What happened
During final approach at 40 knots and 20 feet above ground level (AGL), the helicopter experienced an abrupt left yaw. The pilot reported that the engine RPM suddenly deviated from the gauge. In response, the pilot lowered the collective and moved the throttle to idle. As the aircraft reached approximately 5 feet AGL, the pilot increased the collective and applied slight aft cyclic. The helicopter subsequently struck the ground on the rear section of the right skid, bounced, and rolled onto its left side. There were no injuries reported.
The investigation
An examination of the aircraft at the accident site determined that the clutch cable had separated. Subsequent analysis of the failed cable revealed that it had failed due to high-cycle fatigue crack propagation on the individual wires. The initiation of these cracks was linked to an area where the wires made contact with one another. Investigators found no evidence of manufacturing or material defects within the cable assembly. Furthermore, the inspection indicated that identifying the failing wires would have been difficult without disassembling the component.
Findings
Maintenance records showed the helicopter had accumulated a total of 757.5 flight hours. The time elapsed since the last 400-hour inspection was 398.9 hours. According to the manufacturer's maintenance manual, the clutch cable and clutch assembly must be disassembled and inspected every 400 hours.