After several succesfull long flights with my medium range delta airframe I had a rather strange crash last tuesday. Some facts:
The aircraft was not carrying a camera on its nose, putting the CG slightly further backwards then it was tuned for using autotune. But still the CG was within the envelope of the airframe.
I checked all control surface deflections and directions before take-off, I always do this every flight as it became habit.
The Plane has flown for approximately 5 hours already without any issues or strange behaviour.
Directly after the lauch of this flight i noticed some Pitch oscilations, this is maybe due to the CG and thus the servo trimming not being where it was with the camera on the nose? (my assumption)
I threw the aircraft in stabilzed mode and it flew away fine, made a circle and put the plane into FWBA and then loiter mode which it also performed as expected for some circles. I then put on my FPV goggles and switched into Cruise mode, this is where I first saw the pitch oscilations.
Then switched into Stabilized to check if the oscilations would be the same or dampen out, (should have probably switched into acro at this point though).
At this point the aircraft suddenly nose dived very steeply and crashed into a tree before I could switch to Acro to save it. It took some effort to retrieve the craft and after checking it and plugging it in to retrieve the logfiles I found that my Elevator stick command was suddenly Reversed!?!
Looking a the logfiles the input still seems to be fine during the crash though, I firstly pulled up, then the dive started and then I pushed down since the craft was almost upsidedown and that would have been the best way out at that moment. it however already hit the tree at that point. But that behaviour is clearly visible in the recorded logfile.
To be clear here, after the flight i did not at any moment touch my transmitter so there is no chance I accidentally reversed it. I checked the Parameter list from before the flight with the one currently in the FC and there is nothing reversed as well? If I put the aircraft in stabilized the elevons still compensate in the right direction.
Does anybody have an explanation to this behaviour or had the same issue?
Iam really puzzled about how the pitch input can be reversed suddenly without any changed parameters.
Logfile form the flight as well as param lists before and after the flight can be found in my google drive: Ardupilot logfiles crash austria - Google Drive