Auto takeoff mode, pitch down results in crash

On my 2nd flight on my AtomRC dolphin I used auto takeoff (shake and wake) setup. The plane did do a scary torque roll but did recover and climbed away at 100% throttle. Then at about 100m from me pitched down maybe by 30deg and wouldn’t pull up. The plane hadn’t gained much altitude and was moving very fast. I was not quick enough to move into manual mode. The crash completely destroyed the plane. However the electronics seem to have survived. Unfortunately there was a logging error and there is no log file. I understand this makes it difficult to diagnose problems but please bare with me.

The only change from my Maiden was I have a new radio upgraded to a radiomaster T16s. This radio does have different mapping but i did setup directions correctly and did double check servo direction at the field. Both in manual and in fbwa. In fbwa servos compensated in the current direction.

On my 1st flight did use auto-trim. I also did an autotune for roll. I did have issues with pitch, I didn’t have enough down patch throw, And it was a windy day I had to cut the throttle to get the plane down. So I did increase the Lim pitch values to 3000, and -3000 for pitch. Roll is set to 6500.

I’m using a tbs tracer Rx, it’s set to cut, and failsafe works on the bench. Just before the flight I did update the Rx/Tx firmware. On the day of the flight the yappu script stopped work, but all controls where okay.

However I’m not sure that I correctly did the new radio calibration, (new radio gimbal values are a bit different) could this of lent to the crash? I have five more ardupilot planes. I don’t understand what I’ve done wrong. And I’m scared to fly my other planes.

Oh one thing, I did notice at the crash site that the prop was loose and free to rotate, motor and prop was not damaged. Not sure it it came loose at takeoff or during the crash? But even if the prop was loose and plane lost power even at 100% Throttle would fbwa still pitch down?

If you’ve changed the radio to the TX16S you should also re-do the radio calibration for the flight controller. The neutral trims, and end points of your new radio may not be the same as your old one.

As for the loose prop, if the thrust line of the motor isn’t lined up with the C of G of the plane very well then when you lost power it may cause a significant pitch change. The direction will depend on what way it’s wrong. I’ve had props come loose before and it always makes a crazy noise so I’d expect you’d know if something was wrong.