Incident Overview
A pilot was conducting an underground cable patrol flight at an altitude of 500 to 600 feet above ground level when the accident occurred. The pilot reported that power was set at 20 inches of manifold pressure and 3,000 RPM when a very loud pop was heard. The pilot likened the sound to being hit by ground fire. The aircraft yawed, and the engine oversped.
Emergency Maneuver and Outcome
The pilot performed an autorotation into an abandoned mining pit. During the landing, the aircraft nosed up and rolled over onto its right side. The pilot stated that the engine would respond to throttle inputs, but the main rotor would not follow power changes.
Technical Findings
Examination revealed that the main rotor transmission had a ring gear carrier failure. Additionally, Airworthiness Directive 81-1704 and a related service notice concerning this component had not been complied with.