NTSB Identification: LAX04IA135
14 CFR Part 91: General Aviation
Incident occurred Thursday, February 19, 2004 in Madera, CA
Probable Cause Approval Date: 07/07/2005
Aircraft: Bell 407, registration: N601MT
Injuries: 2 Uninjured.

NTSB investigators used data provided by various sources and may not have traveled in support of this investigation to prepare this aircraft incident report.

The pilot executed an autorotative emergency landing following the failure of the number 6 tail rotor drive shaft hangar bearing while maneuvering. The check pilot and pilot were maneuvering the helicopter when it 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; however, the helicopter rolled to the left again. The check pilot regained control, shut the engine down, and completed an autorotation to a field. The tail rotor drive shaft coupling was supported by the aft hangar bearing, and inspection revealed that it had sheared in torsional overload due to an overheated and seized number 6 hangar bearing. Post accident examination revealed that the hangar bearing inner race and the balls had large heat affected zones from overheating during operation (the metal in the races ball path had been molten and then had resolidified). The grease was also black and dry, with imbedded metal particles. A series of service bulletins, alert service bulletins, and airworthiness directives (including an emergency AD issued by the FAA) had been issued beginning in October 2001 through 2004 concerning the overheating and failure of the hangar bearings. The bulletins and airworthiness directives dealt with, 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 called out if metal particles or "black" grease was observed at any bearing.

The National Transportation Safety Board determines the probable cause(s) of this incident to be:

the failure of the number 6 hangar bearing due to overheating. A factor in the accident was the operator's likely inadequate compliance with service bulletins and airworthiness directives on this issue, along with an inadequate maintenance inspection program for the bearings.

Full narrative available

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