Flight and Incident
Approximately five minutes after takeoff, while in cruise flight, the pilot detected a series of short "clunks" followed by a grinding noise and violent shaking. The pilot elected to perform a precautionary autorotative landing to an off-airport location. During the landing flare, the tail rotor and tail stinger contacted the ground. As a result, the tail rotor blades were damaged, the tail stinger was broken, and the tail rotor gearbox was fractured.
Post-Accident Examination
The vibration experienced by the pilot was traced to a failed bearing at the oil cooler fan forward hanger. The bearing was sent to the helicopter manufacturer for examination; however, due to damage to the fracture surfaces, a fracture mode could not be identified. An airworthiness directive that applied to the bearing had been complied with 11 flight hours prior to the accident.