Autopilot & FMS

022 InstrumentationChapter 5

Air data computer

Key fact

ADC converts probe inputs to electrical signals for EFIS and other systems. Captain and F/O each have separate ADC fed by separate probes. Does not determine true altitude. Advantages: improved displays, reduced lag, error correction, central source, failure warning, modular replacement.

Temperature sensing

Formula — SAT, TAT, RAT

TAT = SAT + Ram rise
RAT = SAT + MRR     where MRR = Kr × RR,  Kr ≈ 0.8
SAT = TAT ÷ [1 + (0.2 × M²)]
SAT = RAT ÷ [1 + (0.2 × Kr × M²)]

Rule of thumb: ΔT ≈ TAS² ÷ 100 (ram rise). Temperature in Kelvin for formulas.

Key fact

TAT probe: radiation shield, de-icing, ratiometer thermometer (supply voltage variations affect both coils). Helical bimetallic sensor for small aircraft without ram rise correction. SAT = static air temperature wanted; RAT = what probe senses.

Inertial navigation

Key fact

INS — distance and direction from known start; accelerometers measure movement. IRS adds attitude, heading, display. Alignment: INS ~15–20 min · IRS ~3–10 min. INS errors: grow exponentially (~1.5 NM or 0.5–2 NM). IRS errors: 0.5 NM or less. ADIRU = IRS + ADC (wind triangle complete).

Key fact

1st gen: stabilised platform, rate integrating gyros (RIGs), torque motors. 2nd gen: wander platform (polar regions). 3rd gen / IRS: strapdown — box of 3 accelerometers + 3 ring laser gyros (RLG); no caging; dither motor prevents laser lock. Alignment not possible above 70° N/S.

Formula — IRS corrections

Coriolis · Centripetal · Schuler oscillation (84.4 min period) · Transport wander · Earth rotation

Key fact

MSU modes: STBY (caging, POS ENTER) · ALIGN · NAV · ATT (attitude only) · BATT (internal battery). AUTO/MAN heading — magnetic away from poles, true near poles.

FMS

Key fact

FMC at heart; dual FMCs (DUAL / Independent / Single / ALTN modes). Composite fix from IRS, GPS, DME, VOR — Kalman filter weightings; early flight favours IRS/GPS, later favours GPS/VOR/DME. ANP (Actual Navigation Performance) must be less than RNP.

Key fact

B-RNAV RNP 5 NM · P-RNAV RNP 1 NM. Cost index = operating cost ÷ fuel cost — high CI = fast/expensive fuel; low CI = max range. FMS init sequence: IDENT · POS INIT · RTE · PERF INIT · TAKEOFF REF (IPRPT).

Exam trap

AFCS = A/P + A/T + FD — does not include EFIS; does not use GPS (FMS uses GPS for position). FADEC: no auto shutdown, no airspeed/yaw modification.

AFCS and autopilot

Key fact

Closed loops: Outer = FD (guidance) · Inner = A/P (pitch/roll attitude). CWS: AP without FD. Autothrottle: controls speed/thrust; at 64 kt servos disengage, THR HOLD (RTO protection). Take-off: N1 hold · Landing: IAS hold · Climb: AP altitude/VS, A/T IAS.

Key fact

Speed reversion (too low): cautionary range (yellow, top = 1.3 VS) → A/T N1, A/P MCP SPD, pitch to 5 kt above cautionary. Speed limiting (too high): red bricks VMO/MMO → clacker, RETARD, pitch 5 kt below red line.

Key fact

Preventing runaway: authority weighting (manual overrides AP), comparators, rate triggers. Triplex — majority vote. Auto-trim: pitch only when AP engaged; manual trim disengages AP. Mach trim above predetermined Mach prevents Mach tuck.

GPWS and radio altimeter

Key fact

GPWS modes mnemonic SEND THE DUCK TO GO BUY WINE: Sink rate/Pull up · Terrain · Don’t sink · Too low gear/flaps/terrain · Glideslope · Bank angle · Windshear. Operating height 0–2500 ft (same as radio altimeter). Warning 20–30 s before obstacle · Caution 40–60 s.

Key fact

Radio altimeter: FMCW, carrier 4300 MHz (SHF), FM 50 MHz (4250–4350 MHz), sweep rate ~0.01 s, range 2500 ft, precision ±20 ft (0–500 ft). Two antennas (TX/RX continuous). Cone emission — lowest return used; accurate to ±40° bank, ±25° pitch.

Stall warning and protection

Key fact

Stall warning: stick shaker, audible, display — activates at 5% CAS or 5 kt, whichever greater (inputs: AOA and flap/slat only). Stall protection: stick push, auto slats — operates on stall warning activation. FEP rolls level for stall/overspeed — no auto manoeuvres or pitch trim inhibiting.

TCAS priority

Key fact

Priority: Stall – Windshear – GPWS – TCAS. RAs have priority over ATC. Mode C minimum for 3D TA/RA · Mode A for 2D TA only · Mode S for datalink coordinated RA.

EFIS

Key fact

Main purpose: electronic ADI and HSI. Boeing EADI/EHSI · Airbus PFD/ND. Magenta = commanded values · Green = low priority · Red = warnings · Yellow = cautions. FPV displays glide angle.