I took out my Tarot FY680 Pro hexcopter with me for an evening flight. Met a buddy of mine who has a Phantom 3 - we were going to get some video shots of the town square. My hexcopter is outfitted with an APM 2.6 flight controller/autopilot and I had changed the flight modes from a previous flight. Normally I have the following flight modes: Stabilize (for manual take offs & landings), Loiter, and RTL (Return To Launch).
But on the previous flight, I had switched RTL to Altitude Hold as I was going to do a video shoot of the neighbohood Christmas lights. I had forgotten that I made this change when I went out for the evening flight over the town square.
I took off, shooting video of the town square and let the copter climb really high. I wasn't worried as I was going to switch to RTL to bring it back. But when I hit the switch, the copter didn't return - it did was it was commanded - held altitude, but drifted away with the prevailing breeze.
Immediately, I switched back to Stabilize, but had trouble figuring out the aircraft's orientation - spend a lot of time trying to lose altitude, and coax it back. Finally after about 5 minutes, I got it fairly close to the takeoff site and I decided to switch off the transmitter, and let the failsafe kick in and the copter would land itself. Instead, the motors went to idle and the aircraft fell out of the sky! It fell from about 90 feet and crashed in the parking lot - a total loss.
To this day, I don't know why it became a rock when I powered off the transmitter. If I had more faith in myself, I could have brought it in manually.
But I have salvaged parts and I'm building a quadcopter from spare parts I have and parts salvaged from the crash.
The lesson here is always check everything before going out to the flying field - do not assume anything! Check and double check. I now have a preflight checklist that I use every time from now on.