The only reason I can come up with is that the battery was loose and it slid backward causing the PIDP.I to gradually increase.
It first happened within RTL and the plane lost altitude and the PIDP.I increased to compensate for the effect or the battery slid back to its place in extreme front of the plane. The second time it again happened in RTL but this time it was unable to recover.
I need confirmation whether my reasoning is correct or not?
Reasoning seems to be incorrect. PIDP.I should increase for a nose heavy plane. That could not be the case due to the fact that the battery was in the maximum front position and if the battery had moved, the pidp.i should have decreased.
I have however observed that pitch was decreasing even with the desired and output pitch calling for increase. Does that indicate loss of a V-tail control surface?
The situation is really confusing.
What could be the cause of increasing PIDP.I? autopilot being unable to achieve the desired pitch?
Another theory could be that the plane weighed almost 6kg. and due to the airspeed decreasing to almost 11 m/s. the aircraft tried to regain speed by increasing throttle but could not recover from stall caused by going into headwind during RTL.