Why sudden movement after AP disconnect?

I can’t get my head around this. Why we get after AP disconnect sudden nose dive or lift? I understand you need calibrate. Is it hard to take current accelerometer data XYZ and Flight parameter combine it to recalibrate automatically? As I seen in real aircraft AP disconnect it doesn’t move much out of the current path for while. Any thoughts on this?

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Usually when this happens its because your speed / attitude / V/S / trim combination is not right… In principle, if you have the proper speed / flaps settings / attitude then the handover is smooth. It takes a bit of practice to get there as you have no “real” feedback from the stick obviously.

Let’s take an example: if you approach way too fast with lots of flaps, the A/P will continously “fight” the plane trying to pitch up by applying some down pressure on the commands. But as soon as you disengage the A/P, if you stay in a “neutral” yoke position then the plane will pitch up. The opposite example would be if you approached too slow for your current flaps / V/S Settings: in this instance the A/P would “fight” the natural attitude of the plane by continuously pulling back the yoke. If you disengage the A/P at this point whilst keeping a neutral yoke position, then the aircraft will plunge.

So the only way to have a smooth A/P to manual handover is to:

  • make sure the flaps / speed / V/S settings of your plane are naturally fitting its flight enveloppe for a neutral yoke position.
  • apply some positive trim on descent (20-30%)

This way, when you disengage, you shouldn’t experience such an issue.
It takes a bit of practice and is airplane specific.

Apologies for my long explanation, I hope this helps!

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Probably difficult to keep track of the changes when a set of data is stored for a current autopilot setting and then pass whatever is the current setting to the relevant parts. Calibrating would probably be a much simpler solution and it doesn’t require significant enough distraction for the user, so it’s functional enough to use. That’s what I’d wager.

Yes 20%-30% trim does compensate nearly 90% effect. Trim has to be very precise. I did fly without recalibrating and if everything falls in place, it’s smooth handover from AP to manual.

Nope, that’s not the reason @lollip

In IF, once you calibrate, it’s like the Yoke is in the “zero” position (it’s perfectly in its whole).
So it’s impossible to prevent this sudden movement without the Yoke in the flight control panel.
You may have noticed that the yoke is hardly ever in the zero position.
Now there are two options:
• Correct the trim (only trim, not speed or VS or anything) in order to get the yoke in the zero position, calibrate, and disengage the A/P.
• If you don’t want to change your trim or you can’t (like in 777s), you have to look carefully at the yoke position. Usually it comes out of its whole. So you have to calibrate first, then pull up your device in order to get the same yoke position, and finally disengage the A/P. This needs a little practice.

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When you’re flying on autopilot and you want to deactivate, you have to recalibrate your device (just in case) and then hold your device in a steady position while you set the trim until the yoke is neutral. Then you should be able to turn autopilot off and the plane will stay steady.

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@lollip you are actually correct, contrary to what Laurens says - it’s just that you can cheat in IF to recalibrate your device to how your holding it to neutralise that difference and incorrect speed / flaps / trim settings.

Even a 10 knot over speed with a certain level of flaps and no trim set with throw the nose into the air - autopilot was counteracting that incorrect setting without you noticing, and now autopilot is off, you suddenly have to manually adjust and do it

@MitchellJames yep, that’s what I believe - this is what happens on a real plane at least… But I didn’t want to fight over this.

I have never re calibrated prior to switching off AP, I make sure my settings are correct and I get a perfectly smooth handover atleast 90% of the time - the other 10% just takes a minor pitch up / down etc and I’m perfectly balanced without any major movements.

Cheating is the easy way - or you can do it like in the real world and have your aircraft correctly set up

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