Drone crash suddenly after a stable flight

ArduCopter Copter 4.6
1

Hi all,
My Drone is flying well, but suddenly it decreased its altitude and crashed in loiter mode. From visual aspect it is like it loosed altitude due to a motors stops suddenly/ speed decreased that’s why it crashed. I might be wrong. But the point why the motors are behaving like this? We have faced couple of this type of incident frequently, could not find any remedy. Went through certain link, all are suggesting there might be a esc motor issue. The questions that is the only problem or anything else we missed to find. and we found this type of problem many times.
I am attaching the link of the log

Kindly help me to find out the reason for the crash.
Thanks in advance.

2

There is a sudden drop in voltage seen. Did you check the battery connections? loose connections maybe.

3

Thrust loss on Motor 4, Motor 3 drops to compensate and down it goes. Motor, ESC, broken prop, hard to say why.

BTW-Battery voltage goes to zero when it hits the ground well after the thrust loss event.

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4

Something else found ? Kindly suggest… Any other thing happened so that crash occurred? Any other reason? kindly help

5

…….

No other reason.

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6

Like if I re-emphasize correctly that we have seen this happen repeatedly in the recent past with a variety of components, basically we use different brand’s motors with relevant ESC also.
All showing the same signature that one motor thrust drops and the diagonally opposite one goes down to compensate. Could there be something systematic in our flight parameter settings which could be causing this? Or some other generic thing that we’re systematically doing wrong that could be affecting all drones in the same manner?

Kindly suggest

7

Thrust loss could be caused by several different things that are not clear from a log. Failed motor, ESC, prop, solder joint, etc. Or it could be a de-sync. Poor quality build and/or the components perhaps.
Got any of this junk?
Hardware to avoid

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