Sudden decent in altitude, full throttle did nothing?

Today i was flying over sea, i did this 2 days prior without issues.

I had a sudden decsend almost touching water, i tried full throttle and the quad still did not raise in altitude.
Eventually i panic and just hit RTL, the quad looked like it stopped just above the water and stood still for couple seconds the began to gain altitude very very slowly.

It Did not return to home, i took over back control and flew it back to land once it gained some altitude on its own.

In the gopro footage i heard the pixhawk beep during the decent.

I was flying in poshold/hybrid mode, i never tried to go into stabilize mode during the problem. I just panicked…

Any ideas?

Attached is my log, zipped because log was 9mb

So hard to get support sometimes…

Did I post in the wrong forum?

Try to post on apm:copter maybe. I will try to have a look as well

It looks you had a short term GPS failure.

On line 77338 your satellite count went to zero and flatlined there for 2.4 seconds at which point the GPS started to come back to life. Maybe bad GPS connection? Ironically, that was the fine part. It’s when your GPS started to come back on that the problems began.

You have your GPS Failsafe to continue in AltHold. Seems wise, and it’s what I do. However as the GPS was starting up its vertical elevation was apparently way off. If you look at your BaroAlt against your RelAlt the RelAlt was much higher than the BaroAlt until the GPS got settled back in. So your aircraft was trying to hold its altitude, and it thought it was too high, so it descended.

It saw your RTL command but looks like it refused to enter it because your HDOP was nudging 5 around then. It appears to have sorted things out just about the time it would have hit the ground. I suspect if you has switched to Stabilize at any point you would have regained control of it right away, but you were still in PosHold, which is very GPS-dependent.

But I’d take a close look at your GPS connections. That was a full GPS failure.

Thank you very much for the response.