Accident
A public-use helicopter was responding to an emergency when it experienced a vibration and yawed to the right during cruise flight. Subsequently, there was a loss of tail rotor thrust. The pilot reduced engine power and executed an autorotation to a park along a lakeshore. During the landing flare, the helicopter yawed left as the pilot increased collective prior to touchdown. The pilot reported that the touchdown seemed smooth and occurred on the aft portion of the skids. However, as the helicopter slowed, the left yaw tendency increased. The helicopter rolled over and came to rest inverted at the completion of the run-on landing.
Examination
Examination of the helicopter revealed that the number-one tail rotor driveshaft hanger assembly had separated, resulting in a discontinuity of the tail rotor driveshaft. The machine bolt that held the assembly together was not installed with the required cotter pin. Further examination of the hanger assembly showed that the splined areas did not contain grease, while the remaining tail rotor driveshaft hanger bearings were properly greased and assembled.
Maintenance
The mechanic who worked on the helicopter reported that due to other workload items, the previous tail rotor driveshaft inspection was interrupted several times before he could complete all required tasks. To avoid intermixing the driveshaft components, he temporarily reassembled the hanger assemblies before working on other projects. His intention was to disassemble each hanger bearing when he had the opportunity to complete the inspection. The mechanic stated that he must have forgotten to disassemble, inspect, and grease the number-one hanger assembly. At the time of the accident, the number-one hanger assembly had accumulated about 33 hours since the inspection.