, when a load is suspended from the lifting hook Do not allow the load to swing, especially when it is being hand-towed Do not using a hoist or crane on soft ground Do not use a crane or hoist if the lifting tackle shows signs of damage.

Wire rope, chain or fibre rope may be used for lifting purposes. Before use, the tackle should be inspected to ensure that it is serviceable, is of the correct type and, when used, that the Safe Working Load (SWL) is not exceeded. The SWL should be stated on an identification plate, attached to the lifting sling, and should never be removed from the sling. Wire Rope is used with cranes, hoists, gantries and various slings. Before use, the wire rope, splices and attachments should be inspected for damage such as wear, corrosion and broken wires. In use, care should be taken that the rope does not kink under load. Before multiple leg wire rope slings are used, they should be laid out on the floor to ensure shackles are correctly attached and the fittings are not twisted. Knotting of ropes, to shorten them, is prohibited. Wire rope slings may be treated against corrosion by immersion in oil and the surplus oil wiped off, but this treatment must not be applied to slings used for oxygen cylinders. They must always be free from oil or grease. Chains are used with cranes and various types of sling. Before use, all chains must be inspected for damage such as cracks, distortion, excessive wear and ‘socketing’. Socketing is the name given to the grooves, produced in the ends of links, when the links wear against each other. Any reduction in diameter will render the chain unserviceable. Fibre rope slings may be used for lifting lighter components, and are made from natural fibres such as sisal, hemp or nylon fibres. They must be inspected for frayed strands, pulled splices, excessive wear and deterioration.


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When not in use, fibre rope slings should be hung on pegs, in a sheltered position, and free from dampness. Immediately before use, the rope should be opened up, by slightly untwisting the strands, to ensure they are not damaged or mildewed internally. A damaged or mildewed fibre rope sling should not be used, and it must be destroyed, by cutting into small, unusable sections, before final disposal. In addition to before-use checks on the rope, all loaded components such as pulley blocks, shackles, pins, spreader bars and hooks are to be inspected for excessive wear, cracks and flaws. Moving parts must be lubricated periodically. 16.4 Parking and Mooring Aircraft When an aircraft is out of service and in the open it should be secured against inadvertent movement and protected against adverse weather conditions. The operations recommended in the relevant Maintenance Manual depend on the type of aircraft, the length of time it will be out of service and the prevailing or forecast weather conditions. 16.4.1 Parking Between flights it is usually sufficient to apply the parking brakes, lock the control surfaces and chock the wheels but, in a strong wind, light aircraft should be headed into the wind. Light aircraft without wheel brakes should be headed into wind and their wheels chocked front and rear. Flying controls, on many aircraft, are locked by movement of a lever in the cockpit/cabin. The lever is connected to locking pins at convenient positions in the control runs or at the control surfaces. When this type of control lock is not provided, locking attachments may have to be fitted to the control column and rudder pedals. A more positive method entails the use of external control surface locks, that prevent control surface movement and, thus, prevent strain on the control system. All external locks should have suitable streamers attached, to make them more visible. If an aircraft is to be parked overnight or for longer periods in the open, then additional precautions should be taken, to guard against the effects of adverse weather. The undercarriage ground locks should be fitted, and all openings, such as static vents, engine and cooling air intakes, should be blanked, to prevent ingress of dirt, birds, insects and moisture. Items such as pitot head and incidence indicators should also be covered.

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When severe weather is anticipated it is recommended that covers for cockpit, canopy and wheel are fitted if available. Blanks and covers should not be left in position when the aircraft is prepared for service. Servicing instructions should include a pre-flight check to ensure that all covers etc, are removed. 16.4.2 Mooring (Picketing) In certain weather conditions, particular in high winds, it would be recommended that the aircraft be parked in a hangar. If they must be left outside, then smaller aircraft may need to be tied down. The aircraft may be provided with picketing rings or attachment points at the wings and tail or adjacent to the undercarriage legs (refer to Fig. 2).

B

A

C

View A

View B Aircraft Picketing Points Fig. 2


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If outside, the aircraft should always be parked nose into wind and secured, from the picketing points to suitable ground anchor points such as heavy concrete blocks or specialised screw pickets. Cable or nylon rope of adequate strength should be used where possible but, if a natural fibre rope is used (sisal or hemp), then sufficient slack must be left to allow for shrinkage in damp conditions. Additional picketing from the undercarriage legs may be recommended in strong winds and, if so, care should be taken not to damage any pipelines or equipment attached to the legs or wheels. 16.4.3 Typical Small Aircraft Procedures When mooring small aircraft in the open, the aircraft, if possible, should be parked head into the wind. The control surfaces should be secured with the internal control lock and the brakes applied. Care must, however, be exercised in extremely cold weather and parking brakes must not be set if there is a danger that accumulated moisture may freeze the brakes. Another danger, in cold weather, exists when the brakes are overheated, because, if they are set in this condition, serious distortion and cracking of the brake (and wheel) components can occur as they cool down. Ropes, cables, or chains should be attached to the wing mooring (tie-down) points, and their opposite ends secured to ground anchors. A tie down rope (no chains or cables) should be fastened to the exposed portion of the engine mount and the opposite end of the rope also secured to a ground anchor. The middle of a rope should be attached to the tail tie-down ring and each end of the rope pulled, at a 45º angle, and secured to a ground tie-down point either side of the tail. A control lock should be applied to the pilot’s control column. If a control lock is not available, then the control may be tied back with a front seat belt. These aircraft are usually equipped with a spring-loaded steering system that affords protection against normal wind gusts. However, if extremely high winds are anticipated, additional external locks may be installed. 16.4.4 Large Aircraft Procedures These may only require picketing in very strong wind conditions. The maximum wind-speed will normally be stated in the Maintenance Manual (including gusting winds). The aircraft should be headed into wind and the parking brakes applied.

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Cables or chains should be attached from the aircraft picketing points to prepared anchorages. In some instances the picketing cables are special components and include a tension meter that is used to apply a pre-load to the cable. If an aircraft is to be parked for a longer period, then additional precautions must be taken. Landing gear down-locks must be installed (if so equipped) and all openings such as static vents and engine intakes should be covered or blanked off (refer to Fig. 3) to prevent the ingress of dirt, birds, insects and all forms of precipitation.

Intake Blank Pitot-Static Blanks

Exhaust Blank Nose Wheel Covers

Main Wheel Covers Typical Aircraft Blanks Fig. 3

16.4.5 Chocking of Aircraft When aircraft are parked, it is normal to place a chock ahead and behind at least one wheel set. The parking brakes are usually left in the ‘off’ position once chocks are in position, to allow the heat, generated by the brakes, to dissipate evenly. At high wind speeds, it is normal to chock all the wheels and apply the brakes (if they have cooled). Some aircraft chocks can be chained together, to give a more secure hold. During ground runs (and especially those involving high power), it is common sense to place chocks at the front of all main wheel sets, to reinforce the parking brake.


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16.5 Aircraft Storage If an aircraft is de-activated for an extended time it will need to be protected against corrosion, deterioration and environmental conditions during its period of storage. The following notes are based on the storage procedures applicable to BAe 146 aircraft that have been de-activated for periods in excess of 30 days and up to a maximum of 2 years. It is not intended for the information given here to be complete, but merely to give the student examples of some of the activities performed. Specific details of an aircraft’s storage procedures can be found in Chapter 10 of the relevant Maintenance Manual. A list of equipment and materials is normally given. This will, typically, include: • • • •

Hydraulic fluid and lubricating oils and greases Specialised water-displacing fluids and corrosion-preventative compounds Aircraft covers and blanks Plastic sheeting and adhesive tape.

Prior to the storage period certain tasks are completed. These may include replacing the tyres with ‘dummy’ tyres (those not suitable for flight), or the raising of the pressures of the normal ones. The various tanks are either filled (water), drained (toilet), or part-filled (fuel). If the aircraft has propellers, they must be feathered, to prevent them rotating in the wind. (they may also be restrained by straps). Generally there would be an initial procedure, this being repeated at specified intervals, as shown in Tables 1 (a) and 1 (b). If no repeat interval is given, then the item is only done initially. Once the aircraft has been prepared, there are routine, weekly checks to keep it in good condition.

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Table 1 (a) TYPICAL AIRCRAFT STORAGE TASKS ITEM

Repeat Intervals (days)

Landing Gear Clean and dry main and landing gear bays Check landing gear for hydraulic leakage Lubricate main & nose landing gear Clean/Check Shock struts for leaks. Wipe sliding tube with hydraulic fluid Clean Gear & Door Uplock Mechanisms. Protect with grease Clean and apply thin coat of hydraulic fluid to actuator and piston rods Spray micro-switches and proximity switches with water dispersion fluid Check tyre pressures and mark position of tyres with date Rotate wheels one quarter of a term and mark tyre with date Should aircraft be stored in a hangar, deflate the shock absorbers. The aircraft may be manoeuvred in the hangar with deflated shock absorbers

7 30 60 7 15

Flight Controls Fully extend flaps Open and tag flap valves and airbrake circuit breakers Fully extend lift spoilers and install safety sleeves to all spoiler jacks Depressurise hydraulic system Lubricate the flight controls Protect flap carriages, upper surfaces of flap tracks with grease Protect all control cables accessible with oil Check for corrosion and where found repair affected areas

7 7 7 30 30 90

Power Plants Carry out special long term storage procedure for engines Note: Renewal of engine long term storage is preceded by engine run

Oxygen System Check test date of oxygen cylinders. Disconnect distribution lines from oxygen cylinders, blank off pipelines and cylinder outlet connection Check cylinder pressure is above 50 p.s.i. Remove crew masks for storage


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Table 1 (b) TYPICAL AIRCRAFT STORAGE TASKS ITEM

Repeat Intervals (days)

Water Waste Drain potable water system Purge potable water system with dry air or nitrogen Fuel System Refuel aircraft with fuel treated with an approved biocidal agent Check fuel tanks for water accumulation Air Conditioning System Install blanks in the ECS ram air inlet, exhaust, APU intake, APU oil cooler, front and rear discharge valves Hydraulic System Check system for leaks Replenish system Coat all unpainted hydraulic pipe-work with preservative compound

7 7

7 180

Aircraft Exterior Wash aircraft Coat all unpainted metal surfaces with preservative compound Aircraft Interior Remove passenger seats and carpets for bay storage Remove, service and store all galley portable equipment Remove, check and store windshield wiper arms complete with blades Remove rain repellent cannisters Electrical/Electronic System Remove and service batteries Remove for bay service, all rack mounted electronic equipment Apply power to and function installed electronic equipment

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To allow the circulation of air around the inside of the aircraft, all the doors and curtains are fixed open, whilst all the external doors and panels are shut. The battery will be removed from the aircraft and kept in the battery bay. More active checks might be done on the two-weekly checks. These checks will probably involve re-installing the battery, running the engines for a period and functionally testing a number of the aircraft’s systems that require the engines operating. The flight controls might require cycling throughout their ranges and, if dummy tyres are not fitted, the aircraft must be moved slightly to prevent ‘flat spots’ forming on the tyres. In addition, when power plants are stored separately, their fuel and oil systems must be inhibited and all their external mechanisms protected with grease or other suitable preservative. They must be stored in a clean, warm, dry atmosphere with inspections at intervals to check for deterioration. Some engines are stored in an airtight bag, which has moisture-absorbent crystals (a desiccant) inside. After the storage period all of the covers, blanks and preservative compounds will need to be removed. All of the systems will need to be restored to their original condition prior to aircraft use. A further set of procedures will be followed, similar to those previously discussed. When the aircraft is to be returned to service, it is simply a case of initially removing all covers, blanks and tie-downs. Once access to the inside of the aircraft is obtained and the battery re-installed, all of the systems must be checked and tested. All the tanks must be replenished to their correct levels and all pressure vessels will require their gases charging to their normal operating pressures. If the cabin furnishings, such as seats, carpets and galleys have been removed, they are to be inspected and, when serviceable, re-installed in the cabin. As already stated, the foregoing summaries are only examples of the form that a basic aircraft storage procedure might take. If the aircraft is smaller or larger and more complex it will require a different form of inspection and routine checking. The correct storage procedures will be found in Chapter 10 of the relevant aircraft’s Maintenance Manual.


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16.6 Aircraft Fuelling Procedures The use of the term ‘fuelling’ can include both refuelling and defuelling procedures and these notes contain examples of the essential points to be considered when refuelling and defuelling aircraft. There may, however, be some further, local instructions, regarding the responsibilities of the various personnel involved in fuelling procedures and these will always take precedence in conjunction with the relevant Maintenance Manual. 16.6.1 Fuelling Safety Precautions Particular care must be taken when fuelling aircraft, so that the operation may be accomplished in the safest possible manner. Whenever possible, aircraft should be fuelled in the open, and not in a hangar (although this is, sometimes, necessary as part of a maintenance programme). This will minimise the fire risk from high concentrations of flammable vapours. Fire appliances should be readily available during all fuelling operations. Carbon dioxide, or foam, extinguishers are recommended but, if there is a perceived increased fire risk, then fire-fighting vehicles should be standing by. Within the specified danger area, around an aircraft being fuelled, no sources of ignition or sparks should be allowed and no electrical power should be switched on or off during the fuelling operation. It is vital that the correct type and grade of fuel is used for the fuelling operation. Use of a turbine fuel in a piston aircraft will certainly cause an engine malfunction, or failure, that could lead to loss of an aircraft. The correct type and grade of fuel is always detailed in the Maintenance Manual and marked adjacent to the aircraft’s fuelling point(s). Care should also be exercised so as to avoid contamination of the fuel system with water or other contaminants. The fuel supply should be regularly checked for water contamination and a sample of fuel drained off after refuelling, so that a water check may be done. It will sometimes be necessary to filter the fuel during over-wing refuelling, particularly in dusty climates.

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Electrical bonding of the fuel system is vital during fuelling operations, as when fuel flows through the refuelling hose, static electricity may be generated. This may lead to potential differences at adjacent metal parts and initiate a spark, fire or explosion. To minimise this risk the following actions should be completed before fuelling operations commence • • •

The aircraft should be earthed The refuelling tanker should be earthed The nozzle of the fuel hose should be electrically bonded to the fuelling point.

16.6.2 Refuelling When refuelling the AMM should always be consulted so that the positions and capacities of the fuel tanks and also the type of fuel, position of the refuelling point(s) and refuelling procedures are known. There are two general re-fuelling methods: •

Gravity or over-wing refuelling: which is, essentially, the same method as used to refuel a motor car (automobile), with a similar type of refuelling hose being used. As the name suggests, the filler points are, generally, on the upper surface of the wing and the tank is open when refuelling is done

•

Pressure refuelling: in which the fuel may be pumped into the aircraft via a pressure refuelling coupling at very high rates. The refuelling pressures and the rates of fuel delivery may be quite different for individual aircraft types, so great care must be taken, to ensure no damage occurs to an aircraft through incorrect refuelling settings.

16.6.3 Checking Fuel Contents This is normally done, using the aircraft fuel gauges, which may be calibrated in kilograms (kg), gallons (Imperial or US) or pounds (lb). If a double check is required, then the contents may be determined, on the ground, by using ‘dip sticks’ (installed into the top of the tanks) or by ‘drip sticks’ (or magnetic ‘drop sticks’) which are installed in the bottom of some aircraft tanks. The aircraft fuel gauges will normally be positioned on the flight deck, but they can, on some aircraft, be duplicated at a fuelling panel, adjacent to the pressure refuel coupling.


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The Relative Density (RD) of fuel will vary with temperature and so the weight of a certain quantity of fuel will also vary. For example, ten gallons (Imperial) of fuel, with an RD of 0.8, will have a weight of 80 lb, while ten gallons (Imperial) of fuel, with an RD of 0.78, will weigh 78 lb. It is crucial, for balance purposes, that the weight of fuel is known and this is the reason why many aircraft fuel gauges are calibrated in units of weight rather than in volume. When fuelling aircraft, it is essential that the technician is aware of the RD of the fuel, so that the necessary weight calculation may be done, if necessary. 16.6.4 Defuelling. Occasionally, it is necessary to remove fuel from an aircraft, to facilitate fuel tank maintenance, or because the aircraft is too heavily loaded for the next flight. Removing fuel from an aircraft can be accomplished by either the gravity or by the pressure defuelling method. The gravity method entails draining the fuel into a suitably earthed container, and this is typical of light aircraft, which are normally ‘gravity’ refuelled. The fuel removed must be disposed of in the correct manner, with regard to local instructions and to the environment. Aircraft that are normally pressure refuelled are normally equipped with a pressure defuelling facility. Pressure defuelling is achieved by utilising a small negative pressure (suction), which effectively draws the fuel out of the tank and returns it into the fuel tanker (bowser). Current rules will normally only allow the fuel, removed from an aircraft, to be placed into a dedicated defueller vehicle and the fuel will not be permitted to be used in another aircraft. This ensures that any contamination such as water or debris will not be transferred to other aircraft.

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16.7 Ground De-Icing/Anti-Icing of Aircraft 16.7.1 Ice Types There are three main types of ice/frost that can effect an aircraft’s performance, Hoar Frost, Rime Ice and Glaze Ice. The temperature and weather conditions will determine the type of ice that forms, but all three types can have a detrimental effect. The Dew Point is the temperature at which moist air becomes saturated and deposits dew if in contact with a colder surface or the ground. Above ground, condensation into water droplets takes place. Hoar Frost is a deposit of ice crystals that form on an object when the dew point is below freezing point. High humidity will normally produce hoar frost, as these are similar to conditions that produce dew. Hoar frost can form when the air temperature is greater than 0°C, but the aircraft skin temperature is less than 0°C. This type of frost produces a very rough surface which leads to turbulent airflow. Rime Ice is a light coloured opaque rough deposit that has a porous quality. At ground level it forms in freezing fog from water droplets with very little spreading. It adds very little weight but it can disrupt the smooth flow of air over the wing, and block pitot and static vents. Glaze Ice can be either transparent or opaque and can form into a glassy surface due to liquid water flowing over a surface before freezing. It is the most dangerous type of ice found on an aircraft and is dense, heavy and tough. It adheres firmly to a surface, is difficult to shake off, and if it does breakaway, it does so in large chunks. During cold weather operations, it may be necessary to remove ice and snow from the aircraft, while it is on the ground, and to keep it clear long enough, to allow the aircraft’s systems to cope with snow or ice removal. This may not occur until the aircraft is flying.


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On the ground, the aircraft must be cleared of all snow and ice from its wings, tail, control surfaces, engine inlets and other critical areas (refer to Fig. 4) before the aircraft can take-off.

Rudder VHF Antenna

Ailerons

TCAS Antenna

Elevator Flaps

Pitot and Static Heads

Slats Engine Nacelle Critical Surfaces for De-icing and Anti-icing Fig. 4 Ice formation on an aircraft on the ground may result from a number of causes: • • •

Direct precipitation from rain, snow and frost Condensation freezing on external surfaces of integral tanks following prolonged flight at high altitude After taxing through snow or slush, ice may accumulate on landing gear, forward facing surfaces and under-surfaces.

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The formation of ice on aircraft structures can produce many adverse effects, and if allowed to remain may result in some or all of the following: • • • • • • •

Decreased aerofoil lift Increased aerofoil drag Increased weight Decreased engine thrust Freezing of moisture in control hinges Freezing of micro-switches that affect systems such as the landing gear retraction Ingestion of ice into the engine.

16.7.2 Definitions The terms ‘de-icing‘ and ‘anti-icing’ have specific definitions, and it is essential to know the differences. • •

De-icing is the removal of ice that has already formed Anti-icing is the prevention of initial ice formation.

16.7.3 De-Icing and Anti-Icing Methods The de-icing procedure for removal of ice, frost and snow from an aircraft’s surface can be achieved by mechanical or chemical methods. Mechanical methods use blowers, brushes and rubber scrapers whilst chemical methods utilise de-icing fluids. The anti-icing procedure provides protection against the formation of ice, frost and snow on aircraft surfaces for a short period known as the ‘Holdover Time’. This is achieved by applying an anti-icing fluid, but the aircraft must be either clean or de-iced prior to this anti-icing fluid application. There are two ways of aircraft de-icing and anti-icing: • •

One Step Method Two Step Method.


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The One Step method utilises hot fluid to de-ice the aircraft, and this fluid remains on the aircraft surfaces to give a limited anti-icing capability. The Two Step method consists of two separate fluid application procedures. The first step is the de-icing part and the second step the anti-icing. This second step must be done within three minutes of starting the first step, surface by surface if necessary. The second anti-icing step protects the aircraft surfaces for a holdover period. Whilst the AMM will detail the exact areas for de-icing and anti-icing, particular attention should be paid to areas around probes, antennas, and pitot/static ports as well as control surfaces, landing gear and inlets and exhausts. 16.7.4 Chemical De-Icing Freezing Point Depressant (FPD) compounds are often used in conjunction with mechanical methods, and there are two main types of FPD compounds: •

Type 1 (unthickened)

These fluids have a high glycol content and a low viscosity. They provide good de-icing performance but only a limited protection against re-freezing. •

Type 2 (thickened)

These fluids have a minimum glycol content of approximately 50%, and due to a thickening agent, are able to remain on the aircraft surfaces for longer periods. The de-icing performance is good and provides protection against re-freezing and/or build up of further accretion, when exposed to freezing precipitation. 16.7.5 Treatment of Frost Deposits Frost deposits are best removed by the use of a de-icing fluid such as Kilfrost ABC (Aircraft Barrier Compound). These fluids usually contain either: • • •

ethylene glycol and isopropyl alcohol di-ethylene glycol and isopropyl alcohol propylene glycol and isopropyl alcohol.

This process is not lengthy and one application is usually sufficient provided it is applied within two hours of flight. Only fluids recommended by the manufacturer should be used and any instructions for their use should be strictly observed. Use of incorrect de-icing fluids may adversely affect glazed panels or paint finish.

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Alcohol based de-icing fluids, may cause dilution or complete removal of oils and greases from joints or bearings. This may allow water ingress, which can subsequently freeze and jam controls. De-icing spray nozzles should not be directed at lubrication points or sealed bearings. Hot air blowers may be used to remove snow, ice or frost, and the liquid residue should be dried and not allowed to accumulate in places such as hinges or microswitches as any re-freezing may cause damage. 16.7.6 Removal of Ice and Snow Deposits There are several methods of removing snow and ice from an aircraft, prior to applying liquids if required. Removal by hand can be accomplished by the use of soft brooms, hand brushes or rubber scrapers. The aircraft can be de-iced using cold air from a pressure supply unit, or by using hot air from a hot air blower designed for the purpose. Deep wet snow should be removed with a brush or rubber scraper, taking care not to damage components such as aerials and pitot probes, which may be covered in snow. The snow should also be cleared from items like vents and control hinges. Light dry snow should be blown off using a cold air blower. Hot air is not recommended as it may melt the snow which may accumulate and freeze requiring further treatment. Moderate to heavy ice deposits or residual snow should be cleared with de-icing fluid applied by spraying. The two methods of fluid spraying involve the: • •

Cold Fluid Spray Hot Fluid Spray.

When using these sprays, it is necessary to observe certain precautions, because of the risk of damage to the aircraft structure and associated components. With this in mind it is important to know that: • • • •

High-pressure sprays may cause damage to pitot-static probes and other sensing devices Covers and bungs should be fitted during de-icing operations to prevent ingress of fluid into intakes and exhausts High-pressure sprays may cause erosion of the aircraft skin. Consult the AMM for recommended maximum impingement pressure No attempt should be made to remove ice by using an impact force to break the bond


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De-icing should proceed symmetrically, to prevent excess weight on one side of the aircraft.

The Cold Fluid Spray method is the simplest method of applying de-icing fluid, but in severe conditions one application may not be sufficient to remove all deposits. Brushing followed by a second or third application may be required. The Hot Fluid Spray method has been adopted specifically to reduce turn-round time. The FPD fluid is mixed with water in proportion to suit prevailing weather conditions, and heated to between of 60ºC (minimum) and 85ºC (maximum). The fluid is normally sprayed onto the aircraft at a pressure of 100 psi (689.5 kN/m2) by use of spray lances. The nozzle of the lance is held close to the aircraft skin, to prevent heat losses. The heat transfers to the skin of the aircraft, breaking the ice bond, and large areas of ice may be flushed away by turning the nozzle sideways. The fluid film remaining on the skin, has only been slightly diluted beyond its original dilution and is effective in preventing further ice formation. Hot water de-icing is a method that must not be used below -70°C and may need to be performed in two steps. • •

Step 1: Snow and ice are normally removed initially with a jet of hot water not exceeding 95°C Step 2: If necessary a light coating of de-icing fluid is then sprayed on immediately (within 3 minutes) to prevent re-freezing.

On some aircraft, not equipped with aerofoil or propeller de-icing systems, the use of a de-icing paste may be specified. The paste is spread evenly, by hand, over wing, tail and propeller leading edges. It provides a chemically active surface on which ice may form but not produce a bond. Any ice, which forms, is blown away by the airflow. The paste should be re-applied before each flight in accordance with the AMM.

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When used for anti-icing, the FPD fluid should be sprayed onto the aircraft cold and undiluted, before the onset of icing or after any hot de-icing. The fluid film will prevent ice and snow from sticking to the aircraft skin and, given time, will melt any fresh precipitation. Typical times for which the fluid remains effective are known as the ‘Hold Over’ time (refer Table 2). Table 2 TYPICAL HOLD OVER TIMES Ambient Temp °C

Above 0

Frost

Weather Conditions Freezing Steady Freezing fog Snow Rain

Rain on cold soaked wing

60/40 (hot)

50/50 (hot)

*

5 hrs 2 hrs 45 min 10 min

4 hrs 1¾ hr 35 min 7 min

3 hr 1½ hr 30 min 5 min

45 min 30 min 15 min 5 min

*

8 hr 1½ hr 45 min 20 min

5 hr 1 hr 30 min 10 min

4 hr 50 min 20 min 5 min

3 hr 45 min 15 min 3 min

30 mins 15 mins 15 mins 3 mins

*

8 hr 1½ hr 45 min

5 hr 1 hr 30 min

4 hr 50 min 20 min

*

8 hr 1½ hr 45 min

5 hr 1 hr 30 min

*

8 hr 1½ hr 45 min

* *

*

* * *

-8 to -10

-11 to -14

-15 to -25

Type I Fluids (See note 2)

75/25 (hot)

*

0 to –7

Type II (AEA) fluids De-Icing

AntiIcing 100% Cold See Note 1 8 hrs 3 hrs 1 hr 20 min

* *

* *

* *

Under extreme cold conditions it may be necessary to heat the fluid (60°C max) to give it sprayability. No significant increase in hold over time is achieved by strengthening the mix of type I (AEA) fluids. Stations using Kilfrost will normally provide a mix of 50/50 or 60/40. It may be difficult to get stronger mixes at short notice unless the temperature conditions at the stations involved are below limits for that mix.


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Certain precautions should be observed when applying chemical anti-icing fluids, and these are: • • • • •

Anti-icing fluid must NOT be applied on top of a similar, earlier coat If possible, the engines or the APU should not be operated during snow/ice removal The fluid should not be sprayed directly onto windscreens, windows, vanes, pitot heads or probes The minimum quantity of fluid should be used in the air conditioning intake areas If possible the fluid should not be sprayed onto lubricated parts, such as landing gear legs

16.7.8 Inspection after De-Icing/Anti-Icing Procedures The following inspections should be done on completion of a de-icing procedure: • • •

• • • • • • •

External surfaces, for signs of residual snow or ice, particularly in the vicinity of control surface gaps and hinges All protrusions and vents, for signs of damage Control surfaces for full and free movement by hand. Where this is not possible the pilot's controls should be used, bearing in mind that poweroperated controls exert large forces and could cause damage if any part of the control surface is frozen Landing gear mechanisms, doors, bays and wheel brakes, for snow and ice deposits Up-locks and micro-switches, for correct operation Tyres to ensure that they are not frozen to the ground. They should be freed by the application of hot air to the ice (not the tyre) and the aircraft moved to a dry area Engine air intakes for ice and snow deposits Gas turbine engines for freedom of rotation by hand. Restriction may indicate icing in the compressor region and the engine should be blown through with hot air immediately before starting until the rotating parts are free Shock absorber struts and hydraulic jacks for leaks caused by contraction of seals and metal parts Tyre pressures and shock absorber pressure and extension

Following the inspections an entry should be made in the Tech. Log, indicating that the De-Icing/Anti-Icing procedure has been completed.

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16.8 Ground Electrical Supplies Ground electrical supplies are normally limited to either 28 volts dc or 115 volts ac, depending upon the systems of the aircraft. Most modern aircraft have at least one 115 volt ac system (as well as a 28 volt dc one), so they will normally be supplied with 115 volts ac from an external power supply. Airfields normally supply electricity to aircraft through external generators called Ground Power Units (GPUs), or have underground supplies, which are connected to the aircraft via the air-bridge, or from beneath the ramp surface. When an external electric supply is required inside the hangar, its generation will normally be through transformer rectifier units. An external power control box may be installed on the hangar wall and the required output for a particular aircraft can be selected. To prevent accidentally connecting-up of incorrect supplies, all aircraft have separately-shaped plugs and sockets. The 28 volt dc supply usually has a threepin connection whilst the 115 volt ac utilises a much larger, six-pin plug and socket (refer to Fig. 5). The 28 volt dc connection has two pins which are longer than the third. The longer pins are the supply connections whilst the shorter pin acts as a safety interlock, to ensure that the power is cut-off, if the cable is inadvertently pulled out without the power being switched off first. The 115 volt ac connection has six pins, with four pins being longer than the other two. The four longer pins provide the three phases and the neutral connection whilst the short pins provide the safety interlock.


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3 PIN EXTERNAL POWER RECEPTACLE EARTH

EXTERNAL SUPPLY SOCKET POSITIVE D.C. 3 PIN PLUG POSITIVE D.C.

ACCESS DOOR

dc Power Socket and Receptacle

EXTERNAL POWER READY LIGHT

SERVICE INTERPHONE CONNECTION

NOSE WHEEL WELL LIGHTS

A.C. PHASE “A” A.C. PHASE “B”

A.C. PHASE “C”

A.C. NEUTRAL D.C.

ac Power Receptacle

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16.9 Ground Hydraulic Supplies Hydraulic test rigs are available, to supply aircraft with a source of hydraulic power without the need for running the engines or APU. These test rigs are normally powered either by internal combustion, or by electric, motors. They must use the same type of hydraulic pump and fluid as the aircraft under test, to allow testing of items such as the timing of system operations. The aircraft has an access panel, behind which are a set of ‘quick-connect’ couplings, allowing the rig hoses to be easily connected to the aircraft’s system without the need for ‘bleeding’ the system of air. This is achieved by use of nonreturn valves, which only open when the couplings are fully tightened. Before connecting a hydraulic testing rig to an aircraft, it must be ensured that all of the lines and couplings are thoroughly clean, so that no dirt can get into the aircraft’s system. 16.9.1 Safety, Health and Servicing Precautions Phosphate ester-based hydraulic fluids constitute a major health risk. Extreme care should be taken when handling this fluid and the following precautions should be taken: A mask must be worn when the possibility of inhaling the fluid in an atomised form exists. The fluid irritates the respiratory passages and can cause sneezing and coughing • Eye protection is essential when the possibility of atomised spray exists. If fluid contacts the eyes, they should be flushed with large quantities of clean/sterile water and medical advice sought promptly • Hands must be washed thoroughly after working with these fluids and particularly before eating or smoking • Hydraulic fluid must not be allowed to contact the skin for excessive periods. Barrier cream and protective gloves must be put on before starting work • Contaminated overalls should be changed as soon as possible after contact with the fluid. •

A typical hydraulic test rig might have a 75kW (100 hp) electric motor, driving the pump through a gearbox, clutch and a flexible coupling. The output could be in the region of 175 litres per minute (38 gallons per minute) at 200 kPa (3000 psi). The oil would be filtered to the standard required by the aircraft system, typically, 3 microns.


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Most hydraulic rigs have a small header tank of system oil. It would utilise the aircraft’s oil for the majority of operations, with the header tank keeping the system primed during coupling and uncoupling operations. The flow valves, which are often integral parts the rigs, must be kept closed until all the hoses have been connected and the rig is ready to run. The motor is started and once the operating pressure is indicated on the rig gauges, the valves can be opened and the rig then forms part of the aircraft system. This will enable the functional testing of the aircraft’s hydraulic systems using the aircraft’s selector valves. The rigs may also be provided with special gauges, such as flow meters, which will allow the testing for internal system leakages. 16.9.2 Rig Maintenance The rigs must have an equal or better filtration level than the aircraft being serviced. Oil samples of the rig are taken on a regular basis, and the following checks must be completed on a regular basis: • • • •

The rig must be kept clean and all hoses blanked when not in use The filters must be changed or cleaned All the gauges should be calibrated Any electrical equipment on the rigs should be checked.

16.10

Ground Pneumatic Supplies

Pressurised aircraft usually require an adequate supply of low-pressure air, for such tasks as engine starting, ventilation, heating and cooling, anti-icing and pressurisation testing. This air supply is, normally, provided by the aircraft’s engine/s or APU but, when these are unavailable, a ground supply unit can be used. Pneumatic units can supply air at the required pressure and flow rate and are powered by turbine engines, diesel engines or electrically powered units. The compressors used by these units are normally axial flow, centrifugal flow, or of the screw or lobe type. Depending on the size of the aircraft being serviced and the air requirements, the compressor can be mounted on a trailer chassis or on a self-propelled vehicle. To ensure the air produced is of a suitable quality, it is normally filtered and cleaned before being fed to the external air supply connection, which is located on the outside of the airframe.

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Some aircraft have two separate connections for air supplies at different points on the airframe. The forward connection may be for low-pressure air, which is then fed directly to the conditioning system, allowing testing of the air conditioning system and also of the pressure hull. The aft connection may be for a higherpressure bleed air supply that is primarily used to start the engines if the APU is unserviceable. Whilst some units are dedicated air starter rigs, some can be used both for starting and also for functional testing of the air conditioning and de-icing systems. As with the electric and hydraulic ground power supply rigs, the output of a pneumatic unit must match the aircraft’s system for pressure and flow. 16.11

Effects of Environmental Conditions on Handling

Previous notes have mentioned a range of precautions that need to be applied when the weather is anything less than perfect. This section will cover actions that the technician will need to take for prevailing situations when various weather conditions exist. 16.11.1 Cold and Wet When the ramp is cold and wet, the friction between the aircraft’s tyres and the ramp can be reduced. This also applies to all self-propelled vehicles and, hence, all movements on the ramp should be at a slower speed than normal, with quick access to chocks, in the event of an emergency. During engine ground running, it is possible that there may be a maximum power limitation if the ramp is very wet or flooded. This will be covered in either the Airfield Operations Manual or the Ground Handling Procedures Manual (as will most other precautions and procedures). If large amounts of protective clothing are worn on the ramp, it is the technician’s responsibility to ensure that nothing can get sucked into a running engine. Also, during ground running, it is important that extra chocks are placed at the wheels of the aircraft to prevent slippage at the higher power settings. Falling rain (and fog) will demand that more care be taken, due to the reduced visibility, especially when towing is in progress. The use of all normal lights, day or night, when moving vehicles in rain, is most important. Where there is a risk of rain and the aircraft is to be parked, then the appropriate aircraft blanks and covers must be used. It is also inadvisable to re-fuel aircraft by the ‘open line’ (over wing) method in rain, due to the high risk of water getting into the tank whilst the filler cap is removed. Great care must be taken, to protect the filler neck orifice, so that very little water enters the tank.


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If a task needs to be completed on the upper surface of a wet wing, it would be advisable to use a ‘safety raiser’ or ‘cherry picker’. This mobile craning device will allow safe access to the upper surfaces of a high wing and also provide the technician with a safety device, to hook onto, should the need arise. 16.11.2 Snow and Ice Many of the precautions, already mentioned, also apply in conditions of snow and ice. Aircraft towing and taxiing may be restricted until all standing precipitation has been cleared from the area to be used. The loss of visibility during falling snow can be severe, especially at times of low light, so great care must be shown if it is considered essential that an aircraft movement must take place. This may require a larger than normal towing team and the use of extra lights. Most airfields that operate continuously have a plan to deal with excessive amounts of snow. This plan might include the application of heater units or allowing APUs to run for extended periods to keep the inside of the aircraft warm. For aircraft, which are to be left out on the ramp, in sub-zero temperatures, it may be necessary to drain the potable water tanks, to prevent them freezing overnight. This will involve some care, as they should not be drained onto the ramp, due to the risk of personnel slipping on the ice. Other items of equipment that use water, such as heaters and pipe-work, may also need protection in cold temperatures. 16.11.3 High Winds High winds can cause loose objects to move across the ramp and strike the aircraft. These can be light items such as twigs and branches but, on occasions, heavy pieces of ground equipment, that have not been secured correctly, have been pushed into aircraft, causing major damage. During very high wind conditions, the smallest objects can be lethal, due to the energy they contain. In certain environments, such as desert climates (or at airfields near seashores), sand and dust, driven by the wind, can enter small crevices, causing problems with aircraft systems and may also block filters. Where extreme conditions exist, such as during a sand storm, then the blanking of all orifices may have to be augmented with tape or other methods, to prevent the ingress of dust and sand. Great care must be taken, to ensure suitable entries are made in the Technical Log, for the complete removal of all blanking material, after the storm has abated.

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16.11.4 High Temperature Certain items of equipment are temperature-sensitive and, when aircraft are operated in environments of extreme high temperature (+55°C), then several extra precautions have to be taken. Some form of cooling must be provided to ensue that the crew does not suffer from heat exhaustion, and reduce their efficiency. The operating temperature electronic equipment must also be kept below a critical level, to ensure its continued serviceability. Most of the larger aircraft have an auxiliary power unit (APU), which can provide a supply of bleed air to allow the air conditioning system of the aircraft to operate on the ground. If an APU is not available, then external air conditioning units can be connected to the aircraft to keep the inside cool. These cooling rigs should have an air conditioning unit of suitable capacity for the size of the aircraft that requires cooling. Some turbo-propeller passenger aircraft have the facility to run an engine, without the propeller turning, to provide air conditioning on the ground. This facility is known as the ‘Hotel Mode’ and, effectively, enables an engine to operate in a similar manner to an APU, without the need to carry extra weight.


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17 PREVENTATIVE MAINTENANCE TECHNIQUES Preventative maintenance is concerned with the early detection of defects (using whatever inspection techniques are specified by the aircraft or component manufacturers) and the repair or modification of the defective parts. The inspection techniques may call for the disassembly of components (before or after cleaning) so that more detailed inspections can be done. Assessment, of the effect of the defect on the continued integrity of the part, will also be required and, following the repair, modification or rejection of the part, reassembly techniques will be used to restore the aircraft to the appropriate level of serviceability. Troubleshooting techniques are used in the process of identifying the cause of a fault, eliminating the fault and returning the aircraft to service. 17.1 Types of Defects An operational aircraft can suffer from many defects and these can be defined as any event or occurrence, which reduces the serviceability of the aircraft below 100%. The manufacturer should specify the inspection areas and the faults, which are expected to be found. In most instances the inspector is looking for indications of abnormality in the item being inspected. Typical examples are: •

Metal Parts: as applicable to all metal parts, bodies or casings of units in systems and in electrical, instrument and radio installations, metal pipes, ducting, tubes, rods and levers. These would be inspected for: • • • • • • • • • • • • •

Cleanliness and external evidence of damage Leaks and discharge Overheating Fluid ingress Obstruction of drainage or vent holes or overflow pipe orifices Correct seating of panels and fairings and serviceability of fasteners Distortion, dents, scores, and chafing Pulled or missing fasteners, rivets, bolts or screws Evidence of cracks or wear Separation of adhesive bonding Failures of welds or spot welds Deterioration of protective treatment and corrosion Security of attachments, fasteners, connections, locking and bonding.


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Rubber, Fabric, Glass Fibre and Plastic Parts: such as coverings, ducting, flexible mountings, seals, insulation of electrical cables, windows. These parts would, typically, be inspected for: ♦ Cleanliness ♦ Cracks, cuts, chafing, kinking, twisting, crushing, contraction – sufficient free length ♦ Deterioration, crazing, loss of flexibility ♦ Overheating ♦ Fluid soakage ♦ Security of attachment, correct connections and locking.

•

Control System Components: cables, chains, pulleys, rods and tubes would be inspected for: ♦ ♦ ♦ ♦ ♦ ♦ ♦

•

Correct alignment – no fouling Free movement, distortion, evidence of bowing Scores, chafing, fraying, kinking Evidence of wear, flattening Cracks, loose rivets, deterioration of protective treatment and corrosion Electrical bonding correctly positioned, undamaged and secure Attachments, end connections and locking secure.

Electrical Components: actuators, alternators and generators, motors, relays, solenoids and contactors. Such items would be inspected for: ♦ Cleanliness, obvious damage ♦ Evidence of overheating ♦ Corrosion and security of attachments and connections ♦ Cleanliness, scoring and worn brushes, adequate spring tension after removal of protective covers ♦ Overheating and fluid ingress ♦ Cleanliness, burning and pitting of contacts ♦ Evidence of overheating and security of contacts after removal of protective covers

17.1.1 External Damage Damage to the outside of the airframe can occur by interference between moving parts such as flying controls and flaps, although this is quite rare. The most common reasons for airframe damage is by being struck by ground equipment or severe hail in flight.

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During ground servicing many vehicles need to be manoeuvred close to the airframe and some have to be in light contact with it to work properly. Contact with the airframe by any of these vehicles can cause dents or puncturing of the pressure hull, resulting in a time-consuming repair. 17.1.2 Inlets and Exhausts Any inlet or exhaust can be a potential nest site for wildlife. The damage done by these birds, rodents and insects can be very expensive to rectify. Other items that have been known to block access holes include branches, leaves and polythene bags. A careful check of all inlets and exhausts, during inspections, must be made, to ensure that there is nothing blocking them. A blocked duct can result in the overheating of equipment, or major damage to the internal working parts of the engine. 17.1.3 Liquid Systems Liquid systems usually have gauges to ascertain the quantity in that particular system. A physical quantity check is often done in addition to using the gauges, as the gauges are not always reliable. These systems usually include oil tanks for the engine, APU and Integrated Drive Generators (IDG), and also the hydraulics, fuel and potable water tanks. The cause of a lower-than-expected level should be immediately investigated, bearing in mind, that some systems consume specific amounts of fluids during normal operation. The consumption rate must be calculated before instigating any trouble-shooting. A low hydraulic system should not be replenished without first investigating the cause of the leak. External leaks of oil and fuel systems are normally easy to locate. The rectification of an external leak is usually achieved by simply replacing the component, seal or pipe work at fault, and completing any tests required by the AMM. If the leak is internal, then a much more thorough inspection of the component must be made, as the problem is more difficult to find. The symptoms are usually signalled by a slower movement of the services or by the erratic operation of services, due to the return line being pressurised. Some hydraulic oils, especially the phosphate ester based fluids, are very toxic and require personnel protection when working on and replenishing their systems. Some oils used are slightly toxic so care must be taken if there is a large leak.


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Potable water tanks are often permanently pressurised, so that a leak that starts somewhere between the tank and the services will continue, even if the aircraft is not flying. Once the pressure is removed, the leak can be investigated, cured and the tank re-filled. The physical signs of water inside the aircraft or dripping from the hull should be the signs of a leak that requires investigation. The unpredictable passenger consumption of water means that the tank level is no indication of a leak in the system. Windscreen de-icers are usually in the form of a pressurised container, which supplies fluid on demand to the spray nozzles. If the fluid leaks onto the flight deck it will give off a distinctive odour in the enclosed space. As the containers are replaced when low, it is more likely that the pipe work will be the likely cause of the leak. 17.1.4 Gaseous Systems These include gases such as oxygen, nitrogen and air. If the gas is to be used from a system during flight, a leak will be very hard to confirm unless a physical check is carried out using a leak detector such as ‘Snoop’ or ‘Sherlock’. A leak from an oxygen system is extremely dangerous, due to the chances of an explosion, if it comes into contact with oil or grease. Once the leak has been cured, the system can be re-charged and leak tested. Nitrogen, used in hydraulic accumulators, can leak into the liquid part of the hydraulic system. This will make the hydraulic system feel spongy and reduce the response of the operating actuators. If the gas leaks into the atmosphere, the system will not function correctly and the efficiency of the system may be reduced. The main cause of accumulators leaking externally is due to faulty seals or gauges. Accumulators assist the hydraulic system as an emergency backup, which only works correctly if it is charged to the correct pressure. Pneumatic systems contain high-pressure air of a stated pressure, and should have the same pressure at the end of the flight as at the start. If the pressure is low at the end of the flight, then the compressor could be suspected. If the pressure falls between flights, it is probably due to a slow leak in the storage system, and this can be investigated using leak-detecting fluids.

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17.1.5 Dimensions There are a number of places where checking the measurement of a component can establish its serviceability. Landing gear oleo shock struts can be checked for correct inflation, by measuring their extension. If the dimension is less than quoted in the manual, then it may be low on pressure and further checks will be required. These checks are usually only done during line maintenance, with checking of the pressure being required for trouble shooting or hangar maintenance. Combined hydraulic and spring dampers, fitted to some landing gears, often have one or more engraved lines on the sliding portion of the unit. This can indicate whether the hydraulic pre-charge is correct or requires replenishment. 17.1.6 Tyres Tyres have their serviceability indicated by the depth of the groove in the tyre tread. The AMM gives information of what constitutes a worn or damaged tyre. Apart from normal wear, other defects, that can affect a tyre, are cuts, blisters, creep and low pressure. Most tyres can be re-treaded a number of times after they have reached their wear limits, but the retread can only be completed if the complete tyre has not been damaged badly. Creep is the movement of a cover around the rim, in very small movements, due to heavy braking action. This movement is dangerous if the tyre is fitted with a tube, as the movement can tear the charging valve out of the tube, causing a rapid loss of pressure. To provide an indicator, small white marks are painted across the wheel rim and the tyre side wall cover so, if creep takes place, the marks will split in half and indicate clearly that the tyre cover has moved in relation to the wheel rim. The installation of tubeless covers has reduced the problem of creep, as the valve is permanently fitted to the wheel. It is still possible for tyres to creep a small amount, but the air remains in the tyre as the seal remains secure. Tyre-inflation devices usually consists of high-pressure bottles fitted with a pressure-reducing valve or a simple air compressor. The pressure a tyre should be inflated to depends on various factors such as the weight of the aircraft.


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The correct pressure for a specific aircraft is given in the relevant AMM for the aircraft in question. It is possible for a tyre to lose a small amount of pressure overnight. A pressure drop of less than 10% of the recommended pressure is not unusual, but the exact figures are given in the AMM. If a tyre is completely deflated with the weight of the aircraft on it, or is one of a pair on a single landing gear leg, which has run without pressure, all the tyres concerned must be replaced due to the possible, unseen damage within the cover. Again the AMM will dictate the conditions. 17.1.7 Wheels Defects to aircraft wheels are usually due to impact damage from heavy landings or from items on the runway hitting the wheel rim. Other problems can arise from corrosion starting as a result of the impact damage and the shearing of wheel bolts, which hold the two halves of a split wheel together. Wheels are usually inspected thoroughly during tyre replacement and it is very unusual for serious defects to be found during normal inspections of a wheel. 17.1.8 Brakes Brake units are normally attached onto the axle of an undercarriage leg, and located inside the well of the main wheels. During braking operation they absorb large amounts of energy as heat. This results in the brake rotors and stators wearing away and, if they become too hot, the stator material may break up. Inspection of brake units between flights is essential, to check for signs of excessive heating and to ensure that they have not worn beyond their limits. Wear results in the total thickness of the brake pack being reduced, which means that by measuring either the thickness of the pack, the amount of wear can be monitored. Once the amount of wear reaches a set figure, the brake pack will be overhauled. If the pads are breaking up there will be signs of debris, excessive amounts of powder and, in extreme cases, scoring of the discs. This will require immediate replacement of the complete brake unit. A rejected take-off at maximum weight will produce the maximum possible amount of heat and wear. It is usual to replace all brake units and main wheels after this has happened, but again the AMM will give the required information on what must be changed and when.

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17.1.9 Landing Gear Locks These items are normally fitted to the aircraft’s undercarriage as a safety device to prevent them inadvertently collapsing. They are usually fitted when the aircraft is to stay on the ground for some time, and removed before the next flight. The most likely defects will be damage to the locking pin ball bearing device or the loss of the high visibility warning flags. These flags will, hopefully, attract attention to themselves to ensure that they are not left in position when the aircraft next goes flying. 17.1.10 Indicators The most common type of indicator is the ‘blow-out’ disc used in fire extinguishing and oxygen systems. This shows that a high-pressure gas bottle has discharged its contents overboard, blowing the disc from its flush housing in the aircraft’s skin. The reason for the ruptured disc (refer Fig. 1) could be due to a fire extinguisher having been operated or the extinguishant having been discharged due to an excessive pressure being reached.

Gas Bottle and Pressure Relief Valve

Retaining Ring

Frangible Disc

Gas Bottle Bursting Disc Fig. 1 71811175.doc Issued March 2002

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17.1.11 External Probes There are several different types of probe, projecting into the airflow, to send information to the flight deck. These can include the pitot/static probes and the angle-of attack (AOA) probes. To prevent these from freezing they have electrical heating elements built into them and, occasionally, they can become overheated. Usually this is when they are left switched ‘on’ on the ground with a faulty weigh-on-wheels (WOW) switch. This switch is designed to reduce or remove power to the probes when on the ground, and to increase or restore it in flight. On smaller aircraft there is no WOW switch and it is up to the pilot to turn them off after landing. If the elements overheat they can burn out and the probes will show this by discoloration. Probes are designed to project out from the aircraft skin, and this makes them vulnerable to physical damage. Probes need to be regularly inspected for signs of physical damage or discoloration. 17.1.12 Handles and Latches Handles and latches usually wear through constant use. The handles and latches of cargo bays and baggage holds, which are operated every time the aircraft lands, are particularly prone to wear. Technicians have to be aware that all panel fasteners will wear slowly and these panels must be secured in flight. Most fasteners have a ‘positive’ form of closing or locking, whilst the more important installations use an indication system (such as painted lines and flush fitting catches) to ensure correct closure. These must be regularly checked and, when found worn, they should be repaired or replaced. Losing a panel in flight is dangerous enough, but may be more so if it is drawn into one of the engines, and causes its destruction. 17.1.13 Panels and Doors These items can be of any size and can be faulty for several reasons. They can be damaged by excessive use and their frames can become damaged where items have to be passed through them (such as with baggage hold doors). If the latches are poorly designed or badly adjusted, they may have been operated with incorrect tools during service and may have been damaged.

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17.1.14 Emergency System Indication Some systems use protective covers, to prevent inadvertent operation of a switch. These covers are usually held closed by some form of frangible device that will indicate the system has been operated when it is broken. Thin copper wire is, sometimes, used to hold the protective cover closed on fire extinguisher switches. A broken wire will indicate that the cover has been lifted and the system may have been operated. Any indication like this must be thoroughly investigated. 17.1.15 Lifed Items There are a number of items on the aircraft that have a specific length of time in service (known as a ‘life’). They would be major airframe and engine components with finite fatigue lives. The company technical department monitors these and they will be replaced during major servicing. The components which can become unserviceable due to life expiry may include, engine fire bottles, cabin fire extinguishers, first aid kits, portable oxygen bottles and emergency oxygen generators. 17.1.16 Light Bulbs These have to be checked regularly, to ensure they remain serviceable at all times. Most bulbs with important functions like fire warning lights and undercarriage indication will be duplicated. This can be achieved either by using two separate bulbs or by a single, twin-filament type. The bulb covers can also be damaged, leading to broken glass or plastic on the flight deck, with its subsequent foreign object damage (FOD) hazard. 17.1.17 Permitted Defects All aircraft have a list of permitted defects that do not have to be immediately corrected. These defects can be left outstanding by the operator until a more convenient time can be found to rectify them.


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17.2 Locations of Corrosion in Aircraft Certain locations in aircraft are more prone to corrosion than others. The rate of deterioration varies widely with aircraft design, build, operational use and environment. External surfaces are open to inspection and are usually protected by paint. Magnesium and aluminium alloy surfaces are particularly susceptible to corrosion along rivet lines, lap joints, fasteners, faying surfaces and where protective coatings have been damaged or neglected. 17.2.1 Exhaust Areas Fairings, located in the path of the exhaust gases of gas turbine and piston engines, are subject to highly corrosive influences. This is particularly so where exhaust deposits may be trapped in fissures, crevices, seams or hinges. Such deposits are difficult to remove by ordinary cleaning methods. During maintenance, the fairings in critical areas should be removed for cleaning and examination. All fairings, in other exhaust areas, should also be thoroughly cleaned and inspected. In some situations, a chemical barrier can be applied to critical areas, to facilitate easier removal of deposits at a later date, and to reduce the corrosive effects of these deposits. 17.2.2 Engine Intakes and Cooling Air Vents The protective finish, on engine frontal areas, is abraded by dust and eroded by rain. Heat-exchanger cores and cooling fins may also be vulnerable to corrosion. Special attention should be given, particularly in a corrosive environment, to obstructions and crevices in the path of cooling air. These must be treated, as soon as is practical. 17.2.3 Landing Gear Landing gear bays are exposed to flying debris, such as water and gravel, and require frequent cleaning and touching-up. Careful inspection should be made of crevices, ribs and lower-skin surfaces, where debris can lodge. Landing gear assemblies should be examined, paying particular attention to magnesium alloy wheels, paint-work, bearings, exposed switches and electrical equipment. Frequent cleaning, water-dispersing treatment and re-lubrication will be required, whilst ensuring that bearings are not contaminated, either with the cleaning water or with the water-dispersing fluids, used when re-lubricating.

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17.2.4 Bilge and Water Entrapment Areas Although specifications call for drains wherever water is likely to collect, these drains can become blocked by debris, such as sealant or grease. Inspection of these drains must be frequent. Any areas beneath galleys and toilet/wash-rooms must be very carefully inspected for corrosion, as these are usually the worst places in the whole airframe for severe corrosion. The protection in these areas must also be carefully inspected and renewed if necessary. 17.2.5 Recesses in Flaps and Hinges Potential corrosion areas are found at flap and speed brake recesses, where water and dirt may collect and go unnoticed, because the moveable parts are normally in the ‘closed’ position. If these items are left ‘open’, when the aircraft is parked, they may collect salt, from the atmosphere, or debris, which may be blowing about on the airfield. Thorough inspection of the components and their associated stowage bays, is required at regular intervals. The hinges, in these areas, are also vulnerable to dissimilar metal corrosion, between the steel pins and the aluminium tangs. Seizure can also occur, at the hinges of access doors and panels that are seldom used. 17.2.6 Magnesium Alloy Skins These, give little trouble, providing the protective surface finishes are undamaged and well maintained. Following maintenance work, such as riveting and drilling, it is impossible to completely protect the skin to the original specification. All magnesium alloy skin areas must be thoroughly and regularly inspected, with special emphasis on edge locations, fasteners and paint finishes. 17.2.7 Aluminium Alloy Skins The most vulnerable skins are those which have been integrally machined, usually in main-plane structures. Due to the alloys and to the manufacturing processes used, they can be susceptible to intergranular and exfoliation corrosion. Small bumps or raised areas under the paint sometimes indicate exfoliation of the actual metal. Treatment requires removal of all exfoliated metal followed by blending and restoration of the finish.


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17.2.8 Spot-Welded Skins and Sandwich Constructions Corrosive agents may become trapped between the metal layers of spot-welded skins and moisture, entering the seams, may set up electrolytic corrosion that eventually corrodes the spot-welds, or causes the skin to bulge. Generally, spotwelding is not considered good practice on aircraft structures. Cavities, gaps, punctures or damaged places in honeycomb sandwich panels should be sealed to exclude water or dirt. Water should not be permitted to accumulate in the structure adjacent to sandwich panels. Inspection of honeycomb sandwich panels and box structures is difficult and generally requires that the structure be dismantled. 17.2.9 Electrical Equipment Sealing, venting and protective paint cannot wholly obviate the corrosion in battery compartments. Spray, from electrolyte, spreads to adjacent cavities and causes rapid attack on unprotected surfaces. Inspection should also be extended to all vent systems associated with battery bays. Circuit-breakers, contacts and switches are extremely sensitive to the effects of corrosion and need close inspection. 17.2.10 Control Cables Loss of protective coatings, on carbon steel control cables can, over a period of time, lead to mechanical problems and system failure. Corrosion-resistant cables, can also be affected by corrosive, marine environments. Any corrosion found on the outside of a control cable should result in a thorough inspection of the internal strands and, if any damage is found, the cable should be rejected. Cables should be carefully inspected, in the vicinity of bell-cranks, sheaves and in other places where the cables flex as there is more chance of corrosion getting inside the cables when the strands are moving around (or being moved by) these items.

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17.3 Corrosion Removal, Assessment and Reprotection Due to the high cost of modern aircraft, operators are expecting them to last much longer than perhaps even the manufacturer anticipated. As a result, the manufacturers have taken more care in the design of the aircraft, to improve the corrosion-resistance of aircraft. This improvement includes the use of new materials and improved surface treatments and protective finishes. The use of preventative maintenance has also been emphasised more than previously. Preventative maintenance, relative to corrosion control, should include the: • • • • • • • • •

Adequate and regular cleaning of the aircraft Periodic lubrication (often after the cleaning) of moving parts Regular and detailed inspection for corrosion and failure of protective treatments Prompt treatment of corrosion and touch-up of damaged paint Keeping of drain holes clear Draining of fuel cell sumps Daily wiping down of most critical areas Sealing of aircraft during foul weather and ventilation on sunny days Use of protective covers and blanks.

General treatments for corrosion removal include: • Cleaning and stripping of the protective coating in the corroded area • Removal of as much of the corrosion products as possible • Neutralisation of the remaining residue • Checking if damage is within limits • Restoration of protective surface films • Application of temporary or permanent coatings or paint finishes. • 17.3.1 Cleaning and Paint Removal It is essential that the complete suspect area be cleaned of all grease, dirt or preservatives. This will aid in determining the extent of corrosive spread. The selection of cleaning materials will depend on the type of matter to be removed. Solvents such as trichloroethane (trade name ‘Genklene’) may be used for oil, grease or soft compounds, while heavy-duty removal of thick or dried compounds may need solvent/emulsion-type cleaners. General-purpose, water-removable stripper is recommended for most paint stripping. Adequate ventilation should be provided and synthetic rubber surfaces such as tyres, fabrics and acrylics should be protected (remover will also soften sealants). 71811175.doc Issued March 2002

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Rubber gloves, acid-repellent aprons and goggles, should be worn by personnel involved with paint removal operations. The following represents a typical paint stripping procedure: • Brush the area with stripper, to a depth of approximately 0.8 mm – 1.6 mm (0.03 in – 0.06 in). Ensure that the brush is only used for paint stripping • Allow the stripper to remain on the surface long enough for the paint to wrinkle. This may take from 10 minutes to several hours • Re-apply the stripper to those areas which have not stripped. Non-metallic scrapers may be used to assist the stripping action • Remove the loosened paint and residual stripper by washing and scrubbing the surface with water and a broom or brush. Water spray may assist, or the use of steam cleaning equipment may be necessary. Note: Strippers can damage composite resins and plastics, so every effort should be made to 'mask' these vulnerable areas. 17.3.2 Ferrous Metals Atmospheric oxidation of iron or steel surfaces causes ferrous oxide (rust) to be deposited. Some metal oxides protect the underlying base metal, but rust promotes additional attack by attracting moisture and must be removed. Rust shows on bolt heads, nuts or any unprotected hardware. Its presence is not immediately dangerous, but it will indicate a need for maintenance and will suggest possible further corrosive attack on more critical areas. The most practical means of controlling the corrosion of steel is the complete removal of corrosion products by mechanical means. Abrasive papers, power buffers, steel wool and wire brushes are all acceptable methods of removing rust on lightly stressed areas. Residual rust usually remains in pits and crevices. Some (dilute) phosphoric acid solutions may be used to neutralise oxidation and to convert active rust to phosphates, but they are not particularly effective on installed components. Corrosion on high-stressed steel components may be dangerous and should be removed carefully with mild abrasive papers or fine buffing compounds. Care should be taken not to overheat parts during corrosion removal. Protective finishes should be re-applied immediately. 17.3.3 Aluminium and Aluminium Alloys Corrosion attack, on aluminium surfaces, gives obvious indications, since the products are white and voluminous. Even in its early stages, aluminium corrosion is evident as general etching, pitting or roughness.

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Aluminium alloys form a smooth surface oxidation, which provides a hard shell, that, in turn, may form a barrier to corrosive elements. This must not be confused with the more serious forms of corrosion. General surface attack penetrates slowly, but is speeded up in the presence of dissolved salts. Considerable attack can take place before serious loss of strength occurs. Three forms of attack, which are particularly serious, are: • Penetrating pit-type corrosion through the walls of tubing • Stress corrosion cracking under sustained stress • Intergranular attack ,characteristic of certain improperly heat treated alloys. Treatment involves mechanical or chemical removal of as much of the corrosion products as possible and the inhibition of residual materials by chemical means. This, again, should be followed by restoration of permanent surface coatings. 17.3.4 Alclad WARNING: USE ONLY APPROVED PAINT STRIPPERS IN THE VICINITY OF REDUX BONDED JOINTS. CERTAIN PAINT STRIPPERS WILL ATTACK AND DEGRADE RESINS. USE ADEQUATE PERSONAL PROTECTIVE EQUIPMENT WHEN WORKING WITH CHEMICALS. USE ONLY THE APPROVED FLUIDS FOR REMOVING CORROSION PRODUCTS. INCORRECT COMPOUNDS WILL CAUSE SERIOUS DAMAGE TO METALS. Obviously great care must be taken, not to remove too much of the protective aluminium layer by mechanical methods, as the core alloy metal may be exposed, therefore, where heavy corrosion is found, on clad aluminium alloys, it must be removed by chemical methods wherever possible. Corrosion-free areas must be masked off and the appropriate remover (usually a phosphoric acid-based fluid) applied, normally with the use of a stiff (nylon) bristled brush, to the corroded surface, until all corrosion products have been removed. Copious amounts of clean water should, next, be used to flood the area and remove all traces of the acid, then the surface should be dried thoroughly. Note: A method of checking that the protective aluminium coating remains intact is by the application of one drop of diluted caustic soda to the cleaned area. If the alclad has been removed, the aluminium alloy core will show as a black stain, whereas, if the cladding is intact, the caustic soda will cause a white stain. The acid must be neutralised and the area thoroughly washed and dried before a protective coating (usually Alocrom 1200 or similar) is applied to the surface. Further surface protection may be given by a coat of suitable primer, followed by the approved top coat of paint. 71811175.doc Issued March 2002

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17.3.5 Magnesium Alloys The corrosion products are removed from magnesium alloys by the use of chromic/sulphuric acid solutions (not the phosphoric acid types), brushed well into the affected areas. Clean, cold water is employed to flush the solution away and the dried area can, again, be protected, by the use of Alocrom 1200 or a similar, approved, compound. 17.3.6 Acid Spillage An acid spillage, on aircraft components, can cause severe damage. Acids will corrode most metals used in the construction of aircraft. They will also destroy wood and most other fabrics. Correct Health and Safety procedures must be followed when working with such spillages. Aircraft batteries, of the lead/acid type, give off acidic fumes and battery bays should be well ventilated, while surfaces in the area should be treated with antiacid paint. Vigilance is required of everyone working in the vicinity of batteries, to detect (as early as possible) the signs of acid spillage. The correct procedure to be taken, in the event of an acid spillage, is as follows: • Mop up as much of the spilled acid, using wet rags or paper wipes. Try not to spread the acid • If possible, flood the area with large quantities of clean water, taking care that electrical equipment is suitably protected from the water • If flooding is not practical, neutralise the area with a 10% (by weight) solution of bicarbonate of soda (sodium bicarbonate) with water • Wash the area using this mixture and rinse with cold water • Test the area, using universal indicating paper (or litmus paper), to check if acid has been cleaned up • Dry the area completely and examine the area for signs of damaged paint or plated finish and signs of corrosion, especially where the paint may have been damaged. • Remove corrosion, repair the damage and restore the surface protection as appropriate. 17.3.7 Alkali Spillage This is most likely to occur from the alternative Nickel-Cadmium (Ni-Cd) or NickelIron (Ni-Fe) type of batteries, containing an electrolyte of Potassium Hydroxide (or Potassium Hydrate). The compartments of these batteries should also be painted with anti-corrosive paint and adequate ventilation is as important as with the lead/acid type of batteries. Proper Health and Safety procedures are, again, imperative.

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Removal of the alkali spillage, and subsequent protective treatment, follows the same basic steps as outlined in acid spillage, with the exception that the alkali is neutralised with a solution of 5% (by weight) of chromic acid crystals in water. 17.3.8 Mercury Spillage WARNING: MERCURY (AND ITS VAPOUR) IS EXTREMELY TOXIC. INSTANCES OF MERCURY POISONING MUST, BY LAW, BE REPORTED TO THE HEALTH AND SAFETY EXECUTIVE. ALL SAFETY PRECAUTIONS RELATING TO THE SAFE HANDLING OF MERCURY MUST BE STRICTLY FOLLOWED. Mercury contamination is far more serious than any of the battery spillages and prompt action is required to ensure the integrity of the aircraft structure. While contamination from mercury is extremely rare on passenger aircraft, sources of mercury spillage result from the breakage of (or leakage from) containers, instruments, switches and certain test equipment. The spilled mercury can, quickly, separate into small globules, which have the capability of flowing (hence its name ‘Quick Silver’) into the tiniest of crevices, to create damage. Mercury can rapidly attack bare light alloys (it forms an amalgam with metals), causing intergranular penetration and embrittlement which can start cracks and accelerate powder propagation, resulting in a potentially catastrophic weakening of the aircraft structure. Signs of mercury attack on aluminium alloys are greyish powder, whiskery growths, or fuzzy deposits. If mercury corrosion is found, or suspected, then it must be assumed that intergranular penetration has occurred and the structural strength is impaired. The metal in that area should be removed and the area repaired in accordance with manufacturer’s instructions. Ensure that toxic vapour precautions are observed at all times during the following operations: • •

Do not move aircraft after finding spillage. This may prevent spreading. Remove spillage carefully by one of the following mechanical methods: • Capillary brush method (using nickel-plated carbon fibre brushes). • Heavy-duty vacuum cleaner with collector trap. • Adhesive tape, pressed (carefully) onto globules may pick them up • Foam collector pads (also pressed, carefully, onto globules). • Alternative, chemical methods, of mercury recovery entail the use of: • Calcium polysulphide paste • Brushes, made from bare strands of fine copper wire • Neutralise the spillage area, using ‘Flowers of Sulphur’ • Try to remove evidence of corrosion


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• The area should be further checked, using radiography, to establish that all globules have been removed and to check extent of corrosion damage • Examine area for corrosion using a magnifier. Any parts found contaminated should be removed and replaced. Note 1: Twist drills (which may be used to separate riveted panels, in an attempt to clean contaminated surfaces) must be discarded after use. Note 2: Further, periodic checks, using radiography, will be necessary on any airframe that has suffered mercury contamination. 17.4 Permanent Anti-Corrosion Treatments These are intended to remain intact throughout the life of the component, as distinct from coatings, which may be renewed as a routine servicing operation. They give better adhesion for paint and most resist corrosive attack better than the metal to which they are applied. 17.4.1 Electro-Plating There are two categories of electro-plating, which consist of: • Coatings less noble than the basic metal. Here the coating is anodic and so, if base metal is exposed, the coating will corrode in preference to the base metal. Commonly called sacrificial protection, an example is found in the cadmium (or zinc) plating of steel. • Coatings more noble (e.g. nickel or chromium on steel) than the base metal. The nobler metals do not corrode easily in air or water and are resistant to acid attack. If, however, the basic metal is exposed, it will corrode locally through electrolytic action. The attack may result in pitting corrosion of the base metal or the corrosion may spread beneath the coating. 17.4.2 Sprayed Metal Coatings Most metal coatings can be applied by spraying, but only aluminium and zinc are used on aircraft. Aluminium, sprayed on steel, is frequently used for hightemperature areas. The process (aluminising), produces a film about 0.1 mm (0.004 in) thick, which prevents oxidation of the underlying metal. 17.4.3 Cladding The hot rolling of pure aluminium onto aluminium alloy (Alclad) has already been discussed, as has the problem associated with the cladding becoming damaged, exposing the core, and the resulting corrosion of the core alloy.

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17.4.4 Surface Conversion Coatings These are produced by chemical action. The treatment changes the immediate surface layer into a film of metal oxide, which has better corrosion resistance than the metal. Among those widely used on aircraft are: • Anodising of aluminium alloys, by an electrolytic process, which thickens the natural, oxide film on the aluminium. The film is hard and inert • Chromating of magnesium alloys, to produce a brown to black surface film of chromates, which form a protective layer • Passivation of zinc and cadmium by immersion in a chromate solution. Other surface conversion coatings are produced for special purposes, notably the phosphating of steel. There are numerous proprietary processes, each known by its trade name (e.g. Bonderising, Parkerising, or Walterising).


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17.5 Non-Destructive Testing/Inspection (NDT/NDI) Techniques Among the many inspection tasks, done by aircraft serving technicians, are those involving Scheduled Maintenance Inspections (SMIs). SMI's are special inspections, detailed by the manufacturer, to be done at a specified time period. When doing these inspections the ultimate aim is to ensure that the aircraft (or part) being inspected, remains in a safe condition or that it complies with the original design specification. The common factor, in all the inspection/test procedures is that they entail techniques that do not affect the continued serviceability of the components under inspection. They are, in fact, non-destructive testing/inspection techniques. Non-destructive testing (NDT) or, in America, Non-destructive inspection (NDI) techniques, involve the use of such methods as: • • • • • • •

Visual and Assisted Visual Inspections Remote Viewing Instruments Penetrant Flaw Detection (PFD) Magnetic Particle Flaw Detection (MPFD) Eddy Current Flaw Detection (ECFD) Ultrasonic Flaw Detection (UFD) Radiographic Flaw Detection (RFD).

It is incumbent on all aircraft servicing technicians, regardless of trade or level of certification, to be constantly vigilant and to use their eyes to detect the slightest imperfection in and around the areas of aircraft or component parts on which they are working. When approaching an aircraft, a perfunctory glance may reveal the fact that one wing is lower than the other, which could indicate a difference in the fluid levels of the respective landing gear struts, different tyre pressures or, perhaps, a deflated tyre. Missing or badly secured panels have often been discovered by such alert observations, as have potentially catastrophic structural failures, and the student is urged to adopt this vigilant attitude as quickly as possible to ensure the safety of all aircraft and the people that fly in them. While all aircraft servicing technicians can, therefore, do visual and assisted visual inspections, only those who have received appropriate training will be authorised to do certain PFD techniques. The more sophisticated MPFD, ECFD, UFD, and RFD techniques will be done by specially trained and approved NDT (NDI) technicians.

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17.5.1 Visual/Assisted Visual Inspections The appropriate visual or assisted visual inspection techniques will be detailed in the relevant servicing manuals but, generally, they will depend on such factors as: • • •

• •

•

The nature of the item being inspected (i.e. the material from which it is made): It may be metallic, plastic, rubber or any other type of material The purpose of the inspection: It may be to establish whether the item is suffering from a known fault or to confirm the integrity of a previous repair The location of the item to be inspected: It may be installed in an aircraft or removed from an aircraft. In most cases the maintenance schedule will specify that an item is always inspected without removal from the aircraft. The term ‘in-situ’ has previously been used to describe this instance The inspection surface: Whether it is an internal or an external surface. The normal convention is that inspections are external unless otherwise stated The time available for the inspection: This is often dictated by circumstances, in that, if a tyre needs to be inspected for wear, it should be able to be checked in a few minutes. A major aircraft inspection, on a large aircraft, is however, normally planned to take many days The degree or depth of the inspection: Depending on the ‘criticality’ of the component, or its adjacent structure, to the safety of the aircraft.

It should be stressed here that, whenever a visual inspection is being done, there must be adequate illumination of the inspection site, to ensure that small defects are able to be detected. Some visual inspections may dictate that a specific amount of illumination (in a stated number of lux) be available during the inspection. To assist in visual inspections, use is frequently made of such aids as: • •

Inspection Mirrors Magnifying Glasses.

Inspection mirrors enable the technician to see the remote surface of components and into places that normal vision is restricted. Selections of inspection mirrors are available, mounted on the end of a handle or rod. Such mirrors should be mounted by means of a universal joint so that they can be positioned at various angles. A development of this device has the ability to change the angle of the mirror by remote control. A rack and pinion mechanism passes through the stem and is controlled by a knob on the handle.


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This permits a range of angles to be obtained, after insertion of the instrument into the structure. Some instruments come equipped with integral non-dazzle illumination. Magnifying glasses are most useful instruments, to assist with the close inspection of an airframe. They are capable of clarifying details, when normal visual inspection only produces a suspicion of a crack or corrosion. Magnifying glasses vary in design from the pocket type, with a magnification factor of ‘times two’ (x2), to the stereoscopic type with a magnification of up to x32. The magnification factor relates to the size of an object, seen through the magnifying lens, compared with the size of the object, viewed with the naked eye, at a distance of 250 mm (10 in). For day-to-day inspection of structures, a hand instrument with a x8 magnification and integral illumination could be used. Magnification above this value should not be used unless specified, because the limited area of observation does not reveal the surrounding area. A higher magnification lens can be used, once the lower powered lens has identified a problem. Note: Magnifying glasses and similar inspection instruments will provide the best results only when the area under inspection is well illuminated. 17.5.2 Remote Viewing Instruments These instruments have a variety of different names, although they all, basically, operate on similar principles. Whether they are called borescopes or fibrescopes, (or, collectively, introscopes), they are optical instruments used for the inspection of the remote areas of structures, components or engines, which would be, otherwise, not directly viewable. Note: A detailed knowledge of the internal structure of the component under inspection is essential, and proper training in their use should be obtained, before inspections involving remote viewing instruments are attempted. Borescopes consist of ostensibly rigid tubes of nickel-plated brass or of stainless steel. The outer diameters of the tubes may range from approximately 5.5 mm (0.22 in) to 11 mm (0.43 in) with lengths from 230 mm (9 in) to 1 750 mm (69 in). While they do possess a degree of rigidity, they can be very easily bent if too much sideways force is applied to them, so great care must be taken in their use. Inside the thin metal tube is a complex series of precision optical lenses and mirrors, surrounded by a bundle of very fine glass fibre filaments, which guide light to the viewing end of the tube.

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The light is provided by a box, containing an electrical transformer, a highintensity, light bulb of quartz-iodine, Xenon or something similar (which is mounted in front of a reflector), and a cooling fan. The light source box is usually connected to a mains outlet and the powerful light is transmitted to the borescope by means of a connecting flexible cable which also contains a guide bundle of glass fibres. In this way ‘cold’ yet brilliant light is provided at the viewing area, to give the necessary high quality illumination without the hazards associated with heat and any flammable fluids which may be present in the viewing area. Rigid borescopes are provided with several versions of viewing ends, which allow either a forward view, a lateral view (normal to the longitudinal axis of the tube), a forward oblique or a retrograde (reverse) view of the inspection area. With the exception of those with a forward view end, all the other borescopes may also have the capability of rotating the tube around the longitudinal axis, so that a full 360º internal view of the area is possible. They also have adjustable focus of the eyepiece, to minimise eye strain on the viewer and to accommodate the various levels of acuity of the inspectors’ eyesight. Fibrescopes are flexible and, probably because of this, they are extremely prone to abuse and damage. As the name implies, they rely on fibre optic cables rather than a rigid tube and lenses/mirrors to provide the image of the inspection area. The image is viewed through a bundle of fibre optic strands, while the object is illuminated by light transmitted through another surrounding bundle of fibre optic strands. Diameters and lengths of fibrescopes are similar to those of rigid borescopes and they are also provided with the various viewing ends and focussing arrangements. Some fibrescopes have a controllable ‘distal’ viewing end, to allow articulation through almost 360º on both an X and Y lateral axis. These (refer to Fig. 2) are most often used (in addition to borescopes) to inspect the inside of gas turbine engines, but can also be used for many other inspections such as; loose article checks, fuel leaks etc. The images, presented by borescopes and fibrescopes, may be viewed directly through an eyepiece, as stated, or they may be displayed on a TV screen via a video camera, which can be attached to the eyepiece. The results of the inspection can also be recorded, by means of a video tape, and retained, for future comparisons of possible deterioration of the inspection area.


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JAR 66 CATEGORY B1

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MODULE 7

engineering Eye Piece Operating Knob to Control Articulating Distal End


Focus

Control Fibre Optic Cable from Light Box Igniter Plug Hole

By-Pass Duct

Distal End

Burner Combustion Chamber

Turbine Blades and Nozzle Guide Vanes

Use of a Fibrescope inside a Gas Turbine Engine Fig. 2 Borescopes and Fibrescopes may be used for the inspection of gas turbine engine: • Compressors: for damage to Fans, FOD, Interference between Rotors and Stators, Surge damage, and Bearing Oil Leakage • Combustion Sections: for signs of Burning, Cracking, Distortion, and Carbon Build-up • Turbine Sections: for signs of Burning, Cracks, Dents, Deposits of Melted Metals and Nicks. Note: When using remote viewing instruments for engine inspections it must be ensured that: • • • •

The engine must be allowed to cool down before inserting the ‘scopes Windmilling (or inadvertent Starting) of the engine must be prevented by gagging or removing the appropriate fuses/circuit breakers and placing warning placards on the flight deck Contamination of the instruments, by Fuel, Grease and Oil, must be avoided Borescopes do not get bent and Fibrescopes do not get kinked nor crushed.

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Remote viewing instruments may also be used to inspect many other areas of an aircraft. Typical areas would include: • • • • •

Electrical Components Electrical Looms Enclosed Structural Parts Fuel System Components Hydraulic System Components.

Wherever they are used, there are certain difficulties involved with the interpretation of what is seen through the instruments. When using remote viewing instruments, it is recommended that the inspecting technician should: • • • • •

Be fully trained in the use (and care) of the instruments being used Be familiar with the layout of the structure or component under inspection If possible, have a spare or an example of the part near at hand with which to compare the images from the inspection area Use the experience of other inspectors where doubt exists (or consult previous video recordings etc.) Refer to the appropriate servicing manual for guidance whenever necessary.

17.5.3 Penetrant Flaw Detection (PFD) Before discussing the application of PFD techniques it is necessary to highlight the health hazards associated with working with PFD materials and to consider the recommended First Aid treatments and the Safety Precautions, which need to be observed, during their use. The hazards include: •

•

• •

Contact with the eyes: to prevent the possibility, chemical proof goggles should be worn. If, despite this, eye contamination occurs, then the eyes must initially be irrigated with copious amounts of water and proper medical assistance sought Contact with the skin: due to the de-fatting action of the chemicals, barrier cream should be applied to the hands before work commences and, where prolonged contact is probable, protective PVC-type gloves should be worn. Contaminated skin should be thoroughly washed with warm soap and water and, after drying, a lanolin-based cream applied. If irritation persists then medical attention is needed Ingestion: food must not be consumed while doing PFD procedures and hands should be carefully washed before eating. If chemicals are ingested then medical help must be sought. VOMITING SHOULD NOT BE INDUCED Inhalation: face masks should be worn where concentrations of fumes or particles are high and there must always be adequate ventilation. Victims who become nauseous, dizzy or drowsy should be moved to fresh air and medical advice sought. Resuscitation methods should be used where asphyxiation occurs and breathing has stopped and the Emergency Services summoned.


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Fire: all the necessary fire precautions must be observed (CO2 , Foam and Dry Powder extinguishers are the recommended types) and, in the event of a fire, any ventilation should be switched off first Storage: PFD chemicals should be stored in a dry area, away from heat and direct sunlight Spillage: any spillages should be soaked up with absorbent materials Transport: appropriate precautions, depending on the flash point of the particular chemicals should be observed Disposal: materials should be treated as oily waste and, where large quantities are involved, must not be discharged into public sewers or waterways.

Penetrant flaw detection may be used to detect surface-breaking discontinuities in any non-porous materials, including ceramics metals, and plastics. It may also be used to detect porosity in those materials that should not be porous, leaks in tanks and cracking of internal bores. The basic principle of penetrant flaw detection is that a liquid (usually oil-based) is applied, to the pre-cleaned surface of the material under inspection, and is then allowed to dwell on the surface for a specified time (the ‘dwell’ or ‘contact’ time). During the dwell time, and due to its characteristics, some of the liquid penetrates into any fine surface discontinuities by capillarity (or capillary action). Capillarity is associated with the surface tension of a liquid, which causes it to rise (or fall) in fine capillaries or tubes. It is the action, which causes moisture, in the ground, to be transmitted to the topmost leaves of a tree. The height (or depth) to which a liquid can travel in a capillary tube is given by the formula: h = 2T Cos θ ρgr Where T = The surface tension of the liquid θ = The angle of contact (wetting angle) of the liquid with the capillary ρ = The density of the liquid g = The acceleration due to gravity r = The radius of the capillary. From the formula it can be seen that a liquid with a low wetting angle and a comparatively high surface tension (but lower than that of water) will constitute a good penetrant. After the allotted dwell time the excess penetrant is carefully removed from the surface so that the surface is, again, clean. A white, fine powder is next applied to the surface and this is also allowed to remain on the surface for a given time.

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During this time any penetrant, which has entered a surface discontinuity, will (through capillarity) seep out into the fine particles of the powder, in a similar fashion to the action of blotting paper, and cause a stain in the powder. This stain will develop as more penetrant is drawn out during the allotted ‘developer’ time and will provide an indication of the site of the surface discontinuity. Indications within the first thirty seconds can be compared to the fully developed indications, observed at the end of the developer time, and an assessment made of the likely size of the discontinuity. A quickly formed stain, with very little subsequent development, could indicate a wide, but shallow discontinuity, while an initially light stain, which gradually spreads over a greater area could indicate a narrow but comparatively deep fissure. In other words: • •

The Rate of staining indicates the Width/Depth of a discontinuity. The Extent of staining indicates the Volume of the discontinuity.

Note: This will only give an approximate assessment and is by no means an accurate method of deciding the actual size of the discontinuity. Properly applied PFD procedures are capable of detecting discontinuities with widths of only 0.000004 mm (0.0001 in) but great care is needed in their application and technicians will need to attend an approved training course before they will be authorised to apply PFD procedures to aircraft parts. Penetrants are available in two basic types. They are the: •

•

Type 1 - Fluorescent penetrant: to which is added a dye, that gives very little colour when viewed in normal light. However, when viewed in subdued lighting conditions, and illuminated with the rays from a mercury vapour lamp, emitting light in the ultraviolet range of the spectrum (referred to as ‘Black Light’), the penetrant fluoresces brilliantly (yellow/green) Type 2 - Colour Contrast: (or Visible) penetrant: which has a dark red dye added to the penetrant liquid so that, when viewed in normal or (better) enhanced, white light conditions, the red dye contrasts strongly against the white background of the developer powder

The penetrants are further sub-classified by the methods, which are used to remove the excess penetrant from the inspection surface. They are the: •

Water-washable penetrants: in which the penetrant has an added emulsifier, that allows the oil-based liquid to be easily removed from the surface by the use of a water spray or wash. These are used, primarily, on rough surfaces (castings etc.)


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•


Post-emulsification penetrants: which do not have a built-in emulsifier and, thus, it is necessary to apply an emulsifier to the penetrant and allow an ‘emulsification time’ before the excess penetrant will be able to be removed (again by water) from the inspection surface. These are used on the highgrade surface finishes of alloy steels (and aluminium and magnesium alloys) in the search for fine fatigue cracks Solvent-removable penetrants: in which the excess penetrant is removed from the surface by a volatile solvent.

Both the Water-washable and Post-emulsification penetrants are (usually) confined to the Type 1 (Fluorescent) penetrants and are used in the controlled environment of a manufacturing or overhaul establishment, for the final acceptance of critical aerospace materials. The Solvent-removable, Colour Contrast (Type 2) penetrants are those which will be more likely to be used by the aircraft servicing technician, as they are the ones which are used for the manual (‘In the Field’) applications on limited inspection areas of aircraft parts. The solvent is usually trichloroethane-based, which, while being volatile (evaporates readily), it has a low toxicity and is non-flammable. The manual PFD process is done by technicians, with the use of the three prime materials - the Penetrant, the Penetrant Remover (Solvent) and the Developer -contained in their respective aerosol cans, which comprise the usual ‘In the Field’ PFD inspection kit. The PFD procedure can be broken down into the: • • • • • • •

Initial surface preparation and thorough pre-cleaning of the inspection area Masking of the areas adjacent to the inspection site Application of the penetrant (with the stipulated ‘Dwell’ time) Removal of the excess penetrant from the surface Application of the developer (and the specified ‘Developer’ time) Inspection and the recording of any significant indications Cleaning and restoration of the surface protective finish.

Obviously if a defect is found, the fault will need to be rectified, either by an approved repair scheme or by replacement of the defective component.

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In the initial surface preparation the area of the inspection must be completely clean and free from corrosion dirt, grease or oil, paint, surface treatments and water. The presence of any of these contaminants will prevent the penetrant from entering surface-breaking discontinuities. Paint should be removed using an approved paint stripper, taking care not to apply the stripper to areas which might be damaged (e.g. Redux bonded joints) by its action. Similar care is needed with the penetrant solvents and such joints. Care must be taken not to damage the material surface with scrapers as this might appear as a defect. The use of non-metallic scrapers is recommended, to avoid this problem. After paint removal, the surface should be washed with water, dried and finally degreased, by spraying the area with the solvent remover and allowing it to evaporate. Harsh abrasive methods should not be used in the inspection area, as they will tend to cause ‘metal flow’, which will cover the lips of any surface cracks and prevent ingress of the penetrant. If such methods must be used (to remove stubborn corrosion deposits, burnt on carbon or oil etc.), then it may be necessary to employ some form of surface etching process (if approved) to remove the ‘smeared’ metal. Before applying PFD procedures, the areas adjacent to the inspection site should be protected, by masking off, to prevent contamination by the chemicals of electrical components and other incompatible materials, and to restrict the rather messy procedure to the smallest possible area. The penetrant should be applied to the clean (and degreased) surface, using the aerosol spray, a brush or by dipping or swabbing. The penetrant should be left on the surface for the recommended contact time and, during this time, must be kept wet. If the penetrant dries on the surface (and in any crack) it will be extremely difficult to remove and the whole area will have to be thoroughly cleaned before the process can be re-attempted. Note: If the parts are too hot, then premature drying is an obvious danger. Ideally the temperature of the part should be in the range of 5ºC – 40ºC. Outside this temperature range, the application of the PFD procedure will require care, because: • •

At low temperatures, metals ‘sweat’, and the resulting moisture will contaminate the area, as previously stated An increase in the viscosity of the penetrant, at low temperatures, will require an increase in the dwell time


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Evaporation of the solvent (and the solvent carrier of the developer powder) will be retarded during low temperatures. This will result in the blurring of indications At higher temperatures the evaporation rate is much shorter and will require frequent wetting of the surface with penetrant to avoid it drying.

The time will usually depend on the temperature, the surface texture and the size of the suspected defect. A time of 5 - 30 minutes is, normally, recommended. The removal of the excess surface penetrant is another facet where incorrect procedures will cause poor results. The object of the exercise is to remove all of the surface penetrant without removing any of the penetrant that is in any defect. In the past, operators have been known to spray penetrant removers directly onto the surface, thus washing the penetrant out of the defect. The recommended method, with solvent-based, spray removers, is firstly to remove the excess penetrant with a clean, lint-free cloth (or paper wipe). Next a small amount of the remover should be applied to a clean cloth and the surface wiped with the moistened cloth (changing to a clean part of the cloth with each wipe), until the surface is quite clean. Wiping should be done carefully and in one direction only, to avoid over-wiping the area with the risk of wiping the penetrant out of a defect. The application of the developer is also a very critical stage of the PFD process and demands a degree of skill on the part of the technician if small fatigue defects are to be discovered. Developers consist of either dry powders, or of the powder carried in a liquid. Dry powder developers are applied by the use of puffers, electrostatic spray guns or by using a ‘dust storm’ cabinet. Liquid-carried developer powders may be classed as: • • •

Water-Soluble developers: where the fine particles are in solution with the water in the same way as sugar dissolves in tea Water-Suspended developers: where the fine particles are suspended in water in much the same way as sand remains suspended in water and does not dissolve Wet (non-aqueous), solvent-based developers: where the microscopic powder particles are suspended in a volatile solvent.

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The dry powder developers and the first two wet types of developers are, normally, confined to those establishments using the Type 1 penetrants, so further consideration of them is not necessary in this topic. The liquid in the ‘In the Field’ PFD developer aerosol is, however, the same as that in the solvent remover aerosol (Wet, non-aqueous). With this in mind, extreme care must be taken when the developer is sprayed onto the inspection area if the penetrant is not to be washed out of a potential defect. Prior to applying the developer, the aerosol container should be shaken vigourously so that the contents are agitated sufficiently to ensure that the powder is thoroughly mixed with the solvent carrier liquid. Holding the aerosol between 300 mm - 400 mm (12 in - 16 in) away from the inspection surface, the developer is sprayed, in three or four very short bursts (of approximately 1-second duration), with intervals (of approximately 6-seconds) between each burst, to allow the solvent carrier to evaporate. The aerosol should continue to be shaken during the intervals, to maintain the powder in suspension and the result should provide a ‘dusting’ of the developer over the inspection area. The objective of the procedure is to produce a thin, even coverage of the inspection area, - just enough to alter the surface background colour - without giving a layer of such thickness which might completely blanket the penetrant. During the specified ‘developer time’ the area should be monitored for indications and, as previously stated, the inspection should be made, using the appropriate quality of illumination. When using colour contrast penetrants it is recommended that the area of the inspection should be illuminated (using daylight or artificial light) to a level of at least 500 lux. This may be achieved with a fluorescent tube of 80 W at a distance of 1 m (39.3 in) or with a tungsten filament pearl lamp of 100 W at 0.2 m (8 in). Critical inspections may, however, demand higher levels of illumination and, as always, the AMM or SRM will specify the precise requirements. Indications of discontinuities (refer to Fig. 3) will need to be interpreted and assessed for their effect on the serviceability of the part under inspection. It is important that the exact position of any significant indication is recorded (with the aid of a drawing or a photograph if required) because it might not be obvious where the discontinuity is when the area is subsequently cleaned for repair or for eventual return to service.


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Narrow Crack

Very Fine Crack

Porosity

Wide Crack

Typical Penetrant Indications Fig. 3 If there are no defects, then the area should be thoroughly cleaned. It is imperative that all traces of the developer are removed because, being hygroscopic, it will attract moisture, which could lead to subsequent corrosion problems in the area. It is recommended that, after the initial signs of the developer have been wiped away, the area be liberally sprayed with the solvent remover which, on evaporating, will leave the area in a clean and de-greased condition. Restoration of the protective surface finish may involve etch priming, painting and, possibly, additional anti-corrosive treatment. If, however, the area is to be inspected regularly, it may be permitted to apply a coating of protective oil, grease or inhibiting fluid between inspections. There are many different brand names of penetrants, all of which must meet the Process Specification DTD 929 (or MIL-I-25135-C), but the approved process will always be specified in the relevant Aircraft Maintenance Manual. There are available, ‘Dual Mode’ penetrants, which exhibit both visible and fluorescent capabilities. When viewed under white light they provide a contrast with the developer and, when viewed under ‘black’ light, they fluoresce. Another type is the Thixotropic variety, which is a gel that only becomes fluid while it is being brushed on a surface and has the capability of remaining in place on vertical or on overhead surfaces. Special, non-oil-based, penetrants, and others which are also low in sulphur, phosphorus and chlorine are used for the inspection of oxygen system components, plastics and rubbers.

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17.5.4 Ultrasonic Flaw Detection (UFD) This form of Non-destructive Testing is done by specially trained, and approved, technicians, so only brief details of the background and the procedures are given in this course. The student is, however, required to have a basic knowledge of the principles of the techniques involved in Ultrasonic Flaw Detection (UFD). UFD methods may be used to detect sub-surface defects in the majority of solid materials. Ultrasonics can also be used to: • • •

Measure the thickness of materials when it is only possible to get access to one side of the component Test for the delamination (de-bonding) of composite structures Monitor ‘real time’ cracking in spars and struts via Acoustic Emission methods.

The term, ‘ultrasonic’, describes sound oscillations at frequencies too high to be detected by the human ear. Normal, healthy adults are, usually, able to detect sound frequencies in the range between 20 Hz – 20 kHz. For example, the lowest note of a typical, full-size, piano vibrates at approximately 27.5 Hz, while the highest note is in the region of 3.52 kHz. UFD procedures use sound frequencies ranging from as much as 500 kHz to 25 MHz (and, sometimes, more). Sound is caused by the sinusoidal oscillations of the particles in a medium and the speed of sound is fixed in different materials, depending on their elasticity and density. Table 1 shows the speed of sound through some common materials. Table 1 SOUND VELOCITIES IN COMMON MATERIALS Material m/sec ft/sec Air (at 20ºC) 343 1,125 Water (at 20ºC) 1,480 4,854 Perspex 2,680 8,793 Pyrex Glass 5,640 18,500 Steel 5,900 19,351 Aluminium 6,350 20,827 Low-frequency sound travels outwards, from its source, and goes in all directions, whereas the higher the frequency, the more the sound becomes unidirectional until, at the extremely high frequencies employed in UFD, the sound can be considered to be similar to a very narrow beam of light. The principle of UFD is that a narrow beam of sound is introduced into a material and the effects on that beam can indicate the structural state of the material.

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The sound beams, used in UFD, are produced (and detected) by means of a piezoelectric transducer (i.e. a device which converts electrical energy to mechanical energy and vice versa). A piezoelectric ‘crystal’ (formerly quartz but, more commonly, man-made ceramics such as barium titanate or lead zirconate titanate) is made to vibrate when stimulated by electrical energy from the pulse generator of a cathode ray tube (CRT) oscilloscope. At the same time a pulse is generated across the time base of the oscilloscope. The pulse repetition frequency (PRF) is set so that the time base of the oscilloscope appears as a straight line. When the transducer, mounted in a device known as the probe (refer to Fig. 4), is applied to a material, the vibrations cause a narrow beam of ultrasonic waves to be transmitted through the material.

Back Wall Echo

Initial Pulse

Pulse Generator Time Base Controller

Probe Controller

X-plate Amplifier Y-plate

Probe Back Wall Component under Inspection

Couplant between Probe and Inspection Surface Sound Beam and Echo

Simplified UFD System Fig. 4 In a similar manner to radar waves in air (and sonar waves in water) the sound waves travel through the material until they meet an interface with a medium which has a different ‘acoustic impedance’. The acoustic impedance of a material is a function of the density of, and the velocity of sound in, the material.


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At the interface of different acoustic impedances the sound will be reflected (as with the radar and sonar ‘echoes’) in proportion to their differences. It is usual for the majority of sound to be reflected from an interface and the interface can be caused by: • • •

The far face (also called the ‘back wall’), of the component under inspection, with the air on the other side A crack or a void within the material (which will contain air or another gas) An inclusion of a foreign body within the material (such as occurs in welds).

The reflected sound (or echo) returns to the transducer probe, where the energy is converted into an electrical pulse, which is fed (via an amplifier) to the oscilloscope. The amplified pulse causes a peak on the time-base, which is calibrated so that the position of the peak represents the distance the reflected sound has travelled in the material under inspection. Because the transducer crystal is vibrating against the casing of the probe, a great deal of sound is initially reflected within the probe. This is referred to as the ‘initial pulse’ (Americans refer to it as the ‘main bang’) and it is usually placed at the extreme left of the time base, to act as the surface reference, and is not considered as part of the search beam. The face of the probe also creates an interface with the surface of the material under test, due to the microscopic particles of air between them. Because of the vast difference in the acoustic impedance of air compared to other materials, most of the sound would not enter the material, unless a medium, with a closer acoustic impedance to the probe and the material under test, is interposed between them to act as a ‘couplant’. Typical couplants used are fluids in the form of glycerine, silicon grease, petroleum jelly or medium-viscosity oils. With this ‘pulse/echo’ method, the location of a discontinuity in a component can be quite accurately calculated. Unlike the PFD method, it is not only able to detect subsurface flaws but also tight surface flaws which may be filled with oil, grease, paint, rubber or any other contaminants which would create difficulties for the PFD methods. UFD has a greater versatility in that different modes of sound waves are utilised to locate discontinuities occurring in various planes relative to the inspection surface. Those modes of sound include: • • •

Longitudinal or Compressional waves (also called Straight Beam testing) Transverse or Shear waves (also called Angle Beam testing) Surface or Rayleigh waves.

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Because the narrow beams of sound obey some of the physical laws applicable to light, with regard to reflection and refraction, it is necessary to employ Longitudinal sound waves to locate discontinuities which are approximately parallel to the surface which is being ‘scanned’ by the probe (refer to Fig. 5). Compressional Wave Probe

Initial Pulse

Back Wall Echo

(a)

) (a)

)

(b)

)

(c) Echo from Flaw (b)

)

(c)

Reduction of Back Wall Echo

Total Reflection from Flaw with loss of Back Wall Echo

Using Longitudinal Sound Waves (Straight Beam Testing) Fig. 5 Considering the light law, which states that, “the angle of incidence is equal to the angle of reflection” it can be seen that the incident sound will be reflected back to the probe when the maximum dimension of the flaw is in a plane parallel to the scan surface. In instances where flaws are oriented at angles which are either oblique or normal to the scan surface, the sound would not be reflected directly back to the probe and the flaws would be extremely difficult (if not impossible) to detect. For the detection of these types of flaws it will be necessary to use Transverse sound waves, to enable the sound beam to be reflected back to the transducer crystal (refer to Fig. 6).


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Shear Wave Probe

Using Transverse Sound Waves (Angle Beam Testing) Fig. 6 Again considering the laws associated with light, the crystal is angled in such a manner that the Longitudinal waves are refracted out of the material under inspection, leaving only Transverse waves for the search. Note: By fixing the transducer to a more acute angle, it is also possible to refract the waves in such a way that they travel along the surface of the material. In this way Surface or Rayleigh waves may be generated, and used, to detect flaws which are in, but which are normal to, the scan surface. In very acoustic absorptive materials it may be necessary to resort to Through Transmission or ‘Pitch and Catch’ methods (refer to Fig. 7) where two probes are used, with one being a Transmitter (Tx) and another a Receiver (Rx).

Tx Rx

Tx

Rx

(a)

(b)

Through Transmission or ‘Pitch and Catch’ Methods Fig. 7

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Using the Through Transmission methods will ensure that enough sound energy is received to provide useful search information in materials which tend to absorb great amounts of sound. In Fig. 7 (a), it can be seen that two Compressional wave probes are being used, where access to two opposite surfaces is possible, while Fig. 7 (b) shows two Transverse wave probes being used, where access is only available to one surface. The disadvantage of the method, however, is that, while the existence of the flaw can be detected (by the loss of the signal to the Receiver probe), unlike the pulse echo method, the actual location of the flaw cannot be determined, because there will be no peak on the time base to indicate its position. As previously stated, UFD techniques can also be used to measure the thickness (or the loss of thickness in the event of corrosion) of metals and most other materials. The de-bonding or delamination of composite structures can be achieved by either the Through Transmission method (already described) or by means of the ‘Resonance’ technique. This technique uses the fact that a material will vibrate at its maximum amplitude when the sound is at the ‘resonant’ frequency for a given thickness of that material. Thus a certain thickness of properly bonded structure will vibrate at a maximum amplitude at a specific frequency when an ultrasonic transducer is applied to its surface. The amplitude and frequency of the sound can be displayed on an oscilloscope and, when the transducer passes over a de-bonded area, the loss of adhesion will be detected as a change in thickness of the material. This results in a change of amplitude and a shift in the frequency range on the oscilloscope, such that the de-bonded area can be quite easily located. Acoustic Emission techniques involve the placing of piezoelectric transducers at critical positions on spars or struts and the monitoring of the sounds being given out as a crack propagates through the metal. The information is electronically processed and, through appropriate circuitry, can be linked to recording devices or ‘real time’ warning lamps to indicate the progress and severity of the cracking or wear in the particular structure.


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17.5.5 Eddy Current Flaw Detection (ECFD) ECFD procedures are, normally, restricted to specially trained and approved personnel but there may be instances where (in aircraft wheel servicing bays for example) locally trained technicians are approved either to do limited and uncomplicated procedures or to monitor automated ECFD equipment. For these reasons it is necessary for the student to understand the fundamentals of ECFD and the techniques involved in its application. Flaw detection procedures, utilising eddy currents, are employed on electrically conductive materials (metals) and are capable of detecting: • • • •

Surface and (to a limited degree) subsurface cracks: which may be closed by metal flow, oil, paint, rubber or other contaminants Variations in the thickness of metal skins: due to internal corrosion of closed areas, to which normal access is difficult (or impossible) due to time/expense constraints Changes in the electrical conductivity of metals: through incorrect heattreatments or fire damage Cracking emanating from rivet and bolt holes in aircraft skins and structures.

Eddy currents are alternating electrical currents, which are induced, into a conductive material, by an alternating magnetic field. They circulate in the material in a plane normal to the field, which produces them. The basic principle is that a probe (refer to Fig. 8), consisting of a small coil, which is tightly wound around a ferrite core and supplied with alternating current (ac), is held in contact with (or in close proximity to) the surface of the component under inspection. ac Supply

Ferrite Core

Alternating Magnetic Field around Coil

Conductive Material under Inspection

Induced Alternating Electrical Currents

Principles of ECFD Fig. 8

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The magnetic field, associated with the current flowing in the coil, is concentrated into a small area by the ferrite core of the probe. When the probe is placed on the surface of the metal, the alternating magnetic field couples with the metal and induces alternating electrical currents into the metal. These induced electrical currents circulate in the metal in a plane normal to the probe’s coil and, as they are electrical currents, flowing in a conductor, they will also create magnetic fields in the metal. In accordance with Lenz’s Law, the induced magnetic fields will oppose the field which produces them, and, in so doing, will modify the coil’s magnetic field, which hence, affects the electrical impedance of the coil. Any event that changes the value of the induced eddy currents will, subsequently, cause a change in the impedance of the coil. If the coil is included in a bridge circuit (refer to Fig. 9), within an inspection instrument, which is provided with a suitable indicating device, it will provide an indication of the condition of the component under inspection.

L1

L2 Set Zero Control

P1

A

P2

Sensitivity Control

L3

L4

Simplified ECFD Impedance Bridge Circuit Fig. 9


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Using the Wheatstone Bridge principle (where the coil is one of the inductances), when the ratio of L1 : L2 equals the ratio of L3 : L4, the bridge will be in balance and the voltages at points P1 and P2 will be equal. The indicating meter will show ‘zero’, because no current is flowing through it. If the probe is now placed on a metal surface, the induced eddy currents will change the value of L4 and cause an imbalance of the bridge. The voltages at P1 and P2 will be different and a current will flow between them. The meter will show an indication of the current flow. The variable inductance L1 is adjusted to regain the balance of the bridge, so that the meter, once more, is set at zero and the probe is moved over the inspection area. The function of the sensitivity control will be explained a little later. Any factors, which alter the value of the induced eddy currents in the inspection area, will result in an indication on the meter. Factors, which affect eddy currents in a metal (and require careful interpretation), include: • • • • • •

Conductivity Permeability Frequency Proximity Probe Handling Discontinuities.

A change in the conductivity and the magnetic permeability of the metal will obviously influence the induced current flow and subsequent magnetic fields. The frequency, at which the ‘driving’ current of the probe coil alternates, will govern the quality of the induced current and influences the depth of penetration of the eddy currents into the material. The higher the frequency of the coil current, then the closer to the surface of the material the eddy currents remain. This is referred to as the ‘skin effect’. High frequencies of ac are used to ‘drive’ the coils when searching for fine fatigue cracks in the surface of metals, while low frequencies are employed for the coils of the probes used to detect sub-surface defects. Incidentally, an increase in conductivity, or permeability, will also result in a decrease in the depth of penetration, of the eddy currents, in the material under inspection.

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The proximity of the probe to the surface affects the induced currents and this phenomenon can be used to provide a ‘coating thickness’ measurement. With the probe placed on a bare metal test specimen, the inspection meter indicator is set at zero. Known thicknesses of plastic films are then gradually interposed between the probe and the metal. As the thicknesses increase, the amount of ‘lift off’ will be indicated on the meter and these indications can be used to compare with readings, obtained when the probe is positioned on a painted surface of similar metal, to provide a measurement of the coating thickness. Probe handling is also very important and the probe should be maintained at a constant angle to the search surface (preferably normal to the surface). To ensure this, probes are often mounted in purpose-made ‘shoes’, which conform to the surface under inspection (particularly in wheel flanges and bead seat areas) to maintain the correct angle of the probe with the surface. Discontinuities will, of course, cause a change in the induced eddy currents but the type (and severity) of flaws cannot be deduced unless the equipment is properly calibrated before (and during) the inspection procedure. In order to calibrate the equipment, standard reference pieces, manufactured from a material similar to that being tested, are necessary. Aluminium alloy skins, used for engine cowlings, will be affected by exposure to elevated temperatures (up to approximately 500ºC) and the resulting ‘heattreatment’ will cause the material to be below strength, with obvious implications. Above this temperature, signs of heat damage, such as melted or charred metal, will become apparent but, below the temperature, the damage is not easily seen. Conductivity changes in the skins, caused by the heat, can however be detected, using ECFD procedures, and below-strength areas identified for subsequent repair or replacement. A ‘Conductivity Meter’ and a surface probe are used for this particular procedure with the meter being calibrated on materials with known levels of conductivity. The meter, in this procedure, registers relative conductivity as a percentage of the International Annealed Copper Standard (% IACS) where commercially ‘pure’ copper is set at 100% and the conductivity of other alloys, containing copper, is compared to it.


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Silver, which is a better electrical conductor than copper, is considered to have a relative conductivity of approximately 106 % IACS.

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Corrosion of the internal surfaces of aircraft skins can be detected, without resort to time-consuming (and expensive) internal trim removal, and stripping, by the use of low frequency ECFD procedures. When searching for corrosion on the remote surface of a single layer skin, a typical calibration standard might be a simple ‘step wedge’ (refer to Fig. 10).

Step Wedge Calibration Standard

Step Wedge used for Corrosion Detection Fig. 10 The probe frequency would be set so that the eddy currents penetrate the full depth of the thick layer (which represents the correct thickness of the skin) and the instrument is ‘zeroed’ with the probe on the thick layer. The probe is next moved onto the thin part of the wedge and the sensitivity control adjusted to give the required indication. Following calibration, the probe is moved onto the inspection area and a series of parallel scans made. Any significant indications are noted and recorded for appropriate action. For inspections, searching for corrosion in the remote surfaces of multiple layers of skins (such as is found in the lap and butt joints of fuselages), it is then necessary to use a calibration standard with the appropriate number of skins. The frequency of the probe would be adjusted (lowered) to provide the required depth of penetration in each plate being inspected. Low frequency probes may also be used to detect cracks emanating from the rivet holes of multi-layered skins. Specially constructed ‘doughnut’ (or ring) probes, which have a clear, plastic centre, to facilitate accurate placement of the probe over the rivet heads, are used in these procedures.


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Low frequency probes operate in the 100 Hz to upper kHz range, while high frequency probes are driven at frequencies of 5MHz (or more) and have a much greater sensitivity, due to the highly concentrated eddy current field at such frequencies. High frequency probes are used in the search for fine fatigue cracks in the surfaces of metals typically used in aircraft structures and components. They are particularly useful for the inspection of such critical items as: • • • • •

Propeller Blades (both the ferrous and the non-ferrous metal types) Compressor and Turbine blades (and guide vanes) of gas turbine engines. Wheels and Landing Gear struts Window and Door surrounds Front and rear Pressure Diaphragms of pressurised aircraft fuselages

The inspections of these items are achieved with small, portable, batteryoperated machines or with automated installations housed in servicing bays or workshops. A calibration standard for the detection of fatigue cracks would necessitate either a previously failed component with a known defect or (typically) a sample of the relevant metal, containing spark eroded, simulated defects (refer to Fig. 11) against which to calibrate the machine. The probe is placed on unflawed material and the meter ‘zeroed’ before moving the probe over the appropriate slit and adjusting the sensitivity control to provide the specified level of sensitivity for the inspection procedure.

Slit Depths

Three Spark Eroded Slits (0.2 mm Wide)

0.2 mm 0.5 mm 1.0 mm Metal of Similar Specification to Area under Inspection

Typical High Frequency EFD Calibration Standard Fig. 11

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Inspections, of critical bolt holes, are achieved by the use of a (high frequency) hand-held, bolt hole probe. Where large numbers of bolt holes are involved, use is made of a specially designed probe which is attached and rotated by a low torque machine resembling an electric rotary drill. The machine is connected to a CRT oscilloscope, which analyses the phase difference of the changing eddy currents and displays them as lissajous curves (also called ‘flying dots’) on the CRT screen. Phase analysis machines provide much more information regarding the cause of the change in the induced eddy currents and are able to discriminate between the various factors (i.e. changes in conductivity, permeability, geometry etc.), whereas the simple impedance change machines cannot. Phase analysis machines can accurately differentiate between the signals received from corrosion, cracks and scores (and even a crack in the bottom of a score!) such that the trend has moved towards the use of computer-controlled phase analysis machines for all but the most simplest of EFD procedures. 17.5.6 Magnetic Particle Flaw Detection (MPFD) MPFD techniques (in the aerospace industry) are restricted to qualified NDT personnel, usually working in purpose-designed workshops. It is a requirement, however, that the student is aware of the fundamental principles and the applications associated with this method of inspection. As the title implies, MPFD procedures are employed on components and structures which are capable of being magnetised. The procedures are used to locate both surface and sub-surface discontinuities in ferromagnetic metals. The principle involves the setting up of a magnetic field in a component such that, if a discontinuity disrupts the lines of magnetic force (flux lines), the resultant flux leakage (refer to Fig. 12) will create local North and South magnetic poles on the surface of the component. Flux Lines

Flux Leakage (Highly Magnified for Clarity) S N

N

S

Magnetic Flux Leakage Fig. 12


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The flux leakage may be caused by internal flaws, such as inclusions, or by extremely tight fatigue cracks on the surface of the component. Both types of defects would be impossible to see with the naked eye but, when a powder, consisting of finely divided ferromagnetic particles, is applied to the surface of the component, the tiny particles will be attracted to the site of the flux leakage and will provide a visible indication of the discontinuity. To aid detection, the powder may be coloured red, grey, yellow or black (or any colour which provides the best contrast against the inspection surface’s finish). Black is the most common colour and, while powders may be applied dry, by puffers or by sprinkling, the particles are normally suspended in a light oil (usually kerosene), to assist their movement to the flux leakage and (as opposed to using water as the carrier) to minimise the onset of corrosion. The resulting blackened fluid has earned the name of ‘magnetic ink’, but this term is used to describe all of the liquid-carried powders, regardless of their colour. Magnetic inks are also available whereby the particles are treated with powerful fluoro-agents so that, in a similar manner to the fluorescent PFD procedures, they can be viewed under black light, to provide superior sensitivity to flux leakages when inspecting Class 1 aerospace materials. Note; Flux leakages can also be caused by abrupt changes of geometry, such as corners, small radii and screw threads. It, thus, demands that great care is needed in interpreting the indications obtained with MPFD procedures and requires that personnel applying the procedures receive appropriate training. There are various methods of establishing a magnetic field in components but they, basically, fall into the two categories of: • •

Magnetic Flow procedures Current Flow procedures.

The choice of method depends on the geometry of the part under inspection and the expected orientation of any flaw. Maximum sensitivity for flaw detection is obtained when the flaw is normal (90º) to the direction of the lines of flux. Sensitivity is not, however, too greatly reduced when flaws lie at angles up to 45º from the optimum angle but, beyond 45º, sensitivity reduces appreciably. For these reasons, complete inspection of a surface will require that the magnetic field be established in at least two directions, mutually at right angles (in separate applications), to ensure full coverage of the area under inspection.

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Establishing a magnetic field, in a component, using Magnetic Flow (MF) procedures (refer to Fig. 13) involves the use of: • • •

Permanent Magnets: using conventional, portable, U-shaped or ‘horse-shoe’ magnets of varying sizes, depending on the strength of magnetic field required. Electro-magnets: or ‘Yokes’ (once again portable and hand-operated devices), with articulating legs, which can be adjusted to accommodate components of different shapes and sizes. Purpose-built installations: used in workshops and consisting of electromagnetic machines, with adjustable ‘heads’, between which components are clamped so that the magnetic flux flows from one head to the other and through the component under inspection. ON/OFF Switch

ac/dc Supply

Articulating Legs Flux Lines Flux Lines

Discontinuities

Discontinuities (b) Electromagnetic Yoke

(a) Permanent Magnet Flux Lines

Discontinuities

Component under Inspection

Adjustable Head

Fixed Head

(c) Simplified MF Installation

Magnetic Flow Methods Fig. 13 71811175.doc Issued March 2002

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Electrical current, used for the electromagnetic methods (portable types or the fixed installations), can be either ac, or half- or full-wave rectified ac. It may also be provided from a bank of dc batteries. The type of current used will depend on the types of defects, which are being sought, and the metal involved. Sub-surface defects will be detected using dc, or rectified ac, to provide magnetic fields which penetrate to different levels below the surface of the metal (straight dc penetrates the deepest). Unrectified ac, in a similar manner to high-frequency ECFD methods, will create a ‘skin effect’ and is used to locate fine fatigue cracks on the surface of high-grade steel alloy parts. Establishing a magnetic field, in a component, using Current Flow (CF) methods involves the previously-described electrical sources and the use of: • •

Direct methods: whereby the electrical current is passed directly through the component under inspection Indirect or induced methods: which involve the use of rigid rods or bars of metal or lengths of flexible, heavy duty (welding grade) cable, through which the various types of previously-mentioned currents may be passed.

With direct methods of CF inspections it is essential that good electrical contact be achieved between the current-carrying electrodes and the surfaces of the component under inspection, in order to avoid damage from arcing or burning. While direct methods may be used in situ on aircraft structures they are more commonly used in workshops, by qualified personnel, using purpose-built installations, with adjustable ‘heads’ (similar to the MF installations). The parts are clamped between the heads and, in this instance, the specified electrical current is allowed to flow through the component (refer to Fig. 14). Flux Lines

Discontinuities

A

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Both the CF and the MF installations are, in fact, contained in one machine, with the capability (through circuit switching) of selecting either current flow before applying magnetic flow or vice versa, depending on the technique requirements. When current flow is applied to the component, a magnetic field is set up in (and around) that component, in a direction normal (90º) to the direction of the current. This ‘transverse’ (or circular) magnetic field will also be at right angles to the field created by the subsequent (or previous) magnetic flow procedure. Once again, maximum sensitivity for flaw detection will be at 90º to the magnetic field (or in the direction of the applied current!) with a lessening of sensitivity up to 45º from the optimum direction. Using such installations, with both CF and MF capabilities, allows full coverage of the inspection area and, thus, maximises the possibility of locating potential defects in critical components. The parts must always be de-magnetised before, between and after all MPFD inspections. This ensures that there are no interfering or spurious fields within the parts before and between each application. Demagnetisation also allows easier cleaning after the inspection, when the ‘lowretentivity’ magnetic particles will be able to be cleaned from the demagnetised parts. Additionally, demagnetisation must be performed after the inspection, to ensure that the parts will not affect magnetic compasses or attract metal swarf (from oil or fuel systems etc.) which could cause damage when the parts are reinstalled in their respective locations. One of the indirect or induced methods of CF inspections takes advantage of the fact that an insulated metal rod may be threaded through annular or cylindrical components (refer to Fig.15). When current is passed through the central conductor (also called a threading bar), the associated magnetic field couples with, and enters, the component. The induced (circular) magnetic field will allow defects to be detected in the inner and outer walls of the component in addition to indicating any flaws in the end surfaces. Maximum sensitivity will, again, be in the direction of the current flow in the central conductor and normal to the circular magnetic field which is induced into the component.


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engineering Direction of Induced Flux

Discontinuities Insulation around Threading Bar

Direction of Current Flow through Threading Bar

Central Conductor (Threading Bar) Fig. 15 Another of the indirect or induced methods of CF inspections entails the use of heavy-duty, insulated cable, formed into coils (e.g. 4 – 6 turns for ac), which may be wrapped around the component under test (refer to Fig. 16). When current is applied to the coil, its associated magnetic field will couple with the component and, in this instance, a longitudinal magnetic field will be induced into the component. Maximum sensitivity for flaws will, of course, be normal to the longitudinal magnetic field and in the direction of the current flow in the coil.

Flux Lines

Discontinuities Current Flow through Coil

Close-Fitting or Wrapping Coil Fig. 16 Coils may also be used in a similar manner to threading bars, where a one- or two-turn coil may be threaded through the lug of a landing gear strut, for example, to search for stress cracks emanating from the centre of the lug.

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Larger, rigid or ‘aperture’ coils are used in workshop situations, where the component under inspection is laid in the coil and the resulting longitudinal magnetisation will assist in revealing flaws in the component. The aperture coils are also the most commonly used methods of achieving the demagnetisation of components, before, between and after MPFD inspections. When ac is being used, the component is placed in the coil and, while the current is applied, the component is slowly withdrawn (maintaining alignment with the coil’s axis) to a distance of approximately 1.5 m (5 ft) from the coil. Demagnetisation may also be achieved, in an ac coil (and, incidentally, by all the other methods using ac to generate the magnetic field), by gradually reducing the value of the current while the component is lying in the coil. Where dc supplies are used to energise coils, there is usually a capability of reducing the amperage in specified increments whilst the direction of the current is alternately switched. This form of demagnetisation is the most thorough as the dc-induced field penetrates deepest in components under inspection. Following demagnetisation a confirmatory test is made, using a special flux meter (or a suitable magnetic compass) before the component is thoroughly cleaned of all traces of magnetic ink. Components should then be stored away from all magnetic sources and any surface protection restored while awaiting reinstallation, repair or replacement. 17.5.7 Radiographic Flaw Detection (RFD) Because the misuse of radiographic equipment could result in the release of physically harmful (ionising) radiation, operators must be trained and aware of the many safety regulations and codes of practice associated with these procedures. Aircraft RFD inspections are, therefore, only done by qualified NDT personnel from organisations approved under BCAR A8, and who are nationally registered as radiation workers. These workers are subject to frequent medical checks and wear sensitive film badges to detect any radiation dosage to which they may become exposed. Should the dosage exceed stringent limits, then the worker is withdrawn from tasks involving ionising radiation. Interpretation of radiographic images is also very important, as incorrect conclusions could result in the acceptance of unserviceable structures or, conversely, in the scrapping of safe structures. Students are, however, required to have knowledge of the fundamental principles of RFD and its applications in aerospace inspections.


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Due to the hazards of radiation, it will be necessary to isolate the aircraft and to keep personnel at a safe distance from the inspection area. The area should be roped off, with radiation warning signs (a black trefoil against a yellow background), clearly shown. Flashing lights and horns (or klaxons) are also employed to signify that ionising radiation sources are in use in the area and that unauthorised personnel should keep away. The main sources of ionising radiation, used in aerospace RFD procedures, involve the use of either X- or Gamma-ray emitters. Both rays have the ability to penetrate materials, which cannot be penetrated by visible light and are identical forms of energy in the electromagnetic spectrum (refer to Fig. 17). Their difference lies only in the way in which they are generated and the names given them by the scientists who discovered them.

Infra-red

Radar

Radio

Rays

TV

UV Rays White Light

Cosmic Rays

X & Gamma Rays

1m 10 m

0.01 m 0.1m

100 µ m

0.001 m

1µ m 10 µ m

10 nm

100 nm

0.1 nm 1nm

10 -3

0.01 nm

nm

10 -5 nm

10 -4 nm

10 -7 nm

10 -6 nm

The Electromagnetic Spectrum Fig. 17 X-rays are electrically produced in a cathode ray tube, (refer to Fig. 18) which accelerates electrons, released at the cathode, towards a dense metal (tungsten) target which is embedded in the anode. The amount of electrons, freed at the cathode, is controlled by adjustment of the tube current, which is measured in milliamps (mA). The force of the collisions and, hence, the penetrating power of the X-rays, is controlled by the potential difference between the cathode and the anode. The tube voltage is measured in kilovolts (kV). Typical aerospace RFD procedures use machines generating X-rays in the range of 10kV to 250kV, depending on the penetrative power required.

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Potential Difference (kV) Free Electrons Cathode -ve

Anode +ve

Tube Current (mA)

Evacuated Glass Envelope

Tungsten Target set in Copper Anode X-Ray Beam

Simplified X-Ray Tube Fig. 18 On striking the target, the electrons collide with the atomic particles of the tungsten and the resulting energy exchange is released as heat, light and a small percentage of ‘packages’ (photons) of energy. A large copper anode, in which the target is mounted, dissipates the heat, generated by the electron stream. The anode and the glass envelope are, in turn, cooled by circulating gas, oil or water being pumped around them. The amount of light is confined to a dull red glow at the target and is insignificant. The photons, however, behave in a similar manner to rays of light in that they travel in straight lines. They also obey the ‘inverse square law’ in that, as they travel from the source, their area of incidence increases but their intensity decreases in a ratio to the square of the distant from the source. One important safety aspect is that X-rays are generated electrically and, hence, can be switched off, either automatically, by a timer switch, or manually, by the NDT person at the control panel. Gamma radiation is the name given to the photons of energy which result from the atomic collisions occurring during the disintegration of radioactive isotopes. An isotope is an element, the nucleus of which gains or loses particles such that the atomic weight changes though the chemical properties remain unchanged.


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The naturally occurring isotopes of many elements are quite stable in their changed state, while others become unstable and, in attempting to regain a stable state, emit photons of energy (become radio-active) due to the nucleic particle collisions. Gamma radiation cannot be switched off (it can only be shielded) and continues until the element reaches a stable condition.

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Gamma radiation sources, used in RFD procedures, include artificially created isotopes such as Cobalt 60, Iridium 192 and Ytterbium 169 (the number following the element name is the atomic weight or relative atomic mass of the isotope). As previously stated, Gamma rays can exist over similar wavelengths as Xrays with similar properties but, generally, the Gamma rays, emitted by the commonly used radio-isotopes, have a greater penetrative power. Their power does, however, decrease with the passage of time as the isotope attempts to gain a stable state. The decrease in power over time is referred to as the ‘half value period’ (also called ‘half life’) and is a measure of the time when the activity of the isotope will decay to half that of its original or former value. Table 2 shows some typical radio-isotopes, their penetrative power (compared to X-rays), the metal thicknesses through which they can penetrate and their half value periods. Table 2 GAMMA-RAY SOURCES COMPARED TO X-RAY GENERATORS RadioPenetrative Power Thickness of Metal Half Value Isotope (compared to X-rays) (capable of penetrating) Period Ytterbium 169 0.15 – 0.40 MeV 2 mm – 30 mm Steel 32 days Iridium 192 0.31 – 1.20 MeV 6 mm – 100 mm Steel 74 days Thulium 170 0.083 – 0.96 MeV 2 mm – 12 mm Aluminium 134 days Cobalt 60 1.17 – 1.33 MeV 50 mm – 200 mm Steel 5.3 years Caesium 137 0.66 – 0.67 MeV 25 mm – 100 mm Steel 26.2 years The advantage of using a radio-isotope as opposed to an X-ray machine lies not only in the fact that the penetrating power is superior but also the equipment is less bulky than the X-ray machines and there is no requirement for a mains electrical supply. The radioactive source is usually so small that it can be placed inside objects such as engine shafts, using small diameter, flexible guide tubes. The disadvantages in their use is the need for greater protection measures, involving larger isolation areas, and the possibility of longer exposures as the isotopes decay in strength with the passage of time. Another drawback, and particularly so with the more powerful Cobalt 60 source, is that the resulting radiographs tend to be less sensitive than those created with low-power X-rays. The radiograph is produced when the appropriate radiation source is directed at the components under inspection and a light-proof envelope (or cassette), containing a sheet of radiographic film, is placed on the remote side of the component (refer to Fig. 19). Depending on the size of the inspection area there may be several sheets of film envelopes, placed simultaneously, to provide adequate coverage at one exposure. 71811175.doc Issued March 2002

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Radiation Source (X- or Gamma Rays)

Metallic Step Wedge Plastic Film Base (Emulsion on Both Sides

Production of a Radiograph Fig. 19 The films, used in radiography, are very similar to those used in photography except that the emulsion is applied to both sides of the transparent, plastic base. The emulsion contains microscopic grains of silver halides, which are sensitive to the photons of energy (and light). When exposed to the rays, a change takes place in the silver particles such that, when the film is developed and ‘fixed’, they turn into different degrees of dark metallic compounds, depending on the amount of radiation they have received. The silver which has not received radiation is unaffected and is washed away in the developing and fixing processes. A negative-type image of the component is thus formed, the darkness (density) of which depends on the quantity of radiation passing through the specimen. The thicker the specimen, the more radiation it will absorb and, consequently, it will provide a lighter (less dense) image. The accurate interpretation of defects, indicated on a radiograph, requires a great deal of skill and a good knowledge of the aircraft structure. Without the knowledge it would be easy for the NDT technician to overlook faults such as distorted or missing parts. Even the presence of leaded fuel in the tank of a piston-engined aircraft can mask defects. The interpretation may be simplified if radiographs of serviceable structure are available for comparison.

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Radiographic inspections are often done during the manufacturing stages, to check for such faults as: • •

Metallurgical defects in castings and welds: these produce patterns recognisable by an experienced viewer. Porosity will reduce the amount of material through which the rays must pass and a darker image will result Cracks in welds: these are difficult to detect, as the angle at which the radiograph is taken is important. The source should be absolutely normal to the direction of the suspected crack.

Typical RFD procedures, done during routine aircraft maintenance, include the search for: •

•

• •

Corrosion: this will show up as a fuzzy image, but the presence of paint and jointing compound will make it difficult to detect. Inter-granular corrosion may not be detected until it has reached an advanced state and affects the metal surface. A corrosion pit, where there is a change in thickness, is more readily detected Cracks: stress cracks often run along a line of rivets, but the edge of jointing compounds, used during the wet assembly of riveted joints, often gives a false indication. Radiographs may show indications of cracks, which, eventually, are found to be cracks in tank sealant. It is sometimes possible to open up cracks, before inspection, by applying a tension load by jacking Loose articles, riveting faults and poor assembly techniques Levels of fluids in accumulators and the presence of water in composite materials.

17.5.8 Miscellaneous Radiation Techniques Other techniques, which involve the use of radiation of one type or another, include: •

Fluoroscopy: using either of the previously mentioned sources the standard sheet film is replaced by a fluorescent screen. This enables moving images to be captured. For safety reasons a video camera is focused on the screen and the image viewed at a safe distance. An example of 'fluoroscopy' is where oscillation in a turbine shaft of gas turbine engine being 'run' can be observed. A more common example of its use is provided by the low-energy X-raying of luggage, at airport departure security 'check-ins'

•

Thermography: using heat radiation, a heat-sensitive camera is used to inspect areas of aircraft in particular composites. In the passive mode the aircraft is inspected shortly after landing and temperature 'cold' spots will indicate de-lamination or osmosis. The active mode consists of microwave radiations being targeted at suspect components with the area being inspected by the camera in the same way as the passive mode.


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17.6 Disassembly and Re-assembly Techniques Disassembly and re-assembly, in the terms of aircraft, can cover a range of activities from complete airframes down to component maintenance, with several steps in between. The reasons for dismantling and re-assembly may include: • • • • •

Complete airframe disassembly for road/air shipment Replacement of major components/modules Replacement of minor components/modules Disassembly & re-assembly of major components Disassembly & re-assembly of minor components

17.6.1 Complete Airframes It may be necessary to dismantle a complete aircraft for the purpose of transportation by road or by air. This could be for recovery from an accident site, remote from the airfield or for movement of the aircraft when it is totally nonairworthy, due perhaps to severe corrosion or an unknown maintenance history. Because many larger, modern aircraft are manufactured at several different locations, the completed modules are assembled in the final build hall of the primary manufacturer. The joining points are often known as ‘transportation joints’, and, in extreme instances, can be the points where the aircraft may be dismantled again to allow transportation (refer to Fig. 20).

Typical Manufacturer’s Joining Points Fig. 20

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The instructions for the dismantling operation will be found either in the aircraft’s Maintenance Manual or in a special dismantling procedure, issued by the manufacturer. During the dismantling operation, precautions must be taken to prevent injury and damage. General precautions would include such items as: • •

•

•

The aircraft should, if at all possible, be dismantled within a hangar. If this is not possible, then level and firm ground will suffice Sufficient clearance in the hangar must be available, both to clear the airframe when on jacks, and to allow heavy lifting cranes enough room to manoeuvre over the aircraft All precautions, in accordance with the manufacturer’s instructions, must be taken prior to the aircraft entering the hangar; such as de-fuelling and the removal of devices such as emergency oxygen canisters When the aircraft is jacked-up, all trestles must also be placed in position. This allows the aircraft to be climbed upon and, later, ensures that it will not overbalance when a major part (such as a wing), is removed.

Whilst the manufacturer’s instructions will give the details for a specific aircraft, the following sequence of dismantling gives an idea of the correct order of disassembly: • • • • •

Main planes Tail unit Undercarriage units Centre section Fuselage.

Re-assembly is in the reverse order of disassembly, and all parts must be cleaned, protected and serviceable, prior to their installation. 17.6.2 Replacement of Major Components/Modules This type of operation will normally be completed at a large maintenance base, where all the required equipment is available. An example could be the replacement of a wing that has suffered major damage. Other types of similar work might be the replacement of damaged wing tips, empennage surfaces and nose cones. If the aircraft is at an ‘outstation’ when the damage occurs, confirmation should be sought as to whether the aircraft can be flown back to base for repair, or repaired where it is.


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17.6.3 Replacement of Minor Components/Modules Some components have to be repaired by replacement. Repairs to flying control surfaces, for example, are often done in a repair bay. The component may be replaced on the aircraft by a serviceable item, or reinstalled once the repair in the bay has been completed. Other components, which are replaced first and repaired later, might include some structural items such as doors of all types, and most fairings and cowlings. Most of these tasks are simple removal and replacement operations that are covered in the AMM. 17.6.4 Disassembly and Re-assembly of Major Components Most of the work done, during this phase of maintenance, is scheduled in with normal aircraft maintenance. The components may not only be removed and reinstalled at different times during the maintenance, but work will also be done on the items whilst they are removed. They may also be removed to allow access to other parts of the airframe during the maintenance. Items such as engines, propellers, landing gears and wheels require some form of maintenance. This may include a simple condition check, or a full overhaul of its component parts, allowing checks on internal component parts for wear, damage and corrosion. The full procedure for this type of work will be carried out in accordance with the CMM. This book will give all the operations required to dismantle the component and will advise what to look for whilst the item is undergoing maintenance. It will also state the re-assembly method, including the fitting of new parts such as seals, gaskets, oil and other consumables that have to be replaced, during overhaul. 17.6.5 Disassembly and Re-assembly of Minor Components A typical passenger aircraft can contain hundreds of small components that work together as parts of a larger system. This can include a wide range of hydraulic and pneumatic components that can be mechanical, electromechanical or electrical in operation. Other components might include those installed into fuel, air conditioning, pressurisation, electrical and electronic systems. These components have their own CMM to allow maintenance and trouble-shooting to be done. Some components are only removed once they fail (On-Condition), while others receive regular maintenance.

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Instruments, electric and electronic components can be dismantled and serviced by the aircraft operator. It normally requires the use of a dedicated overhaul facility, which can provide the correct environmental conditions and is equipped with the special test equipment required to carry out maintenance and repair. Operators of smaller aircraft, or those who operate only a few aircraft, will usually send components requiring repair or maintenance to a ‘third party’ maintenance organisation. This company will have the special facilities, equipment and personnel, to complete the required work on components from a number of different customers. 17.6.6 Basic Disassembly and Re-assembly Techniques All of the previously mentioned procedures require the use of the correct techniques over a wide range of working practices. These techniques will ensure that the components are removed, dismantled, re-assembled and re-installed in accordance with both the relevant manuals and using the correct ‘standard practices’. An AMM and CMM dictate the correct type and size of locking wire or split pin to be used during overhaul or maintenance of a component. These publications also stipulate exact detail of items such as the lock wire angle of approach and the correct positioning of a split pin. AMM chapters 20 and 70 list the standard practices that should be used during overhaul. Other locking devices include items such as single tab washers, shake-proof washers, circlips and locking rings. Some can only be used once only whilst others are re-used provided they are still serviceable. The replacement of spring washers is ‘advised’ during overhaul and repair, especially on engines and pumps. Other devices used for locking or holding fasteners in position, such as multi-tab washers and locking plates can normally be reused. Stiff nuts with fibre or nylon inserts can be checked to ascertain if a certain degree of stiffness is still available. If the nut can be run along a thread by hand it should be replaced. They should not be used in high temperature areas. In all matters relating to aircraft, the manufacturer has the final say on which fasteners can be reused and which must be replaced. Because friction is essential to keep the fasteners secure, sometimes it is necessary to do a ‘torque check’ on the bolt/nut combination, in order to confirm their continuing serviceability. This is especially true of all metal fasteners that can normally be re-used.


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The majority of nuts, bolts and set-screws, on an aircraft, are subject to a standard torque value. This depends on their material, finish, lubrication, thread type and size, although the manufacturer’s torque value will be the correct one to use. The correct torque loadings are normally applied using a torque wrench that has been previously calibrated to the correct value. In some special instances, preload indicating (PLI) washers may be specified. When assembling any component or major airframe part, the manufacturer will specify whether the torque value is ‘lubricated’ or ‘dry’. Lubricated values are measured with the threads and all mating surfaces lightly lubricated with oil, sealant or anti-seize compound as appropriate. When assembling some components, it may be vital that certain alignments, dimensions or profiles are achieved. During initial production, most of the airframe and many of the components are assembled in a jig. A jig is device that allows the manufacture, repair or rigging of components to a high dimensional accuracy. This guarantees consistency over a number of components. The jig holds all of the items securely, so that, when assembled, the whole component is exactly the shape that the designer has stipulated. Jigs are used to build fuselage and wing sections in the factory. They are also used to ensure that small actuators are pre-set to the exact length, to assist in ‘rigging’ the controls containing the actuator. 17.6.7 Small Part and Component Identification When disassembling or removing any component, it is vital that all small items such as bolts, screws, nuts, washers and shims are clearly identified. This involves not only identifying the items by part number, but also recording their correct location and which aircraft they have been removed from as, in some hangars, more than one aircraft may be in a state of disassembly at any one time. Some items may simply be attached to the major assembly using many small ‘tie on’ bags with identification labels. If a number of different sized fasteners are removed from a component such as a windscreen, they can be located in a locally-made holding jig which keeps the different parts in their same relative position to the original item. This should allow all the screws to be returned to their original locations when the screen is reinstalled. Any part which is removed must have its identity and location retained until it is reinstalled.

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17.6.8 Discarding of Parts A number of items, when they are removed from their original position, have to be discarded. The ‘once only’ policy is a combination of the manufacturer’s recommendations together with normal engineering practice. Items that are usually discarded at removal are filters, sealing rings, desiccants, fuels and oils of all types. There are many other items that have a given ‘life’. This may be counted in flying hours, calendar time or operating cycles, which will mean that items have to be replaced throughout the life of the aircraft. If aircraft, or major components of them, have been disassembled, it may be policy to replace components with ‘zero life’ items prior to re-assembly. This will allow the aircraft to fly for considerable time before any parts become due for replacement. 17.6.9 Freeing Seized Components When dismantling any part of an aircraft, it is not unusual for the technician to encounter a seized fastener. Depending upon its location, the AMM may recommend a range of actions to assist in the removal of the item(s). These actions may involve use of penetrating oil, which works its way down through the seized threads, providing both an anti-corrosion action and lubrication for the threads. Other actions may involve the application of heat or cold to a specific part, so that their relative diameters change, thus lowering the friction between the parts. 17.6.10 Use of Correct Tools It is normal for technicians to own a comprehensive tool kit, containing tools recommended for the work planned and which are of the highest quality. In a number of situations, it may be possible for a technician to use an incorrect tool that may appear to be the correct one for the task. It is most important that only the correct tools are used for each and every maintenance task. When, for example, using a cross point screwdriver, it is possible to find at least six different types of screw bits, each of which only fits its own respective screw head slot. The removal of nuts from bolts is normally accomplished using a socket and wrench set and these sockets can be of the twelve-point or six- point type. When spanners need to be used, preference should be given to a ring spanner rather than an open-ended spanner. Adjustable spanners or ‘mole grips’ should never be used on aircraft. 71811175.doc Issued March 2002

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The manufacturer of the aircraft often specifies special tools, when standard tools are unable to complete the task. Unless approved to do so, the technician should never substitute conventional tools for the special tools that are called for in the manual. Damage to the part being worked upon will almost always result from the use of incorrect tools. 17.6.11 ‘Murphy’s Law’ This ‘law’ states that: ‘If a part or component can be installed incorrectly, someone, somewhere will install it that way.’ There are numerous solutions in the fight against this problem. For example, when a pair of pipes or hoses are to be joined, there is the risk of the two pairs of couplings being ‘cross connected’. This could result in serious damage if the pipes carried fuel and hydraulic oil. To prevent this happening, pipes and couplings usually have different diameters. Alternatively, the two sets of couplings would be located at different places, so the pipes could not be wrongly connected under any circumstances. The same logic is applied to control cables that, of course, must also never be cross-connected. In this instance, the turnbuckles are located at slightly different locations at each cable break, again making it impossible to connect the wrong pair of cables together.

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18 ABNORMAL EVENTS All aircraft are designed to withstand the normal flight and landing loads expected during a typical flight cycle. These loads will include the normal manoeuvres the aircraft is expected to make. The designer will build in a safety factor to compensate for loads slightly larger than normal. Sometimes extreme circumstances occur which cause stresses outside the normal design limits. If the design limits are exceeded, then damage may occur to the aircraft. If it is known or suspected that the aircraft has been subjected to excessive loads, then an inspection should be made, to ascertain the nature of any damage that may have occurred. The manufacturer will normally have anticipated the nature of some of these occurrences and detailed special checks for these ‘Abnormal Occurrences’. 18.1 Types of abnormal occurrences The aircraft maintenance manual will normally list the types of abnormal occurrences needing special inspection. The list may vary, depending on the aircraft. The following items are a selection from a typical aircraft: • • • • • • • • • • •

Lightning strikes High-intensity radiated fields penetration Heavy or overweight landing Flight through severe turbulence Burst tyre Flap or slat over-speed Flight through volcanic ash Tail strike Mercury spillage Dragged engine or engine seizure High-energy stop.

18.2 Types of damage It is not intended to describe the types of damage applicable to every type of occurrence. It is more important to understand that, often, the damage may be remote from the source of the occurrence. In many cases the inspection would be made in two stages. If no damage is found in the first stage then the second stage may not be necessary. If damage is found, then the second stage inspection is done. This is likely to be a more detailed examination.


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18.3 Lightning Strikes Both lightning strikes and high-intensity radiated fields (HIRF) are discussed in Module 5. Consideration is given in this topic to their effects and the inspections required in the event of their occurrence. Lightning, of course, is the discharge of electricity in the atmosphere, usually between highly charged cloud formations, or between a charged cloud and the ground. If an aircraft is flying in the vicinity of the discharge or it is on the ground, the lightning may strike the aircraft. This will result in very high voltages and currents passing through the structure. All separate parts of the aircraft are electrically bonded together, to provide a lowresistance path to conduct the lightning away from areas where damage may hazard the aircraft. 18.3.1 Effects of a Lightning Strike Lightning strikes are likely to have two main effects on the aircraft: •

Strike damage where the discharge enters the aircraft. This will normally be on the extremities of the aircraft, the wing tips, nose cone and tail cone and on the leading edge of the wings and tailplane. The damage will usually be in the form of small circular holes, usually in clusters, and accompanied by burning or discoloration. • Static discharge damage at the wing tips, trailing edges and antenna. The damage will be in the form of local pitting and burning. Bonding strips and static wicks may also disintegrate, due to the high charges. 18.3.2 Inspection The maintenance schedule or maintenance manual should specify the inspections applicable to the aircraft but, in general, bonding straps and static discharge wicks should be inspected for damage. Damaged bonding straps on control surfaces may lead to tracking across control surface bearings, this in turn may cause burning, break up or seizure due to welding of the bearings. This type of damage may result in resistance to movement of the controls, which can be checked by doing a functional check of the controls. Additional checks may include: •

Examine engine cowlings and engines for evidence of burning or pitting. As in control bearings, tracking of the engine bearings may have occurred. Manufacturers may recommend checking the oil filters and chip detectors for signs of contamination. This check may need to be repeated for a specified number of running hours after the occurrence.

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• •

Examine fuselage skin, particularly rivets for burning or pitting. If the landing gear was extended, some damage may have occurred to the lower parts of the gear. Examine for signs of discharge. • After the structural examination it will be necessary to do functional checks of the radio, radar, instruments, compasses, electrical circuits and flying controls. A bonding resistance check should also be done. 18.4 Example of a Post Lightning Strike Procedure This procedure is an extract from the Boeing 757 Maintenance Manual. It is included to give an idea of a typical aircraft inspection procedure. Not all of the details have been supplied, but there is enough information to provide a general idea. The student will not be examined in detail on this procedure, but should be able to identify specific checks that highlight the previous notes. This procedure has these three tasks: • • •

Examination of the External Surfaces for Lightning Strike Examination of the internal Components for Lightning Strike Inspection and Operational Check of the Radio and Navigation Systems.

18.4.1 Basic Protection The aircraft has all the necessary and known lightning strike protection measures. Most of the external parts of the aircraft are metal structure with sufficient thickness to be resistant to a lightning strike. This metal assembly is its basic protection. The thickness of the metal surface is sufficient to protect the internal spaces from a lightning strike. The metal skin also gives protection from the entrance of electromagnetic energy into the electrical wires of the aircraft. The metal skin does not prevent all electromagnetic energy from going into the electrical wiring; however, it does keep the energy to a satisfactory level. If lightning strikes the aircraft, then all of the aircraft must be fully examined, to find the areas of the lightning strike entrance and exit points. When looking at the areas of entrance and exit, this structure should be carefully examined to find all of the damage that has occurred.


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18.4.2 Strike Areas Lightning strike entrance and exit points (refer to Fig. 1) are, usually, found in Zone 1, but also can occur in Zones 2 and 3. Lightning strikes can, however, occur to any part of the aircraft, including the fuselage, wing skin trailing edge panels. wing-body fairing, antennas, vertical stabiliser, horizontal stabiliser, and along the wing trailing edge in Zone 2.

A

A&B

Zone 1. High Possibility of Strike Zone 2. Average Possibility of

Strike

Zone 3. Low Possibility of Strike A = Aerials and Protrusions B = Sharp Corners of Fuselage and Control Surfaces

Risk Areas for Lightning Strikes Fig. 1

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18.4.3 Signs of Damage In metal structures, strike damage usually shows as pits, burn marks or small circular holes. These holes can be grouped in one location or divided around a large area. Burned or discoloured skin also shows lightning strike damage. In composite (non-metallic) structures, solid laminate or honeycomb damage shows as discoloured paint. It also shows as burned, punctured, or de-laminated skin plies. Hidden damage can also exist. This damage can extend around the visible area. Signs of arcing and burning can also occur around the attachments to the supporting structure. Aircraft components made of ferromagnetic material may become strongly magnetised when subjected to large currents. Large currents, flowing from the lightning strike in the aircraft structure, can cause this magnetisation. 18.4.4 External Components at Risk A lightning strike usually attaches to the aircraft in Zone 1 and goes out a different Zone 1 area. Frequently, a lightning strike can enter the nose radome and go out of the aircraft at one of the horizontal stabiliser trailing edges. External components most likely to be hit are the: • • • • • • • • • • •

Nose Radome Nacelles Wing Tips Horizontal Stabiliser Tips Elevators Vertical Fin Tips Ends of the Leading Edge Flaps Trailing Edge Flap Track Fairings Landing Gear Water Waste Drain Masts Pitot Probes


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18.4.5 Electrical Components at Risk Lightning strikes can cause problems to the electrical power systems and the external light wiring The electrical system is designed to be resistant to lightning strikes but a strike of unusually high intensity can possibly damage such electrical system components as the: • • • • •

Fuel valves Generators Power Feeders Electrical Distribution Systems Static Discharge Wicks

NOTE: Should inaccuracies in the standby compass be reported, after a lightning strike, then a check swing will be necessary. Frequently, a lightning strike is referred to as a static discharge. This is incorrect and may create the impression that the metal static discharge wicks, found on the external surfaces of the aircraft prevent lightning strikes. These static discharge wicks are for bleeding off static charge only; they have no lightning protection function. As the aircraft flies through the air, it can pick up a static charge from the air (or from dust/water particles in the air). This static charge can become large enough to bleed off the aircraft on its own. If the charge does not bleed off the aircraft on its own, it will usually result in noise on the VHF or HF radios. The static discharge wicks help to bleed the static charge off in a way that prevents radio ‘noise’. The static discharge wicks are frequently hit by lightning. Some personnel think static dischargers are for lightning protection. The dischargers have the capacity to carry only a few micro-Amps of current from the collected static energy. The approximate 200,000 Amps from a lightning strike will cause damage to the discharge wick or make it totally unserviceable. 18.4.6 Examination of External Surface Examine the Zone 1 surface areas for signs of lightning strike damage. Do the examinations that follow: • •

Examine the external surfaces carefully to find the entrance and exit points of lightning strike. Make sure to look in the areas where one surface stops and another surface starts.

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Examine the internal and external surfaces of the nose radome for burns, punctures, and pinholes in the composite honeycomb sandwich structure. Examine the metallic structure for holes or pits, burned or discoloured skin and rivets. Examine the external surfaces of the composite components for discoloured paint, burned, punctured, or de-laminated skin plies. Use instrumental NDI (NDT) methods or tap tests to find composite structure damage which is not visible. Note: Damage, such as de-lamination can extend to the areas around the damage area which is not visible. De-lamination can be detected by instrumental NDI methods or by a tap test. For a tap test, use a solid metal disc and tap the area adjacent to the damaged area lightly. If there is delamination, it will produce a sound that is different to the sound of a solid bonded area.

• • • • • •

Examine the flight control surfaces for signs of strike damage. If the control surfaces show signs of damage, examine the surface hinges, bearings and bonding jumpers for signs of damage. If the ailerons show signs of a lightning strike, examine the surface hinges, bearings, and bonding jumpers for signs of damage. If the speed brakes show signs of a lightning strike, examine the surface hinges, bearings, and bonding jumpers for signs of damage. If the trailing edge flaps show signs of a lightning strike, examine the surface hinges, bearings, and bonding jumpers for signs of damage. If the leading edge flaps/slats show signs of a lightning strike, examine the surface hinges, bearings, and bonding jumpers for signs of damage. Examine the nose radome for pin-holes, punctures and chipped paint. Also ensure bonding straps are correctly attached. Examine the lightning diverter strips and repair or replace them if damaged. If there is radome damage, examine the WXR antenna and wave-guide for damage.

18.4.7 Functional Tests Functional tests will need to be done as follows: • • • • •

Ensure the navigation lamps, rotary lights and landing lights operate. If the previously mentioned control examinations show signs of damage: Do an operational test of the rudder if there are signs of lightning strike damage to the rudder or vertical stabiliser. Do an operational test of the elevator if there are signs of lightning strike damage to the elevator or horizontal stabiliser. Do an operational test of the ailerons if there are signs of lightning strike damage to the ailerons. Do an operational test of the speed brakes if there are signs of lightning strike damage to the speed brake system.


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Do an operational test of the trailing edge flaps if there are signs of lightning strike damage to the trailing edge flaps. Do an operational test of the leading edge flap/slats if there are signs of lightning strike damage to the trailing edge flap/slats. If there are signs of strike damage to the landing gear doors, disengage the main gear door locks and manually move the doors to ensure they move smoothly. Visually examine the door linkage, hinges, bearings and bonding jumpers for strike damage. Ensure the proximity switch indication unit gives the correct indication.

18.4.8 Examination of Internal Components If a lightning strike has caused a system malfunction, do a full examination of the system. • • • •

• • • • • • • • • •

Do a check of the stand-by compass system if the flight crew reported a very large compass deviation. Make sure the fuel quantity system is accurate. This can be achieved by a BITE test. Examine the air data sensors for signs of strike damage. Do an operational test of the pitot system if there are signs of damage to the probes. Do a test of the static system if there are signs of damage near the static ports. Do an operational check of any of the following systems that did not operate following the strike, or if the flight crew reported a problem, or if there was any damage found near the system antenna: HF communications system VHF communications system ILS navigation system Marker beacon system Radio altimeter system Weather radar system VOR system ATC system DME system Automatic Direction Finder (ADF) system

If one or more of the previous systems have problems with their operational checks, examine and do a test of the coaxial cables and connectors.

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18.4.9 Return the Aircraft to Service After all areas have been inspected and lightning damage has been repaired, components replaced as necessary and tests completed if necessary, the aircraft may be returned to service. 18.5 High Intensity Radiated Fields (HIRF) Penetration Module 5 discusses electromagnetic phenomena, in particular the problem of electromagnetic interference. HIRF may be generated by airborne transmitters such as high-powered radar or radio. to commercial aircraft. Increased use of digital equipment has increased the problem. HIRF can be generated from an internal (within the aircraft and its systems) or external source (i.e. HIRF may be transmitted by military aircraft in close proximity). All of the systems which might cause, or be affected by, HIRF, must be suitably protected. Electronic developments have yielded greater miniaturisation and complexity in integrated circuits (IC) and other electronic circuitry and assemblies, increasing the probability of electromagnetic interference. Rapid advances in technology and the increased use of composite materials and higher radio frequency (RF) energy levels, from radar, radio, and television transmitters, have substantially increased the concern for electromagnetic vulnerability of flight critical systems, relative to their exposure to HIRF. Environmental factors such as corrosion, mechanical vibrations, thermal cycling, damage and subsequent repair and modifications can potentially degrade electromagnetic protection. Continued airworthiness of these aircraft requires assurance that the electromagnetic protection is maintained to a high level by a defined maintenance programme. HIRF can interfere with the operation of the aircraft’s electrical and electronic systems by coupling electromagnetic energy to the system wiring and components. This can cause problems relating to the control systems, both of the aircraft and its power- plants, the navigation equipment and instrumentation. Design philosophies in the area of aircraft bonding for protection against HIRF can employ methods that may not have been encountered previously by maintenance personnel. Because of this, the HIRF protection in the aircraft can be unintentionally compromised during normal maintenance, repair and modification. It is critical that procedures, contained in the AMM/CMM, reflect reliable procedures, to detect any incorrect installations, which could degrade the HIRF protection features.


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There are three primary areas to be considered for aircraft operating in HIRF environments: • • •

Aircraft Structure (Airframe Skin and Frame). Electrical Wiring Installation Protection (Solid or Braided Shielding Connectors). Equipment Protection (LRU case, Electronics Input Output Protection).

Visual inspection is the first and generally most important step in HIRF maintenance. If errors have been made that do degrade the protection (paint over spray and incorrect assembly of connectors for example), then they should be found during inspections. Whilst the visual inspection may suffice for observation of the deterioration of the protective features, any time that this method is found to be insufficient or inefficient, then specific testing may be required. These techniques should make use of easy-to-apply, quick-look devices that can be readily integrated into the normal maintenance operations. 18.5.1 Specific Testing – HIRF The milliohmmeter is often used to measure the path resistance of earthing straps or other bonding. This technique is limited to the indication of only single path resistance values. The Low-frequency Loop Impedance testing method complements dc bonding testing and it can be used together with visual inspection. It can give good confidence in the integrity of the shielding. This loop impedance testing can be used to check that adequate bonding exists between braiding/conduits and the aircraft structure, especially where there are multiple earth paths, when the dc resistance system will not indicate which earth has failed. The frequency of any maintenance tasks selected for the HIRF protection features should be determined by considering the following criteria: • • • • •

Relevant operating experience gained. Exposure of the installation to any adverse environment. Susceptibility of the installation to damage. Criticality of each protective feature. (within the overall protection scheme) The reliability of protective devices fitted to equipment.

Table 1 gives some indication as to the maintenance tasks that may be applied to certain types of electromagnetic protection features. ‘Raceway’ conduits are separate conduits containing individual cables to the various aircraft systems while ‘RF gaskets’ have conducting properties.

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Table 1 HIRF PROTECTION MEASURES Applicable Maintenance Tasks for HIRF Protection Measures Aircraft Structure Shielding

Protection Type

Cable Shielding

Description

Over braid shield, critical individual cable shield Metallic conduit braid

Raceway conduits

RF gasket

Shield for nonconductive surfaces

Structural bonding

Raceway conduits

Removable Panels

Conductive coatings

Contact bonds, rivet joints

Bonding lead/straps, pigtails

Corrosion, damage

Corrosion, damage

Damage, erosion

Corrosion, damage

Visually inspect and measure cable shielding or bonding

Visually inspect and measure bonding

Corrosion, damage, deformation Visually inspect gaskets, bonding leads and straps

Visually inspect and measure shielding effectiveness

Visually inspect and measure bonding

Corrosion, damage, security of attachment Visually inspect for corrosion, attachment and condition, measure bonding

Examples

Degradation or failure modes Maintenance operations

Circuit Protection Devices HIRF protection devices

Resistors, Zener diodes, EMI filters & filter pins Short circuit, open circuit Check at test/repair facility iaw maintenance or surveillance plans

18.5.2 Protection against HIRF Interference The manufacturer will normally protect the aircraft against HIRF. Bonding, shielding and separation of critical components usually achieve this. It is difficult to know when the aircraft has been subjected to HIRF; consequently protection is best achieved by regular checks of: • • • •

Bonding of the aircraft Correct crimping Screens correctly terminated and earthed All bonding terminals correctly torque loaded.


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18.6 Heavy Landings A heavy or overweight landing, can cause damage to the aircraft both visible and hidden. All damage found should be entered in the aircraft’s Technical Log. An aircraft landing gear is designed to withstand landing at a particular aircraft weight and rate of descent. If either of these parameters was exceeded during a landing, then it is probable that some damage has been caused to the landing gear, its supporting structure or elsewhere on the airframe. Over-stressing may occur if the aircraft is not parallel to the runway when it lands or if the nose- or tail-wheel strikes the runway before the main wheels. Some aircraft are provided with heavy landing indicators, which give a visual indication that specified ‘g’ forces have been exceeded. Long aircraft may have a tail scrape indicator fitted, as a scrape is more likely. In all instances of suspect heavy landings, the flight crew should be questioned for details of the aircraft’s weight, fuel distribution, landing conditions and whether any unusual noises were heard during the incident. Primary damage, that may be expected following a heavy landing, would normally be concentrated around the landing gear, its supporting structure in the wings or fuselage, the wing and tailplane attachments and the engine mountings. Secondary damage may be found on the fuselage upper and lower skins and on the wing skin and structure. Different aircraft have their own heavy landing procedures. For example, some aircraft, which show no primary damage, need no further inspection, whilst others require that all inspections are made after every reported heavy landing. This is because some aircraft can have hidden damage in remote locations whilst the outside of the aircraft appears to be undamaged. 18.6.1 Example of Post Heavy Landing Inspection The following items give an example of a typical post heavy landing inspection: Landing Gear • Examine tyres for creep, damage, and cuts. • Examine wheels and brakes for cracks and other damage. • Examine axles, struts and stays for distortion. • Check landing gear legs for leaks, scoring and abnormal extension. • Examine gear attachments for signs of cracks, damage or movement. Some aircraft require the removal of critical bolts and pins for NDT checks. • Examine structure in vicinity of gear attachment points. • Examine doors and fairings for damage. • Carry out retraction and nose wheel steering tests Page 12


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Mainplanes • Examine the upper and lower skins for wrinkles and pulled rivets, particularly if the engines are mounted on the wings. • Check for fuel leaks. • Check the root attachments and fairings for cracks. • Function the flying controls for freedom of movement. • Examine wing spars. Fuselage • Check skin for damage and wrinkles. • Examine pressure bulkheads for damage. • Check all supporting structures of heavy components like galleys, batteries, water tanks and APUs. • Ensure no inertia switches have tripped. • Check instruments and their panels are functional. • Ensure pipes and ducts for security. • Check all doors and panels fit correctly. Engines • Check controls for freedom of movement. • Examine all mountings and pylons for damage and distortion. • Check turbine engines for freedom of rotation. • Examine all cowlings for wrinkling and distortion. • Check all fluid lines, filters and chip detectors. • On propeller installations, check for shock-loading, propeller attachments and counterweight installations. Tail Unit • Check flying controls for freedom of movement. • Examine all hinges for distortion or cracks especially near balance weights. • Examine attachments, fairings and mountings of screw jacks. There are numerous other checks that need to be done, depending on the damage found (or not found), during the inspections. This can include engine runs and functional checks of all the aircraft systems. Signs of some damage and distortion could be a reason to do full rigging and symmetry checks of the airframe.


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18.7 Flight through Severe Turbulence If an aircraft has been flown through conditions of severe turbulence, the severity of the turbulence may be difficult to assess and report. For aircraft that utilise accelerometers, flight data recorders or fatigue meters, the records obtained can give an overall picture of the loads felt by the aircraft. They cannot, however, give a full picture and so must only be used for guidance. Turbulence can be too fleeting to record on some forms of load instrumentation. As a general guide only, loadings greater than – 0.5g and + 2.5g on transport aircraft could indicate some damage to the airframe and engines. Aircraft, which have no recording devices installed, must have reports of flight through severe turbulence thoroughly investigated. Severe turbulence may cause excessive vertical or lateral forces similar to those felt during a heavy landing. The forces felt may be increased by the inertia of heavy components such as engines, fuel and water tanks and cargo. Damage can be expected at similar points to those mentioned previously concerning heavy landings. It is also possible for damage to occur in those areas of the wings, fuselage, tail unit and flying controls where the greatest bending moment takes place. Pulled rivets, skin wrinkles or other similar structural faults may provide signs of damage. As with a ‘heavy landing’ report, further inspection, involving dismantling of some major structural components, may be necessary if external damage is found during the initial inspection following flight through turbulence. .

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19 MAINTENANCE PROCEDURES An aircraft has to receive regular maintenance, of varying depths, to remain fully airworthy at all times. This is achieved in most circumstances by making various checks, at intervals, throughout the life of the aircraft. These intervals can be stated in quantities of flying hours, calendar time or combinations of the two systems. 19.1 Maintenance Planning The periods of maintenance can be small or large. The aircraft can be in for a short period of maintenance over-night (or perhaps no longer than two days), whilst, on a large maintenance period, the aircraft might be in the hangar for a week or two, depending on the type of aircraft. It is normal to apply what is known as a ‘back-stop’ to each period for safety. For example, if the frequency of each maintenance action is every 100 flying hours, then there will probably be a calendar ‘back-stop’ of one month. This means that if the aircraft is only flown for 25 hours during one month, then it will have its maintenance done on the last day of that month, regardless if its low hours. Equally, if the aircraft is intensively flown day-and-night, it might reach its 100 hours after 19 days. It will then receive its maintenance at that time, as a result of its intensive flying. The decision as to the frequency and depth of this maintenance is controlled by the ‘Type Design Organisation’, the organisation which designed the aircraft. The maintenance programme contains a list of the most significant items and recommendations as to the maintenance actions, recommended frequencies and sampling/inspection points. It will also contain a programme that monitors engine critical parts and the inspections to be done on those parts. All aircraft have a list of critical parts, with which it cannot fly without them being serviceable, or which can be dispensed with, providing other parts can cover for the missing part.


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19.2 Modification Procedures Modifications are changes made to a particular aircraft, including all its components, engines, propellers, radio apparatus, accessories, instruments, equipment and their respective installations. With the exception of modifications which the CAA agree to be of such a minor nature that airworthiness is unaffected, all modifications must be approved in accordance with the relevant parts of JAR OPS. The modifications are approved by the CAA or by the ‘Approved Organisation’ carrying out the modification programme. Modifications must be such that the design of the aircraft, when modified, complies at least with the requirements which applied when the aircraft was originally certified. When a modification is being designed, a decision has to be made as to whether the modification is to be classified as ‘Minor’ or ‘Major’. The installing of a new type of engine would most definitely be a major modification, whilst changing the type of clips holding cables together would be a minor one. It is somewhere in the middle when the decision as to the grading of a modification has to be decided by the CAA. 19.2.1 Major Modifications The organisation sends a form, AD282 to the CAA and, when approved, an approval note is returned to the organisation. This allows the modification to be embodied. 19.2.2 Minor Modifications The organisation writes to the CAA, requesting permission to embody the modification and, when approved, the CAA sends a form, AD261 back, to permit embodiment. If the organisation has CAA approval, it is permitted to approve its own modifications. All the organisation has to do is to keep full records of the design and embodiment of the modification. All modifications are recorded in the aircraft documentation, either inside the Airframe Log Book, if the aircraft weighs less than 2730 kg, or in a separate Modification Record Book if the aircraft weighs more than 2730 kg.

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19.3 Stores Procedures All aircraft and component manufacturing and maintenance establishments will have a stores department, whose object is twofold. Its purpose, firstly, is to ensure that all materials, parts, components etc. used on aircraft are to the correct specification. The second purpose of the stores is to enable the history of any important part to be traced back to its original manufacture and its raw materials. All stores transactions use the same forms throughout the JAA system as well as the USA and Canada. This system ensures that a store in one part of this country will receive a component from within the UK, all JAA countries or North America on the same form. This is known throughout the JAA system as the JAA Form 1. Stores that operate within an organisation that is approved by the CAA to operate, with little control or supervision from the CAA, is known as an ‘Approved Stores’. An ‘Approved‘ Store will contain three main departments: • • •

A quarantine store, which accepts items from other companies and checks that they are satisfactory. A bonded store which takes items from the quarantine store, after approval, and, when requested, issues those components to the servicing technicians. An office or administration centre, which keeps adequate files and records, to enable cross-checking of any transaction through the store system.

19.4 Certification and Release Procedures Any maintenance done on an aircraft that has a Certificate of Airworthiness (C of A), has to be certified by the technician(s) doing the work. Depending on the company they work for, the technicians can have either personal certification or approval by their own company. The legal requirement is quoted as: ‘An aircraft shall not fly unless there is in force a Certificate of Release to Service issued in respect of any overhauls, modifications, repairs or maintenance to the aircraft or its equipment’. Normally the work is either certified by an approved engineer or, completed by a non-approved engineer and certified by another, approved engineer. This certification is known as a Certificate of Release to Service. The wording on the document for signature is to a standard format and certifies that the work has been done in accordance with JAR 145 and that the aircraft is fit for release back to service.


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The certification should also contain particulars of the work done or the inspection completed and the organisation and place at which the work was done. It is also required that the aircraft type and registration or component type, part and serial number shall be recorded as applicable. There are a number of minor maintenance operations that do not require certification/ release to service. This can include minor maintenance, done by the pilot, on a small private aircraft. 19.4.1 Interface with Aircraft Operation There are many links between aircraft maintenance and the flying done by both commercial and private operations. These links, or interfaces, include the legislation that dictates how the two operations are to work together. For the larger commercial companies, all the legislation is currently laid down under JAROPS, produced by the JAA as an approximate replacement for the publication CAP 360 which was the method by which commercial flying companies obtained their ‘Air Operators’ Certificate’. JAR-OPS controls many facets of commercial flying. This can include how the company maintains its aircraft, (or how it sub-contracts the work elsewhere); how the documentation and publications record all the information needed for both the engineers and the flight crew and how the quality of the whole operation is kept to an acceptable standard. The communication of information between maintenance and flying personnel is normally via a number of different publications such as: • • •

The Technical Log Book (Tech. Log) The Log Books (Aircraft, Engine and Propeller) The Modification Records.

The Tech. Log contains all details of the sector by sector flight operations, such as flight times, defects, fuel (on arrival and uplifted), other ground maintenance and replenishments. The Log Books are usually kept within the records department, but they are a long term record of not only the total flying hours, but of the life remaining on engines and propellers and the maintenance checks done on the aircraft. The Modification Records allow all to see what changes, (modifications), have been embodied to the aircraft. These changes might require different flight operations or maintenance actions than prior to their embodiment.

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Other publications that can be used by both sections include the Minimum Equipment List (MEL) and Configuration Deviation List (CDL). These publications inform both the crews and the engineers which components and parts can be unserviceable, and yet allow the aircraft to be dispatched. There are minor, but no less important, systems in place to allow the same form of communication with smaller, private aircraft. They also have Log Books and records of modifications but, because of their lower utilisation and private ownership, most work is done during their annual and three-yearly Certificate of Airworthiness by approved and licensed engineers. 19.5 Maintenance Inspection/ Quality Control and Assurance All maintenance done on the aircraft, from the Pre-Departure Inspection (made before every flight); to the heavy Check ‘D’ inspection (done every four to six years), is controlled from the Maintenance Schedule. This publication is produced by the aircraft manufacturer, and dictates the depth and frequency of work at which each inspection is completed. On light aircraft, the maintenance is normally done in accordance with a Schedule produced by the CAA, called the Light Aircraft Maintenance Schedule, (LAMS). This is a simple schedule, common to all private aircraft below 2730kg, which divides the maintenance into 50 and 150 flying hour, annual and tri-annual inspections. The personnel who do any of the inspections have to be either licensed by the CAA or ‘approved’ by their own company, (if the company is itself approved by the CAA). The types of aircraft being serviced, and their use, will control which type of qualification they require. If a company has CAA approval under JAR-145, it is permitted to control all of the maintenance it does as well as, in some instances (with the additional approval under JAR-147), the ‘in house’ training of its own engineers. An approved company has to introduce a Quality Assurance Department, to the strict rules laid down in JAR-145. This department controls the standards of the company from the lowliest worker on the hangar floor to the Accountable Manager, usually the managing director. It is responsible for all of the engineers and their approvals. It also examines engineers and trainees, prior to their examination by the CAA. The Quality department also makes ‘audits’ throughout the company, at intervals, to ensure all the procedures, laid down in the company manuals, are being followed.


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When certain operations are being done on an aircraft, whereby there might be catastrophic consequences to the aircraft if the work was not done correctly, then a duplicate inspection is required. This involves two engineers; one of whom completes the work while the second (who has had nothing to do with the task), checks the work and signs that it has been completed correctly. 19.6 Additional Maintenance Procedures Apart from the regular maintenance checks, listed in the Maintenance Manual, there are a number of additional maintenance procedures that are done at irregular intervals. These could include washing the aircraft, de-icing it in the winter, weighing it after certain operations and painting it when its condition warrants it. The information and the correct procedures will probably be found in the maintenance manuals. (under Washing, De-icing, Weighing and Painting). Other work done, in addition to the normal regular maintenance, might include an on-going sampling programme or condition monitoring, which is done during the normal day-to-day operation of the aircraft. These tasks would probably be organised at the request of the local CAA office, to comply with an airworthiness request from the manufacturer. 19.7 Control of Life-Limited Components On almost any aircraft, there will be a number of components that have a stated ‘life’, usually quoted in flying hours, cycles, calendar time or operating hours. The correct terminology for ‘life’ is Mandatory Life Limitation. The components will have been given a life for various reasons. For example, a fatigue life on a structural component in flying hours; the landing gear legs due for retirement after 10,000 landings, the batteries due for replacement after 3 or 4 months and a retirement life on an APU measured in hours running time. The control of the replacement of components, on completion of their lives, rests with the Technical Control/Records department, which monitors all of the aircraft documents. When an item is due for replacement, the work is often synchronised with a scheduled maintenance check, so that the aircraft is out of service for the minimum amount of time. It is normal, however, for small items such as batteries, to be changed on the flight line, often at the end of the day’s flying, with the battery replacement being done at the same time as the daily inspection.

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The notification of the correct day for the replacement will be annotated on a document called the Maintenance Statement, which gives all items due for replacement, in between scheduled maintenance checks. In the front of the Maintenance Manual is a chapter, variously entitled ‘Retirement Lives’; ‘Long Life Items’ or ‘Fatigue Lives’. This chapter lists the retirement lives of many components and parts with long lives, which can include such items as engine ‘hot-end’ components, landing gear legs and major structural items that have retirement lives in the thousands of flying hours/cycles. This list will be monitored by the Technical Records department, and the aircraft documents will be annotated and the work cards etc., raised when the task is required to be done.


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MODULE 07A. MAINTENANCE PRACTICES

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