Incident Overview
A helicopter was maneuvering with a check pilot and pilot on board when it suddenly shuddered momentarily, followed by a heavier airframe vibration. The helicopter rolled left as it climbed to 500 feet. The check pilot attempted to turn right towards the airport, but the helicopter rolled left again. The check pilot regained control, shut the engine down, and completed an autorotation to a field.
Examination Findings
Post-accident examination revealed that the tail rotor drive shaft coupling, supported by the aft hangar bearing, had sheared in torsional overload due to an overheated and seized number 6 hangar bearing. The hangar bearing inner race and the balls had large heat-affected zones from overheating during operation, with metal in the races ball path having been molten and then resolidified. The grease was black and dry, with embedded metal particles.
Prior Service Bulletins and Airworthiness Directives
A series of service bulletins, alert service bulletins, and airworthiness directives (including an emergency AD issued by the FAA) had been issued from October 2001 through 2004 concerning the overheating and failure of the hangar bearings. The bulletins and airworthiness directives addressed, in part, the modification of the helicopter to include cooling ducts and changes to existing bearings to prevent overheating and eventual bearing failure. The ADs mandated inspections of the hangar bearings every 25 hours of flight; bearing replacement before further flight was required if metal particles or "black" grease was observed at any bearing.