In the most commonly flown GA aircraft – the C172 Skyhawk – fitted with the S-TEC 55, there is an annoying behaviour which can catch the most seasoned X-Plane pilot unawares:
The autopilot is supposed to be “left in peace” when engaged, and disconnected (via a button on the yoke) before making manual corrections.
However, the autopilot is often sufficiently “forgiving” that we get lazy and sometimes make a manual course adjustment with the yoke, without disconnecting the autopilot. The autopilot will then “fight” you by opposing your action, and then happily just carry on once you have released the yoke. We often try this when we see that the wind has blown us a little off-track and we want to briefly move left or right and then allow the autopilot to resume our course.
This works fine, until it doesn’t. At some point the autopilot stops fighting our manual input and simply disconnects. The problem is that unless we are watching the STEC-55 display we will not see that the autopilot has disconnected. This can have a disturbing result when we release the yoke and expect the autopilot to resume course.
The autopilot often disconnects without warning:
- If you override the heading by trying to turn too sharply
- If you override the heading too long
- If you move the throttle so that your speed approaches a stall
- If you pitch up too steeply
The problem is that you never quite know what you can “get away with”
Moral of the story… Always disconnect the autopilot before making manual changes and trim the aircraft before restoring the autopilot.