Bell 407 N6040Y accident near Fort Worth, Texas

Probable cause

The flight instructor's failure to prevent the engine/rotor overspeed condition during the demonstration maneuver and the excessive descent rate during the emergency landing, which resulted in a hard landing. Contributing to the engine/rotor overspeed condition was the flight instructor's failure to comply with the demonstration procedure.

NTSB analysis

The flight instructor was demonstrating the emergency procedure for a full-authority digital engine control (FADEC) failure that would cause it to switch from auto to manual mode as part of the helicopter manufacturer's training program. The instructor stated that the helicopter was in a stable 5-ft hover when he was demonstrating the maneuver. To begin the demonstration, the flight instructor intentionally switched the FADEC to the manual mode. After about 10 seconds, the engine speed began to increase, which resulted in a corresponding increase in the main rotor speed. The instructor increased the collective control input in an attempt to control the engine and rotor speeds. The helicopter subsequently climbed and began to shake violently. The instructor initiated a descent for landing with an excessive descent rate, which resulted in damage to the landing skids and tailboom upon touchdown.

The helicopter flight manual emergency procedure for a FADEC failure and the training program procedure for the FADEC manual mode demonstration both noted that the engine and rotor speeds must be maintained between 95% and 100% with the collective and throttle controls. The demonstration procedure also noted that the throttle control should be in the fly position when the FADEC is placed into manual mode. A review of engine data recovered from the FADEC indicated that the throttle was not in the fly position before beginning the demonstration, which likely resulted in a greater increase in engine and rotor speeds than would have been expected. The flight instructor did not promptly manage the increase in the engine and rotor speeds, which resulted in an inadvertent overspeed condition and subsequent airframe vibrations that prompted the need for an emergency landing.

Text from the NTSB. Open the NTSB docket for CEN17LA188 for the full record.