Instant Altitude Loss / Almost Crash

Hi there, we’re making a quadcopter with ArduPilot 3.6.9. We’re having an issue (probably electronic or mechanical but not sure) that makes our quad drop almost to ground. We changed motors and ESCs with new ones but no cure.



Edit: Copter weighs about 1.9 kilograms
And here is the last log. Flight were performed at night so the video is not that good but if anyone demand it I can upload that too.
00000005.BIN (640.7 KB)

Before continuing, update to at least ArduCopter 3.6.12. Previous versions are know to crash.

Updated to ArduCopter 3.6.12 and calibrated. Here is the flight. There is a little drop as you can see in the video around the 50th second. I ran AutoAnalysis on the log, here is the output;

Log File C:\Users\balta\AppData\Local\Temp\tmpF6F5.tmp.log
Size (kb) 3657.931640625
No of lines 44137
Duration 0:02:10
Vehicletype ArduCopter
Firmware Version V3.6.12
Firmware Hash cb570c06
Hardware Type
Free Mem 0
Skipped Lines 0
Test: Autotune = UNKNOWN - No ATUN log data
Test: Brownout = GOOD -
Test: Compass = WARN - Moderate change in mag_field (25.68%)

Test: Dupe Log Data = GOOD -
Test: Empty = GOOD -
Test: Event/Failsafe = GOOD -
Test: GPS = GOOD -
Test: IMU Mismatch = GOOD - (Mismatch: 0.13, WARN: 0.75, FAIL: 1.50)
Test: Motor Balance = FAIL - Motor channel averages = [1676, 1709, 1555, 1581]
Average motor output = 1630
Difference between min and max motor averages = 154
Test: NaNs = FAIL - Found NaN in CTUN.DSAlt
Test: OpticalFlow = FAIL - FAIL: no optical flow data
Test: Parameters = FAIL - ‘THR_MIN’ not found
Test: PM = GOOD -
Test: Pitch/Roll = UNKNOWN - ‘BarAlt’
Test: Thrust = GOOD -
Test: VCC = UNKNOWN - No CURR log data

Here is the video

Here is the log

Good, now follow:

Thanks for that, I’ll tune the copter asap