Pitch oscillation in A321 while on autopilot

I have had two flights where I was in level flight on AP when my A321 developed a progressively more severe pitch oscillation. The first one put me into a stall at FL28. Fortunately I caught it in time to avoid a bunch of violations on the expert server. I figured I left some aspect of the AP off and chalked it up to pilot error.

The second flight immediately after also exhibited the behavior at FL24 although I caught the oscillation much sooner.

I am now in level flight at 16000 and am not experiencing oscillation. Winds are calmer too although I don’t know if that was a factor on the first flight or at all.

Is this a known issue? Would you like more repro steps?

Thanks,
Chris

What was your airspeed? You could also be overweight. If you are overweight, then try step climbing.

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Hadn’t thought about a weight issue. Thanks for raising that as a possibility.

That said I don’t think that is the issue unless the A321 is really sensitive. I typically fly an Citation, A320 or 787 so I don’t have tons of exp with the 321.

I checked the aircraft load and it was at 20% around the time of the oscillation. It was a short flight from KBOS to KJFK and I had 28K lbs of fuel on board (more than needed - was from prior flight).

Airspeed was 325 KIAS I believe.

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I believe this is a a known issue with the A320 family.



I believe they have said a fix is underway, but don’t quote me on that.

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Were your flaps down?

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No, flaps were retracted.

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Hmm. Did you have any trim?

I don’t usually set trim on departure for longer flights because I engage the AP pretty quickly. I can’t be 100% sure but I am reasonably sure they trim would have been at zero.

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Thanks for finding and sharing those! Should have checked the forums for an existing report first.

Chris

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I remember flying with the MD-11 at around 330 kias and I had the same problem with autopilot. Could you try and redo the flight at a speed of around 250-300 kias? If so then it could be easier to discuss after that.

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It’s a known issue with the a321 unfortunately it doesn’t happen too everybody so as yet no fix has been found

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It’s known that the A321 will have oscillations during flight. However, that only happens if your airspeed is close to the speed limit of the aircraft (the red tape).

You should reduce your speed while cruising, preferably at M 0.78 approximately (the cruising speed in real life)

I have used the A321 very often but my flight is always stable cruising at a lower speed.

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I have seen oscillations at speeds below Mach 0.80 but they seem relatively minor and generally seem to converge to stability.

Increasing speed to say Mach 0.84 at 26,000’ indices an oscillation that grows ever more violent.

Higher and faster increase the likelihood of a oscillation that becomes violent.

This doesn’t seem to be an issue with the aircraft though. Hand flying it at that speed / altitude is straightforward and it is easy to arrest the oscillation.

Seems like the autopilot should be able to fly the plane in level flight as well as or, hopefully, better than I can fly manually.

325 is too fast, put below 250 and see how? I know it’s unrealistic, at slower speeds the aircraft tends to react slower. Your load is as light as a feather. My IAS doesn’t exceed M 0.70 when cruising on a A319 (I believe the working principle is the same for an A321.)

Thanks. Yes at 250 things seem fine, but, as you note, that’s unrealistic. It is also risky for players that care about violations because few people would expect the aircraft to self destruct.

The rated cruise speed is M0.78 with a max of M0.82. The autopilot should work 100% at 0.78 and the red tape should start at 0.82.

Note that the 321 isn’t always unstable at 0.78 or 0.82. Faster than 0.82 and good luck, especially if there is weather or altitude changes to induce some initial pitch changes.

This problem has been happening to me for a lloooong time now. I’ve heard that the devs will fix it for over a year now.

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