Pixhawk Flyaway (and fortunate retrieval)

I had a disturbing experience yesterday. I was test flying my Hex and after a few successful hovers, a little PID tuning, and running through a few flight modes, I decided to let it stretch it’s wings a little more.

On a fresh charge, I put it into Loiter, took it up a couple hundred feet, did a slow 360 and then slowly moved it forward.

Then, All of a sudden, the Hex sped up, failed to respond to controller inputs (tried to kill the throttle) headed toward the river, then banked to the right, spun around and flew itself into a field about a quarter mile away.

Fortunately, no-one was hurt, and the Hex itself only suffered a couple broken arms and a prop - but needless to say, I’m quite concerned and want to get to the bottom of this before taking it out again.

I’ve uploaded the flight video to youtube at http://youtu.be/sc4UqOOiUUQ

I’m also attaching the flight log files.

I don’t doubt it’s related to something I did (or failed to do) and any insight would be greatly appreciated.


In case it helps - here the vehicle specs

Tarot 680 (Carbon) Hex frame.
Sunnysky V3508 380KV motors
5S 4000mah Zippy Lipo
3DR Pixhawk, 3DR 915Mhz Telemetry & 3DR GPS unit
Futaba 14GS Radio
Mavlink connection to Droidplanner 2 on a Nexus 7

Your flyaway was caused by a crazy amount of vibration. Have a look at the attached plot. It shows the BarAlt, RelAlt and Z axis vibration.

Everything was fine until around 3 1/2 minutes. You have very low vibration and your BarAlt was tracking perfectly with your RelAlt, as it should. Then “something” happened and all hell broke loose. It was experiencing very high Z axis acceleration, and it thought it was going down fast. You can tell that since the RelAlt (green line) is going down. In response the copter called for more power so in fact it was headed up. That’s your red BarAlt line.

The question is, what suddenly caused the massive vibration? You clearly had a serious failure. Possible things could include a cracked prop, a misfiring ESC or maybe a winding short in one of your motors. Whatever it was seemed pretty blatant though.

I’d suggest repairing the damage, looking for anything obvious. Do a low hover in Stabilize and see if there’s anything weird about it. If not, flip it into Alt Hold and see if it’s stable and fly it around a bit. But have your finger on the flight mode switch and be ready to flip it back into Stabilize if it does anything unsuspected.

Had you done that when it went nuts, you might have been able to save it. Instead it was in Loiter then switched to RTL, both of which are vibration susceptible.


Thanks for your feedback - Having read through several of these threads before, and after looking at the logs I suspected that vibration could have been to blame, and am glad that someone more experienced than I can offer deeper insight into the issue.

After a more thorough review, I have identified a potential factor in the build - I had originally built this hex with the intention of using 3-4S batteries, but when the vendor accidentally shipped me 380KV SunnySky motors rather than the 580KV I ordered, I decided to go with 5S rather than exchange the motors.

What I overlooked was that my ESC’s (Afro slims), though rated plenty high for Amperage, are only recommended for up to 4S Voltage.

Do you have any thoughts on whether the pattern of “hell breaking loose” that the logs show, would be inline with how an ESC driven over voltage might fail?

Thanks again! - As a newcomer to this amazing hobby, I’m truly greatful for the wonderful community than I’ve joined.

I’m not sure of the effects of using 5S on 4S rated ESCs. My guess is that it would cause permanent failure and not something intermittent. But again, I don’t know.

I have used Afros and experienced mixed results. One definitely failed on me and I had another that acted flaky, so I replaced it. But they do seem to run very cool. You might check and see if yours are running warm. In any case I wouldn’t run over their recommended max voltage unless you’re feeling lucky…