Incident Sequence
During a positioning flight, the pilot added power to initiate a slow climb. Shortly after, multiple bangs and yawing were experienced. The pilot responded by reducing power. He concluded that tail rotor thrust had been lost and prepared for a precautionary landing in a wooded area. When the helicopter touched down, it rolled over.
Examination Findings
Post-accident examination revealed that a bolt was missing from the Thomas coupling. This coupling is located between the aft end of the aft short shaft and the forward segment of the tail rotor drive shaft. Additionally, the bracket for the forward hanger bearing had fractured. A metallurgical analysis of the flange from the Thomas coupling, where the bolt was missing, identified thread imprint marks on the interior of the bolt hole bore and lipped material at the edge of the bolt hole bore. The hanger bearing mount had failed due to overstress; no evidence of progressive cracking was observed.
Maintenance History
The helicopter had accumulated 15.2 hours of operation since the Thomas coupling was last disassembled and reassembled. The mechanic who performed the work reported that he reused the bolts and self-locking nuts and tightened them to the specified torque. The missing nut and bolt were not recovered.
Database Review
A check of the FAA Service Difficulty Report (SDR) database for occurrences in the preceding five years found no other similar events.