Air Data Handbook

SENSOR SEN SOR SYS SYSTEM TEMS S Air Data Handbook

TABLE ABLE OF CONTE CONTENT NTS S

CONCEP CONCEPTS TS OF AIR DAT DATA MEASUR MEASUREM EMENT ENT What is Air Data? Types of Air Data Pressure Temperature Accuracy

1 1 1 2 2

AIR DAT DATA SYSTEM SYSTEMS S System Components Air Data System Architecture

3 3

CHAPTER CHAPTER ONE:

CHAPTER CHAPTER TWO:

CHAPTER CHAPTER THREE: THREE:

APPL APPLYING YING THE MEASUR MEASURED ED AIR DAT DATA

Altitude Airspeed CHAPTER CHAPTER FOUR: FOUR:

5 5 AIR DAT DATA EQUA EQUATIONS TIONS AND CALCUL CALCULA ATIONS TIONS

Altitude Impact Pressure Indicated Airspeed Mach Number Speed of Sound Static Temperature True Airspeed

6 7 7 8 9 10 10

GLOSSARY

12

INDEX

15

CHAPT CHAPTER ER ONE ONE

CONCEPTS OF AIR DATA MEASUREMENT CONCEPTS OF AIR DATA DATA MEASUREMENT What is Air Data?

means accurate static pressure measurement must consider a number of factors including:

Air data is a measurement of the physical characteristics of  the air mass that surrounds an aircraft. The two main physical characteristics measured are temperature and pressure. Using these basic measurements individually and in combination allows many other flight parameters to be calculated.

• • • • •

Air data is measured using a variety of sensing devices. The output of these devices provides air data information necessary for safe, effective operation of the aircraft. Basic air data measurements include: • Speed Speed (Mach, (Mach, as well well as indicated, indicated, true, calibrated, calibrated, and equivalent airspeeds) • Altitude • Rates Rates of climb climb or or descen descentt (altit (altitude ude rate) rate) • AngleAngle-ofof-atta attack, ck, angleangle-ofof-sid sidesl eslip ip

Types of Air Data The two broad categories of air data — temperature and pressure — each contain several types of measurements. Pressure measurements consist of static and total pressures. By subtracting static pressure from total pressure (Pt - Ps) a third measurement, impact pressure, qc, can be calculated.

Static Pressure Static pressure is the atmosphere weight over a particular area in a given location. The higher the altitude, the less atmosphere above it, and therefore the lower the measured pressure. At sea level, the static air pressure is sufficient to raise the mercury in a barometer 29.92 inches (or 1013 millibars). But at 18,000 feet above sea level the pressure is only half as great — raising the mercury only 15 inches. In this way, static pressure measurements can give an indication of altitude. Measuring true static pressures from a fixed location on the ground is one thing. Measuring it on an aircraft in flight is quite another. That’s because the aircraft influences and disturbs the atmosphere through which it flies. The altered atmosphere in turn affects the ability to provide an accurate static pressure measurement. A common technique to measure static pressure is to mount pressure inlet ports flush with the aircraft fuselage, but this solution requires finding locations on the aircraft fuselage with clean airflow. In addition, the area around these flush ports must be smooth and uniform to ensure accurate movement. This

Airspeeds Mac Mach numb number er (M) (M) Angl Anglee-ofof-at atta tack ck (AOA (AOA)) Angl Anglee-ofof-si side desl slip ip (AOS (AOS)) Aircraft Aircraft design design (location (location of flaps, landing landing gear gear,, rotor rotor blades, etc.)

Another way to measure static pressure is to place a static port on the body of a pitot probe (see next section). This approach gives better measurements than flush-mounted static ports because the static port is now located away from the aircraft fuselage and away of the influences of the variations in the aircraft skin. The port is not part of the fuselage; it can be manufactured with greater precision to provide a smoother airflow surface. Placing the static port on the pitot probe, therefore, greatly improves accuracy and repeatability of static pressure measurements.

Impact Pressure As aircraft operate they they also encounter impact pressure. This pressure results from force of the moving airstream against the aircraft as it flies. The force of the moving air against the back of the closed tube (called a pitot tube) facing into the airstream creates impact pressure. The airflow disturbances caused by the aircraft movement must be considered in the design and mounting of the pitot tubes.

Pressure Measurement Technology Increasingly, pressure sensors are incorporating advanced silicon technology that provides superior accuracy and reliability compared to non-silicon based sensors. The superior consistency of the solid state pressure sensor combined with it’s unequaled long-term stability performance ensures highly accurate measurements year after year. Solid state pressure sensors use batch fabrication and micromachining processes to provide consistent, high accuracy performance at affordable prices. The sensor’s mechanical design assures uniform thermal expansion of all sensor structures to minimize stresses, which reduces the temperature sensitivity of the sensor device. Silicon, a crystalline material, is used as a diaphragm structure because it is totally elastic to applied stresses. This elasticity enhances the stability and repeatability of the sensor.

1

CHAPT CHAPTER ER ONE ONE

CONCEPTS OF AIR DATA DATA MEASUREMENT Temperature Air temperature information is generated by measurements of  static air temperature (SAT), total air temperature (TAT), or outside air temperature (OAT). Static air temperature is the temperature of the undisturbed air through which the aircraft is about to fly. It is required for calculating true airspeed (the actual aircraft speed moving through the air). The total temperature measurement, on the other hand, is a component of the airstream so it reflects the effects of bringing airflow to rest. It is the only way to accurately measure OAT above 200 KIAS. Typically, total temperature measurements are higher (warmer) than static temperature measurements. Outside air temperature data also helps regulate engine performance at take-off or at cruising altitude to maximize fuel efficiency. Air temperature measurement devices are usually probes incorporating an element which changes its electrical resistance with any air temperature changes. Because moisture and icing can affect the measured temperature, heating elements are included, which must be isolated from the sensing element to ensure an accurate temperature measurement. The measured resistance from the temperature sensor is sent to a signal conditioner for conversion into analog or digital signals. Depending on the application, temperature data may be combined with pressure data in the same transducer.

Accuracy The air data information accuracy that reaches the cockpit or other aircraft destinations is primarily a function of errors that can be encountered while making the air data measurement. Factors affecting measurement accuracy are encountered in either the probe/port or the transducer. Probe/Port Accuracy • • • •

Sensit sitivity Reliabilit lity Location Inst Install allat ation ion repea repeatab tabil ilit ityy

Transducer Accuracy • • • • • • 2

Stati taticc accu accura racy cy Oper Operat atin ing g accu accura racy cy Long Long-t -ter erm m sta stabi bili lity ty Shor Shortt-te term rm sta stabi bili lity ty Meas Measur ured ed data data rang rangee Digi Digita tall reso resolu luti tion on

Static accuracy is defined as the uncorrectable error caused by the combined effects of the following parameters: • • • •

Non-l n-linear neariity Hysteresis Repeat peatab abiility Calib libratio tion

Operating accuracy is the static error combined with uncorrectable error caused by exposure of the transducer to such operational factors as: • Tempe emperat rature ure varia variati tions ons • Vibr Vibrat atio ion n var varia iati tion onss • Acce Acceler lerati ation on varia variati tions ons Long-term stability is a measure of how well the transducer performs to its static and operating accuracy specifications for one year. After that time, factors such as the aging of the electronics, outgassing of components on the vacuum side of  the pressure sensor and degraded integrity of the pressure sensor can affect accuracy. Short-term stability is affected by factors such as signal noise, sensing element response time, conversion speed and filtering time constants and environmental changes. Digital resolution is affected by the number of data bits used when the measured analog wave form is converted into a digital word. A 32-bit digital word can provide more significant bits and pass on more accurate data than a 16-bit card. The air data accuracy needed for a given aircraft and its associated flight envelope will vary greatly. A high-performance supersonic military fighter will have very different operational requirements than a turbo-prop cargo plane. These differences affect the range of the measured data and are often reflected in the required air data accuracy and are categorized as primary or secondary accuracy. In general, accuracy reflects the maximum potential difference between the actual input to a sensor or transducer and the output from that device. Accuracy is typically indicated by a value representing a percentage of the full-scale measurement range of the device. Primary accuracy devices have a narrow range of measurement variance or error to ensure the necessary performance for mission critical applications. Secondary accuracy devices allow a greater measurement variance for missions where the desired performance requires less precise air data inputs.

CHAPT CHAPTER ER TWO TWO

AIR AIR DAT DATA SYSTE SYSTEMS MS AIR AIR DAT DATA SYSTEM SYSTEMS S Air data systems are usually well thought out, deliberate configurations of sensors, transducers, data transmission medium and cockpit displays. The systems provide information about in-flight atmospheric conditions and performance of the aircraft. This information may be “delivered” to the pilot in the cockpit, to the auto-pilot, flyby-wire flight control systems, or other control mechanisms. Air data systems vary in design and architecture depending on the type of aircraft, its flight envelope and mission requirements. Examples of the differences between air data system requirements include: • Commercia Commerciall jet jet — subsonic; subsonic; 50,000 50,000 ft. altitud altitudee ceiling; ceiling; moderate environments, low AOA • Military Military jet jet — supersonic; supersonic; 80,000 ft. altitud altitudee ceiling ceiling;; harsh environment, high AOA • Rotor Rotor — low speed speed;; low altitu altitude de opera operatio tion; n; harsh harsh environment, high AOA

System Components The basic components of an air data system are: • Probe/Port • Trans ansducer • Data Data trans transmi miss ssio ion n med mediu ium m • Displ Display ay and and con contr trol ol dev devic ices es

Probe/Port Pressure or temperature measurements of the air through which an aircraft is flying requires a sensing element be exposed to the ambient air. For pressure measurements, flush ports, pitot probes or pitot-static probes provide access to the air for static or total pressures. Temperatures are measured using using probes inserted into into the airstream that contain temperature sensitive elements that change resistance in response to changes in temperature. All sensing probes/ports may be heated to prevent icing that would compromise the unit’s accuracy.

Transducer  A transducer accepts pneumatic input (for pressure measurements) or resistive input (for temperature measurements) and converts the inputs into the appropriate output signal for communication to a host system such as cockpit instruments, flight control equipment, and/or other aircraft devices. Pneumatic plumbing connects pitot-static probes to their respective transducers. Wiring carries the analog resistive signals from temperature sensors.

The transducer air data output may include any or all of  the following parameters, depending on the transducer’s processing capabilities: • Stat Static ic pres pressu sure re (Ps) • Total otal pres pressu sure re (Pt) • Impa Impact ct pres pressu sure re (qc ) • Pres Pressu sure re alt altit itud udee (h) (h) • • Alti Altitu tude de rate rate (h) • Indi Indica cate ted d airs airspe peed ed (IAS (IAS)) • Mach Mach num number ber (M) (M) • Angl Anglee of of Att Attac ackk (AO (AOA) A) • Angl Anglee of of Sid Sides esli lip p (AO (AOS) S) • True rue air airsp spee eed d (T (TAS)* AS)* • Total otal air air tempe temperat rature ure (TA (TAT)* • Static Static air temper temperatu ature re (SAT)* (SAT)* * Optional with TAT inputs 

Data Transmission Medium The output of a transducer typically is an analog or digital electrical signal. Analog signals are hard-wired to their destination. Digital signals, on the other hand, use communication buses with standardized speeds and protocols. All digital transmission mediums are well documented in Interface Control Documents (ICDs) and are available from the factory if needed. Typical digital communication bus standards for air data applications include: • MIL-STD-15 MIL-STD-1553B. 53B. A bi-dire bi-directiona ctional,l, high-sp high-speed eed data bus. • ARINC ARINC 429. 429. Point Point to multi multi-po -point int commu communic nicati ation on protocol. • RS-422. RS-422. An An Electron Electronics ics Industries Industries Association Association (EIA) standard specifying a two-wire, serial transmit channel, operating in broadcast mode only. • RS-485. RS-485. An An Electron Electronics ics Industries Industries Association Association (EIA) standard specifying a two-wire, bi-directional serial channel.

Air Data System Architectures The term “air data system” architecture refers to the overall functional organization and layout of the total air data system. The two major architectures currently in use are centralized and distributed (Figures (Figures 2-1 and 2-2). 2-2).

3

CHAPT CHAPTER ER TWO TWO

AIR AIR DAT DATA SYSTEM SYSTEMS S Distributed  Distributed air data systems consist of air data probes with co-located integral pressure transducers at each location. The Goodrich Sensor Systems SmartProbe™ combines Pitot, Static, Angle of Attack, and Air Data Computer functions into one LRU. It consists of an air data computer (ADC) combined with a multi-function probe (MFP). The SmartProbe may simply transmit the local conditions on a digital data bus to a central flight control computer for calculation of air data, or by communicating between multiple SmartProbes and optional TAT sensor, full air data can be calculated at each location. Figure 2-1: Centralized Architecture

Centralized  A single (often with a redundant backup) air data computer into which all pressure measurements are fed characterizes centralized architectures. This requires extensive use of  pneumatic tubing running from pressure probes or ports to the central air data computer(s). Centralized air data computers sometimes also bring in electrical signals from other components such as angle of attack transmitters, and discrete switches from landing gear weight-on-wheels, flaps, slats, etc. The units digitize the data and transmit the calculated air data as well as other information on a digital data bus, typically to aircraft avionics and flight control.

SmartProbe distributed air data systems offer many advantages over centralized air data systems, including: • Elimination Elimination of pneumat pneumatic ic tubing tubing (no leak checks, checks, no no drain traps, no tubing installation) • Elimin Eliminati ation on of separat separatee angle angle of attack attack transmi transmitter tterss • Higher Higher reliabil reliability ity due to to active active control of probe probe heaters heaters • Elimin Eliminati ation on of separate separate probe probe heate heaterr current current monitors monitors • Elimin Eliminati ation on of pneum pneumati aticc lag (abou (aboutt 1 msec/ft msec/ft)) • Less we weight • Redu Reduce ced d powe powerr cons consum umpt ptio ion n • Elimination Elimination of “skin “skin effects” effects” on static static measurements measurements Distributed air data systems have been successfully used on many advanced aircraft, including the B-1B, B-2, F-22, Embraer 170/190, and Dassault F7. Goodrich Sensor Systems SmartProbe distributed air data system is the only distributed air data system certified to FAA and JAA standards. Distributed systems may also utilize SmartPort™. Goodrich Sensor Systems is the only company with a production SmartPort. The SmartPort combines a flush static port with one or more transducer channels. The output is typically static pressure.

Figure 2-2: Distributed Architecture

4

CHAPTER CHAPTER THREE THREE

APPL APPLYI YING NG THE THE MEAS MEASUR URED ED AIR AIR DAT DATA APPL APPLYI YING NG THE THE MEAS MEASUR URED ED AIR AIR DAT DATA The measurements of pressure and temperature can be converted, combined and applied to provide many other forms of information useful to the flight crew and aircraft flight control systems. For example, static pressure can be used to derive altitude information. Impact pressure can generate airspeed indications. Combining the altitude and airspeed data can provide Mach number. The altitude and airspeed data combination also contributes to true airspeed calculations when combined with static air temperature. Similar combinations are employed to provide the full spectrum of air data information, but all of the information has its basis in the temperature and pressure measurements made by the air data sensors. The rest of this chapter shows how measured air data is applied to generate key flight parameters for altitude, airspeed and angle-of-attack.

Altitude Two types of altitude indication are generated from pressure measurements. These indications include pressure altitude and altitude rate. Calculation of the indications is based on a “standard atmosphere,” which assumes a known relationship between pressure, temperature and atmospheric density. density. The altitude equations in this handbook are based on the 1962 U.S. Standard Atmosphere. Pressure altitude is the height above a specified reference plane (usually sea level). It is determined by measuring the atmospheric pressure, and it is indicated by the symbol, h. Equations 4.1 through 4.6 in Chapter 4 calculate h for three atmospheric altitude levels. •

Altitude rate (h) is a dynamic parameter calculated using altitude, and time to generate a rate of gain or loss of  height. Altitude rate is usually measured in feet-per-minute. Equation 3.1

Airspeed

Indicated airspeed (IAS) measures the aircraft motion through the surrounding air mass. IAS is a simple indication of speed uncorrected for any installation or instrument errors. It is derived by subtracting static pressure from total pressure. IAS represents true airspeed at standard sea level conditions only o nly.. Equations 4.8 through 4.10 in Chapter 4 calculate IAS for subsonic flight. Equation 4.11 provides a similar calculation for supersonic flight. Equation 4.7 provides a definition for impact pressure which is a key factor in IAS and Mach. Calibrated airspeed (CAS) is simply the indicated airspeed corrected for instrument calibration and position errors. It is most frequently used to judge aircraft performance, particularly in military applications. CAS is represented by the symbol, Vc. True airspeed (TAS) uses static pressure, total pressure and air temperature measurements to derive the actual aircraft speed as it flies through the air. True airspeed can help determine actual flight times and distance traveled. True airspeed is calculated using Equation 4.19 in Chapter 4. Mach is a number representing the ratio of true airspeed to the speed of sound in the air surrounding an aircraft in flig fligh ht (Equation 4.12). 4.12). The speed of sound varies as the square root of average temperature. Mach number is determined using the ratio of impact to static pressure. The Mach number indicates the maximum speed for subsonic and some supersonic aircraft. It also provides a valuable measurement to maximize an aircraft’s operational efficiency, particularly in jets. Mach is indicated by the symbol, M. Equations 4.13 and 4.14 in Chapter 4 calculate Mach for subsonic flight. Equations 4.15 and 4.16 provide a similar calculation for supersonic flight. Angle-of-attack indicates the angle created between between the chord line of a wing and the plane of the oncoming air. Using pneumatic measurement of flow angles eliminates inertia effects and improves response times. In some instances, angle-of-attack measurement can be added to existing pitot probes by simply adding appropriate pneumatic ports.

The four key airspeed indications provide a variety of useful information: • • • •

Indica Indicated ted airsp airspeed eed (IAS (IAS)) Calib Calibrate rated d airspee airspeed d (CAS) (CAS) True airsp airspeed eed (TAS) (TAS) Mach (M (M)

5

CHAPTER CHAPTER FOUR FOUR

BASIC BASIC AIR DAT DATA EQUA EQUATIONS/C TIONS/CALC ALCULA ULATIO TIONS NS ALTITUDE The following altitude equations are based on 1962 U.S. Standard Atmosphere. They are grouped into low, medium, and high altitude ranges.

Low Altitude defined as:

h < 36,089 ft. Ps > 6.6832426 in. Hg To calculate altitude or static pressure in this range, use the following equations: Equation 4.1

Equation 4.2

Mid altitude defined as:

h = 36,089 to 65,617 ft. 6.6832426 > Ps > 1.6167295 in. Hg To calculate altitude or static pressure in this range, use the following equations: Equation 4.3

Equation 4.4

Ps = 6.683246e (1.7345726 - 0.00004806353h)

High Altitude defined as:

h > 65,617 ft. Ps < 1.6167295 in. Hg To calculate altitude or static pressure in this range, use the following equations: Equation 4.5

6

Equation 4.6

CHAPTER CHAPTER FOUR FOUR

BASIC BASIC AIR DAT DATA EQUATIO EQUATIONS/C NS/CALC ALCULA ULATIO TIONS NS IMPACT PRESSURE Equation 4.6

qc = Pt - Ps WHERE: qc = impa impacct pres pressu sure re Pt = total pressure P or Ps = true true sta stati ticc pres pressu sure re

INDICATED AIRSPEED The following equations can be used to calculate indicated airspeed for subsonic and supersonic flight:

For subsonic flight (M<1): Equation 4.8

OR

Equation 4.9

Equation 4.10

WHERE: IAS Ps Pt qc

= = = =

indicated airspeed in knots stat static ic pres pressu sure re in. in. Hg tota totall pre press ssur uree in. in. Hg Pt - Ps = impact pressure in. Hg

For supersonic flight (M>1): Equation Equation 4.11

WHERE: IAS IAS qc

= =

indi indica cate ted d airs airspe peed ed in knot knotss Pt - Ps = impact pressure in. Hg

7

CHAPTER CHAPTER FOUR FOUR

BASIC BASIC AIR DAT DATA EQUA EQUATIONS/C TIONS/CALC ALCULA ULATIO TIONS NS MACH NUMBER In its simplest form, Mach can be defined as follows: Equation 4.12

M = Mach number = TAS/a WHERE: TAS

a

= =

True rue airs airspe peed ed in knot knotss spee speed d of soun sound d in knot knotss

However, for air data applications more precise Mach values are required and can be calculated using pressure measurements, as the following equations demonstrate: For subsonic flight (M<1): Equation 4.13

OR

Equation 4.14

WHERE: qc = Ps = Pt =

Pt - Ps = impact pressure in. Hg stat static ic pres pressu sure re in. in. Hg tota totall pr pressu essure re in. in. Hg Hg

For supersonic flight (Mv1): Equation 4.15

OR

Equation 4.16

WHERE: qc = Ps = Pt =

8

Pt - Ps = impact pressure in. Hg stat static ic pres pressu sure re in. in. Hg tota totall pr pressu essure re in. in. Hg Hg

CHAPTER CHAPTER FOUR FOUR

BASIC BASIC AIR DAT DATA EQUA EQUATIONS/C TIONS/CALC ALCULA ULATIO TIONS NS

Figure 4-1: Mach No. vs Altitude and IAS

SPEED OF SOUND Equation Equation 4.17

WHERE: a Ts

= =

speed of sound in knots stat static ic temp temper erat atur uree in in °K  °K 

9

CHAPTER CHA PTER FOU FOUR R

BASIC BASI C AIR DA DAT TA EQU EQUA ATIO TIONS/C NS/CALC ALCULA ULATIO TIONS NS STATIC STATIC TEMPERATURE Equation Equation 4.18

WHERE: Ts Tt

= =

stat static ic temp temper erat atur uree in in °K  °K  tota totall temp temper eraature ture in °K 

TRUE AIRSPEED Equation Equation 4.19

WHERE: TAS M a Tt

= = = =

true airspeed Mach speed of sound tota totall temp temper eraature ture in °K 

To find the best-fit pressure range for your flight or instrument envelope, plot a data point on the Flight Envelope Chart (Figure 4-2). 4-2).

10

CHAPTER CHAPTER FOUR FOUR

BASIC BASIC AIR DAT DATA EQUA EQUATIONS/C TIONS/CALC ALCULA ULATIO TIONS NS

Figure 4-2. Flight Envelope Chart

• • • • •

Locate Locate the the applic applicati ation’s on’s maxi maximum mum Mach Mach numbe numberr on the horizontal axis. On the vert vertica icall axis, axis, locate locate the lowe lowest st altitu altitude de at which which the aircraft will achieve maximum Mach. Plot the the data data point point where where the the lines lines from thos thosee two value valuess intersect. Identi Identify fy the pres pressur suree range range in which which the the data poin pointt is located. Chec Checkk off off the the total total pres pressu sure re (P (Pt ) range (1-38, 1-50 or 180 "Hg) that corresponds to the location of your data point.

If the location of your data point is on or near the edge of a given Pt range, use the higher of the two ranges.

11

AIR DAT DATA GLOSSARY

Accuracy, Primary: Primary accuracy is a system descriptor and is so-named because it is used to identify systems for mission-critical applications such as primary air data, safetyof-flight air data and/or cockpit display within the air data system. Primary accuracy is typically indicated by a value representing a percentage of the full-scale pressure measurement range of the device. Accuracy, Secondary: Secondary accuracy is a system descriptor and is used to identify systems used for applications applications that tolerate a greater measurement variance, such as flight control gain scheduling, altitude altitude or airspeed hold, or environmental control systems. Secondary accuracy is typically indicated by a value representing a percentage of  the full-scale measurement range of the device. Adiabatic: The thermodynamic change in a system without heat transfer across the system boundary to the surrounding medium (i.e. no gain or loss of heat). Air Data: The mathematical values corresponding to the physical characteristics characteristics of the air mass surrounding a body in flight. These physical characteristics most often include temperature and pressure, measured using a variety of  sensing devices. The output of these devices (the air data) can then be used to generate information such as speed, altitude, rates of climb or descent and other flight parameters. Altitude Rate: The amount of altitude change per period of  time, usually measured in feet-per-minute. Indicated by the • symbol, h. Angle-of-Attack (AOA): The acute angle of an aircraft measured in the XZ plane ( body axis coordinate system ) between the X-axis and the projection of the resultant flight velocity in the XZ plane. Angle-of-attack is positive when the flight velocity vector vector impinges from below below the aircraft. AOA is indicated by the symbol, α (Alpha). Angle-of-Sideslip Angle-of-Sideslip (AOS): The acute angle of an aircraft measured in the XY plane (body axis coordinate system ) between the X-axis and the projection of the resultant flight velocity in the XY plane. It is positive when the flight velocity vector impinges from the left of the aircraft. AOS is indicated by the symbol, β (Beta).

ARINC 429: This communication standard specifies a twowire, digital communications protocol. Often used for sending air data from a transducer to other components of  the air data system, especially for commercial transport aircraft. BIT: Acronym for Built-in Test Test (BIT). A system in which an electronic instrument performs tests internally to determine if  the instrument is operating correctly correctly.. BIT typically has three modes; periodic (automatic), initiated and startup. Calibrated Airspeed (CAS): Indicated airspeed corrected for instrument calibration and position errors. Indicated by the symbol, Vc. Calibration: The comparison of a transducer of unverified accuracy to a measurement standard or device of known or greater accuracy. The purpose of the comparison is to detect and correct any variation from the required performance specifications of the transducer. Data Latency: The time lag between an input measurement and the data message transmission. transmission. ETI: Acronym for Elapsed Time Time Indicator. Indicator. ETI monitors the time of operation for a device. Hysteresis: The tendency of an instrument to give a different output for a given input, depending on whether the input change resulted from an increase or decrease of the previous value. Impact Pressure: Sum of the total pressure minus the local atmospheric pressure. The pressure a moving stream of air produces against a surface which brings part of the moving stream to rest. Subsonically, it is commonly referred to as the compressible dynamic pressure. pressure. It is the sum difference between the total and static pressures, Pt - Ps = qc. Indicated by the symbol, q c. Indicated Airspeed (IAS): The speed of an aircraft with respect to the surrounding air mass. It is uncorrected for any installation or instrument errors. Indicated airspeed represents true airspeed at standard sea level conditions only, and it is a function only of impact pressure, q c. Isentropic: Without change in entropy (the unavailability of  energy in a system) over time.

12

AIR DAT DATA GLOSSARY

Long-term Stability: The ability of the transducer to maintain operation within its static accuracy specifications specifications over a long time period (one year minimum). Factors affecting long-term stability include aging and outgassing of electronic components. LRU: Acronym for Line Replaceable Unit. An assembly that can be replaced on the flight line or local maintenance facility. Mach Number: The ratio of true airspeed to the speed of  sound in the surrounding air. The speed of sound varies as the square root of average temperature. Mach number is determined using the ratio of impact to static pressure. Indicated by the symbol, M. MIL-STD-1553B: This military communication communication standard which uses a bi-directional, high-speed digital data bus. Often used for sending communications within a military air data system. Mmo: The maximum operating Mach number certified for a given aircraft. Non-linearity: The departure from a desired linear relationship between corresponding input and output signals. NOVRAM: Acronym for Non-volatile Random Access Memory. A digital memory device that maintains data when power is removed. Operating Accuracy: The uncorrectable error caused by exposure to external operational conditions conditions (primarily the ambient temperature operating range). Pitot Tube (also Pitot Probe): An open-end tube facing forward into the air flow to measure total pressure (P t). Pitot-Static Tube (also Pitot-Static Probe): An open-end tube facing forward into the air flow for measuring total pressure (Pt), and with ports to measure local static pressure (P s). Pneumatic Lag: The time elapsed between the sensing of a pressure and when that pressure is pneumatically pneumatically transmitted through piping and received by the transducer.

Recovery Error: The per unit or fractional total temperature error. Recovery Factor: The proportion of kinetic energy converted to heat. A recovery factor of one means all kinetic energy is converted to heat. In such a case, the recovery temperature is equal to the total temperature. Recovery Temperature: The equilibrium temperature of a surface with a given recovery factor or recovery error. Indicated by the symbol, Tr. Repeatability: The ability of an instrument to duplicate, with exactness, the measurements of a given value. Resistance Range: In temperature sensors, the range of  resistances (in ohms) corresponding to the range of desired temperature measurements. Indicated by the variable, R0. Resolution: The exactness of the numbers used to portray the measurement. It is usually affected by the number of data bits in a digital system. RS-422/RS-485: Electronics Industries Association (EIA) standards that use a two-wire, signal path for high-speed, binary serial communication. RS-422 and RS-485 interface hardware and protocols are identical. However, RS-422 specifies a transmit channel operating in broadcast mode only. RS-485 specifies a bi-directional transmit and receive channel. Set Point or Relative Accuracy: The error between actual performance and expected or operational set point, independent of absolute accuracy. Typically applied to flight control “hold” functions. Short-term Stability: A system describer which is affected by factors such as signal noise, sensing element response time, conversion speed and external environment. Standard Atmosphere: A well-defined relationship between static air pressure, temperature and altitude. It is calculated from the hydro-static equation using a standard variation of  temperature with height from a fixed pressure datum point (usually taken above mean sea level for the earth).

Pressure Altitude: The height above a specified reference plane (usually sea level), determined by measuring the atmospheric pressure. Indicated Indicated by the symbol, h.

13

AIR DAT DATA GLOSSARY

Standard Sea Level Conditions: Conditions: The term used for sea level values of the standard atmosphere, specifically specifically 15° 15°C (59.0° (59.0° F) and 29.92126 inches of mercury (in. ( in. Hg). Static Ports: An opening in a plate carefully placed to be flush with the aircraft skin used, in most flight conditions, conditions, to measure true static pressure (P s). Sometimes called static vents. Static Pressure: The absolute pressure (total pressure above that of a vacuum) of still air surrounding a body. Put another way, it is the absolute air pressure that would have existed at the aircraft’s location in the atmosphere, if the aircraft had created no pressure disturbances. disturbances. Indicated by the variable, Ps. Static Accuracy: The uncorrectable error caused by the combined effects of non-linearity non-linearity, hysteresis, repeatability and calibration. calibration. This is the combination of all errors in the absence of transient conditions. Static Air Temperature (SAT): The temperature of undisturbed undisturbed air through which the aircraft is about to fly. It is the local temperature of the air with no element due to the velocity of  the air. Static temperature is lower than recovery or total temperature. Indicated by the symbol, T. Temperature Transient: A dynamic temperature condition not periodically periodically repeated. The term “transient” often implies an anomalous, temporary departure from a steady-state temperature condition. condition. The departure may be either constant or cyclic. Total Pressure: The sum of local atmospheric pressures plus dynamic (operating) pressures. Total pressure is the sum of  static and impact pressures. Indicated by the variable, P t. Total Air Temperature (TAT): The temperature of an airflow measured as the airflow is brought to rest without removal or addition of heat. Total temperature is higher than static or recovery temperature because of adiabatic compression of air going to zero velocity. Indicated by the symbol, T t. Transducer: Device for translating a physical phenomena from one form to another. In air data, transducers are most commonly used to translate physical measurements of  pressure or temperature into electrical signals (analog or digital) for transmission to the aircraft’s control or display instruments.

14

True Airspeed (TAS): Indicated airspeed corrected for nonstandard temperatures that can be determined using Mach number and total temperature information. It is the actual aircraft speed through the air mass. Indicated by the symbol, V.

Asssume Freestream Static Temperature Temp erature = 15 degrees C

Measure Actual Total Air Temperature

Update Rate: The transmit intervals for each item of  information transferred from a transducer on a digital communication bus. Vertical Speed: The aircraft’s rate of change in height. Also referred to as rate of climb, rate of descent, or altitude rate. Vmo: The maximum permitted operating true airspeed for a given aircraft under any condition.

INDEX

angle of attack

5

angle of sideslip

1

altitude rate

5

altitude

3

architectures

3

data transducers

3

data bus

3

data communications

2

error

2

impact pressure

1

Mach

1

operating accuracy

2

pneumatic lag

4

ports

1

pressure altitude

3

pressure

1

primary accuracy

2

probes

2

resolution

2

secondary accuracy

2

sensors

1

stability, long-term

2

stability, short-term

2

static accuracy

2

static air temperature (SAT)

2

total air temperature

2

total pressure

1

total air temperature (TAT)

2

transducers, see data transducers true airspeed

5

15

WE’RE HERE TO HELP Some air data questions related to specific applications may not be directly addressed in the handbook. After five decades of providing the finest air data measurement devices and systems for commercial and military aircraft worldwide, we know listening to your questions, comments, and suggestions - whether you are a current customer or not - helps us deliver superior performance and value in all of our air data products, systems and services. If you don’t find the information you need, simply call a Goodrich air data specialist at 952 892 4000. We will be happy to answer your questions and discuss your air data application. If you did not receive copies of Goodrich brochures with this handbook, or if you’d like more information about how the Goodrich Air Data Computers can solve your air data problems, contact a Goodrich air data computer expert today at: Sensor Systems Goodrich Corporation Attn: Pressure Product Marketing 14300 Judicial Road Burnsville, MN 55306-4898 USA Tel: 952 892 4000 Fax: 952 892 4800 www.aerospace.goodrich.com

16

Sensor Systems Systems Goodrich Corporation 14300 Judicial Road Burnsville, MN 55306-4898 USA Tel: 952 892 4000 Fax: 952 892 4800 www.aerospace.goodrich.com

4081 LIT 08/02 © Rosemount Aerospace Inc., 2002 Printed in the USA The Goodrich name, logotype and symbol are trademarks of Goodrich Corporation.