Airbus A321 Autopilot Failure

Just recently, I was attempting to fly from Zurich, Switzerland, to London Heathrow, England, in a Swiss Airbus A321. I found that the autopilot, specifically the ALT autopilot, appeared to be faulty. Once activated, the aircraft began to bob it’s nose up and down almost immediately. This motion became progressively worse, until the aircraft stalled. Had I not been present to recover from the stall, I would have crashed, and racked up several speeding violations in the process. I found the issue to be extremely reproducible, occurring 100% of the time the autopilot was engaged. I was initially flying at 34000 ft, which was the Simbrief suggested altitude. In the full knowledge that Simbrief can sometimes make mistakes, I tried again at 30,000ft. Same problem. This has happened to me before, whilst flying from Manchester, also in an A321. I have never had autopilot issues whilst flying any other A320 family aircraft. I recorded the issue, and it is being uploaded to YouTube. Any ideas what may be happening here? If you want device specs, or other info, ask me in the comments and I will provide.

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What was your speed? What was the weather like because the weather has caused this for me.

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Ive had this several times as well. Ive tried speeding up, slowing down, playing with trim and flaps but nothing helps. This happened when I had a 4kt headwind. Was in a 787-8.

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There was a 111 knot headwind, and some turbulence. The speed in the recording was Mach 0.82, although the problem occurs at higher speeds as well. I think I tried Mach 0.85, with no improvement.

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Now that I think of this, I remember someone had a similar problem. what were your fuel settings?

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111kts headwind seems really huge, maybe thats whats causing the problem. Are there huge gusts?

My makeshift solution is just to disengage ALT and manually control pitch until VS is near 0 (+/-10). It doesn’t work all the time, but it helps alleviate the bobbing.

I’m just thinking this may be the cause of some of the numerous “I woke up and I had crashed” type support topics, with people trying to get violation reversals. If this is the cause, the only way around it is to constantly monitor the flight, which could become more than a little tiring on a 10+ hour flight.

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Um, I just asked what your fuel was lol. Anyways, If you had a full tank and you went straight up to 34,000 that might be the cause as you should have climbed to maybe 27k, then leveled out for 20 minutes, then climb a few thousand feet and stop and so on. Im not sure if this goes for planes as small as the a321 though.

What was your weight, sometimes that can affect the A/P at higher altitudes.

I don’t know exactly how much fuel I had at the time that the issue struck, but it started during my ascent, and occurred every time I engaged the autopilot from that point onwards. At Zurich i took on 8055kg of fuel. Aircraft load during the recorded incident was 52%, although the issue happened multiple times.

Can you try at a much lighter weight, say 20% fuel with no pax or cargo and see what happens?

I will go on solo mode, and try multiple weight and wind settings, to see what happens.

So the nose kept going up and down you say?

A: You climbed too fast then slowed your climb really fast at the final second

B: You’re too heavy for whatever cruise altitude you would like to be at

C: Weather ;)

Set the turbulence setting to none when you go and see if the problem still persists.

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It could possibly be because you are above the maximum takeoff weight during takeoff. The plane is just too heavy to remain in the air. This has happened to me once and I just had to dump some fuel to get below the MTOW and I was fine. The weather was still the same from before I dumped fuel and then after dumping fuel to below the MTOW.

Was your VS at a normal rate when you engaged Autopilot?

Well then, it seems its all down to the weather. A flight at the very low altitude of 10,000ft, at 5% load, but with a strong headwind and turbulence, caused the bobbing issue. I didn’t stall, but I wasn’t far from it. This is the worst possible outcome. What if I’m flying long haul, say, from KLAX to EGLL, and during the middle of the flight, when I’m not around to monitor the situation, I hit headwinds and turbulence. This causes the bobbing issue, which leads to a stall, and speeding violations. Then I’ll get back to my doomed flight, see the dreaded word “CRASH” on my screen, and find numerous violations against my name. When I go onto the forums to enquire about my violations, I’ll simply be told something along the lines of, “You must have done something wrong, you don’t get violations for nothing.” How am i supposed to control the weather? This is starting to sound like a rant, i know, but surely its more than a little unrealistic for an autopilot to just give up in bad weather. It doesnt happen in real life, so it shouldnt happen in a flight simulator like infinite flight. Note that this is only the result of testing in the A321 at this moment, but Etrain claims to have suffered the issue in a B787-8, so the issue may not be isolated to the A321. I will conduct further testing, and publish the results of the tests on this page.

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I eventually resorted to climbing manually to the desired altitude, levelling off manually at that altitude, and then only once the aircraft was stable at the required altitude, I would engage autopilot. The problem began almost immediately.

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What VS were you climbing at during your climb? Also, were you using any trim while climbing or were you keeping it at 0%?

Was climbing at 2000fpm, when the problem started. Rate of climb doesnt seem to be the issue though, as I experienced the same issue multiple times, even when engaging AP in cruise, at the desired altitude, at a vertical speed of 0 +/- 50fpm.