Accident Sequence
The pilot reported that the helicopter's engine failed during the performance of a power assurance check. Prior to beginning the check, the pilot had established straight and level flight at 90 knots and 8,000 feet mean sea level (approximately 5,000 feet above ground level). As the pilot increased collective pitch, the helicopter began to climb. At an altitude between 8,300 and 8,400 feet msl and a torque reading of 95%, a loud grinding noise was heard, and the engine lost power. The pilot immediately entered an autorotation. During touchdown in an open field, the main rotor struck and severed the tail boom.
Examination Findings
Examination of the engine revealed internal damage; the turbine would not rotate. Disassembly of the turbine section showed rub damage to all blade tips on both the first and second stage turbine wheels. Corresponding rub damage was noted on the blade paths in the second stage turbine nozzle for these wheels. A single blade had 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 Analysis
Metallurgical examination determined that the second stage turbine nozzle assembly's 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 the forward and aft ends. The defects in the coating resulted in blade tip rub, which led to the fatigue failure of a blade. The separated blade then destroyed the downstream turbine wheels and nozzles.
Maintenance History
Review of the helicopter's maintenance records indicated that the engine's turbine section had been 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's second stage blade path had been performed.