Need help on Crash Analysis

Hello All , im new this forum but i have been using arducopter for many years , recently i had a crash of a copter running ArduCopter
Firmware Version V4.1.3. on a cuav v5 nano .Prior to the crash i had many good flights with the copter doing more than 50mins air time of each flight . The crash was very quick , happened just after auto mode .
I need help in analysing the log file and find out the reason for crash

log file

Hi Emma,
just before it was switched to Auto vibrations started to increase and got really bad shortly after.
This can be because:

  • Broken / cracked propeller
  • Loose Motor mount
  • Loose propeller mount
  • FC coming loose
  • Something else like payload suddenly causing excessive vibrations
  • Part of the frame coming loose

I would check everything closely for anything that could cause those vibrations

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Graph the RC outputs. Thrust loss on motor 4, 3 drops to stabilize attitude, down it goes.

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Hi Karl , Thanks a lot for the feedback

Hi Dave , is that happened in flight ?

Dave is correct,
for some reason one motor dropped out whilst another tried to make up for it. This send the vibrations high.
…Checked various things but haven’t been able to find why that motor stopped.
And yes, it was at the max. height during the flight when this occurred. (red line)

Right, as Karl’s graph shows. It’s difficult to determine from a log why thrust is lost. Having ESC telemetry can sometimes give a clue. But in any case case closely inspect Motor 4 and it’s ESC for damage. Perhaps a prop broke but usually unless you actually see it you can’t tell if it happened in the air or when it hit the ground.

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Thanks a lot Karl and Dave , I will power up and check the esc