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 pilot reported that he was in straight and level flight over mountainous, tree- covered terrain when he heard a loud noise followed by an abrupt jolt and right yaw. The pilot initiated a descent into a valley when he heard a second loud noise followed by severe vibrations and activation of the low rotor RPM horn. The helicopter continued to descend before it impacted the trees and ground below. During an on- scene examination, control continuity was established throughout the helicopter with the exception of the 90- degree gearbox and tail rotor assembly, which were separated from the helicopter and not found.
Postaccident examination and run of the helicopter's engine revealed no anomalies that would have precluded normal operation. The gear box is positioned in between the tail rotor drive shaft and tail rotor assembly; it takes the input from the tail rotor drive shaft (through a quill assembly), and adjusts and transfers the energy to the tail rotor assembly. Because the gearbox and tail rotor separated in flight, the investigation was not able to examine them to determine their condition or their proper installation. However, During the postaccident examination of the airframe, the tail rotor gearbox input quill assembly, vertical driveshaft clamp, and two remaining gearbox attachment studs were removed for additional examination. Fatigue damage was noted along the fracture surface of the two gearbox attachment studs that were still attached to the vertical fin side of the mounting flange. The remaining four attachment holes were empty;, however, helical rubbing marks were found around their perimeter along with an adjacent lip of deformed material, which is indicative of the studs being present at one point. Postaccident examination and a test run of the helicopter's engine revealed no anomalies that would have precluded normal operation.
The helicopter's most recent tail rotor gearbox inspection occurred about one month prior to the accident;, however, it was unable to bethe investigation could not determined if the fatigued area the fatigue was found was would have been included in the inspection, or whether any evidence of fatigue would have been detectable at the time of the inspection.