NTSB investigators either traveled in support of this investigation or conducted a significant amount of investigative work without any travel, and used data obtained from various sources to prepare this aircraft accident report.
The instructor and student were conducting a night orientation flight. According to a witnesses who worked for the operator, about 15 minutes after the helicopter departed, he heard what sounded like an engine rollback and the helicopter making an autorotation. This was followed by the sound of an increase in engine rpm and the drive belts squealing, culminating with the sound of the helicopter making a loud thud-type noise. Another witness stated that the engine sounded rough and that the helicopter was making a high pitch whining/squealing sound, after which it went silent. A third witness also heard the helicopter making high pitch noise just before the accident. The helicopter was subsequently located in an open field near the departure airport; a postcrash fire erupted, which consumed the helicopter.
A postaccident examination of the lower coupling drive shaft showed evidence of severe wear completely around the forward spline that extended beyond the root of the spline teeth. Severe wear of the forward spline teeth could have been caused by a loss of alignment between the engine and the drive shaft or an inflight loss of lubrication in the rubber boot. The rubber boot that retains grease for the forward spline portion of the drive shaft was not recovered and was presumed missing. Loss of grease coverage for the forward spline, either from a rupture of the rubber boot or a loss of the clamp for the rubber boot, could cause sudden inflight wear and overheating of the spline teeth. Severe wear of the forward spline portion of the lower coupling drive shaft most likely led to sudden and complete loss of translational/rotational power between the engine and the transmission. 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.