Accident Overview
A converted military helicopter was engaged in a hovering operation to lift an external load of logs when the main rotor pylons separated in flight. The separation resulted in an uncontrolled inverted descent into the ground.
Examination Findings
On-scene examination determined that the transmission rotor input gear shaft, part number K774515-5F, had fractured. Subsequent metallurgical examination by the safety board indicated that the fracture was characteristic of fatigue failure and showed signs of repetitive high loads.
Configuration Details
The helicopter's FAA type certificate, based on its military configuration, specifies an approved engine of 1,100 shaft horsepower (SHP) and an external load limit of 2,300 pounds. However, the FAA had approved the installation of a 1,400 SHP engine and operations with an increased external load limit of 4,000 pounds.