Instrumentation: Measurement of Air Temperature, Pressure, and Speed

Table ojCOl1lelllS CHAPTER 1 Air Temperature Measurement Introduction................................................................................................................................. 1-1 Direct Reading Thermometer.................................................................................................. 1-3 Electrica l Thermometer.......................................................................................... 1-3 Static Air Temperature Sensor............................................................................ 1-4 Total Air Temperature Probe.............................................................................................. 1-5 CHAPTER 2 Pitot-Static System Introduction.................................................................................................... 2-1 Pilot Tube................................................................................................................................... 2-1 Sialic Source................................................................................................................................... 2-2 Alternale Stalic Source.............................................................................................................................. 2-3 Combined Pilol-Slalic (Pressure) Head.............................................................................................. 2-4 Operating Problems.................................................................................................................................2-5 PitoVStatic System Errors.........................................................................................................................2-6 CHAPTER 3 Pressure Altimeter Introduction................................................................................................................................................ 3-1 Pressure Altitude.........................................................................................3-2 Density Altitude............................................................................3-2 The Simple Altimeler........................................................................................................................ 3-2 Datum Sub-Scale Settings................................................................................................ 3-3 The Sensitive Altimeter...................................................................................................... 3-4 Altimeter Displays..................................................................................................................................... 3-4 Design Errors.......................................................................................................................................... 3-7 Errors due to Calibration...................................................................................................3-7 Blockages and Leakages.......................................................................................................... 3-9 Servo Altimeters......................................................................................................................... 3-9 Operation of a Servo-Altimeter................................................................................................................. 3-10 Servo-Altimeler Power Failure............................................................................................ 3-11 Altitude Encodi ng........................................................................................................................... 3-1 1 Advantages of Servo-Altimeters........................................................................... 3-11 CHAPTER 4 Vertical Speed Indicator Inlroduction....................................................................................................................................4-1 Principle of Operation.....................................................................................................................4-1 Operation of the VS I..................................................................................................................4-2 Errors of Ihe VSI..................................................................................................... 4-2 Lag................................................................................................................................................4-2 Instrume nt Error.....................................................................................................................................4-2 Manoeuvre Induced Error........................................................................................................................4-2 Faults of the VSI................................................................................................... 4-2 Blockages.....................................................................................................................................4-2 Breakage or leakage in the Stalic Line................................ ;.....................................................................4-2 Inslantaneous Ve rtical Speed Indicator (IVSI).....................................................................4-2 Operation of Ihe IVSI I ILVSI........................................................................ 4-3 instnImentation V II Table of Contents CHAPTER 5 Airspeed Ind icator Introduction................................................................................................................................ _. __ 5-' Principle of the Airspeed Indicator (ASI).............................................................................................. __ 5- ' Operation of a Simple ASI................................................................................................................ _ _ 5- ' Sensitive and Servo Airspeed Indicators......................................................................... _.. 5-1 Calibration of the ASI.................................................................................................... 5-2 Colour Coding of the ASI.................................................................................................................. 5-2 ASI Errors................................................................................................................................. 5-3 ASI Faults................................................................................................................................................. 5-5 Calculation of CAS to TAS (up to 300 knots).............................................................................................. 5.£ CHAPTER 6 Machmeter Introduction..................................................................................................................................... 6-1 Critical Mach Number (M"it) "..................................................................................................... 6-1 Principle of Operation............................................................................................................................... 6-2 Machmeter Construction and Operation........................................................................................ 6-4 Calibration............................................................................................................................................. 6-4 Errors............................................................................................................................................. 6-5 Blockages and Leakages.................................................................................................................... 6-5 Accu racy.................................................................................................................................. 6-5 Serviceability Checks....................................................................................................... 6-5 CHAPTER 7 Mach Airspeed Indicator Introduction................................................................................................................................. 7-1 Display................................................................................................................................... 7-1 VMO Pointer.................................................................................................................................. 7-1 Drive n Cursor.............................................................................................................................................. 7-2 Bugs.............................................................................................................................................. 7-2 Linkages............................................................................................. 7-2 E~ffi...............................................................................................................................................M CHAPTER 8 Central Air Data Computer Introduction............................................................................................................. 8-1 The Central Air Data Computer................................................................................................................. 8-1 Conversion of Sensing Pressures..................................................................................................... 8~2 Digital Air Data Computer.......................................................................................................................... 8-4 V 1I1 Instrumentation Table o/COl11el11s CHAPTER 9 Basic Magnetism Introduction.................................................................................................................................................9-1 Magnetic Properties................................................................................................................................... 9-1 Fundamental Laws of Magnetism........................................................................................... 9-3 Characteristics of Lines of Magnetic Flux........................................................................... 9-4 Magnetic Materials......................................................................................................................9-5 Ferromag netic.................................................................................................................................. 9-5 Paramagnetic......................................................................................................................................... 9-5 Diamagnetic........................................................................................................................................ 9-5 Permeability............................................................................................................................................... 9-6 Electromagnetism............................................................................................................................ 9-6 An Electromagnet...................................................................................................................................9-7 Magnetic Moments....................................................................................................................... 9-8 Period of Oscillation of a Suspended Magnet..............................................................................................9-8 CHAPTER 10 Terrestrial Magnetism Introduction.............................................................................. 10-1 Magnetic Dip.............................................................................................1 0-2 Earth's Total Magnetic Force................................................................................................... 10-4 Examples..................................................................................................................................... 10-4 Magnetic Variation....................................................................................................................... 10-5 CHAPTER 11 Aeroplane Magnetism Introduction............................................................................................................................................11 -1 Types of Aeroplane Magnetism............................................................................................................... 11-1 Components of Hard Iron Magnetism.............................................................................................. 11-2 Components of Soft Iron Magnetism...................................................................... 11 -5 Determination of Deviation Coefficients.............................................................. 11-6 Minimum Deviation................................................................................................................................11-7 Maximum Deviation........................................................................................................................ 11-7 Joint Airworthiness Requirements (JAR) Limits............................................................. 11-8 Compass Swinging.................................................................................................... 11 -9 The Compass Swing Procedure............................................................................................................. 11-9 An Example of a Compass Swing..................................................................................... 11-1 0 Deviation Compensation Devices......................................................................................................... 11 -11 CHAPTER 12 Direct Reading Magnetic Compass Introduction.............................................................................................................. 12-1 Properties of a Direct Reading Compass.................................................................12-2 'E' Type Compass........................................................................ 12-3 Pre-fiight Checks............................................................................................................. 12-4 Principle of a Pendulum........................................................................................................... 12-4 Acceleration Erro rs.......................................................................................................... 12-4 Turning Errors...........................................................i................................................................... 12-7 Instrumentation IX Table of Contents CHAPTER 13 Remote Indicating Compass Introduction........................................................................................................................................ 13-1 RI C Architecture....................................................................................................................... 13-1 Principle of a Flux Detector Elemenl.................................................................................................... 13-1 Flux Detector Unit................................................................................................................................. 13-4 Operation of the Remote Ind icating Compass System......................................................................... 13-5 Gyroscope Element........................................................................................................................... 13-8 Heading Indicator................................................................................................ 13-8 Modes of Operation.................................................................................................. 13-9 Synchro nising Indicators................................................................................ 13-9 Manual Synchro nisation............................................................................................... 13-10 Operation of an RIC in a Turn................................................................................................................ 13-1 0 Advantages of a Remote Indicating Gyro Magnetic Compass............................................... 13-11 Disadvantages of the Remote Indicating Gyro Magnetic Compass....................................................... 13-11 C HA PTER 14 Gyroscopic Principles Introduction.............................................................................................................................................. 14-1 Principle of Construction.................................................................................................. 14-2 Gyroscopic properties.............................................................................................................................. 14-2 Types of Gyroscopes....................................................................................................................... 14-4 Power Sources for Gyroscopes....................................................................................... 14-6 The Disadvantages and Advantages of Air Driven Gyros............................................................... 14-7 The Disadvantages and Advantages of Electrically Driven Gyros.............................................. 14-7 Gyro Wander................................................................................................................................ 14-8 Horizontal Axis Gyro......................................................................................................................... 14-8 Vertical Axis Gyro................................................................................................................. 14-10 Transport Wander....................................................................................................... 14-10 Examp les of Gyro Wander.................................................................................................................... 14-11 CHAPTE R 15 Direction Indicator Introduction............................................................................................................................................. 15-1 Basic Description of the Direction Indicator........................................................................................ 15-1 Operation of the Direction Indicator................................................................................................... 15-2 Errors Associated with the Ai r Driven Direction Indicator......................................................................... 15-4 Use of the Direction Indicator (DI).......................................................................................... 15-4 Advanced Use of the Direction Indicator................................................................................................ 15-4 Sample Calcu lation................................................................................................................... 15-5 x Instrumentation Table ojCol1lel1ls CHAPTER 16 Artificial Horizon Introducti on........................................................................................................................................ 16-1 Ai r driven (Classic) Artificial Horizon................................................................................................ 16-3 Construclio n..............................................................................................................................................16-3 Operation.................................................................................................................................................16-4 Erection Syslem............................................................................................................................ 16-5 Errors..................................................................................................................................... 16-6 Eleclrically Driven Artificial Horizon.................................................................................. 16-7 Con struction.............................................................................................. 16-7 Torqu e Motor and Levelling Switch System....................................................................... 16-8 Fast Erecti on................................................................................................................................. 16-8 Errors................................................................................................................................................ 16-9 Remote Vertical Gyro............................................................................................................................... 16-9 Siandby Attilude Indicator...................................................................................................................... 16-10 CHAPTER 17 Turn and Balance Indicator Introduction............................................................................................................................. 17-1 Turn Indicator.............................................................................................17-1 Construclion and Principle of Operation.................................................................................. 17-1 Operation.................................................................................................................................... 17-3 Errors..................................................................................................................................................... 17-4 Pre-flight Check............................................................................................................... 17-4 The Balance Indicator.......................................................................................................... 17-4 Construction and Operation................................................................ 17-4 Limitations and Errors.............................................................................................................................. 17-5 Pre-Flight Ch eck...................................................................................................................... 17-5 Electrically Driven Turn and Balance Indicators.................................................................................. 17-5 Typica l Indications on a Turn and Balance Indicator...................................................................... 17-6 CHAPTER 18 Turn Co-ordinator Introduction............................................................................................................................ 18-1 Principle of Operalion.............................................................................................................................18-2 Instrumentation xi Table o/Contents CHAPTER 19 Inertial Navigation System InlmducIion....................................................................................................................................... 19·1 The Principle and Construction of an Accelerometer.............................................................................. 19· 1 Performance............................................................................................................................................. 19·3 Operation of a Gyro-Stabilised Platform.................................................................................................. 19-3 Setting-up Procedures........................................................................................................................... 19-6 Levelling.................................................................................................................................................. 19-6 Alignmenl............................................................................. 19-7 Levelling and Alignment................................................................................................... 19-7 Corrections.................................................................................................................... 19·7 Coriolis............................................................................................................... 19-8 Centripetal Acceleration................................................................................................................. 19-8 Wander Azimulh Syslem......................................................................................................... 19-9 The Schuler Tuned Platform................................................................................................................... 19-9 Errors................................................................................................................................... 19-11 The Advantages and Disadva nlages of an INS..................................................................................... 19-12 Mode Selector Panel............................................................................................................................. 19·13 Control Display Unit............................................................................................................................ 19-14 CHAPTER 20 Inertial Reference System Introduclion......................................................................................................................... 20·1 Description of the Sirap-Down Syslem.......................................................................... 20-1 Solid State Gyros............................................................................................................ 20-2 Ring Laser Gyro......................................................................................................................... 20-2 Fibre Optic Gyro...................................................................................................................................... 20·3 Advantages and Disadvantages of RLG s................................................................................................ 20·3 Alignment of the Inertial Reference System....................................................................................... 20-4 Performance.............................................................................................................................................. 20-4 The Control, Display, and Output from an IRS.................................................................................... 20·4 Description of a Typical IRS.................................................................................................................... 20-5 IRS Transfer Switch..................................................................................................................... 20-7 IRS Alignment...................................................................................................................... 20-7 Loss of Alignment in Flight.................................................................................... 20-7 CHAPTER 21 Radio Altimeter Introduction............................................................................................................................ 21-1 The Radi o Altimeter System.................................................................................................. 21-1 Principle of Operation of a Radio Altimeter.......................................................................... 21-3 Performance and Accuracy of a Radio Altim eter............................................................................21-4 Errors Associated with a Radio Altimeter......................................................... 21-4 Leakage Errors........................................................................................................................... 21-4 Mushing Errors.......................................................................................................................... 21-4 The Advantages of a Radio Altimeter..................................................................................................... 21-5 xii Instrumentation Table o/Contents CHAPTER 22 Electronic Flight Instrument System Introduction....................................................................................................................... 22-1 EFIS Architecture......................................................................................................................22-1 Symbol Generator (SG ).............................................................................................................................22-2 Instrument Comparator Unit (ICU).............................................................................................................22-2 Compression Mode......................................................................................................................... 22-2 Temperature Sensing Units........................................................................... 22-3 Mode Control Panels (MCP)...................................................................................................................... 22-3 Light Sensors.................................................................................................... 22-3 Attitude Director Indicato r (ADI ).................................................................................................... 22-3 Radio All itude................................................................................................................................ 22-5 Decision Height....................................................................................... 22-5 Localiser and Glide Slope Indication.........................................................................................................22-5 The Horizontal Siluation Indicator (HSI).......................................................................................... 22-5 Plan Mode..................................................................................................................................22-8 VOR and ILS Modes............................................................................................ 22-8 EFISIIRS Interface............................................................................................................... 22-10 Heading Reference Switch....................... 22- 11 HSI Symbology..................................................................................................................................... 22-11 CHAPTER 23 Flight Management System Introd ucti on................................................................................................................................ 23-1 The Flight Management Computer System.................................................. 23-2 Command Di splay Unit............................................................................ 23-4 Control Panels.................................................................................................................................. 23-5 CDU and FMC Terminology............................................................................................................ 23-7 The Flight Management Computer Memory......................................................................... 23-9 General FMS Operation........................................................................................................................ 23-11 Pre-Flight....................................................................................................................... 23-12 Enro ute.......................................................................................................................................... 23- 13 Lateral Navigation (LN AV)........................................................................................................... 23- 13 Vertica l Navigation (VNAV)............................................................................... 23-14 Operational Notes....................................................................................................................23-15 Fuel Monitoring.......................................................................................... 23-1 6 Flight Control and Management Summary............................................................................................ 23- 16 CHAPTER 24 Flight Director System Introduction....................................................................................................................................... 24-1 Flight Directo r Architecture....................................................................................................... 24-1 Flight Director Control Inputs.............................................................................................. 24-3 Th e Flight Director Computer (FDG).................................................................................................. 24-4 Mode Control Unit................................................................................................................................... 24-4 Flight Director Displays...................................................................................................... 24-4 Flight Director Modes of Operation................................................................................ 24-6 Attitu de Mode.......................................................... 24-6 Heading............................................................................................. 24-7 Altitude Hold......................................................................................... 24-9 LocaliserNOR (LOCNOR)........................................................................................... 24-9 Glideslope (GS)................................................................................................... 24- 11 Go-Around (GA).................................................................................................................................. 24-12 Mode Annunciator................................................................................................................................24-12 Operation of the Attitude Director Indicator....................................................................24-12 The Horizontal Situation Indicator (HSI) Flight Director Comman ds............................. 24- 14 In strumentation xii i Table o/Contents CHAPTER 25 Automatic Flight Control System Introduction................................................................................................................................... 25-1 Stability and Control Augmentation........................................................................................... 25-1 Attitude Hold........................................................................................................................................... 25-1 Flight Control............................................................................................................................................ 25-1 Classification of an AFCS........................................................................................................................ 25-2 Control Channels................................................................................................................. 25-2 Inner Loop Control (Stabilisation).......................................................................................................... 25-2 Operation of an Inner Loop Pitch Stabilisation System........................................................ 25-3 Outer Loop Control................................................................................................................................. 25-4 Roll Modes............................................................................................................................... 25-5 Pitch Modes............................................................................................................................................. 25-7 Combined Roll and Pitch Modes............................................................................................................... 25-8 Attitude Sensing................................................................................................................................. 25-9 The AFCS Computer (Signal Processor)................................................................................................ 25-10 Servomotor Actuators................................................................................ 25-11 Autopilot Terminol ogy................................................................................................................... 25-12 Cross Coupling............................................................................................................................... 25-14 ILS Coupling.................................................................................................................. 25-14 VOR Coupling......................................................................................................................... 25-15 Stability Problems................................................................................................................ 25- 16 Yaw Damper.................................................................................................................................. 25-16 Mach Trim System.................................................................................................. 25-17 Automatic Pitch Trim Control....................................................................................................... 25-18 Interlocks......................................................................................................................................... 25-20 Synchronisation................................................................................................................................ 25-20 Instinctive Cut Out............................................................................................................ 25-20 CHAPTER 26 Automatic Landing System Introduction............................................................................................................................................. 26-1 Basic Requirements for an Automatic Landing Syste m................................................................. 26-1 Automatic Landing System Terminology.................................................................................................. 26-2 Automatic Landing System Equipment Requirements......................................................................... 26-3 Automatic Approach , Flare, and Landing Sequence................................................................................ 26-4 Weather Minima.............................................................................................................................. ,. 26-5 ICAO Categorisation for Low Visibility Landing Capabilities............................................................ 26-5 The Fundamental Landing Requirement............................................................................................. 26-6 System Reliability and Integrity...................................................................................................... 26-7 CHAPTER 27 Thrust Management Systems Introductio n...................................................................................................................... 27-1 Determi ning the Thrust Required............................................................................. 27-1 Calculation of Climb and Cruise Thrust...................................................................................................... 27-2 Cruising Methods........................................................................................................................... 27-3 Electro nic Engine Control (EEC)................................................................................. 27-3 Full Authority Digital Engine Control (FADEC)................................................................................... 27-3 Autothrottle (A/T).................................................................................................... 27-6 Thru st Lever Operation........................................................................................................................... 27-7 Thru st Management via the Autothrottle.................................................................................................. 27-7 Thrust Management Computer (TMC )............................................................................................. 27-8 Thru st Mode Select Panel (TMSP)................................................................................................. 27-8 XI V Instrumentation Table o/Contents CHAPTER 28 Central Warning System Introduction................................................................................................................................................ 28-1 Central Wa rning System Annunciator Panel............................................................................................. 28-1 Aural Warni ngs................................................................................................................................. 28-2 CHAPTER 29 Altitude Alerting System Introduction.............................................................................................. 29-1 Altitude Alerting System Operation..................................................................................................... 29-1 CHAPTER 30 Ground Proximity Warning System Introduction.................................................................................................................................... 30-1 GPWS System Arch itecture........................................................................... 30-1 GPWS Modes.......................................................................................................................................... 30-2 Warni ng System................................................................................................................................... 30-3 GPWS Control Panel..................................................................................................................30-4 Discretionary Response...........................................................................................30-5 Wa rn ing Inhibition............................................................................................................................. 30-6 The Reporting of GPWS Events....................................................................................................30-6 Operation of th e GPWS...................................................................................................................... 30-6 Mode 1............................................................................................. 30-6 Mode 2.......................................................................................................................... 30-7 Mode 3.....................................................................................................................................................30-8 Mode 4......................................................................................................... 30-8 Mode 5...............................................................................................................................30-10 Mode 7................................................................................................................................................... 30-10 Joint Aviation Req uireme nts...................................................................................................... 30-11 CHAPTER 31 Traffic Collision Avoidance System Introduction.......................................................................................................................................... 31-1 TCAS I................................................................................................................................................ 31 -1 TCAS II.................................................................................................................................................... 31- 1 TCAS III.............................................................................................................................................. 31-1 Aeroplane Installation................................................................................................... 31-2 Operation of TCAS II.................................................................................................................. 31 -4 TCAS Aural Wa rnings........................................................................................................................ 31 -4 Inform ation Displ ay........................................................................................................ 31-5 Resolution Advisory I Vertical Speed Indicator (RA I VS I)................................................................. 31-7 TCAS Control Panel..................................................................................................31 -8 Operating Restrictions................................................................................................................ 31 -8 Instru mentation xv Table o/Conten/s CHAPTER 32 Mach/Airspeed Warning System Introduction.......................................................................................................................................... 32-1 System Architecture and Operation........................................................................................................ 32-1 Maximum Operating Ai rspeed Schedule.............................................................................................. 32-2 CHAPTER 33 Stall Warning Introduction............................................................................................................ 33-1 light Aeroplane Stall Warning Device..........................................................................33-1 Transport Category Aeroplane Stall Warning Device.............................................................................. 33-2 CHAPTER 34 Recording Devices Introduction............................................................................................................................................. 34-1 Flight Data Recorder (FDR) Requirements............................................................................................. 34-1 FDR Design............................................................................................................................................... 34-3 Cockpit Voice Recorder (CVR) Requirements........................................................................................... 34-3 CVR Design..................................................................................................................................... 34-4 CHAPTER 35 General Engine Instrumentation Introducti on........................................................................................................................... 35-1 Pi ston Engines.................................................................................................................. 35-1 Turbo Propeller Engines.................................................................................... 35-2 Gas Turbine Engines.............................................................................................................................. 35-2 CHAPTER 36 Pressure and Temperature Sensors Introd uction................................................................................................................................ 36-1 Pressure Measurement............................................................................................................... 36-1 Temperature Measurement..................................................................................................................... 36-3 CHAPTER 37 Pressure and Temperature Indicators Introduction................................................................................................................. 37-1 Pressure Indicators................................................................................................. 37-1 Manifold Pressure (MAP)........................................................................... 37-1 Engine Pressure Ratio (EPR)................................................................................................................ 37-1 Fuel and Oil Pressures..................................................................................................... 37-3 Temperature Indicators............................................................................................................. 37-3 Cylinder Head Pressure..................................................................................... 37-3 Exha ust Gas Temperature.........................................................................................................37-3 Fuel and Oil Temperatures..................................................................................................... 37-4 xvi I nstmmentation Table of Con/ems CHAPTER 38 RPM Indicators and Propeller Synchroniser Systems Introduction............................................................................................................................................... 38-1 Tachometers.................................................................................................................. 38-1 Magnetic Drag Tachometer......................................................................................................................... 38-1 Tacho-Generator and Indicator System...................................................................................................... 38-2 Tachometer Probe and Indicator System.................................................................................................... 38-3 Propeller Auxiliary Systems.............................................................................................................. 38-6 Synchronisation System......................................................................................................................... 38-6 Synchrophasing System............................................................................................38-7 Operation of a Synchrophasing System................................................................................................... 38-8 CHAPTER 39 Engine Torque Measurement Introduction................................................................................................................................ 39-1 Torque Meter........................................................................................................ 39-1 Negative Torque Sensing....................................................................... 39-3 CHAPTER 40. Vibration Monitoring Introduction.......................................................................................40-1 Vibration Monitoring System.....................................................................................................................40-2 CHAPTER 4 1 Fuel Gauge Introduction.............................................................................................................................................41-1 Measurement of Fuel Quantity............................................................................................41-1 Float Type...........................................................................................................................41-1 Ratio Metre Type Fuel Gauge..................................................................................................................4 1-2 Capacitance Type of Fuel Gauge.............................................................................................................41-2 Fuel Totatiser.........................................................................................................................41-5 Fuel Flow........................................................................................................................................41-5 CHAPTER 42 EICAS Introduction................................................................................................................42-1 EI CAS Architecture................................................................................................................42-1 Engine Displays.........................................................................................42-2 Crew Alerti ng.......................................................................................................................................42-3 Warnings (Levet A)....................................................................................................................................42-3 Cautions (Level 8)....................................................................................................................................42-4 Advisories (Level C)..............................................................................................................42-4 Master Warning/Caution Light..........................................................................................................42-4 Inhibits.....................................................................................................................................................42-4 Display Messages..........................................................................................................42-4 Status.................................................................................1...............................................................42-6 Maintenance............................................................................................................................................42-7 EICAS Failure Modes.......................................................................................................................42-7 Instrumentation xvii Table of Contents CH APTER 43 ECAM Introduction............................................................................................................................. 43-1 Engine I Warni ng (EIW) CRT Display............................................................................................ 43-2 The System I Status (SIS) CRT Display................................................................................................... 43-3 ECAM System Arch itectu re..................................................................................... 43-3 Flight Warn ing Compute rs (FWCS)......................................................................................................... 43-4 Display management Computers (DMC)...................................................................................... 43-4 System Data Acqu isition Concentrators (SDAC )...................................................... 43-4 ECAM Control Panel (ECP)........................................................................................................ 43-4 Attention Getters........................................................................................................................ 43-5 ECAM System Failure............................................................................. 43-6 Failure Categorisation............................................................................................................ 4 3-6 System Operation................................................................................................... 43-6 XV II I Instrumentation INTRODUCTION The temperature of air under pure static conditions at the various fiight levels is one of the basic parameters required to establish data that is vital to the performance monitoring of modern aeroplanes. The measurement of Static Air Temperature (SAT) by direct means is, however, not possible for all types of aeroplanes, because the measurement is potentially affected by adiabatic compression of the air at increased airspeeds. The boundary layer over the outer surface of an aeroplane flying below Mach 0.2 is very close to SAT, but at higher Mach numbers the boundary layer slows or even stops relative to the aeroplane. This in turn results in adiabatic compression, which causes the air temperature to rise well above SAT. This increase is known as RAM Rise , and the temperature indicated under such conditions is known as Ram Air Temperature (RAT). RAT = SAT + RAM Rise or SAT = RAT - RAM Rise The majority of RAM Rise is due to adiabatic compression because of the airflow coming to rest, with only a relatively small amount due to friction between the surface of the aeroplane and the high-speed airflow. The RAM Rise is always pre-calculated and is tabulated or graphed as a function of Mach number in the Operations or Flight Manual for each type of aeroplane , a sample of which is shown below. Instmmentation 1-1 Chapler 1 Air Temperature j\lleasliremel1l INDICATED "ACH N~ER.301.401.501.601. 701.731. 761.781.801.821.84 tHD TAT - 'e TRUE OUTSIDE AIR TE"PERATURE - DEGREES C 70 47 39 37 35 33 31 29 27 65 49 42 35 33 30 28 26 25 23 60 49 44 37 30 28 25 24 22 21 19 55 49 45.40 33 26 24 21 19 18 16 14 50 45 40 35 28 21 19 17 15 13 10 45 40 35 30 23 17 15 12 11 9 7 "5 40 35 30 25 19 12 10 8 6 4 3 1 35 30 26 20 14 8 6 3 1 0 - 2 - 3 30 25 21 16 10 3 1 - 1 - 3 - 5 - 6 - 7 25 20 16 11 5 - 2 - 3 - 6 - 7 - 9 -11 -12 20 15 11 6 0 - 6 - 8 -10 -12 -13 -15 -16 15 10 6 2 - 5 -11 -13 -15 -16 -18 -19 -21 10 5 1 - 3 - 9 -15 -17 -19 -21 -22 -24 -25 5 0 - 3 - 8 -14 -20 -21 -24 -25 -27 -28 -29 0 - 5 - 8 -13 -18 -24 -26 -28 -30 -31 -33 -34 - 5 -10 -13 -18 -23 -29 -31 -33 -34 -35 -37 -38 -10 -15 -18 -22 -28 -33 -35 -37 -39 -40 -41 -43 -15 -20 -23 -27 -32 -38 -39 -42 -43 -44 -46 -47 -20 -24 -27 -32 -37 -42 -44 -46 -47 -49 -50 -51 -25 -29 -32 -36 -42 -47 -49 -51 -52 -53 -55 -56 -30 -34 -37 -41 -46 -51 -53 -55 -57 -58 -59 -60 -35 -39 -42 -46 -51 -56 -58 -60 -61 -62 -63 -65 -40 -44 -47 -51 -56 -61 -62 -64 -65 -66 -68 -69 The proportion of RAM rise measured is dependent on the ability of the sensor to sense (or recover) the temperature rise. The sensitivity is expressed as a percentage and is known as the recovery factor. For example, if a sensor has a recovery factor of 0.80, it measures SAT + 80% of the RAM Rise. A useful approximation of RAM Rise is: ( TAS kt )2 100 For example, for an estimated TAS of 460 kt deduct (460 kt)2 from the indicated air 100 temperature, which is approximately equal to 21 "C. If this figure is subtracted from the Indicated Outside Air Temperature (IOAT), it gives the Corrected Outside Air Temperature (COAT), which is the best determinable val ue of the temperature of the air through which the aeroplane is flying , and is that which is required for navigational purposes: COAT = IOAT - RAM Rise 1-2 Instrumentation Ai,. Temperature Measurement Chapter / Alternatively written as: Where: Ts =SAT in degrees absolute. TM = Indicated temperature in degrees absolute. K = Recovery Factor. M = Mach Number. Various types of air temperature sensors are fitted to aeroplanes, although the particular type is dependent on whether SAT or RAT is required, and the normal operating speed of the aeroplane on which it is fitted. DIRECT READING THERMOMETER This is the simplest type of thermometer, and it only indicates SAT. It consists of a bi-metallic element in the shape of a helix, which expands and contracts when subjected to temperature changes....-_.... Pointer Scal. As the temperature changes, the helix winds or unwinds, and causes a pointer to rotate against a scale calibrated in degrees Celsius. This type of thermometer is normally only fitted on low-speed light aeroplanes, and passes through the fuselage or canopy with the sensitive element projecting into the airflow. The element is also shielded so that it is not directly affected by solar radiation. Another method of making a direct read ing thermometer is to use a liquid filled bulb and capilla ry tube, with the bulb mounted externally in the airflow and the capillary tube transmitting the liquid pressure to a Bourdon tube. As the temperature to which the bulb is subjected changes, the liquid expands or contracts, and causes the Bourdon tube to wind or unwind. This movement of the Bourdon tube is connected to a pointer, which moves around a scale calibrated in temperature. ELECTRICAL THERMOMETER The resistance of an electrical conductor is dependent on temperature , and its magnitude changes due to thermal changes , thus altering its ability to oppose current flow when connected in a circuit. The Electrical (Wheatstone Bridge) Thermometer commonly uses the temperature/resistance technique. In strumentation 1-3 Chapter 1 Air Temperature Measurement TEMPERATURE SENSITIVE R4 RESISTANCE A c WHEATSTONE BRIDGE CIRCUIT FOR MK. 1 INDICATOR F ANDOR MK. 1 BASIC CIRCUIT AND MK. 1A THERMOMETERS MK. 2 THERMOMETERS (1 ) (2) (3) The bridge consists of four resistance arms, which connect across a low voltage source as shown above. When the system is switched on, current flows in the circuit and divides at point A before flowing through R1 and R2 at strengths that vary as the temperature of R4 (positioned outside the fuselage) alters. At point C the currents re-unite and flow back to the voltage source. If the Bridge is balanced resistance-wise , no current flows and the moving coil galvanometer (meter) read s zero. If the temperature (resistance) of R4 changes, the bridge becomes unbalanced, and the resulting current flowing through the galvanometer registers as a change in temperature. STATIC AIR TEMPERATURE SENSOR The majority of temperature sensors use a platinum wire element contained either in a probe mounted in what is termed a flush bulb configuration , or in a specially designed probe shielded from solar radiation. A flush bulb configuration is subjected to both air temperature and skin temperature and is, therefore , less accurate than a probe type sensor. In the type of sensor shown above the probe protrudes through a hole in the aeroplane skin with the orifice facing forward into the airfow. Air enters the probe and comes to rest where a sensing element forms one leg of a Wheatstone bridge. The air flows out from the probe through the spill ports allowing a limited rate of change of air measured at the element. 1-4 Instrumentation Air Temperature Measurement Chapter I Sometimes known as a total temperature head, this probe is less accurate than the total air temperature probe described below and is usually only fit to simpler and or slower aircraft where RAM rise is limited. The ability to sense RAM rise accu rately is known as the recovery factor, and this type of probe has typical values of between 0. 75 and 0.95. TOTAL AIR TEMPERATURE PROBE For aeroplanes operating at high Mach numbers, it is usual to sense and measure the maximum temperature rise possible, which is Total Air Temperature (TAT) if the recovery factor is 1, and Ram Air Temperature (RAT) if the recovery factor is less than 1. TAT or RAT is the temperature of the air when it is brought to rest (or nearly so) without the addition or remova l of heat. The Rosemount probe shown below has a recovery factor of virtually 1, and is, therefore , known as a TAT probe , and is commonly used on transport-category aeroplanes. AIR FLOW ,; , - / ,." =".\ == ---------.;: - -d., - - AI~ SC~OP 'I I f.-lWOCO NCENTRIC PLATI NU M TUBES I" I --- = RADI ATION DE·ICING V.....-- SHIE LD HEATERS ;; SENSING / ELEMENT 1= 7 ' ;-- LOCKIN G SCREW 10 / c ENTREBOD~ 0 / ---r- AIRCRAFT SKIN./\\..M II -- ~~ CEMEN T '"[ ia---- ~-POLE CONNE CTOR The advantages of this type of thermometer over the flush bulb type are that it has a virtually zero time lag , and also has a recovery factor of approximately 1. This type of probe is normally connected directly to a flight deck indicator, and also to the Mach number module of an Air Data Computer (ADC). Instrumentation 1-5 Chapfer J Air Temperature Measurement An air intake, which is mounted on top of a small streamlined strut, is secured to the aeroplane skin at a predetermined location around the nose , where it is free from any boundary layer activity. Air flows through the probe and over the sensing elements before it is expelled through a vent at the back of the strut, which allows the probe to continuously sample new air. Separation of water droplets from the air is achieved by causing the air to turn through goo before it passes over the sensing element. Boundary layer air is drawn off by bleed holes in the casing. This layer of air is due to the pressure differential which exists across the casing. A pure platinum resistance wire, which is sealed within two concentric platinum tubes, is used to sense the temperature , and a heating element is mounted on the probe to prevent any ice forming. The heater has a minimal effect on the indicated temperature readings, with typical values of O.goC at Mach 0.1 and 0.15°C at Mach 1.0. \-6 Instrumentation INTRODUCTION The pitot and static systems on an aeroplane measure the total pressures created by the forward motion of the aeroplane, and the static pressure surrounding the airframe. These pressures are fed to instruments, which convert the pressure differentials into speed , altitude , and rate of change of altitude. The system is alternatively referred to as a manometric or air data system. PITOT TUBE The Pitot tube (pitot probe) senses the total or pitot pressure, which is the combined static and dynamic pressure of the airflow. The tube is fitted to the airframe with its opening facing directly into the airflow, and the airflow comes to rest (i.e. stagnates) inside the tube entrance. Dynamic + PITOT.. ; r - - t - - - - - - - - - - -..... Static Pressure HEATER SWITCH A pinhole drain allows any moisture to leak away to atmosphere without significantly affecting the sensed pressure. Because moisture can accumulate within the pipeline due to condensation , drain traps are positioned at various intervals throughout the pipe work of the aircraft, usually at low points along certain pipe runs. These drain traps contain any accumulation of water in a bowl which can be released by operating a spring loaded plunger on the underside of the bowl. After operations of these drain traps , the integrity of the pitot system requires verification by carrying out a leak test on the system. Therefore, drain traps are only operated by ground technicians during maintenance. The probe is mounted on a part of the aeroplane where there is minimal disturbance to the airflow, and is designed to extend well forward into the airflow. These probes are typically mounted close to the nose, at the wing tips, on a pylon extending well below the wing , or at the top of the fin. The probe is also fitted with a heater, which is powered from the aeroplane electrical supply (usually 28 Volt DC or 115 Vol t AC), and is switched on as required by the flight deck crew to prevent the formation of ice. An indicator light gives the operative state of the system. Some types show an amber light when switched OFF, or alternatively with the system switched ON and the heating element failed. Most transport category aeroplanes have at least two pitot tubes. Instrumentation 2-1 Chapter 2 Pilot-Static System STATIC SOURCE The ambient pressure of the air mass surrounding the aeroplane, or static pressure, is obtained via a static source. The static source, or static vent, senses the static pressure of the atmosphere, which is unaffected by the airflow. To achieve this , the source (vent) is located on a part of the aeroplane where the airflow is undisturbed by its passage, and is positioned with its entrance perpendicular to the airflow. The vent is manufactured and attached to the surface of the aeroplane so that it does not create local disturbances in the airflow. A typical design is shown below. " \, \ \ \ \ \ \ I I DRAIN ALTERNATE STATIC PORT Vent pipe connections are installed with a slight downward angle to ensure adequate drainage, and as with the pitot system, drain traps are provided to contain any water condensed into the system. It is important that the ven t plates are not painted as this would impair their thermal efficiency and a placard is often placed next to the static vent saying "do not paint". The direction of the airflow around the static vent may vary as the airspeed and configuration of the aeroplane changes, and may induce errors known as position (or pressure) errors. Minimisation of these errors is achieved by carefully positioning the static vent, or by using multiple vents to average out the errors. This is known as static balancing, and is achieved by fitting vents on either side of the aeroplane fuselage. The purpose of this is to even out any differences of pressure that are caused by the sideways motion of the static vents, which occurs during a yaw or sideslip condition. Any residual position (pressure) errors are recorded during initial flight tests and a correction table is produced , for various airspeeds and configurations. These readings are incorporated into the Aeroplane Operating Manual (AOM). If failure of the primary pitot/static pressure source should occur, for example icing up of a pitot or pressure head due to a failed heater circuit, introduction of errors may occur in the instrument readings and other areas dependent on such pressure. The installation of a standby system in some aeroplanes acts as a safeguard against partial failure, whereby static pressure andlor pitot pressure from alternate sources is usually selected and connected into the primary system. A blockage of the pitot source only affects the ASI. A blockage of the static source affects all of the instruments, and it is thus common practice to provide an alternate static supply. 2-2 Ln strumentati on Pilot-Static System Chapter 2 ALTERNATE STATIC SOURCE The changeover to an alternate static source is norm ally achieved by selector valves located in the static lines, which are located on the fiight deck, within easy reach of the flight crew. A typical internal alternate static source installation is shown below. Such a system only operates satisfactorily if the cabin is unpressurised and the air within the cabin is relatively undisturbed. When calibrating the pre ssure/position errors of the alternate system, the manufacturer lays down the conditions required in respect of the position of such items as windows, heating/ventilation , and doors, all of which must be observed if the system is to work correctly. TO CAPTAIN'S TO FIRST OFFICER'S AIR DATA AIR DATA INSTRUMENTS NORMAL =-= == PITOT STATIC NORMAL STATIC ALTERNATE STATIC VENT ~ ALTERNATE STATIC Instrumentation 2-3 Chapter 2 Pi/Of-SIalic System Moderate or large aeroplanes normally have a minimum of two separate static systems, and a pair of balanced static vents, as shown below, feeds each. ALTITUDE ALTERNATE WARN AIR LEFT SELECTOR VALVES SIDE STATIC PORT~S~~~~~~~~~~~~~;;~~~~~~~~~~~~~~~~~RIGHT FIRST OFFICER AUXILIARY SIDE STATIC PORTS CABIN 01FF. PRESSURE AUXILIARY PITOT TUBE The second pair of static vents is normally referred to as the auxiliary static ports, whose precise location is determined during prototype development. COMBINED PITOT-STATIC (PRESSURE) HEAD In some light aeroplanes the complication and expense of separate pitot head(s) and static sources is often avoided by incorporating both fun ctions into a combined pressure head. The combined head is usually mounted on a pylon below the wing toward the tip, thus reducing the position/pressure error to an acceptable level although accura cy is well below the standards achieved by systems that are more sophisticated. A typical combined pressure head is in the following illustration. 2-4 Instrumentat ion Pilol-Slalic Syslem Chapler 2 P. PRESSURE LINE 1 PRESSURE LINE r==-- The lines of flux have direction or polarity. They flow from the north pole to the south pole outside the magnet, but flow from the south pole to the north pole within the magnet. These lines are continuous and always form complete loops. >- The lines of flux do not cross one another, like poles repel , as shown on the next page, and >- Lines of flux tend to form the smallest possible loops (i.e. unlike poles attract). >- Establishment of magnetic flu x is easier in some materials than in others. All materials, whether magnetic or not, also have a property called reluctance that resists the establishment of magnetic flux, and equates to the resistance found in an electrical circuit. 9-4 In strumentation Basic Magnetism Chapter 9 » Lines of magnetic fiux can distort by interaction with other lines of fiu x, as shown below. MAGNETIC MATERIALS Theoretically, magnetic fields affect all materials to some extent, and are categorised as follows : FERROMAGNETIC This is the property of a material that enables it to become a permanent magnet (i.e. when placed in a magnetic field the material becomes magnetised). The most common ferromagneti c materials are iron, cobalt, and nickel , and alloys of these materials. Ferromagnetic materials can divide into two further categories: Hard Iron This material, known as a permanent magnet, is difficult to magnetise , but when removed from a magnetic field , it retains the magnetism for a considerable length of time , unless it is subject to a strong demagnetising force (e.g. cobalt and tungsten steel). Soft Iron This material, known as a temporary magnet, is easily magnetised , but once removed from the magnetic field easily loses its magnetism (e.g. silicon iron). The above terms also describe the magnetic effects that occur in aeroplanes. PARAMAGNETIC This is the property of a material, which when placed in a magnetic field slightly attracts the lines of magnetic force, but once removed loses its magnetism. The most common materials are platinum, manganese, and aluminium. DIAMAGNETIC This is the property of a material , wh ich when placed in a magnetic field slightly repels the lines of magnetic force. The most common materials are copper and bismuth. In strumentation 9-5 Chapter 9 Basic Magnetism PERMEABILITY Permeability (11) is the ease by which magnetic flux induces into a material , and is comparative to conductance in an electrical circuit. It is the ratio of BtH , where B is the induced magnetic flux, and H is the magnetising force. ELECTROMAGNETISM When current flows through a conductor, a magnetic field forms around it, with the magnitude directly proportional to the amount of current flow. MAGNETIC FIELDS /...-..., ,,, / - \ ,,' ',' 1/'- ',' I ,I ' _ "\ \ \ '/'-',' 1"-,,, I'" 1--.1 "",1 II"" ,I , I", ,11--. '" \.Lf1:=~~':':'\TTI:::;::' 1 1 ' \ : ril 1,1, ;ril ~"::i\ ~I_l _ _ 1 \ II : 1 - ( 1 - - \1\,...... ' " ',',',,,":,',',' \1\......,'," I'" II \,- '__ / ,," ",1 "_,, CONDUCTOR \...-, ,...... -, , ,,' LINES OF ~ , MAGNETIC FLUX The direction of the conventional current flow determines the direction of the field around the conductor. To establish the direction, use the Right Hand Grasp Rule, as shown below. By pointing your thumb in the direction of the current flow, and gripping the conductor, your fingers indicate the direction of the magnetic field. FINGERS IN DIRECTION OF LINES OF ____ MAGNETIC FLUX........... DIRECTION OF. - CURRENT FLOW 9-6 Instrumentation Basic Magnetism Chapter 9 The magnetic field produced by a straight piece of wire is relati vely weak and is of no practical use. It has direction, but no north or south poles and , unless the current is extremely high , the resulting magnetic field has little useful strength. However, its magnetic characteristics greatly improve by shaping the wire into a loop. This causes: ~ The lines of magnetic flux to move closer together ~ The majority of the lines of magnetic flux to concentrate at the centre of the loop ~ Creation of north and south poles at the ends of it, and assumption of the magnetic characteristics of a permanent magnet. Lines of magnetic flux emerge from the north pole and return via the south pole. AN ELECTROMAGNET A strong magnetic field is produced if the wire is formed into a coil , which is commonly known as a solenoid, as shown on the next page. Also the larger the magnitude of the current, or the hig her the number of turns of the coil, the greater the magnetic strength. If a soft iron bar is inserted into a coil , this further increases the strength of the magnetic flux around the coil. This has the affect of concentrating the lines of magnetic flux, and the polarity of the coil can be determined if the direction of the current through the coil is Known, by using the Right Hand Grasp Rule. Your thumb points in the direction of the North Pole if the fingers of your right hand are wrapped around the coil in the direction of the current fl ow. Instrumentation 9-7 Chapter 9 Basic Magnetism MAGNETIC MOMENTS The magnetic moment of a magnet is the tendency for it to turn, or be turned by another magnet. It is a requirement of any aeroplane compass design that the strength of the moment is such that the magnetic detection system rapidly responds to the directive force of the magnetic field. N N Fie!d H 5 L--- Field H The diagram above shows a pivoted magnet of pole strength S and length of magnetic axis L, which is positioned at right angles to a uniform magnetic field H. In this situation, the field distorts and "passes through" the magnet. The distortion resists and the field tries to align the magnet with the magnetic field. The forces applied to the magnet act in opposite directions, and the = resulting magnetic moments (m S x L) produce a couple , which swings the magnet into line with the magnetic field. The greater the pole strength and the longer the magnetic moment, the greater the magnet's tendency is to quickly align itself with the applied field. PERIOD OF OSCILLATION OF A SUSPENDED MAGNET If a suspended magnet deflects from its position of rest in a magnetic field , the magnet is immediately subjected to a couple urging the magnet to resume its original position. When the deflecting influence is removed, the magnet swings back, and if undamped , the system continues to oscillate about its equilibrium position before coming to rest. The time taken for the magnet to swing from one extremity of oscillation to the other and back again is known as the period of the magnet. As the magnet approaches its aligned position, the amplitude of the oscillations gradually decreases, but the period remains the same, and is not altered by simply adjusting the amplitude. The period of a magnet depends upon its shape and size or mass (the factors that affect its moment of inertia) , the magnetic moment, and the strength of the field in which it is oscillating. The period of oscillation increases if the magnet's mass is increased , and becomes shorter if its field strength increases. 9-8 Instrumentation INTRODUCTION The Earth is itself a huge magnet surrounded by a weak magnetic field that cul minates in two internal magnetic poles situated near the North and South geog raphic poles. The Earth's magnetic field is similar to that produced at the surface of a short but very powerful bar magnet, and is why the magnetic poles cover relatively large geographic areas, as the lines of magnetic force spread out. This is why the l

Instrumentation: Measurement of Air Temperature, Pressure, and Speed

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