Autopilot Disengaged

  • Hello,

    I am learning how to use the autopilot, on the Q400, and I tried the Learjet as well. I am understanding most of it, but I am still facing "AP Disengaged" about a second after I engaged it sometimes without really understanding why. I am clearly not going too fast or too slow or having a pitch or a roll to high, and there are no specific warning on anywhere, and I am not touching the commands. Are there somewhere a list of all the reasons why an autopilot would disengage?

    A more general question, out of curiosity, my understanding is that the autopilot, even as advanced as the one of the Q400, does not control the thrust. It does control the pitch in order to stabilize the speed OR the altitude for a given thrust. Is it a general behavior? Are there any plane where the thrust is controlled by the autopilot?

    Thanks.

  • The Boeing 747, 737 and the A320 have an autothrottle system that controls the engine thrust for you. The Q400, LJ45 and C90 don't have this feature.

    The autopilot disengages due to several reasons:

    - overspeed (flaps or gear extended?)

    - unusual attitude (too high bank or pitch attitude)

    - controls deflected (release control inputs once you engaged AP)

    - stall

    - too close to ground (use AP at least 50ft above ground)

    - go around button pressed (disengages AP in Learjet, q400, C90)

  • Thanks a lot for all these explanations. I am still not sure what happened to me, maybe the involuntary use of the go around button, maybe it is an issue with my controls which were registered as moving even though I was not touching them.

    Could you please tell me a little bit more about the go around (toga?) button? The documentation of the Q400 is really not clear about that, I looked on google but what I found was about autothrottle so I am a bit confused about what it is supposed to do in a Q400. Thanks!

  • We have a long tutorial on the q400: https://www.aerofly.com/dokuwiki/doku.…raft:dash8-q400

    The go around button in the Q400 are on the power levers. When you press them you engage the GA vertical mode and the wings level lateral mode. And autopilot disengages if it was on. You can then advance the power to full rating and manually climb away. The Q400 does not have autothrottles.

    In the 747 for example the go around button set the flight director vertical mode to TOGA (takeoff goaround) and when autothrottles are set to armed or already on they go into full thrust mode. Autopilot remains engaged. But not in the Q400...

  • OK, I got it. I use an old Microsoft For Feedback Pro that I generally do not power, so it does not autocenter when released. Which mean that if I do not center it when I press autopilot, it considers it as "deflected" and disengage the autopilot. So I guess I have either to power it or to pay attention to manually center it when engaging the autopilot. From my first experiments the second options works fine, it does not have to be very precisely centered.

    Anyway, thanks for your help, you gave me some nice pieces of information.