Quadcopter gained height really quick in AltHold and then crashed

Hello ardupilot community,

I’ve just build my first quadcopter and did my first really low level flights (around 5 meters) successfully, so I’ve wanted to get a little bit (around 10 meters) higher. After flying in stabilize a bit, I switched to Alt Hold and flew for a minute or so, when suddenly it goes into full throttle and gaines height really quick. I was totally shocked and switched into Land mode as fast as I could (I’m not sure if the land mode that could be seen in log was actually from my side or if that was a failsafe). It then was drifted away by the wind a larger distance and I found it crashed, with a broken arm and 3 broken rotors.

I tried to analyse the log a bit for myself and I’ve made the following observations:

  • VFR_HUD_Throttle goes to 100% suddenly, RC input was transmitting ~ 50 % throttle (See above pic)
  • Altitude (VFR_HUD_alt as well as GPS_relative_alt) dropped below zero shortly AFTER throttle went to 100%. I wasn’t landing or something, it was still in the air
  • Zmag drops really low BEFORE Throttle went to 100%
  • Pic off the log.

Other observations, they don’t seem so important though:

  • For me it looked like it dropped really fast after that, but in the log you can see a smooth declination. I’m pretty sure that the log is cut of at the end unfortunaly. I’m not sure how to see events in this log though PIC
  • Relative alt and VFR_HUD_alt have an offset after switching to land
  • The throttle goes really far down after initiating Land and it doesn’t goes up a little bit to inititiate a smoother landing. Seems to be the cut off log as well.
  • Vibration is higher than allowed, but i can’t seem to find a correlation between zacc, yacc & xacc with the Full Throttle PIC

Infos about my build:

  • APM 2.6 with sponge on barometer
  • external compass with GPS, but it isn’t working in most cases
  • EMAX 4in1 ESC
  • Picture of my quadcopter when it was still fine

Unfortunaley, I wasn’t able to get a complete .log/.bin file of it, I only had the *.tlog from the “3dr Services” app on my android tablet.

I was flying in windy conditions as well, maybe it has something to do with that. I also have to add that the apm is not really vibration resistant, as described above, because the foam isn’t delivered yet and I only wanted to make a low level test flight.

Album with the logs

Could you help me with this, what exactly was causing the full throttle? I’m assuming it’s because of the dropped Zmag, but I’m not sure what caused it (and what exactly it is).

Do you need the tlog I have? Or are the pics enough? I’m also trying at the moment to get the *log file, but it’s a little bit bugged.

Thank you very much in advance.


The best way to get the log file is to remove the microSD card from the PixHawk and copy the files (bin extension) from the card to a PC/Mac and attach’ed here. Also the tlogs might be useful.

I was actually using a APM, so it isn’t possible. Logs aren’t working, no idea why; but that’s another problem.

My quadrocopter was just a little bit misconstructed (I build him myself; no xp with rc before); calibrating the compass and APM after that fixed the problem, I’ve never experienced such a thing again. I also learned that you always should be able to switch to Stabilize on your remote, which helped me a lot.