Robinson R44 II N74868

Summary

On 11 February 2024 at about 20:30 local time, a Robinson R44 II registered N74868, operated by Sheep Mountain Air LLC, was involved in an accident near Glacier View, Alaska, United States. One person was on board and nobody was injured. The aircraft was substantially damaged. The NTSB has published a probable cause for this accident; it is quoted in full below.

Date

11 February 2024 at 20:30

Classification

Accident

Location

Glacier View, Alaska, United States

Nearest airport

Ridgeline (NONE)

Coordinates

61.6613, -147.5822

Aircraft

Robinson R44 II

Registration

N74868

Category

Helicopter

Engines

1

Operator

Sheep Mountain Air LLC

Operating rule

Part 91: General Aviation

Purpose of flight

Business

Phase of flight

Not recorded

Route

Glacier View → Unknown

Aircraft damage

Substantial

Weather

VMC

Light

Day

NTSB number

ANC24LA013

Nobody was injured.

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

Quoted verbatim from the NTSB record. This site does not paraphrase or interpret it.

Read the full NTSB narrative

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.

Quoted verbatim from the NTSB record.