What happened
During a maintenance test flight intended to perform a functional flight check for vibration and autorotation, the helicopter entered a rapid descent. Witnesses observed the rotor system slowing down and the rotor blades "folding" over the top of the aircraft during the maneuver. As the rotor system slowed, fragments identified as pieces from the transmission cowling were seen exiting the helicopter. The aircraft subsequently descended into the ground while performing a right-hand turn.
The investigation
An examination of the wreckage showed no evidence of any structural or system failure or malfunction. Investigators found that engine power had been applied while the rotor system was at a low rotational speed, though there was no evidence of engine failure or malfunction prior to impact.
According to the manufacturer, when the main rotor system slows under aerodynamic load, the main gearbox can undergo horizontal orbital movement, known as "walking," on its mounting structure. This movement can cause the transmission cowling to make contact with the main rotor control rod ends.
Findings
Investigation revealed that the pilot had received no training from the operator regarding how to conduct a maintenance test flight. It was also noted that the FAA does not require such training.