So I built a large hexacopter last year with a Sony Nex-6 in 3 axis gimbal with a pixhawk as the controller. It sat over the winter and I got it out the other day and updated / installed new firmware etc.
I took it out today and after making sure everything was hooked up and secure I took it up with a 5200mah 6s battery to make sure it was working and responding correctly. After that successful flight I landed and swapped out the battery for my larger 10,000mah 6s battery to try and do an autotune.
Being that it is a 6s setup I couldn't use the 3dr default power module so I have a 6s compatible power module I got online hooked up instead. I just have it sensing voltage but no current to monitor the voltage in flight. I also have a 5v bec hooked up to the servo rail for power for the esc's (no bec on the esc's) and backup.
It was flying fine in stabilize mode but when I would switch it to alt hold it wanted to drop altitude / act a bit weird. When that happened I would switch back to stabilize and all is well. After a few tries switching to alt hold it seemed to get the hang of it and actually hold it's altitude at which point I switched to autotune mode.
Autotune seemed to be working fine just was taking a long time to get through roll and pitch and i kept guiding it back to the center of the field as needed and when I would let off stick it would continue autotune like it should. Eventually it finally got to yaw and seemed alright till it started spinning in a circle in one direction for awhile which I had not seen in autotune videos but kept guiding it to open area as needed and it kept going. Before it finished though I noticed the voltage was getting too low on the battery and I would have to put it down before autotune finished.
I switched modes back to stabilize and phonescreen showed "autotune canceled" or stopped whatever the exact message was, tablet wouldn't work with 3dr radio but phone did go figure. Anyways, went to go land and it was going fine but then started dropping fast and right before touching down the hexacopter pitched forward hard on its own, I wasn't pitching forward or back at all and it ended up flipping over as it came down from the sudden forward pitch. Only thing I could think was maybe didn't have enough power left as I tried to land but doesn't explain the pitching forward and I checked the battery when I got home and it was over 20 volts but that is with no load obviously so I don't know.
My understanding is since I switched modes back to stabilize before autotune finished and you have to land and let it disarm in autotune mode or after autotune finished to get the adjusted pids to save it should have just gone back to the default pids and back to stabilze. If anyone can help me out with checking out the log and seeing if you see anything that might explain what happened there at the end much appreciated. I'd like to try to autotune again after I make repairs, got lucky wasn't a ton of damage, just one arm rotated and piece of carbon fiber where the rc receiver was mounted broke and the landing gear broke at the bottom. I think I'll have to autotune with the gimbal off next time or just do one axis at a time (underestimated how long autotune takes / thought 10,000mah battery would have been enough) but yea, I'd be a little hesitant / worried to try again after repairs without trying to figure out what happened this time around.
Sorry for the long post, just wanted to give as much info / background as to what happened. Log attached, had to zip it to get it under 7MB attachment limit and guess I'll add a clip from the camera gimbal though not sure how useful it is or not but it with the log might help I guess. Thanks much.
10 second clip at end of flight, can hear the beep switching modes to stabilize then try to start landing and bam on youtube.