N2096W - 2004 Aircraft Aircraft Registration

The loss of translation/rotational power between the engine and the transmission due to the severe wear of the forward spline portion of the lower coupling drive shaft. The reason for the severe wear of the forward spline could not be definitively determined due to fire damage and the loss of associated components, which were not located during the investigation.

The reduction of the throttle to idle at a high density altitude during a practice autorotation, which resulted in the engine stoppage. Factors contributing to the accident included the instructor pilot's improper planning by not adhering to the flight manual's warning to avoid reducing the throttle to the idle position at high density altitudes, and the uneven terrain.