WPR12TA364 · American Eurocopter LLC AS350B3
Dept of Homeland Security Customs & Border American Eurocopter LLC AS350B3 · Accident: flight control system malfunction/failure while maneuvering
From Sunburst, MT to Sunburst, MT
Event
- NTSB case
- WPR12TA364
- 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
- API-reported fatalities
- 0 · all aircraft and ground
- 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
- Government
- 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
American Eurocopter LLC AS350B3
- Aircraft type
- Eurocopter AS350
- 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
- N3939A
- Onboard fatalities
- Unknown
- Route
- From Sunburst, MTTo Sunburst, MT
- 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
- Public Aircraft
- 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
- Maneuvering · hover
- 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
- Flight control system malfunction/failure
- 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 systems
Approximate · Coordinates as recorded by the NTSB; no uncertainty radius is established.
NTSB narrative
The pilot was maneuvering the helicopter in a 3-foot in-ground-effect hover in preparation for departure on a border patrol flight when he heard a loud "bang," which was immediately followed by a decay in the main rotor rpm and a right yaw. The pilot performed an autorotation, and, after touchdown, he conducted the emergency shutdown procedures.
The postaccident airframe examination revealed that the tail rotor moved independently of the main rotor. Further examination revealed that the aft flange of the main gearbox transmission drive shaft was liberated from its flex coupling at the engine output. The castellated nuts that secured the flex coupling to the aft flange of the shaft were found loose, and the bolts were sheared. An examination of the bolts and flex coupling determined that the nuts were most likely hand-tightened and that cotter pins were not installed on three attachment bolts between the flex coupling and flange portion of the drive shaft.
A review of the maintenance records revealed that, about 75 flight hours before the accident, the engine had been removed so that maintenance personnel could perform a modification. Maintenance personnel removed the bolts to the engine-to-main gearbox flex coupling and then partially reassembled the flex coupling bolts, which was not in accordance with the helicopter's maintenance manual engine removal procedure. Although no comments were found in the engine maintenance logbooks indicating that the flex coupling was disassembled from the tail portion of the helicopter, the maintenance performed involved disassembly of the drive shaft. The cotter pins likely were not installed on the attachment bolts during the most recent maintenance, which allowed the nuts to start to back out of the three attachment bolts.
Probable cause
The improper installation of the engine-to-main gearbox flex coupling, which resulted in the failure of the flex coupling and a loss of power to the rotor system during takeoff.
Verbatim NTSB analysis and probable cause from the NTSB dataset
Sources
NTSB record WPR12TA364
- Event ID
- 20120821X32707
- Case number
- WPR12TA364
- 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. 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: 176e1763bc7cfac6cc584c0fcaa158934d861972356b6277a73b2766880719fa; retrieved 2026-09-15T19:27:42.516Z.