What happened
While performing external load operations in the vicinity of tall trees, the helicopter's main rotor impacted a tree. This impact caused the aircraft to descend nose first into the ground.
During the operation, the crew was utilizing a 78.8-foot-long line for external loads. In certain instances, this length placed the main rotor of the aircraft below the canopy level of the surrounding 80- to 90-foot trees.
The investigation
The Vehicle Engine Monitoring Display (VEMD) recorded specific fluctuations during the rotor impact. Data showed a high torque measurement of 146 percent and a low engine drive speed of 184 rpm, which was immediately followed by a low torque of 5.7 percent, a high engine speed of 511 rpm, and a temperature of 999C.
Investigators determined that the impact with the trees caused the engine's torque to twist apart the shaft connecting the engine to the transmission drive. This mechanical failure resulted in an engine overspeed and overtemperature condition. Physical examination of the aircraft revealed no evidence of any preimpact malfunction or failure regarding the power plant or the control system.