Fly Away! Please help me figure out why

Today I learned a lot, and I know that I likely made some stupid mistakes.  What was intended as a simple test flight, got way out of control, and had the potential to end very badly.  I am doing my best to dissect the logs, but I am still new with a lot of this. I would appreciate your help to figure out what went wrong and figure out what happened, and most importantly I want to learn from this and make sure it NEVER happens again.
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Let me give you some details on what happened….
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I recently completed a DJI 450 ARF build with a TBS Discovery Plate and a Pixhawk FC with GPS Module.  I spent months researching the build and decided on using the Pixhawk. I completed a few basic test flights in the backyard, keeping it eye level and just some basic movement.  Everything seemed in order despite more drifting then I had expected.  So, today I decided to take it to a local park to do another test flight.  The plan was to keep it under 15’, practice some basic flight movement and test out its stability.  I connected the battery, made sure it had a solid GPS lock, armed the copter and took off.  Everything was going as planned, it still wasn’t as stable as I had hoped but in stabilize it was fairly easy to control. Next I tested out loiter which performed pretty well, it didn’t hold position as well as the pixhawk videos online but it was decent.  Then I went to Alt Stabilize mode.  It seemed to have trouble maintaining a constant altitude, nothing crazy but drifting up and down 4’ or so.  Then, out of the blue it shot skyward! I watched in horror as it went straight up in the air, higher, higher and higher.  As you can see in the logs I immediately cut the throttle, but there was no result.  I then switch modes out of Alt Stabilize, I intended to switch to Stabilize but instead switched to Loiter.  Either way this did not have any effect. It still continued to clime, and then drift out of the area of the park and over houses!!  Then in what seemed like an eternity, I watched it start to fall from the sky. I tried switching modes and throttling up, but couldn’t do a thing except watch in horror as it came crashing down.  By the grace of God it landed in a tree in someones yard, and did not do damage to anything but itself.  I was able to quickly and easily find it from the beeping of the pixhawk.  All 4 props broke, one arm snapped, the GPS mast broke, lost the telemetry module and the rest of it is pretty banged up, BUT I have it, and that is the only damage that occurred. I ignorantly did not consider the risk of it throttling up and going skyward, I figured that if I went to  a local park kept it close and low I would be responsible and safe, boy was I wrong! 
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I downloaded the logs and have been trying to make heads and tails of them to figure out why this happened, and most importantly make sure it NEVER happens again. I know for sure I will have to go way farther out than a local suburban park for my next test flight.
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Here is a graph showing altitude, throttle in, throttle out, and mode. You can clearly see what I described above.  It shows that a failsafe fence kicked in, which I assume was the altitude and luckily i think that kept if from getting any higher
 
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Any help and insight you can provided would be GREATLY appreciated.  Thank You!

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  • Thanks for the replies!

    Vibrations, interesting... I remember reading about that in other posts and now that you all have pointed that out, it does appear to have a pretty strong correlation. The thing is pretty broken up to so hard to tell if something came loose. I will also verify that sonar was not selected

    Any suggestions on how to combat the vibration issues? (mounting techniques or piece of hardware?)  Also what should have been my response when that happened? Is there anything I could have done to stop the climb?

    • No, your setup was fine until that happened, so it's not about much better damping, but maybe about something not getting loose - it's not aliasing alone, both your accelerometers experience a achange in Z axis at the same time, but the mail one more than the other.

      A manouver that can stop it, is stabilize, set descend throttle , and give hard yaw to one then the other direction (whatever works).

      This makes it possible for the motor/propeller/arm etc  to stop resonating itself/other parts..

      • Interesting, thanks! Tonight I will try to identify what could have come loose.

        • I just read through this post on the forum

          http://diydrones.com/group/arducopterusergroup/forum/topics/why-naz...

          and the video he posted is exactly how mine was flying during its first two test flights and then right before it flew away.  In reading the comments one thing it made me remember is that when I would arm the copter or after I would land and throttle all the way down, all 4 motors were not "slow" spinning at the same speed.  I remember at least one of them would actually stop.  Perhaps there is something wrong with the ESC or Motors. 

          • From my experience, it seems that the props spin at different speeds if the ground is not level. Highest prop often stops.
            • Thanks, that makes sense

  • Developer

    Sorry about your troubles.  The Inertial nav's weakness is the risk of aliasing getting through and throwing off it's vertical speed estimate.

    In the next version of Arducopter (or the current version of ArduPlane) we mix the two accelerometers that are on the Pixhawk and because one runs at 800hz and the other at 1000hz we think it's much less likely that aliasing will get through.  It hasn't been fully tested yet but in one test this did greatly reduce the vibration aliasing which is what leads to the runaway.

  • Sorry to hear about this. I to have had a few crashes due to my error are something in the software. Also almost hit a freaking house. Try and repost here http://ardupilot.com/forum/viewforum.php?f=3 you might find some more help.

    Good luck.

  • Graph your IMU/AccZ vs CTUN/ALT

    You get crazy vertical vibrations , then it takes off - Z vibrations is a known reason to cause such climbing.

    Most likely, some propeller failed, motor or Pixhawk came loose.   - you can also see a increase in Y vibrations before both Y and Z goes haywire.

  • I notice that CTUN > DAlt steadily increases when your flyaway happens.
    Have you accidentally ticked the box to say that you have sonar, when you do not? Seems this can cause this, example here:
    http://ardupilot.com/forum/viewtopic.php?t=6547&p=12409

    I also notice that your GPS and BarAlt are the same UNTIL the flyaway happens, when they disagree. Seems maybe that caused it to try to correct for an alt issue that didn't exist?

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