Sudden uncommanded pitch wobble

Got a S1000 from DJI, using pixhawk 2.1 cube, AC 3.6.7, here GPS, autotuned and all, flew fine for a test mission after the autotune flight and then this happened. Check the logs and see how att_desired_pitch goes haywire(in loiter) and the multicopter starts pitching up and down very fast and with great amplitude and not even switching back to alt hold/loiter does not help the situation.
After this the issue never repeated in a future flight.
Check the troublesome log here:
Check a future flight that day with no issues:

I`m suspecting the autotuned pids for pitch are too weak compared to the roll ones, check in the second log that the att_desired_pitch and pitch differ quite a lot compared to the roll axis.

Maybe re-autotune ?

I haven’t looked at your logs, but in my experience, strange uncommanded movements are often related to too low PID’s. So, I think your analysis might be correct. Try increasing the pitch PID’s.
If you lowered your autotune aggr setting, I’d put it back to .1 and do it again. Make sure the battery and everything else is secured tightly and don’t wiggle. If you have a gimbal, put it in ‘lock’ mode if you can, where the motors hold everything solidly in place. Or take it off or tape it all together tightly so it doesn’t move.

And otherwise, I’d suggest just manually tuning… If it’s flyable just try higher PID settings. Autotune always gives me much to low of PID’s, but I’m playing w/ very small copters that are probably outside arducopters design parameters.

I’ve increased autotune param by 50% and the pid values found for the pitch axes were about twice as large as before. So far the craft has been flying normally and yeah maybe the weak pids were to blame.

I’m having the same exact issue on my 5kg X8 copter, did it eventually turn out to be weak PIDS ? Mine are set at borderline twitchy, and still encountering it

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I wish I could say for sure that fixed it. In my case, as I forgot to mention, this happened during a partial landing where the craft barely touched the ground and then the pilot wanted to land somewhere else but it was sort of too late to abort the touchdown. Somewhere in between flying and shutting down…