Incident Details
During cruise flight, the instructor pilot reported that the clutch light flickered several times. He pulled the clutch circuit breaker, but the noise continued to worsen, and he suspected the drive belts were loosening. After resetting the clutch circuit breaker, the clutch light illuminated steadily. Immediately afterward, the engine and rotor RPM needles split, indicating a drive train failure, and a loud metal-to-metal grinding noise began. The flight crew, consisting of the instructor and a dual student, initiated an autorotation. At the termination of the autorotation, the helicopter yawed to the left and did not respond to right pedal inputs.
Post-Accident Examination
A post-accident examination revealed a complete failure of the fan shaft. The portion of the engine cooling fan assembly aft of the fracture point had departed the aircraft and was subsequently found approximately 1.2 miles south of the forced landing site. Additionally, the tail rotor blade pitch push-pull tube was completely severed at the forward rod end, with extensive scraping and gouging noted on the rod end fitting. The fan shaft, fan wheel, and bearing were sent to the NTSB Materials Laboratory for examination. The examination noted that the fan shaft was fractured in a "V" shape from a transition in shaft diameter adjacent to the bearing at the aft end, forward to the attachment bolt holes. Fracture surface features were indicative of fatigue propagation.
Maintenance History
According to the dual student, during the delivery flight, one of the drive v-belts broke, and an emergency landing under partial power was made. The helicopter was repaired by Robinson technicians, who replaced the drive belts and clutch actuator. A visual inspection of the fan shaft and lower bearing was completed at that time, and no discrepancies were found. However, the fan shaft and bearing were not disassembled, so the mating surfaces of the fan shaft and bearing race were not inspected.