Stumped in analysis of Pixhawk AltHold crash

So I have built/rebuilt about 10 multicopters. The first 8 were all using APM2.6, while the last 2 are using Pixhawk (or rather Fixhawk, the RCTimer clone). In both of these I have been experiencing crashes, where the machine goes berserk right after take off. In the 1st crash it happened in Stab, while this 2nd crash happened in AltHold. I do not have further info on the 1st crash, but this 2nd one has me stumped.

It is a modified Tarot 680 pro hex. I am using a Neo-6M GPS/Compass (3DR clone) and it is on a sturdy mast. I am on the latest (3.3FW) version of Arducopter.This was 1st test flight, and all initial settings/calibrations were done. I armed it, and after a few seconds took off in Stab (at about line 6500). It felt fine, so I immediately switched to Alt Hold. I did two or so left/right rolls to get a feel of the hexa. It then pitched sharply backward, which I tried to correct. It was at this point that mayhem broke out, with it eventually (after a second or so, turned upside down and crashed).

I am stumped. Since it is in AltHld, compass differences do not come into play. Vibes are all excellent. What I can see is at line 8962, in the ATT section, Pitch starts to deviate sharply from DesPitch. I tried to correct it, but it is from here of on that the hexa is doomed.

So my interpretation is that the described Pitch change is the smoking gun, but I cannot figure out in the logs if there are any reasons as to what could have triggered it.

Of course it could be hardware (the actual flight computer), or??

And this is where I would appreciate some assistance in the analysis of the attached log file. Can any of you spot what could be the cause of this?


I’m inclined to say it was some sort of electrical or mechanical problem in your rear two motors.

If you look at the RCOUT values in your log, at the same time with the pitch, you can see the pitch starting to go very positive (pitch up). As this is happening your controller is dropping the request for power to your 3 and 5 motors (the front two). So it’s seeing the pitch change and trying to fix it, but fails, even though it drops the power to your front motors to essentially zero.

I’d look first at the motors and ESCs for your two rear motors. Could be something intermittent that causes one to lose power. Your controller is asking for more power on both 4 and 6, but especially 4. Another possibility could be sync issues as some have found Pixhawks (and maybe worse for Fixhawks) act oddly with certain models of ESC. And these same ESCs work fine with older APMs. Lastly, it could be some strange hardware problem since it’s a clone as that occasionally comes with the territory.

Thanks for the reply.

Unfortunately it does seem to align with my own suspicions. I had taken care in the assembly, but in this build am using new ESCs - Afro Slim 20A. I had flashed them with BLHeli, but reading various posts on the Net, seem to indicate that some folks do experience issues with Pixhawk and these ESCs.

I have experienced sync issues in past build (on APM) with different ESCs, but those crashes had a totally different behavior. You can hear the motors stutter/screech, with a subsequent rapid drop of an arm

I guess my next step would be to do some “test flight” with the hexa strapped down, and then look at the RCOut logs and see if any of the motors show a repeat . . .