Originally Posted by
rotorspeed
Then he could have had a gyro or autopilot problem which was not noticed. Or instead of having ALT engaged, had VS and that was actually in slight down so caused an unrecognised gradual descent.
Or hit a severe rotor in the Lee of the ridge that he hit and the AP disengaged...
Given that he hit so close to the summit with lots of altitude beneath him to the valley floor it doesn't really look like he got caught at the end of a valley. Looks more like he was trying to get over the mountains in AP when something didn't work out as it should, being terrain higher than he thought (i.e. being somewhere else than expected), or helicopter not able to maintain/reach required altitude quickly enough or the AP kicking out at an inappropriate moment (e.g. due to turbulence/downdraft) and the pilot losing control in IMC or another event roughly along these lines.