Stabilize, Alt-Hold, Loiter and then crash - help needed pls


A week ago I was flying my quad in an open field. It has a pixhawk PIX2 and runs AC 3.2 with GPS and 3DR telemetry. The quad was flying just fine before and did auto tune etc. All went fine and finished 4 packs in the 2 previous weeks.

This time I hooked the battery, armed and took off in Stabilize mode. Then switched to Alt-hold which went a couple of seconds fine. After that I switched to Loiter and the quad immediately revved to the left angled (rolling at 30 degrees) and then crashed in the ground. I tried to control back and get in control but with no luck. It was completely unresponsive.

I looked into the logs and I could not find any reason. This is my first time and I really appreciate if somebody helps me debugging it. Health seems was fine. GPS with >8 Sat and <2 HDOP. Barometer, accl, gyro all seem fine, I think.

Can you please help me look into the dataflash logs? I am including the log file from that flight. Just to learn what is going on!


Anybody can help with this would be really appreciated!

I had a look at your log, at it seems that just after you switch to loiter, the roll and pitch diverge from the desired values (under ATT section). Looking at the RCOU[T] values and assuming you have a QUAD-X configuration, channel 2 immediately jumps to full scale, which would seem to indicate a failure of the rear left motor. Channel 1 (front left) decreases drastically, probably trying to counter the roll induced by the failure above.

I think it’s most like that your ESC went out of sync or the prop failed (assuming that the ESC / motor seem to ok now). The expected result is that it would have tilted left and backwards, which sounds similar to what you described.

Thanks a lot. So rcou the output from the pixhawk to the esc, right?

That is correct, the RCOU (RC out) log variables represent the control signals sent by the controller to each ESC. When a motor or ESC fails, the controller will notice that that motor needs more power, and increase throttle for that motor.

The control is ‘open-loop’, in that the controller doesn’t actually know what the motor is doing, only the vehicle orientation; if the front drops, the front motors need more power etc.