Robinson Helicopter Company R44 II N74868 accident near Glacier View, Alaska

Probable cause

A sudden stoppage of the tail rotor for unknown reasons, which resulted in the tortional overload failure of the tail rotor driveshaft.

NTSB analysis

The pilot stated that he was flying the helicopter solo with the doors removed while scouting potential locations for an aerial photography flight scheduled for later that day. During cruise flight, he heard a loud “boom” and felt a slight airframe vibration. He subsequently lost tail rotor thrust and performed an autorotation to an area of snow-covered, mountainous terrain.

Examination revealed the tail rotor driveshaft was fractured between the forward and aft flex plates. Further examination of the tail rotor driveshaft revealed the fractures were consistent with ductile tension overload. There was no evidence of pre-existing cracks or other damage to the fracture surfaces. The tail rotor blades were not damaged during the accident sequence, and there were no mechanical anomalies found with the tail rotor gearbox assembly.

The torsional twisting of the fractured surfaces of the tail rotor driveshaft is consistent with a sudden binding of the tail rotor gearbox, or a rapid deceleration of the tail rotor blades. There was no damage evident to the tail rotor assembly from a bird strike or impact with a hard object.

Although, if a small rag or towel was overlooked during the preflight inspection and it had blown out of the helicopter during the doors-off flight, it likely would have been struck by the tail rotor, resulting in the rapid deceleration of the tail rotor blades, with little to no detectable damage. Thus, the reason for the torsional fracture of the tail rotor driveshaft could not be determined.

Text from the NTSB. Open the NTSB docket for ANC24LA013 for the full record.