Flight controls, autopilot and landing gear
Scope
This domain covers primary and secondary flight controls, fly-by-wire, autopilot modes and landing gear. Expect control function, trim, envelope protection, autopilot disconnect logic and gear/brake operation.
Flight controls
- Primary: ailerons (roll), elevator (pitch), rudder (yaw). Secondary: flaps, slats, spoilers and trim. Trim reduces control force; Mach trim counters Mach tuck.
- Fly-by-wire replaces cables with computers and actuators; artificial feel is needed because there is no direct feedback. Envelope protection limits pitch, bank and angle of attack.
- Hydraulic actuation is normal; some types provide manual reversion.
Autopilot
- Modes: roll (wings level, heading, VOR/LOC) and pitch (altitude hold, vertical speed, IAS/Mach, glideslope). The flight director gives cues; the autopilot follows them.
- Servos move the controls; disconnect by button, trim input or failure. CWS lets the pilot move the controls without disengaging. Autothrottle controls thrust.
- The autopilot must not be engaged on the ground; it disengages on excessive input or power loss.
Landing gear
- Retraction is hydraulic or electric; uplocks and downlocks hold the gear. A squat switch prevents retraction on the ground. Gear warning sounds with gear up, flaps landing and low thrust.
- Brakes are hydraulic with carbon discs. Antiskid compares wheel speed with aircraft speed and releases brake pressure at a skid. Autobrake selects a deceleration rate. Nosewheel steering uses a tiller or rudder pedals.
Worked example
During landing, a wheel slows faster than the aircraft. The antiskid control unit detects the slip and reduces brake pressure to that wheel, allowing it to spin up. This preserves steering and stops the tyre flat-spotting, but it does not shorten the dry-runway stop.
Traps
Autopilot does not operate the gear. Antiskid is not a traction improver. The squat switch is not the only weight-on-wheels source. Spoilers are not primary roll control on all types.