NTSB Identification: CHI03FA025.
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Accident occurred Wednesday, November 20, 2002 in St. Jacob, IL
Probable Cause Approval Date: 04/28/2004
Aircraft: Robinson R22, registration: N559DD
Injuries: 2 Uninjured.
NTSB investigators either traveled in support of this investigation or conducted a significant amount of investigative work without any travel, and used data obtained from various sources to prepare this aircraft accident report.
The helicopter was substantially damaged when the fan shaft failed causing a loss of power to the main and tail rotors. The instructor pilot reported that while in cruise flight the clutch light flickered several times. He stated that he pulled the clutch circuit breaker, "however the noise continued to worsen and [he] suspected the drive belts were loosening." He reset the clutch circuit breaker and the clutch light illuminated steadily. The instructor pilot reported, "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 (instructor and dual student) immediately initiated an autorotation. At the termination of the autorotation, the helicopter yawed to the left and did not respond to right pedal inputs. 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. This component was subsequently found approximately 1.2 miles south of the forced landing site. In addition, the tail rotor blade pitch push-pull tube was completely severed at the forward rod end. Extensive scraping and gouging was 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 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." According to the dual student, on 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. The fan shaft and bearing were not disassembled. Therefore, the mating surfaces of the fan shaft and bearing race were not inspected.
The National Transportation Safety Board determines the probable cause(s) of this accident to be: Fatigue fracture of the clutch assembly fan shaft and the resulting complete loss of power to the rotor drive system. A contributing factor was the loss of tail rotor authority due to the secondary failure of the control tube assembly. Full narrative available
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