13.3 Autoflight (ATA 22)

Scope

Level 3 for B2 and for the B2L Autoflight rating (exam block 13.3 + 13.7).

• 13.3(a) working principles and terminology; command signal processing; roll, pitch and yaw channels; yaw dampers; helicopter SAS; automatic trim; autopilot–navaid interface.
• 13.3(b) autothrottle and automatic landing: principles and categories; modes; approach; glideslope; land; go-around; monitors and failure conditions.

Building blocks of an automatic flight control system (AFCS)

Block	Function
Sensors	Attitude and heading (vertical gyro/AHRS/IRS), rate gyros, accelerometers, air data (altitude, airspeed, vertical speed), radio altitude, navigation receivers (VOR, LOC, GS), FMS
Computer (FCC)	Compares the commanded value with the actual value; the difference is the error signal; applies gains, limits and filters and outputs a servo command
Servo actuators	Move the control surfaces (electromechanical with clutches, or electrohydraulic)
Feedback	Position/rate feedback from the servo or surface closes the inner loop so the surface deflection is proportional to the error
Controller / mode control panel (MCP/FCP)	Engage switches, mode selection, targets (heading, altitude, speed, V/S)
Annunciators	Flight mode annunciator (FMA) showing armed and engaged/active modes

Inner loop = attitude/rate stabilisation; outer loop = guidance (altitude, heading, navigation path) feeding attitude commands to the inner loop.

Command signal processing

• Error detection (synchro or digital comparison).
• Gain scheduling — control-surface authority varies with airspeed (via air data) so the same error gives smaller deflections at high speed.
• Rate and attitude limiters — keep bank angle (e.g. 25–30° in heading select) and pitch rate within passenger-comfort and safety limits.
• Integration to remove standing errors; washout (high-pass) filters so steady states (e.g. a steady turn) are not opposed.
• Synchronisation — before engagement the computer follows the current attitude so engagement is smooth (no "snatch").
• Servo series/parallel: series servos move the surface without moving the pilot's control (limited authority, used for yaw damping/SAS); parallel servos move the control column/wheel, giving full authority and visible movement.