What happened
The pilot was conducting an underground cable patrol mission at an altitude of 500 to 600 feet above ground level when the incident occurred. At the time of the event, engine power settings were recorded at 20 inches of mercury and 3,000 RPM. The pilot reported hearing a very loud pop, describing the sensation as being hit by ground fire. Immediately following this sound, the aircraft yawed abruptly and the engine oversped.
In response to the mechanical failure, the pilot initiated an autorotation maneuver. The helicopter descended into an abandoned mining pit. During the landing sequence, the aircraft nosed up and rolled over onto its right side. Post-incident assessment indicated that while the engine continued to respond to throttle inputs, the main rotor system failed to follow power changes, confirming a loss of transmission function.
The investigation
Examination of the wreckage revealed a failure of the ring gear carrier within the main rotor transmission. This component failure was identified as the primary mechanical cause of the loss of rotor control. Further review of maintenance records showed that Airworthiness Directive 81-1704 and related service notices concerning this specific part had not been complied with prior to the accident.