Hexacopter/Pixhawk2.1 - Crash


I had a crash today with my hexa falling by chance in a corn field.
A few seconds before crash I got battery unbalanced messages. When I found the hexa, prop on MOT01 had disappeared (maybe got off when landing in the corn field - I only saw it later)!
An auto analysis showed huge magnetic field disturbance (above 500 when during my previous flights it was about 20).
Compass seemed to have failed.
It was the first time I flew with FPV.
Could someone help me find the source of the crash and how to avoid it on my next flight?

2017-04-21 17-36-35.bin (2.6 MB)

hey dude,im sorry to hear that.
Dont think its compass problem,since the desired yaw didnt change a lot.
Hard to understand,it seems that it lost alt and fall on ground.but the alt controller gives the right output.what is the temperature that day?cold ?

your gps signal is very good.what is your gps model?

Yeah… as I remember, it rolled on its right than began to sink. I gave full throttle, it levelled again but continued to sink flat until it hit the ground.

I use this GPS:

It really seems related to a battery loss. After the crash, i checked the battery voltage and 3 cells were about 3.9v and the fourth one at 4.02v. The battery became suddenly unbalanced. I would like to find the reason to avoid it in the future.

Thanks a lot for your help…

Hey ,when the power is not good, the motor input should be higher than before.
ok ,it seems one motor reach its max value,and the another one got its min value.So the other motor cant go higher?
you need a better battery or bigger battery.

Another problem ,the roll isnt very good before it crash.you should check the motors .

You lead me to a good path, thanks.
If I look to the RCOUT graph (C2-C7), I can see that at 17:39:25, C7 begins to rise and C2, by compensating, begins to sink. At 17:39:31, C7 reach its max and will not move from there, C2 staying low.
As I stated before, I found the hexa with a missing propeller on MOT 01 (C7). The graph seems to confirm the inflight loss of propeller on C7 that began to turn on its max to compensate the loss of portability. C2 compensated to keep the hexa levelled.
I will go back to the flying place to try to find the propeller back.