Can anybody help analysing the cause of this crash?

The copter started a test mission without any troubles and after reaching a waypoint, he started to yaw constantly but continued to follow the mission path.

After a couple of seconds the pilot switched out of AUTO mode into PosHold and then down to STAB, but the copter kept on spinning. The resulting oscillation of trying to get the copter stable and dealing with the ongoing spin resulted in a crash.

When I looked at the log-files, I can see the actual yaw following the desired yaw nicely. Until this very certain waypoint where the desired yaw goes up to 360, stays there for 3 seconds and then drops to 0 and stays there - all still during AUTO mode. When the desired yaw goes up to 360, the copter started its constant yaw and did not stop unitil crashed.

We are aware of the z-vibrations being not super nice, but I am pretty sure that this is not the cause of this behaviour. The copter holds it's altitude pretty nicely.

Frame type is a X8.

Any ides or suggestions?

Any help is greatly appreciated!




2016-03-09 17-34-25.tlog

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  • I am experiencing a similar issue.  I have had intermittent yaw issues for the past few months.  So frustrating that it has been so intermittent!  As soon as I think I've solved it I end up back in the shop troubleshooting.  My latest crash followed a perfect test flight a couple days before with no issues.  Immediately on take off it went into a counterclockwise yaw and couldn't recover.  I'm starting to suspect an intermittent bad ESC.  I hate to go to all the trouble of replacing the ESC's if I'm making the wrong conclusion.  If anyone here could take a quick look at my latest crash log I would really be grateful.  Thanks!!!

    PS. I'm flying a tarot 690S with Sunnysky v2814 700v and MS ESC's flashed with SimonK

    • Developer

      Hi Calvin,

      It looks like your number 4 motor is lazy. I would check your wires and make sure there are no breaks. I had an issue where one of my wires broke behind the bullet connector solder joint but because of the heat shrink it was in contact. It would then occasionally move and I would have an intermittent crash. You may also have overheating of a bad esc and it is cutting out.

      • Hi Leonard!

        Thanks so much for taking a look.  Actually a had just fixed a break in the solder joint for the power wire for the number 4 ESC.  I had been doing a compass mot calibration and noticed #4 was intermittent so I investigated and found the break.  After fixing it I repeated the compass mot procedure I found it strange that motor #4 still did not start in sync with the others.  This makes me wonder if something is wrong with the ESC.  I went ahead and did a test flight.  All motors started in sync and it flew perfect.  Then this latest flight it went right into a spin.  Are there any tests I can do on this ESC?  Any idea why it would behave differently in compass mot?  

        The other thing that confuses me about the logs is why does desired yaw match the spin behavior?  Shouldn't desyaw be straight and yaw show the spin?


  • MR60

    If it may help, I had once a quad that flew in a very unstable way. At a certain moment in flight it started to yaw like crazy. The conclusion was two mechanical problems: a loose motor attachment and a too loose Pixhawk mount. How did you mount your Pixhawk? Check it did not came loose even partially from its mount plate.

  • Developer

    Hi Marc,

    This is definitely a mechanical issue of some sort. As has been stated already, the copter is doing everything it can to fight the yaw but is unable to do so. This is why it doesn't matter what mode it is in. The controller has been deliberately setup to sacrifice yaw in order to maintain roll and pitch in these situations. This lets you land safely and correct the problem.

    I can't point to which arm is twisted because you were in a dynamic flight state at the time. However, a twisted arm is the only thing I can think of that would cause this issue. A single lost motor should be clear in the logs and should not result in lost yaw authority (as you demonstrated).

    I have been fooled by this myself once as it doesn't take much of a twist to overcome the limited yaw control of quads.

    • Hello Leonhardt,

      would you mind having a final look at the LOG files after our changes?

      Thank you very much.


      • This link is this page ;)

    • Leonard and all the others,

      thank you for your time and findings. I will inform the owner of the copter about what you all have found out.

      What you say makes sense when I review the logs now, a second and third opinion always helps. The good thing is that the flightcontrol is doing its work without any problems. Mechanical failures can be found and fixed, that part is easy now.

      Thank you again for helping us.



      • Hello altogether,

        i would like to thank you for your input and for the time you spent in investigating and trouble-finding.

        We checked all topics we could, tested with 7 motors running, checked all mechanical issues, optimized electrical system and put some small weight (none magnetic; non iron material) on the pixhawk and reduced the Z-vibration level a little more.

        Yesterday we finally made some more testflights (manual and auto missions) which worked nicely. Vibs are still to be found, but not too high i guess.

        While landing next to a facility building we got a "Bad Gyro Health", but no bad behavior of the copter.
        So, i hope we fixed our crash issue and in total we hopefully brought the whole system to a safer and more reliable level.

        I would really appreciate if you could have a look on our LOGS from yesterday:

        Thanks so far


        • Developer

          Hi Syrphus,

          Could you set your log bitmask to 192509 and do another flight. You have high rate accel logging turned on and your sd card can't handle that, this means there is a lot of data missing.

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