Bell 206 N61PH accident near Fort Mcdowell, Arizona

Probable cause

The separation of the main rotor head from the mast as a result of a sudden displacement of the cyclic stick during a low-G maneuver, leading to mast bumping. Contributing to the accident were the unsecure positioning of the flight test engineer’s laptop computer and the deviation from standard operating procedures to leave the left seat cyclic control installed during the test flight.

NTSB analysis

The pilot and flight engineer were conducting a flight test to evaluate developmental main rotor blades that had been installed on the helicopter. The flight consisted of multiple autorotations at maximum gross weight, entered following a 1-second delay after a simulated loss of engine power. A witness saw the helicopter falling from the sky and saw several other objects descending to the ground before losing sight of it behind trees.

Examination of the wreckage revealed damage to the main rotor mast, consistent with a mast bumping event. Cyclic controls were installed in both the pilot’s seat and the left seat, where the flight test engineer sat with equipment used for recording flight parameters, including an 8-lb laptop computer. In order for the engineer to operate the computer or take notes on the clipboard, he would have to hold the computer with one hand while using the other to enter commands or take notes. The installation of the cyclic in the engineer’s position represented a deviation from company procedures by both the pilot and the flight test engineer. Examination revealed no evidence of mechanical malfunctions or failures that would have precluded normal operation of the helicopter prior to the mast bumping.

The flight engineer’s fatigue of holding the 8-lb computer over his lap, the awkwardness of entering commands with one hand, or taking notes single-handedly may have allowed the computer to suddenly shift and strike or block the cyclic during the simulated loss of power test at the critical low-G flight condition.

Text from the NTSB. Open the NTSB docket for WPR19FA109 for the full record.