Crash in auto/loiter mode; undesired drop in throttle out

Rig: x8+
Firmware: ArduCopter 3.2

During a waypoint navigation flight, my x8+ unexpectedly began rapidly losing altitude. At this point I switched into Loiter mode with the throttle set around 60%, yet the drone continued to fall at the same rate, eventually hitting the ground. I’ve gone through the typical log diagnosis; vibrations seem fine, compass performance seems ok, gps readings are reasonable, etc…but am still unsure what caused the incident.

Some strange things I noticed from the logs:

  1. Throttle out drops to 100 unexpectedly and remains there even after switching to loiter mode with throttle set above midway. (see fig 1)
  2. The throttle output from RCOUT channel 5 deviates from the control tuning’s throttle out packet. (see fig 2)
  3. The desired altitude and inertial altitude estimates are very strange. The drone crashed about 10m below where it started (I was flying downhill), and the barometer and gps readings reflect this. However, the inertial system thinks the drone is still well in the air, even during the crash. The desired altitude behavior is very strange, and what is further perplexing is the inertial estimate tracks it well, despite both being wildly inaccurate. (see fig 3)

-I had flown ~50 missions essentially identical to this one successfully before this incident
-The gps/compass mass snapped off on impact, hence the compass/gps readings going off the charts post-incident.

Any advice around what might of happened would be much appreciated!

Log and figures are attached

Looks like an IMU hiccup.
IMU2 drops by 5 at the point it started to come down.


Hmm, good spot, but even though it drops by 5 it’s still within the ‘acceptable’ range (it drops to -14.85). Could this still cause the seen behavior?