Don't know exactly what happened. Had 3 successful 3-5 minute flights in stabilize, autotune, and loiter modes prior to attempting autotune. During first flights quad was generally stable but much less smooth than my brother's quad with similar configuration.
Every few seconds quad would "glitch" very slightly. Just a few degrees / feet then return to stable flight. I was hoping autotune would eliminate this behaviour. Partway through the autotune process, about 3/4 of the way through the attached tlog the quad began to act erratically, I no longer had control and it flew up about 20 feet and to the side about 20 feet at what seemed like full power. It did not respond to any input.
I cut throttle, switched back to stabilize, and let it come crashing to the ground before it got somewhere anything could be damaged.
After rushing to the quad the back left motor (2) was much much hotter than all the rest of the motors.
Any thoughts? Thanks guys. I'd hate to put this back in the air without fixing the underlying problem.