I have a quad, Iconic-X frame with APM2.5 . I have a very sneaky trouble with quad. Periodically one motor (Motor #4) stops in the middle of the flight. Today it happened after 4 minutes of the flight. When it flipped and fell belly on the grass with no damage, I tried immediately to lift it in the air, and I couldn’t. I also noticed that motor #4 spinning slower than the other three. It was spinning, but regardless of throttle level was spinning with slower (close to idle?) speed, unlike 3 other motors that were reacting to throttle properly. I disarmed the quad, armed it again and it behaved normally. I even flew another 4.5 minutes until my battery died then I landed normally. I flew another battery on minimal altitude directly after that with no troubles or accidents.
I checked THR_MIN and it was set it to 150, (higher than default 130). My THR_MID is 410 (I hover at 39-41% of the throttle). I was flying in stabilize mode when quad fell. I was switching to Loiter and Auto for several seconds before that.
If anyone recognizes the symptoms, let me know! Thank you!