ANC24LA013 · Robinson Helicopter Company R44 II
Sheep Mountain Air Robinson Helicopter Company R44 II · Accident: system/component malfunction/failure (non-power) en route
From Glacier View, AK (private airstrip) to an unrecorded destination
Event
- NTSB case
- ANC24LA013
- Event typeAccident: someone killed or seriously injured, or the aircraft substantially damaged (NTSB definition). Incident: an occurrence that affected or could have affected safety, short of that. Glossary
- Accident
- InvestigationHow far the investigation had got when the record was published: preliminary, ongoing, final or completed. Unknown where the source does not say. Glossary
- Completed
- Event fatalities
- Unknown
- Ground fatalities
- Unknown
- OperationWhat kind of flying it was, grouped by the rules it flew under: airline (US Part 121), air taxi and commuter (Part 135), general aviation (Part 91 and similar), military or government. Glossary
- General aviation
- WeatherVisual meteorological conditions (VMC): good enough to fly by looking outside. Instrument conditions (IMC): cloud or low visibility, flying by instruments. Glossary
- Visual Meteorological Cond
Robinson Helicopter Company R44 II
- Aircraft type
- Robinson R44
- Category
- Helicopter
- RegistrationThe aircraft's tail number, such as N12345 or G-ABCD. Registrations are reissued, so the same one years apart can be a different aircraft. Glossary
- N74868
- Operator
- Sheep Mountain Air
- Onboard fatalities
- Unknown
- Route
- From Glacier View, AK (private airstrip)To not recorded
- Aircraft age
- Not recorded
- Flight rulesThe regulations the flight operated under: in the US, Part 91 (general aviation), Part 121 (airlines), Part 135 (air taxi and commuter) and others; for flights abroad, the NTSB's coarser commercial or non-commercial code. Glossary
- Part 91: General Aviation
- Phase of flightThe stage of the flight when things started to go wrong: standing, taxi, takeoff, initial climb, en route, maneuvering, approach or landing. Glossary
- En route
- Defining eventThe single coded event the NTSB judges best describes what happened (records from 2008 on). Older records name the first occurrence in the sequence instead. Glossary
- System/component malfunction/failure (non-power)
- DamageDestroyed: beyond practical repair. Substantial: damage that affects the structure, performance or handling and normally needs major repair. Minor: less than that. Glossary
- Substantial
Cause areas
- Aircraft › Aircraft propeller/rotor
Approximate · Coordinates from the NTSB case API, which marks them as estimated or does not say; no uncertainty radius is established.
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.
Probable cause
A sudden stoppage of the tail rotor for unknown reasons, which resulted in the tortional overload failure of the tail rotor driveshaft.
Verbatim NTSB analysis and probable cause from the NTSB dataset
Sources
NTSB record ANC24LA013
- Event ID
- 20240214193797
- Case number
- ANC24LA013
- Dataset
- full-current
- Source SHA-256
- 5cf380f0061817c0331a6b2d8cc7e0ee3a79bea469a1001dc5c10e56f35f5ab3
Source notes (4)
- Unreviewed is an editorial label, not an investigation status. API-sourced is not report-checked or human-reviewed. Explicit event totals are used without summing aircraft injury tables; onboard allocation is withheld. Unknown values remain unknown. The operation category is mapped from the NTSB-reported FAR part and has not been reviewed.
- Filled from the NTSB case API where the bulk record had no value: investigation status, coordinates, cause areas. Values present in the bulk record are kept.
- The date is the local date, which is the same as the UTC date the NTSB stores.
- API snapshot SHA-256: 0b3b8981def07a32eaf497e13006be4afaa8adcab51f7e5b48ae26c0a3e9f3ec; retrieved 2026-09-16T18:30:50.622Z.