Flight Director, Autopilot and Autothrust
Domain 7 covers the flight director, autopilot and autothrust - the automatic flight control system. The exam tests the control-loop structure, the modes and their arming/capture/engagement logic, autoland capability, and the interaction between speed control by elevator and by thrust.
Control loops
The autopilot is a closed-loop servo. The inner loop damps short-period aircraft motion using rate feedback; the outer loop removes displacement from the selected path. Control laws are proportional-integral-derivative: proportional for response, integral for zero steady-state error, derivative for damping. Actuators move the surfaces, and the flight director computes the same commands for display as bars on the PFD.
Modes
- Lateral: HDG, NAV/LNAV, LOC, APP, B/C, ROLL, TO/GA.
- Vertical: ALT hold, ALT CAP, VS, FLC/IAS, VNAV path or speed, G/S, FLARE.
- Arming (white) precedes engagement (green) - LOC arms and then captures; G/S arms only after LOC capture in APP. Mode confusion is a classic accident factor, so the FMA is the primary cross-check.
Autothrust
Modes: THR (fixed takeoff/go-around thrust), THR REF, SPD (thrust varies to hold the command speed), HOLD (thrust locked, so speed must be controlled by pitch) and IDLE. On many types the A/THR is armed on the ground and engages in the air; it can be disconnected by the instinctive button, by moving the thrust levers, or by mode degradation.
Autoland
Requires redundancy: multiple autopilots, independent sources and fail-operational capability so that a single failure during approach and flare is survivable. Categories depend on RVR and decision height as well as on aircraft capability.
Worked example
On approach, LOC is captured and G/S is armed. Passing the glideslope, G/S engages and pitch follows the beam while thrust holds speed in SPD mode. If the A/THR drops to HOLD, thrust freezes at its current value - the aircraft will no longer hold speed, so pitch or thrust must be managed manually.
Traps that cost marks
- Saying the autopilot flies terrain or avoids traffic; it only follows selected modes.
- Reversing armed and engaged indications and colours.
- Believing ALT HOLD captures the altitude - ALT CAP does.
- Assuming a single-channel autopilot can perform a CAT III autoland.