Cruise/AP issue

Flying in an a319, got up to FL330 and have the AP set up as I always have, but when the aircraft reaches the cruise alt after a slow climb, it is stable for a short period, then starts to ‘bob’. The weather isn’t anything drastic at all, but the aircraft will slowly begin to rise, then descend, then rise, and so on and so forth. After a while becomes rather profound pitching and nose diving. Ends up to around (-)900ft/m.
Anything obvious I’m missing?

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What was your airspeed or Mach?

You may be overspeeding. The aircraft will bob and then may crash.

I had the same problem last night and I was doing cruise speed

This happened to me the other day also. I ended up fixing it by raising the airplane another 200 feet at 100ft/m. I think you just have to be really gentle with it and not make any drastic changes when climbing.

It’s caused by overspeeding for some aircraft unless it’s a bug in the AP System.

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No I think it’s a bug, my GS was 330.

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Was cruising at Mach .61, so must be a bug?

Was the VS(Vertical Speed) AP changing during this occurrence?

The AP VS was at 0, but indicated was jumping up and down constantly

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.61 on FL330? That’s a bit slow…


That isnt close to cruise speed. Cruise speed should be M.78 bottom line. You were going too slow and at the lower density air you need more speed to maintain sufficient lift.

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M0.61 is very slow at FL330. Try getting closer to M0.8 and you’ll find your fuel will go further due to your attack angle being more level (nose pitch closer to the horizon).

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You where on the verge of a stall


Ahh didn’t read through that again, I meant .81 originally. Pardon me there, thats what was confusing me

I have climbed to 36,000’ and experiencing no issues so far…

This is happening to me in the 777

When this happens (with any plane) speed is usually the culprit. Too slow or too fast and it will bounce with A/P on.

If you are slow, try speeding up and see if it levels out. If you are going fast, slow down and see. Airbus planes seem to be more sensitive than Boeing from my experience.

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If you are flying into a headwind (jetstreams), try descending lower since the winds are usually less intense at lower altitudes.

This used to happen to me some time back in 787 but at much lower altitudes.

How I managed to fix it was remove the ALT hold on autopilot and only engage the VS hold at 0.

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