Y6 (Pixhawk 3.1.2) flip and crash

We have a 3DR Y6B with 850kV motors, SimonK 3DR ESCs, default props, Pixhawk with 3DR GPS/Compass Module, 3DR power module and a 3S battery. We were hovering in loiter at around 30m when the vehicle flipped and fell straight down, pretty much in a free fall. We switched to Stabilize on the way down and put the throttle input to max with no effect. We’d appreciate a review of our log files to help figure out what happened.

Definitely a motor or prop failure, almost certainly motor 2 (front bottom right.)
Possible contributing factor is the stability bug in 3.1.2, but it only took effect well after the initial disturbance. Please upgrade to 3.1.3.

We fixed our Y6, replaced motor 2, and upgraded to 3.1.3. We did a test flight this morning and had a similar crash in loiter. There was a twitch that preceeded the flip by several seconds that did not seem to be caused by wind. Then the vehicle pitched over and did a fast uncontrolled descent. Like before, we switched to stabilize on the way down, put the RC throttle to max with no effect. I see two problems here, why did it lose stability in loiter (since we replaced the motor thought to be bad last time it seems unlikely to be the cause), and why did the Pixhawk not ramp up the throttle signals to the ESCs in response to max throttle from the RC in stabilize. Once again help is appreciated, we’d like to have a usable vehicle at some point.

Once again, channel 2 goes highest during the crash. See fig 1. Blue line is roll error. Red line is channel 2. Green line is average throttle.

The total throttle is not being increased because the copter is physically unable to meet the attitude demand, so there are motors at limits. See figure 2.

Hi there,

Did you manage to find out what was wrong? Is it really a motor problem or something else? Having a conclusion will really help people like us understand APM better!