Autopilot goes crazy

Hello, I was flying an A350 at FL350 with weight just under MTOW. I turned on autopilot with LNAV and the plane flew fine for a while, but then suddenly pitched up sharply and started rapidly banking left and right past 40deg. It ended in a stall and the autopilot swiched off, and at that point I had to manually fly it for about 15 minutes. When I tured autopilot on again, it kept doing the same thing. Does anyone know why?

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Likely you were too heavy for the altitude you were flying at, causing too much AoA (angle of attack) for staying aloft. This at some point creates more drag than available power from your engines.

With a video or image, it would show the higher AoA, and probably speed gradually decreasing.

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Hey there!

This would be because you flew too high for the weight of the plane. If you’re just below MTOW, you’d have to start much lower and step climb along the way. I recommend you check out this awesome topic on how to pick an optimal cruising altitude for your weight with the A350.

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Thank you for the suggestion, but I’m still having the same issues. Now it’s less of a stalling issue and more of a pitching left and right (right now at around 10deg on both sides)… any suggestions?

I’ve seen this behavior before but only when the aircraft thinks it’s going to stall. I tested it at FL350, and 146% load (the maximum the A350 can carry) and it’s not even at 100% throttle so it’s not the weight. It can even gain speed at this altitude so I’m not really sure what’s happening. How fast are you flying?

Actually, I just tested this and it doesn’t become a problem until you get to Mach 0.6 and I highly doubt you’re flying that slow.

This is where there’s really no substitute for a video record (sometimes a screenshot can show strong clues). A rapid enough change in wind direction for example, combined with being heavy and high can trigger full throttle up, which may be insufficient to recover too fast a drop in IAS (the drag shoots higher than power at too low a relative wind speed).

Still, this is just a guess.

It’s also possible to test this by going into solo mode (so you have wind control), going high and heavy, and abruptly shifting the wind sliders.

This indeed will bring down the plane but it has to be 100 knot headwinds directly to 100 knot tailwinds and a 200 knot shift in winds is highly unlikely I feel. I think if this had resulted in a violation there would be a case for its removal because a 200 knot instant shift in winds is a bit egregious.

I tried the following: same weight and altitude as OP (just below MTOW, FL350), and I went from no wind to an 85kts tail wind, but I also increased the temp to 20C (I should probably try without the hotter condition). I get IAS something below 190kts, large AoA, and I keep losing speed rather than gaining (power at full throttle), and my wings are rocking back and forth 10 degrees or so.

edit: now I’m stalling out

edit 2:

It appears from further testing, a change in wind speed and temp together are likely needed.

The main issue is, the higher and heavier you are, the lower your safety margin against weather shocks such as a combined change in wind and temperature.

Turbulence and the onset of wing rocking only intensifies the effect (banking during the wing rock appears to inhibit forward acceleration).

FL350 is a pretty standard initial cruising altitude for a heavy a350 irl. But IF might be different, just look at the A339…

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I’m able to get the A350 to FL350 at MTOW. What speed you are flying can also make a difference - if you are flying too slow that can cause the bank left/right motion…

I flew the New York to Singapore route, with 99% load. I start at FL350 exactly, then FL370 390 410 430. I didn’t experience a stall or banking. Of course weather condition matters, would you @RedBeaver give us more information?

I think that’s likely the key issue. If you get too slow, by whatever cause (when heavy and high), you risk getting trapped by drag increasing greater than the capacity for thrust.

I caused this shock with wind and temp change.

Another possible influence: getting too slow from excessive climb rate.

And combining these obviously compounds effects.

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I just realized that I was going at M.62, but after burning fuel to about 41% it solved the issue itself. Thank you @adit and everyone else who gave suggestions!

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