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, headed toward the river, then banked to the right, spun around and put 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

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Oh, one thing I forgot to mention - once things went haywire - there seemed to be no apparent response to any control inputs from the radio control unit (as well as my attempt to hit the "go home" button on droidplanner)

I've had a similar event happen to me while in Loiter. I discovered my GoPro 3 was interfering with the GPS. I too had very little input. The only thing kept from hitting the ground was I got it out of loiter and into stabilize mode. If loiter isn't working right chances are RTL won't work either.

I'm no expert looking at the log files but at first there doesn't seem to be the normal GPS fail that most people encounter. Also mapping you GPS track it looks nice and stable. At one point the HDOP went up to 6.94 but this is when you where just hovering low to the ground.. It is best when this is below 2. But it looks like at that point you were just hovering in one spot. When . By chance do you have the TLOG file on are tablet. Having the TLOG allows you to replay the flight on Mission Planner.

I also looked at your RCIN channels and see the inputs were being received by the pixhawk but in loiter or RTL your inputs will cause minimal reactions.

BTW where are your speed controllers located? Also is there a chance the pixhawk came loose from the frame?

Here is a quick synopsis of your log file provided by Mission Planner

Log File C:/Program Files (x86)/Mission Planner/logs/HEXAROTOR/1/9.log
Size (kb) 2614.5185546875
No of lines 35566
Duration 0:03:58
Vehicletype ArduCopter
Firmware Version ArduCopter
Firmware Hash
Hardware Type
Free Mem 0
Skipped Lines 0

Test: Brownout = PASS -
Test: Compass = FAIL - Large change in mag_field (38.29%)
Max mag field length (723.70) > recommended (550.00)

Test: Dupe Log Data = PASS -
Test: Empty = PASS -
Test: Event/Failsafe = PASS -
Test: GPS = WARN - Min satellites: 7, Max HDop: 6.94
Test: Parameters = PASS -
Test: PM = PASS -
Test: Pitch/Roll = PASS -
Test: Underpowered = PASS -
Test: VCC = PASS -

What version firmware?

What's strange is when you lose control TH_OUT is pinned max.. 

Did you have fence enabled? What was the setting?

In log alt and baro alt very strange, Alt -150! but baro alt +80.

1, Z accel    (Mechanical?)

2, Alt down to -150

3, ThrOut   Max

4, Descend full speed to hold -150 alt


is that a genuine pix?

Hello Adam, 

I'm sorry for your copter,

had exactly 2 times the same problem, have replaced all ESC. There were priced with inserted cable. Have 6 new soldered. My hexa is still flying healthy :)

sorry for my bad english Greetings from sunny Germany

ciao Klaus

i think this guy is right. as soon as your copter started moving (easy to tell by LAT because you were in loiter so long beforehand) your accelerometers are going nuts. Desired X & Y accel is +/-5 (centered at 0) and Z between -5 and -15 (centered at -9.8). 

Your X hung out around -5, but your Y was drifting between -10 and 20, with your Z fluctuating between 0 and 60! (based on IMU) I would definitely say that your pixhawk came detached and was vibrating, leading to erratic behavior.

public service announcement: always switch to stabilize when in doubt! stabilize doesnt care about acceleration, just orientation.

Help me out here guys, I'm not familiar with the pixhawk. There is also IMU2 - in the 'high stress' region the readings differ significantly from IMU. is this due to the nature of vibration, or could this be indicative of something else?

I agree, it sounds like the flight controller came loose.  Those are not X and Y values that make any sense with it firmly attached.

Well, that's very scary and I'm sorry for your loss.

The issue appears to be vibration.  I suspect some kind of mechanical malfunction of a motor or propeller because both accelerometers on the vehicle go nuts.  That's 6G to 8G of vibration on the MPU6k accelerometers (top) and a much more reasonable 1G ~ 2G on the backup accelerometers.  This completely messes up the inertial nav position and loiter does very bad things.

So clearly we need to make ArduCopter more capable of dealing with this kind of situation. AC3.2 will use the accelerometer that is more reasonable.  So in this case it would use the backup instead of the main one.  That would have helped but it still would have flown off I suspect.  We will run the logs through the Extended Kalman Filter "replay" to see if it would have done any better.

Really sorry for your troubles.

My gut instinct would be to consider a 'failsafe' that decides when to ignore the accelerometers altogether. I understand that this could be problematic... how do you detect intense vibration? time averaging? min/max? And you can't just switch back into STAB, because of potential throttle differences. And how would you alert the pilot that they are in full control? Is GPS accurate / fast enough to attempt to bring the copter down by itself?

And of course, while the developers feel the need to compensate for this risk, there is always the possibility that the simplest solution is better education about any automatic mode's dependence on acceleration and the importance of good vibration dampening.

1) I'll check on the firmware version when I get home, it was installed via APM Planner as the latest stable version about 2.5 weeks ago.

2) Nope, no fence enabled (I haven't started playing with those features yet) 

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