4081 Air Data Handbook

Air Data Handbook

SENSOR SYSTEMS

CHAPTER ONE: CONCEPTS OF AIR DATA MEASUREMENT

What is Air Data? 1

Types of Air Data 1

Pressure 1

Temperature 2

Accuracy 2

CHAPTER TWO: AIR DATA SYSTEMS

System Components 3

Air Data System Architecture 3

CHAPTER THREE: APPLYING THE MEASURED AIR DATA

Altitude 5

Airspeed 5

CHAPTER FOUR: AIR DATA EQUATIONS AND CALCULATIONS

Altitude 6

Impact Pressure 7

Indicated Airspeed 7

Mach Number 8

Speed of Sound 9

Static Temperature 10

True Airspeed 10

GLOSSARY 12

INDEX 15

TABLE OF CONTENTS

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CONCEPTS OF AIR DATA MEASUREMENTWhat is Air Data?Air data is a measurement of the physical characteristics ofthe air mass that surrounds an aircraft. The two mainphysical characteristics measured are temperature andpressure. Using these basic measurements individually andin combination allows many other flight parameters to becalculated.

Air data is measured using a variety of sensing devices. Theoutput of these devices provides air data informationnecessary for safe, effective operation of the aircraft. Basicair data measurements include:

• Speed (Mach, as well as indicated, true, calibrated, and equivalent airspeeds)

• Altitude• Rates of climb or descent (altitude rate)• Angle-of-attack, angle-of-sideslip

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

Static PressureStatic pressure is the atmosphere weight over a particulararea in a given location. The higher the altitude, the lessatmosphere above it, and therefore the lower the measuredpressure. At sea level, the static air pressure is sufficient toraise the mercury in a barometer 29.92 inches (or 1013millibars). But at 18,000 feet above sea level the pressure isonly half as great — raising the mercury only 15 inches. Inthis way, static pressure measurements can give anindication of altitude.

Measuring true static pressures from a fixed location on theground is one thing. Measuring it on an aircraft in flight isquite another. That’s because the aircraft influences anddisturbs the atmosphere through which it flies. The alteredatmosphere in turn affects the ability to provide an accuratestatic pressure measurement. A common technique tomeasure static pressure is to mount pressure inlet portsflush with the aircraft fuselage, but this solution requiresfinding locations on the aircraft fuselage with clean airflow.In addition, the area around these flush ports must besmooth and uniform to ensure accurate movement. This

means accurate static pressure measurement must considera number of factors including:

• Airspeeds• Mach number (M)• Angle-of-attack (AOA)• Angle-of-sideslip (AOS)• Aircraft design (location of flaps, landing gear, rotor

blades, etc.)

Another way to measure static pressure is to place a staticport on the body of a pitot probe (see next section). Thisapproach gives better measurements than flush-mountedstatic ports because the static port is now located away fromthe aircraft fuselage and away of the influences of thevariations in the aircraft skin. The port is not part of thefuselage; it can be manufactured with greater precision toprovide a smoother airflow surface. Placing the static porton the pitot probe, therefore, greatly improves accuracy andrepeatability of static pressure measurements.

Impact PressureAs aircraft operate they also encounter impact pressure.This pressure results from force of the moving airstreamagainst the aircraft as it flies. The force of the moving airagainst the back of the closed tube (called a pitot tube)facing into the airstream creates impact pressure. Theairflow disturbances caused by the aircraft movement mustbe considered in the design and mounting of the pitottubes.

Pressure Measurement TechnologyIncreasingly, pressure sensors are incorporating advancedsilicon technology that provides superior accuracy andreliability compared to non-silicon based sensors. Thesuperior consistency of the solid state pressure sensorcombined with it’s unequaled long-term stabilityperformance ensures highly accurate measurements yearafter year. Solid state pressure sensors use batch fabricationand micromachining processes to provide consistent, highaccuracy performance at affordable prices.

The sensor’s mechanical design assures uniform thermalexpansion of all sensor structures to minimize stresses,which reduces the temperature sensitivity of the sensordevice. Silicon, a crystalline material, is used as a diaphragmstructure because it is totally elastic to applied stresses. Thiselasticity enhances the stability and repeatability of thesensor.

CHAPTER ONE

CONCEPTS OF AIR DATA MEASUREMENT

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TemperatureAir temperature information is generated by measurements ofstatic air temperature (SAT), total air temperature (TAT), oroutside air temperature (OAT). Static air temperature is thetemperature of the undisturbed air through which the aircraft isabout to fly. It is required for calculating true airspeed (theactual aircraft speed moving through the air).

The total temperature measurement, on the other hand, is acomponent of the airstream so it reflects the effects of bringingairflow to rest. It is the only way to accurately measure OATabove 200 KIAS. Typically, total temperature measurements arehigher (warmer) than static temperature measurements.

Outside air temperature data also helps regulate engineperformance at take-off or at cruising altitude to maximize fuelefficiency.

Air temperature measurement devices are usually probesincorporating an element which changes its electrical resistancewith any air temperature changes. Because moisture and icingcan affect the measured temperature, heating elements areincluded, which must be isolated from the sensing element toensure an accurate temperature measurement.

The measured resistance from the temperature sensor is sentto a signal conditioner for conversion into analog or digitalsignals. Depending on the application, temperature data maybe combined with pressure data in the same transducer.

AccuracyThe air data information accuracy that reaches the cockpit orother aircraft destinations is primarily a function of errors thatcan be encountered while making the air data measurement.

Factors affecting measurement accuracy are encountered ineither the probe/port or the transducer.

Probe/Port Accuracy

• Sensitivity• Reliability• Location• Installation repeatability

Transducer Accuracy

• Static accuracy• Operating accuracy• Long-term stability• Short-term stability• Measured data range• Digital resolution

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

• Non-linearity• Hysteresis• Repeatability• Calibration

Operating accuracy is the static error combined withuncorrectable error caused by exposure of the transducer tosuch operational factors as:

• Temperature variations• Vibration variations• Acceleration variations

Long-term stability is a measure of how well the transducerperforms to its static and operating accuracy specifications forone year. After that time, factors such as the aging of theelectronics, outgassing of components on the vacuum side ofthe pressure sensor and degraded integrity of the pressuresensor can affect accuracy.

Short-term stability is affected by factors such as signal noise,sensing element response time, conversion speed and filteringtime constants and environmental changes.

Digital resolution is affected by the number of data bits usedwhen the measured analog wave form is converted into a digitalword. A 32-bit digital word can provide more significant bits andpass on more accurate data than a 16-bit card.

The air data accuracy needed for a given aircraft and itsassociated flight envelope will vary greatly. A high-performancesupersonic military fighter will have very different operationalrequirements than a turbo-prop cargo plane. These differencesaffect the range of the measured data and are often reflected inthe required air data accuracy and are categorized as primary orsecondary accuracy.

In general, accuracy reflects the maximum potential differencebetween the actual input to a sensor or transducer and theoutput from that device. Accuracy is typically indicated by avalue representing a percentage of the full-scale measurementrange of the device. Primary accuracy devices have a narrowrange of measurement variance or error to ensure the necessaryperformance for mission critical applications. Secondaryaccuracy devices allow a greater measurement variance formissions where the desired performance requires less preciseair data inputs.

CHAPTER ONE

CONCEPTS OF AIR DATA MEASUREMENT

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CHAPTER TWO

AIR DATA SYSTEMS

AIR DATA SYSTEMSAir data systems are usually well thought out, deliberateconfigurations of sensors, transducers, data transmissionmedium and cockpit displays. The systems provideinformation about in-flight atmospheric conditions andperformance of the aircraft. This information may be“delivered” to the pilot in the cockpit, to the auto-pilot, fly-by-wire flight control systems, or other control mechanisms.

Air data systems vary in design and architecture dependingon the type of aircraft, its flight envelope and missionrequirements. Examples of the differences between air datasystem requirements include:

• Commercial jet — subsonic; 50,000 ft. altitude ceiling; moderate environments, low AOA

• Military jet — supersonic; 80,000 ft. altitude ceiling; harsh environment, high AOA

• Rotor — low speed; low altitude operation; harsh environment, high AOA

System ComponentsThe basic components of an air data system are:• Probe/Port• Transducer• Data transmission medium• Display and control devices

Probe/PortPressure or temperature measurements of the air throughwhich an aircraft is flying requires a sensing element beexposed to the ambient air. For pressure measurements,flush ports, pitot probes or pitot-static probes provideaccess to the air for static or total pressures. Temperaturesare measured using probes inserted into the airstream thatcontain temperature sensitive elements that changeresistance in response to changes in temperature. Allsensing probes/ports may be heated to prevent icing thatwould compromise the unit’s accuracy.

TransducerA transducer accepts pneumatic input (for pressuremeasurements) or resistive input (for temperaturemeasurements) and converts the inputs into theappropriate output signal for communication to a hostsystem such as cockpit instruments, flight controlequipment, and/or other aircraft devices. Pneumaticplumbing connects pitot-static probes to their respectivetransducers. Wiring carries the analog resistive signals fromtemperature sensors.

The transducer air data output may include any or all ofthe following parameters, depending on the transducer’sprocessing capabilities:• Static pressure (Ps)• Total pressure (Pt)• Impact pressure (qc)• Pressure altitude (h)• Altitude rate (h• )• Indicated airspeed (IAS)• Mach number (M)• Angle of Attack (AOA)• Angle of Sideslip (AOS)• True airspeed (TAS)*• Total air temperature (TAT)*• Static air temperature (SAT)** Optional with TAT inputs

Data Transmission MediumThe output of a transducer typically is an analog ordigital electrical signal. Analog signals are hard-wired totheir destination. Digital signals, on the other hand, usecommunication buses with standardized speeds andprotocols. All digital transmission mediums are welldocumented in Interface Control Documents (ICDs) andare available from the factory if needed. Typical digitalcommunication bus standards for air data applicationsinclude:

• MIL-STD-1553B. A bi-directional, high-speed data bus.• ARINC 429. Point to multi-point communication

protocol.• RS-422. An Electronics Industries Association (EIA)

standard specifying a two-wire, serial transmit channel, operating in broadcast mode only.

• RS-485. An Electronics Industries Association (EIA) standard specifying a two-wire, bi-directional serial channel.

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

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CentralizedA single (often with a redundant backup) air data computerinto which all pressure measurements are fed characterizescentralized architectures. This requires extensive use ofpneumatic tubing running from pressure probes or ports tothe central air data computer(s). Centralized air datacomputers sometimes also bring in electrical signals fromother components such as angle of attack transmitters, anddiscrete switches from landing gear weight-on-wheels, flaps,slats, etc. The units digitize the data and transmit thecalculated air data as well as other information on a digitaldata bus, typically to aircraft avionics and flight control.

DistributedDistributed air data systems consist of air data probes withco-located integral pressure transducers at each location.The Goodrich Sensor Systems SmartProbe™ combines Pitot,Static, Angle of Attack, and Air Data Computer functionsinto one LRU. It consists of an air data computer (ADC)combined with a multi-function probe (MFP). TheSmartProbe may simply transmit the local conditions on adigital data bus to a central flight control computer forcalculation of air data, or by communicating betweenmultiple SmartProbes and optional TAT sensor, full air datacan be calculated at each location.

SmartProbe distributed air data systems offer manyadvantages over centralized air data systems, including:

• Elimination of pneumatic tubing (no leak checks, no drain traps, no tubing installation)

• Elimination of separate angle of attack transmitters• Higher reliability due to active control of probe heaters• Elimination of separate probe heater current monitors• Elimination of pneumatic lag (about 1 msec/ft)• Less weight• Reduced power consumption• Elimination of “skin effects” on static measurements

Distributed air data systems have been successfully used onmany advanced aircraft, including the B-1B, B-2, F-22,Embraer 170/190, and Dassault F7. Goodrich SensorSystems SmartProbe distributed air data system is the onlydistributed air data system certified to FAA and JAAstandards.

Distributed systems may also utilize SmartPort™. GoodrichSensor Systems is the only company with a productionSmartPort. The SmartPort combines a flush static port withone or more transducer channels. The output is typicallystatic pressure.

CHAPTER TWO

AIR DATA SYSTEMS

Figure 2-1: Centralized Architecture

Figure 2-2: Distributed Architecture

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CHAPTER THREE

APPLYING THE MEASURED AIR DATA

APPLYING THE MEASURED AIR DATAThe measurements of pressure and temperature can beconverted, combined and applied to provide many otherforms of information useful to the flight crew and aircraftflight control systems. For example, static pressure can beused to derive altitude information. Impact pressure cangenerate airspeed indications. Combining the altitude andairspeed data can provide Mach number. The altitude andairspeed data combination also contributes to true airspeedcalculations when combined with static air temperature.Similar combinations are employed to provide the fullspectrum of air data information, but all of the informationhas its basis in the temperature and pressuremeasurements made by the air data sensors.

The rest of this chapter shows how measured air data isapplied to generate key flight parameters for altitude,airspeed and angle-of-attack.

AltitudeTwo types of altitude indication are generated from pressuremeasurements. These indications include pressure altitudeand altitude rate. Calculation of the indications is based on a“standard atmosphere,” which assumes a knownrelationship between pressure, temperature and atmosphericdensity. The altitude equations in this handbook are basedon the 1962 U.S. Standard Atmosphere.

Pressure altitude is the height above a specified referenceplane (usually sea level). It is determined by measuring theatmospheric pressure, and it is indicated by the symbol, h.Equations 4.1 through 4.6 in Chapter 4 calculate h for threeatmospheric altitude levels.

Altitude rate (h•) is a dynamic parameter calculated usingaltitude, and time to generate a rate of gain or loss ofheight. Altitude rate is usually measured in feet-per-minute.

Equation 3.1

Airspeed The four key airspeed indications provide a variety of usefulinformation:

• Indicated airspeed (IAS)• Calibrated airspeed (CAS)• True airspeed (TAS)• Mach (M)

Indicated airspeed (IAS) measures the aircraft motionthrough the surrounding air mass. IAS is a simple indicationof speed uncorrected for any installation or instrumenterrors. It is derived by subtracting static pressure from totalpressure. IAS represents true airspeed at standard sea levelconditions only.

Equations 4.8 through 4.10 in Chapter 4 calculate IAS forsubsonic flight. Equation 4.11 provides a similar calculationfor supersonic flight. Equation 4.7 provides a definition forimpact pressure which is a key factor in IAS and Mach.

Calibrated airspeed (CAS) is simply the indicated airspeedcorrected for instrument calibration and position errors. It ismost frequently used to judge aircraft performance,particularly in military applications. CAS is represented bythe symbol, Vc.

True airspeed (TAS) uses static pressure, total pressure andair temperature measurements to derive the actual aircraftspeed as it flies through the air. True airspeed can helpdetermine actual flight times and distance traveled. Trueairspeed is calculated using Equation 4.19 in Chapter 4.

Mach is a number representing the ratio of true airspeed tothe speed of sound in the air surrounding an aircraft inflight (Equation 4.12). The speed of sound varies as thesquare root of average temperature. Mach number isdetermined using the ratio of impact to static pressure. TheMach number indicates the maximum speed for subsonicand some supersonic aircraft. It also provides a valuablemeasurement to maximize an aircraft’s operationalefficiency, particularly in jets. Mach is indicated by thesymbol, M. Equations 4.13 and 4.14 in Chapter 4 calculateMach for subsonic flight. Equations 4.15 and 4.16 provide asimilar calculation for supersonic flight.

Angle-of-attack indicates the angle created between thechord line of a wing and the plane of the oncoming air.Using pneumatic measurement of flow angles eliminatesinertia effects and improves response times. In someinstances, angle-of-attack measurement can be added toexisting pitot probes by simply adding appropriatepneumatic ports.

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ALTITUDEThe following altitude equations are based on 1962 U.S. Standard Atmosphere. They are grouped into low, medium, andhigh 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



Ps = 6.683246e (1.7345726 - 0.00004806353h)

High Altitude defined as:h > 65,617 ft.Ps < 1.6167295 in. Hg



CHAPTER FOUR

BASIC AIR DATA EQUATIONS/CALCULATIONS

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IMPACT PRESSURE

Equation 4.6

qc = Pt - Ps

WHERE:qc = impact pressurePt = total pressure P or Ps = true static pressure

INDICATED AIRSPEEDThe 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 = indicated airspeed in knotsPs = static pressure in. HgPt = total pressure in. Hgqc = Pt - Ps = impact pressure in. Hg

For supersonic flight (M>1):

Equation 4.11

WHERE:IAS = indicated airspeed in knotsqc = Pt - Ps = impact pressure in. Hg

CHAPTER FOUR


MACH NUMBERIn its simplest form, Mach can be defined as follows:

Equation 4.12

M = Mach number = TAS/a

WHERE:TAS = True airspeed in knotsa = speed of sound in knots

However, for air data applications more precise Mach values are required and can be calculated using pressuremeasurements, as the following equations demonstrate:

For subsonic flight (M<1):


WHERE:qc = Pt - Ps = impact pressure in. HgPs = static pressure in. HgPt = total pressure in. Hg

For supersonic flight (Mv1):


WHERE:qc = Pt - Ps = impact pressure in. HgPs = static pressure in. HgPt = total pressure in. Hg

CHAPTER FOUR


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

CHAPTER FOUR


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SPEED OF SOUND

Equation 4.17

WHERE:a = speed of sound in knotsTs = static temperature in °K

CHAPTER FOUR


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STATIC TEMPERATURE

Equation 4.18

WHERE:Ts = static temperature in °KTt = total temperature in °K

TRUE AIRSPEED

Equation 4.19

WHERE:TAS = true airspeedM = Macha = speed of soundTt = total temperature 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).

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CHAPTER FOUR


• Locate the application’s maximum Mach number on the horizontal axis.

• On the vertical axis, locate the lowest altitude at which the aircraft will achieve maximum Mach.

• Plot the data point where the lines from those two valuesintersect.

• Identify the pressure range in which the data point is located.

• Check off the total pressure (Pt ) range (1-38, 1-50 or 1-80 "Hg) that corresponds to the location of your data point.

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

Figure 4-2. Flight Envelope Chart

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Accuracy, Primary: Primary accuracy is a system descriptorand is so-named because it is used to identify systems formission-critical applications such as primary air data, safety-of-flight air data and/or cockpit display within the air datasystem. Primary accuracy is typically indicated by a valuerepresenting a percentage of the full-scale pressuremeasurement range of the device.

Accuracy, Secondary: Secondary accuracy is a systemdescriptor and is used to identify systems used forapplications that tolerate a greater measurement variance,such as flight control gain scheduling, altitude or airspeedhold, or environmental control systems. Secondary accuracyis typically indicated by a value representing a percentage ofthe full-scale measurement range of the device.

Adiabatic: The thermodynamic change in a system withoutheat transfer across the system boundary to the surroundingmedium (i.e. no gain or loss of heat).

Air Data: The mathematical values corresponding to thephysical characteristics of the air mass surrounding a body inflight. These physical characteristics most often includetemperature and pressure, measured using a variety ofsensing 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 flightparameters.

Altitude Rate: The amount of altitude change per period oftime, usually measured in feet-per-minute. Indicated by thesymbol, h•.

Angle-of-Attack (AOA): The acute angle of an aircraftmeasured in the XZ plane (body axis coordinate system)between the X-axis and the projection of the resultant flightvelocity in the XZ plane. Angle-of-attack is positive when theflight velocity vector impinges from below the aircraft. AOA isindicated by the symbol, α (Alpha).

Angle-of-Sideslip (AOS): The acute angle of an aircraftmeasured in the XY plane (body axis coordinate system)between the X-axis and the projection of the resultant flightvelocity in the XY plane. It is positive when the flight velocityvector impinges from the left of the aircraft. AOS is indicatedby the symbol, β (Beta).

ARINC 429: This communication standard specifies a two-wire, digital communications protocol. Often used forsending air data from a transducer to other components ofthe air data system, especially for commercial transportaircraft.

BIT: Acronym for Built-in Test (BIT). A system in which anelectronic instrument performs tests internally to determine ifthe instrument is operating correctly. BIT typically has threemodes; periodic (automatic), initiated and startup.

Calibrated Airspeed (CAS): Indicated airspeed corrected forinstrument calibration and position errors. Indicated by thesymbol, Vc.

Calibration: The comparison of a transducer of unverifiedaccuracy to a measurement standard or device of known orgreater accuracy. The purpose of the comparison is to detectand correct any variation from the required performancespecifications of the transducer.

Data Latency: The time lag between an input measurementand the data message transmission.

ETI: Acronym for Elapsed Time Indicator. ETI monitors thetime of operation for a device.

Hysteresis: The tendency of an instrument to give a differentoutput for a given input, depending on whether the inputchange resulted from an increase or decrease of the previousvalue.

Impact Pressure: Sum of the total pressure minus the localatmospheric pressure. The pressure a moving stream of airproduces against a surface which brings part of the movingstream to rest. Subsonically, it is commonly referred to as thecompressible dynamic pressure. It is the sum differencebetween the total and static pressures, Pt - Ps = qc. Indicatedby the symbol, qc.

Indicated Airspeed (IAS): The speed of an aircraft with respectto the surrounding air mass. It is uncorrected for anyinstallation or instrument errors. Indicated airspeedrepresents true airspeed at standard sea level conditions only,and it is a function only of impact pressure, qc.

Isentropic: Without change in entropy (the unavailability ofenergy in a system) over time.

AIR DATA GLOSSARY

AIR DATA GLOSSARY

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Long-term Stability : The ability of the transducer to maintainoperation within its static accuracy specifications over a longtime period (one year minimum). Factors affecting long-termstability include aging and outgassing of electroniccomponents.

LRU: Acronym for Line Replaceable Unit. An assembly thatcan be replaced on the flight line or local maintenance facility.

Mach Number: The ratio of true airspeed to the speed ofsound in the surrounding air. The speed of sound varies asthe square root of average temperature. Mach number isdetermined using the ratio of impact to static pressure.Indicated by the symbol, M.

MIL-STD-1553B: This military communication standardwhich uses a bi-directional, high-speed digital data bus. Oftenused for sending communications within a military air datasystem.

Mmo: The maximum operating Mach number certified for agiven aircraft.

Non-linearity : The departure from a desired linearrelationship between corresponding input and output signals.

NOVRAM: Acronym for Non-volatile Random AccessMemory. A digital memory device that maintains data whenpower is removed.

Operating Accuracy: The uncorrectable error caused byexposure to external operational conditions (primarily theambient temperature operating range).

Pitot Tube (also Pitot Probe): An open-end tube facingforward into the air flow to measure total pressure (Pt).

Pitot-Static Tube (also Pitot-Static Probe): An open-end tubefacing forward into the air flow for measuring total pressure(Pt), and with ports to measure local static pressure (Ps).

Pneumatic Lag: The time elapsed between the sensing of apressure and when that pressure is pneumaticallytransmitted through piping and received by the transducer.

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

Recovery Error: The per unit or fractional total temperatureerror.

Recovery Factor: The proportion of kinetic energy convertedto heat. A recovery factor of one means all kinetic energy isconverted to heat. In such a case, the recovery temperature isequal to the total temperature.

Recovery Temperature: The equilibrium temperature of asurface with a given recovery factor or recovery error.Indicated by the symbol, Tr.

Repeatability: The ability of an instrument to duplicate, withexactness, the measurements of a given value.

Resistance Range: In temperature sensors, the range ofresistances (in ohms) corresponding to the range of desiredtemperature measurements. Indicated by the variable, R0.

Resolution: The exactness of the numbers used to portray themeasurement. It is usually affected by the number of databits 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 interfacehardware and protocols are identical. However, RS-422specifies a transmit channel operating in broadcast modeonly. RS-485 specifies a bi-directional transmit and receivechannel.

Set Point or Relative Accuracy: The error between actualperformance and expected or operational set point,independent of absolute accuracy. Typically applied to flightcontrol “hold” functions.

Short-term Stability: A system describer which is affected byfactors such as signal noise, sensing element response time,conversion speed and external environment.

Standard Atmosphere: A well-defined relationship betweenstatic air pressure, temperature and altitude. It is calculatedfrom the hydro-static equation using a standard variation oftemperature with height from a fixed pressure datum point(usually taken above mean sea level for the earth).

AIR DATA GLOSSARY

Standard Sea Level Conditions: The term used for sea levelvalues of the standard atmosphere, specifically 15°C (59.0° F)and 29.92126 inches of mercury (in. Hg).

Static Ports: An opening in a plate carefully placed to be flushwith the aircraft skin used, in most flight conditions, tomeasure true static pressure (Ps). Sometimes called staticvents.

Static Pressure: The absolute pressure (total pressure abovethat of a vacuum) of still air surrounding a body. Put anotherway, it is the absolute air pressure that would have existed atthe aircraft’s location in the atmosphere, if the aircraft hadcreated no pressure disturbances. Indicated by the variable,Ps.

Static Accuracy: The uncorrectable error caused by thecombined effects of non-linearity, hysteresis, repeatability andcalibration. This is the combination of all errors in theabsence of transient conditions.

Static Air Temperature (SAT): The temperature of undisturbedair through which the aircraft is about to fly. It is the localtemperature of the air with no element due to the velocity ofthe air. Static temperature is lower than recovery or totaltemperature. Indicated by the symbol, T.

Temperature Transient: A dynamic temperature condition notperiodically repeated. The term “transient” often implies ananomalous, temporary departure from a steady-statetemperature condition. The departure may be either constantor cyclic.

Total Pressure: The sum of local atmospheric pressures plusdynamic (operating) pressures. Total pressure is the sum ofstatic and impact pressures. Indicated by the variable, Pt.

Total Air Temperature (TAT): The temperature of an airflowmeasured as the airflow is brought to rest without removal oraddition of heat. Total temperature is higher than static orrecovery temperature because of adiabatic compression of airgoing to zero velocity. Indicated by the symbol, Tt.

Transducer: Device for translating a physical phenomenafrom one form to another. In air data, transducers are mostcommonly used to translate physical measurements ofpressure or temperature into electrical signals (analog ordigital) for transmission to the aircraft’s control or displayinstruments.

True Airspeed (TAS): Indicated airspeed corrected fornonstandard temperatures that can be determined usingMach number and total temperature information. It is theactual aircraft speed through the air mass. Indicated by thesymbol, V.

Update Rate: The transmit intervals for each item ofinformation transferred from a transducer on a digitalcommunication bus.

Vertical Speed: The aircraft’s rate of change in height. Alsoreferred to as rate of climb, rate of descent, or altitude rate.

Vmo: The maximum permitted operating true airspeed for agiven aircraft under any condition.

Asssume Freestream StaticTemperature = 15 degrees C

Measure Actual Total Air Temperature

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INDEX

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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

WE’RE HERE TO HELPSome air data questions related to specific applications may not be directly addressed in the handbook. After five decadesof providing the finest air data measurement devices and systems for commercial and military aircraft worldwide, we knowlistening to your questions, comments, and suggestions - whether you are a current customer or not - helps us deliversuperior 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 toanswer 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 theGoodrich Air Data Computers can solve your air data problems, contact a Goodrich air data computer expert today at:

Sensor SystemsGoodrich CorporationAttn: Pressure Product Marketing14300 Judicial RoadBurnsville, MN 55306-4898USATel: 952 892 4000Fax: 952 892 4800

www.aerospace.goodrich.com

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SSeennssoorr SSyysstteemmssGGooooddrriicchh CCoorrppoorraattiioonn14300 Judicial RoadBurnsville, MN 55306-4898USATel: 952 892 4000Fax: 952 892 4800

www.aerospace.goodrich.com

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