This is not my aircraft, rather I am asking this on behalf of a client for whom I am doing some flight testing. I attempted transition on his Believer VTOL, which is a tiltrotor setup (front 2 motors tilting) but it ended in a crash. Details as follows:
I had set ARSPD_FBW_MIN conservatively (17m/s) as the airspeed sensor had not been calibrated and did not want to risk transition too close to stall speed. Initially the aircraft was struggling to reach 17m/s when switching to FBWA, as the recommended Q_TILT_MAX setting (45 degrees) was probably a bit low. So I gently pushed the nose down and the aircraft then made transition (597 line number) at which point I neutralized the elevator and also reduced the throttle input. Suddenly at 604 it rolls and pitches down and is pretty much inverted at 607, whilst still pitching nose down. At some point in this sequence of events, the right stabilizer (it is a V-tail) can be seen coming off. See graph below, which shows attitude, airspeed and RC inputs for throttle and elevator.
At this stage I am leaning towards airframe failure, ie. the right stabilizer/V-tail. Here are some pictures of the rear of the fuselage where the tailplane was bonded to, as well as the plastic tailplane root itself. I am not going to say, but is there anything that jumps out from these photos?
Here is the log for anyone who wants to take a further look: https://www.dropbox.com/s/y9txddzpsu3k8kz/2019-01-31%2010-36-17.bin?dl=0
I did try save the aircraft in QStabilize at 605 but it seems to have been over-ridden to Manual. EDIT: as Rolf noted in his reply below, it looks like I invoked Manual after QStabilize. This was probably instinctive, as a fixed wing RC flyer.
Any thoughts, opinions will be appreciated.