ROBINSON HELICOPTER COMPANY R44 II

Full Narrative

On February 11, 2024, about 1130 Alaska standard time, a Robinson Helicopter R44 II, N74868, was substantially damaged when it was involved in an accident near Glacier View, Alaska. The pilot was not injured. The helicopter was operated as a Title 14 Code of Federal Regulations Part 91 business flight.
According to the pilot, while in cruise flight with both aft doors removed, about 700 ft above an area of snow-covered glacial terrain, he heard a loud “boom” noise and felt a slight airframe vibration. The pilot stated that he subsequently lost tail rotor thrust and that he then performed an autorotation to an area of snow-covered, mountainous terrain.
A postaccident inspection, after the helicopter was recovered, revealed that the tail rotor driveshaft was fractured between the forward and aft flex plates, which resulted in substantial damage to the tail rotor driveshaft system. 
The fractured pieces of the tail rotor driveshaft were sent to the NTSB Materials Laboratory for further examination. The examination revealed the fractures were consistent with ductile torsion overload. There was no evidence of pre-existing cracks or other damage to the driveshaft surfaces. The tail rotor blades were not damaged.

Figure 1. View of the tail rotor driveshaft fracture surfaces
The tail rotor gearbox assembly was removed from the accident helicopter and no anomalies were discovered.
The pilot stated that he did not have a passenger, but that he did have the doors off in preparation for a passenger/photographer later that day. He also stated that during his preflight inspection, he checked for loose items that could blow out and strike the tail rotor.
The pilot did not submit the National Transportation Safety Board Pilot/Operator Aircraft Accident/Incident Report Form 6120.1.