Accident Overview
A helicopter conducting a maintenance test flight experienced a rapid descent observed by witnesses. The test flight was required to perform a functional check for vibration and autorotation following maintenance.
Sequence of Events
During the descent, the rotor system was observed to slow down, and the rotor blades folded over the top of the helicopter. Pieces later identified as fragments from the transmission cowling exited the helicopter as the rotor slowed. The helicopter descended into the ground while executing a right-hand turning maneuver.
Wreckage Examination
Post-crash examination of the wreckage revealed no evidence of structural or system failure or malfunction. Examination of the engines showed that engine power was applied when the rotor system was at low rotational speed, but there was no evidence of preimpact engine failure or malfunction.
Manufacturer Information
According to the helicopter's manufacturer, when the main rotor system slows while under aerodynamic load, the main gear box can experience horizontal orbital movement ("walking") on its mounting structure. This movement can result in contact between the transmission cowling and the main rotor control rod ends.
Pilot Training
According to available information, the pilot had not received training from the operator regarding conduct of a maintenance test flight. The FAA does not require such training.