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Thread: Uncommanded Dive

  1. #1
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    Default Uncommanded Dive

    OK, so doesn't matter what aircraft or what airport, I get the same problem. I take off normally, rotate at a high enough speed select gear up, retract flaps, engage the autopilot and autothrottle, and heading. At this point everything's normal. But as soon as I select Altitude Hold the aircraft goes into a dive. I've tried selecting the elevator trim up max but, for some reason, whilst taxiing and/or whilst taking off, the elevator trim returns to neutral of it's own accord. It's driving me nuts.

  2. #2

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    Quote Originally Posted by ac103010 View Post
    OK, so doesn't matter what aircraft or what airport, I get the same problem. I take off normally, rotate at a high enough speed select gear up, retract flaps, engage the autopilot and autothrottle, and heading. At this point everything's normal. But as soon as I select Altitude Hold the aircraft goes into a dive. I've tried selecting the elevator trim up max but, for some reason, whilst taxiing and/or whilst taking off, the elevator trim returns to neutral of it's own accord. It's driving me nuts.
    Depending on the aircraft, it might be a mistake to click A/P Alt Hold. What you MAY be doing is clicking Alt Hold and that seeks to `hold` the alt at the moment you clicked the Hold. This will see you diving to the previously selected altitude...

    If it's the elevator trim then you may have inadvertently checked a duplicate assignment.

  3. #3

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    It's likely the aircraft your using has an altitude bug that has to be set or an altitude entry that has to be made. When you start it's probably at zero and that's where it is trying to fly.

  4. #4
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    I've tried selecting the elevator trim up max but, for some reason, whilst taxiing and/or whilst taking off, the elevator trim returns to neutral of it's own accord. It's driving me nuts.
    That's normal operation. The AP in FSX makes heavy use of the elevator trim, so that you effectively are fighting the AP if you try to adjust the trim and it WILL fight back.

    I expect that Ray has the answer for the unexpected (not uncommanded) dive. You'll need to find what altitude the AP is set for or use CTRL-Z which should both engage the ALT HOLD and SET the current altitude hold value. Engaging ALT HOLD with the AP button(s) will try to maintain whatever altitude was already set in the AP.

    So either use CTRL-Z or change the setting (perhaps an altimeter bug, or IAS hold window) of the AP.

    Larry N.

    As Skylab would say:
    Remember: Aviation is NOT an exact Science!

  5. #5
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    Hi,

    I've read all 3 replies.

    First, let me say that this happens with a number of different aircraft: B747, A320, B737, A350.

    Second, it's always been my understanding that when A/P Alt Hold are selected the aircraft would climb to the altitude set in the A/P window. I'm selecting the A/P passing, say, 3,000ft, and the A/P Alt Hold is set at 12,000ft, for example. And this is the way it always worked for me for about 3 years previously. This issue has only happened since I've installed FSX again after 5 years being away.

    Then, when I say the elevator trim resets when taxiing, the A/P is not engaged at that point. I can view the trim operating when taxiing but without changing anything when I start my takeoff run it's reset.

    Allan

  6. #6
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    Quote Originally Posted by ac103010 View Post
    OK, so doesn't matter what aircraft or what airport, I get the same problem. I take off normally, rotate at a high enough speed select gear up, retract flaps, engage the autopilot and autothrottle, and heading. At this point everything's normal. But as soon as I select Altitude Hold the aircraft goes into a dive. I've tried selecting the elevator trim up max but, for some reason, whilst taxiing and/or whilst taking off, the elevator trim returns to neutral of it's own accord. It's driving me nuts.
    Allan - Check all your AP settings on climb out. Make sure you haven't assigned a negative vertical speed assignment by mistake! You could have designated 12,000' for your desired altitude, but if you have a -2000' vertical speed assignment, and at 3,000' you engage the AP, you are going to go down! Oh, and make sure you are rolling the trim wheel in the right direction also! Like Larry says, the trim wheel will go wild on it's own responding to the needs of the autopilot!

    It's happening on too many of your aircraft, something common you are doing wrong in all of them!

    Rick
    Last edited by Downwind66; 11-24-2020 at 09:32 PM.

  7. #7

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    I don't have a clue after all of the above.

    Alternatively I did read this on another site:

    "Whenever I have seen this sort of thing, it was due to the elevator or ailerons being randomly assigned to some other joystick axis (in ADDITION to its normal assignment). I use the CH Products yoke & rudder peddles and when I make changes (which is rare), my computer has a bad habit of assigning random axis to the elevator or ailerons (the propeller pitch axes on the yoke gets assigned to the ailerons for example). Everything will function fine until you bump the pitch handle, and boom, it activates the control and the airplane flies out of control. And YES, you can have TWO axis assigned to the elevators for example (like the yoke and the prop pitch mixture lever). This means it may function normally for some time until you bump the other control also assigned to it - and then all #### breaks loose. You have to go to Settings/Controllers and check to see if they are assigned to more than one joystick axis. Not sure if this is your problem or if this will help."

  8. #8
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    And Ray, that too could be a common denominator in his problem also, seeing that he has the same problem with numerous aircraft! He has a lot to double check!

    Rick

  9. #9
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    Update.

    I deleted fsx.cfg and restarted. Went to joystick assignments and deleted every one except the 3 I knew were working previously. In settings I set everything as they were before deleting fsx.cfg

    So no aircraft settings were changed. on the same B744.

    Began my takeoff roll. At aroung 120 kts the aircraft tried to takeoff. I held it until I could engage the A/P but then the nose kept lifting until the aircraft stalled.

  10. #10
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    Began my takeoff roll. At aroung 120 kts the aircraft tried to takeoff. I held it until I could engage the A/P but then the nose kept lifting until the aircraft stalled.
    Am I understanding this correctly? Are you trying to engage the AP to do the takeoff? Did the nose lift before or after you engaged the AP? What were you doing with the trim at that point, if anything?

    You say the aircraft tried to take off -- Was it too slow? Is that why you fought the take off?

    As you might gather, I'm a bit puzzled about your procedure, both what and why.

    Larry N.

    As Skylab would say:
    Remember: Aviation is NOT an exact Science!

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