Module 11A.5 — Instrument systems (ATA 31) and avionics systems overview

Levels

11.5.1 Instrument systems: level 1 A1, level 2 B1.1. 11.5.2 Avionics systems: level 1 for everyone (fundamentals of layout and operation only — B2 covers them in depth in Module 13).

11.5.1 Instrument systems (ATA 31/34)

Pitot-static system

• Pitot (total) pressure = static + dynamic pressure, sensed by a forward-facing pitot probe (heated).
• Static pressure sensed at flush static ports on both sides of the fuselage (cross-connected to cancel sideslip errors) or on pitot-static probes.
• Drain traps/water drains at low points; leak checks with a pitot-static test set after any disturbance (static leak typically checked by rate of altitude change over 1 minute per AMM limits).
• Large aircraft use air data computers (ADC) or ADIRUs fed by air data modules to supply digital data to displays and systems.

Instrument	Inputs	Principle
Airspeed indicator (ASI)	Pitot + static	Diaphragm/capsule expands with dynamic pressure (pitot inside capsule, static in case)
Altimeter	Static only	Sealed aneroid capsules expand as static pressure falls; subscale (Kollsman window) set to QNH/QFE/1013.25 hPa
Vertical speed indicator (VSI)	Static only	Capsule fed directly by static; case fed through a calibrated (metered) leak — the pressure difference during a climb/descent indicates rate; has a lag. IVSI adds a dashpot/accelerometer to remove lag
Machmeter	Pitot + static	Airspeed capsule + altitude capsule combined

Errors: position (pressure) error, instrument error, compressibility error, density error (IAS vs TAS), blocked pitot (ASI acts like an altimeter when drain and inlet blocked), blocked static (ASI under-reads in climb, altimeter frozen, VSI zero).

Speeds: IAS (indicated) → correct instrument/position error → CAS → compressibility → EAS → density → TAS.

Key fact: Only the ASI and Machmeter need pitot pressure; the altimeter and VSI use static pressure only.

Gyroscopic instruments

Gyro properties: rigidity in space (resists change of plane of rotation — greater with higher mass, speed and radius) and precession (an applied force acts 90° later in the direction of rotation).

Instrument	Gyro type	Notes
Artificial horizon (attitude indicator)	Earth (vertical) gyro, two gimbals, spin axis vertical	Erection system (pendulous vanes or electrical torque motors) keeps it vertical; topple limits on older types
Attitude director indicator (ADI)	Attitude display + flight director command bars	Fed by vertical gyro/IRS and FD computer
Direction indicator (DI)	Horizontal tied gyro, spin axis horizontal	Drifts (apparent drift due to earth rotation ≈ 15° × sin latitude per hour, plus mechanical drift); reset to the magnetic compass
HSI	Combines heading (slaved compass) with VOR/ILS deviation and course
Turn indicator	Rate gyro, one gimbal, spin axis lateral	Precession against springs indicates rate of turn (rate 1 = 3°/s)
Turn coordinator	Rate gyro with gimbal canted ~30°	Senses both roll rate and yaw rate
Slip indicator	Inclinometer ball in curved tube	Ball shows balance: slip (ball inside), skid (ball outside)

Gyros are driven by vacuum/air (light aircraft) or electric motors. Modern aircraft use ring-laser gyros or MEMS in IRS/AHRS — no spinning mass, no precession.

Compass systems

• Direct-reading magnetic compass (standby): magnet system floating in liquid (damping); errors: variation (true vs magnetic north — not an instrument error), deviation (aircraft magnetic fields — corrected with compensator and recorded on a deviation card after a compass swing), acceleration and turning errors due to magnetic dip (northern hemisphere: accelerate on east/west heading → apparent turn north, "ANDS"; turning through north lags, through south leads — "UNOS").
• Remote-reading (slaved) compass: flux valve/flux detector in a wing tip or tail senses the earth's field; slaves a directional gyro so heading is stable and accurate; output to HSI/RMI and other systems.
• Compass swing required after installation, significant electrical/structural changes, lightning strike, periodically per AMP.

Stall warning and angle-of-attack systems

• Stall warning must give clear warning before the stall (aural, stick shaker, sometimes lights). Light aircraft: leading-edge reed (vane) switch or a reed horn.
• Angle-of-attack sensors: rotating vanes or conical probes, heated, feeding computers that drive stall warning, stick pusher (stall protection), FBW protections and AoA indicators.
• Stall warning margin (CS-25.207): warning must begin at a speed exceeding the stall speed by at least 5 kt or 5 % CAS, whichever is greater.

Glass cockpit

• EFIS: PFD (attitude, speed, altitude, vertical speed, heading, FMA) and ND (map, route, weather, TCAS).
• Engine/system display: EICAS (Boeing) or ECAM (Airbus) — engine parameters, warnings and cautions, system synoptics, maintenance pages.
• Display units are LCD; data from display management computers/IMA; reversion switching if a display fails.
• Indications of other aircraft systems: synoptic pages for hydraulics, fuel, electrics, doors, etc.

11.5.2 Avionics systems — fundamentals

Autoflight (ATA 22)

• Autopilot: servos move control surfaces (or command FBW) to hold/achieve attitude, heading, altitude, speed; modes such as heading select, altitude hold, V/S, LNAV/VNAV, approach and autoland (CAT II/III).
• Flight director: computes commands shown on the ADI/PFD for the pilot (or autopilot) to follow.
• Autothrottle / autothrust: controls engine thrust. Yaw damper, Mach trim and auto trim are related functions.

Communication systems (ATA 23)

System	Key facts
VHF comms	118.000–136.975 MHz, line of sight, AM voice; 8.33 kHz channel spacing in Europe
HF comms	2–30 MHz, long range via sky-wave (ionosphere), SSB; antenna coupler
SATCOM	Satellite voice/data (e.g. Inmarsat, Iridium) — oceanic/remote
CPDLC	Controller–pilot data link communications — text clearances via VHF data link or SATCOM
Audio systems	Audio control panels, interphone, PA, SELCAL, flight/service interphone
ELT	Emergency locator transmitter: 406 MHz digital signal to COSPAS-SARSAT satellites, plus 121.5 MHz homing; activated by g-switch or manually
CVR	Cockpit voice recorder — records crew voice and flight-deck audio; underwater locator beacon (37.5 kHz); for EU CAT aeroplanes over 27 000 kg MCTOM first issued with a CofA from 1 January 2021 the recording duration is 25 h (older installations 2 h)

Navigation systems (ATA 34)

System	Key facts
VOR	108–117.95 MHz; bearing from the station (radials)
ADF	Receives NDB/broadcast stations (about 190–1 750 kHz); loop and sense antennas; relative bearing on RMI
ILS	Localiser 108.10–111.95 MHz (lateral), glideslope UHF 329–335 MHz (vertical, ~3°), marker beacons 75 MHz
MLS	Microwave landing system (5 GHz) — rare
DME	Slant range; aircraft interrogates, ground replies; 962–1 213 MHz
Flight director systems	See autoflight
RNAV / FMS	Area navigation using GNSS, IRS, DME/DME; FMS combines navigation, flight planning, performance and guidance
Satellite navigation	GNSS (GPS, Galileo); GPS L1 1575.42 MHz; SBAS (EGNOS) augmentation
Inertial (IRS/INS)	Ring-laser gyros + accelerometers; aligned on ground before movement
ATC transponder / SSR	Interrogation 1030 MHz, reply 1090 MHz; Mode A (squawk), Mode C (altitude), Mode S (addressed, data link, ADS-B out)
TCAS / ACAS	Uses transponder signals to show traffic; TA (traffic advisory) and RA (resolution advisory, vertical)
Weather radar	X-band (~9.3 GHz)
Radio altimeter	4.2–4.4 GHz, height above terrain, used for GPWS/TAWS, autoland
GPWS / TAWS	Terrain warnings (modes 1–7, enhanced with terrain database)

Other: FDR (flight data recorder, at least 25 h), ACARS data link, ADS-B.

Key fact: Transponder: ground interrogates on 1030 MHz, aircraft replies on 1090 MHz. ELT: 406 MHz to satellites, 121.5 MHz for homing.