The pilot reported that the helicopter's engine failed during performance of a power assurance check. Prior to beginning the power check, the pilot established straight and level flight at 90 knots and 8,000 feet msl (about 5,000 feet agl). He increased collective, the helicopter began to climb, and at approximately 8,300 to 8,400 feet msl and a torque reading of 95%, there was a loud grinding noise and the engine lost power. The pilot entered an autorotation. During touchdown in an open field, the main rotor struck and severed the tail boom. Examination of the engine determined that the turbine exhibited internal damage and would not rotate. Disassembly of the turbine section revealed rub damage to all the blade tips on the first and second stage turbine wheels. Corresponding rub damage was noted to the blade paths in the second stage turbine nozzle for these turbine wheels. A single blade was separated from the second stage turbine wheel. There was severe damage to the turbine wheels and nozzles located downstream of the second stage turbine wheel. Metallurgical examination found that the second stage turbine nozzle assembly second stage blade path had been dimensionally restored using thermal spray material. The coating exhibited unmelted particles, oxides, and was separated from the base metal at both forward and aft ends. The defects in the coating resulted in blade tip rub and led to the fatigue failure of a blade, which separated and destroyed the downstream turbine wheels and nozzles. Review of the helicopter's maintenance records indicated that the engine's turbine section was replaced with a repaired unit approximately 13 flight hours prior to the accident. During the repair of the turbine, dimensional restoration of the second stage turbine nozzle second stage blade path was performed.

The National Transportation Safety Board determines the probable cause(s) of this accident as follows:

The improper overhaul of the turbine assembly by maintenance personnel, which resulted in failure of the turbine and a total loss of engine power. A contributing factor was the pilot's misjudgment of the landing flare at the termination of the autorotation.