Automatic Flight Control and Flight Director Systems
Modes and operation
The autopilot (AP) and flight director (FD) work together. The FD computes a command bar display; the AP follows the same commands through servos. Modes divide into lateral (heading, nav, approach, localiser, VOR) and vertical (altitude hold, vertical speed, IAS/Mach, glideslope, go-around). Autothrottle, if fitted, controls thrust for speed or Mach targets.
Key concepts
- CWS (control wheel steering): the pilot can manoeuvre with the AP engaged; the AP follows and then holds.
- Mode annunciation: the armed and engaged modes are shown on the FMA (flight mode annunciator); always verify what is armed and what is active.
- Capture and track: an approach mode arms and then captures the localiser and glideslope; capture criteria and intercept angles matter.
- Disconnect and reversion: know the aural warning, the disconnect button and the manual reversion.
Limitations and traps
Autopilots have limited authority, may disconnect in turbulence or at low speed, and may not be certified for all approaches (e.g. low-visibility autoland requires specific equipment and crew qualification). The FD does not move the controls; only the AP does. A common trap is confusing "armed" with "engaged". Another is assuming the AP will level the aircraft after a failure - it may not.
Worked example
On an ILS approach, the approach mode is armed; when the localiser is captured the annunciation changes from armed to engaged. If the glideslope is captured before the localiser, the mode logic may not engage correctly - always verify the FMA before descending below the approach gate.
Rule of thumb
Before every mode change, announce and verify the FMA: what is engaged, what is armed, and what the aircraft is actually doing.