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Hello all,
Sorry for my English, I’m Chinese.

Pixhawk autopilot running AC 3.2
550 frame
2216-800KV
20 amp esc
Futaba t8fg
Futaba 6303 Receiver 
3DR GPS 
Power module
3s 5300mah battery
1147 Propellers

I recently had 2 crashes in a morning. 2 crashes are same issues.
The second crash:
I pushed Throttle stick slowly up to taking off and switch to AltHold mode. I was flying about 5 meters in front of me and about 30 meters high in AltHold mode. During the flight the copter was flying fine for the first two minutes, but then started losing altitude and sudden falling, I immediately increased the throttle stick position to full (and hold it in that position almost till the end). No reaction at all. I noticed that it didn't listen to the Throttle stick and did not respond to throttle increase resulting in an ugly crash, fell down horizontal from about 30 meters....
Because crash was horizontal, the Gopro3+, gimbal was a total wreck. Frame, motors, propellers remained intact without damage. Suddenly the copter was skimming the ground with full throttle automatically bashed against the garden railing more than a dozen meters away. (at this time the transmitter’s throttle stick keep bottom)

What is strange is that in all the cases why the Pixhawk make ThrottleOut going down, while ThrottleIn kept on the top?

The first crash was same issue, the Pix shut off the power during AutoTune, and Fortunately there was no damage in the first crash.

I first thought of a battery issue but there appear to be plenty of juice left in the battery after crash. The problem still occurs when using a different battery after first crash.

Now I'm sure the Battery,ESC,Power module,Propellers here is not the fact of the crash, during the whole flight there is no brownout or power interruption. The battery is an ‘old battery’, the internal resistance gets higher than a new one, so the voltage during flight is lower.Although voltage drops to 9.97v, according to the factory datasheet, even at 7.4v each motor can generate 510g thrust, sum of four motors are 510*4=2040g, My copter weight is just 1560g. At that time I push full throttle stick.

For the test I discharger this battery with 20a current 8 minute can draw more than 3500 mah. Then measure the open circuit voltage is about 3.72/cell, each cell has good balance.

I had also tested many times in stabilize mode with same battery it worked perfectly.
I also changed the Flight controller to ‘Dji naza lite’ everything is doing what it should and functional.

Here is the second crash video from the Gopro3+, log file and a graph are attached to the post,

https://www.youtube.com/watch?v=XXTHElKVnUE

PS:
The year of Gopro3+ set wrong, it should be 2015, not 2014
Log bitmask: Default+IMU



Could you please take a look on what I wrote here and help me, if you see the reason of the issue? I'm really scared now to have any flight, until I'm 100% sure that the issue is resolved.

Looking forward to your assistance. Thanks in advance!

Firstcrash.rar

SecondCrash.rar