Help!!! All 8 motors of quad co-axial multi-rotor drone stopped spinning in mid-air

Hi there,

I have a quad co-axial multi-rotor drone and crashed during loitering mode. The drone was hovering about 10 m from the ground in loiter mode and then all of a sudden ALL motors stop spinning and finally crashed.

About this drone,

- MTOW = 25 kg

- U-10 T-motors

- running on 10s Li-Po Battery (made from China)

What we have investigated so far

- Flight controller was still working even when the motors stopped spinning. The data log shows that the flight controller was giving command to the motor to go full throttle at the time when the motor stopped spinning. The flight controller stopped working during impact.

- The Li-Po battery voltage per cell seems normal after crashed (showing about 4V for a cells). However, the Li-Po battery was fully charged and left it fully charged for about 6 months though...

- No signs of burn marks on all electronics.

- 4 flight tests were performed 4 days before the crash and no signs of problems at all. The day of the crash, we did a morning flight test for about 5 mins and no signs of problems at all. But on that same day in the afternoon, the drone crashed.

- We are in the stage of checking the ESC and motors, we checked two of them so far and no signs of problems.

You guys have any idea on this? 

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I think it may have been the aliens, or maybe an act of god, or maybe you want to post the dataflash log if you expect anything else than crazy guessing :) 

A df log would be nice. If I were to speculate, total power failure and the FC still worked for a while. Was the impact seen in the accel data just before the end or did it just stop?  There are things to look for. I have looked at quite a few crash logs. I like the challenge. If you post the log I would be happy to look at it.

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