I've come across what I suspect may be a bug in AP 2.5 and also maybe in 2.6. What happens is the APM will, shortly after takeoff, start showing an incorrect attitude which is always a pitch up relative to the real attitude. I've seen this happen several times in my flying wing with AP2.5.
In the attached log, which is a short flight that followed a completely successful flight, the wing takes off and gains altitude in FBW-A. Then I switch to RTL and it seems to be working at first. Almost as soon as it gets to 100m, the actual plane starts pitching down and losing altitude but the attitude indicator still shows level flight. I then switch to FBW-A and pull back all the way. The plane continues diving down at about 30-35* down but the HUD shows a huge pitch up. Then I switch to manual and the sudden control surface change makes the plane roll around a lot but it actually recovers to more or less right up in real life - I didn' tthink I could fly it manually so I switched back to FBW-A and it just kept diving until it crashed.
I have the APM secured in the plane with 3-4 square inches of velcro and it takes some muscle to get it off, I'm very confident it did not come loose in flight. It was still securely in place even after the crash as well. Whatever is causing this doesn't happen every flight, I did have several successful flights but I've also seen this issue 2-3 times so far.
Has anyone else seen this? If it's not something in the software, any ideas what might cause the APM to sense a false positive pitch?