American Eurocopter LLC AS350B3 accident near Sunburst, Montana, August 18, 2012
On August 18, 2012 at about 6:15 pm local time, a American Eurocopter LLC AS350B3 (helicopter), registered N3939A, was substantially damaged in an accident during maneuvering (hover) near Sunburst, Montana. It was a public-use flight (federal) under public-use (government) rules. No one was hurt; 2 people were on board or involved. The weather was visual conditions (good weather).
The NTSB's probable cause their words, unchanged
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.
Source: NTSB aviation accident database, copy made October 5, 2026. Docket and reports at the NTSB.
What the record shows
- Date
- August 18, 2012 · about 6:15 pm local time
- Place
- Sunburst, Montana · map
- Type
- Accident
- Injuries
- No one was hurt; 2 people were on board or involved.
- Weather
- visual conditions (good weather)
- Aircraft
- American Eurocopter LLC AS350B3
- Registration
- N3939A · registry record · serial 4806
- Damage
- Substantial damage
- Flight
- Public-use flight (federal) · public-use (government) rules
The NTSB's narrative final · quoted from the NTSB record
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.
The complete narrative as the NTSB published it. The NTSB's docket holds the report as a PDF and any photographs, statements and other documents from the investigation.
The factual record from the NTSB's investigation tables, in plain English
What happened, in order
- Flight control sys malf/fail during maneuvering (hover) defining event
- Flight control sys malf/fail during takeoff
The NTSB's findings
- cause Aircraft › Aircraft systems › Flight control system › (general) › Incorrect service/maintenance
Pilot
- Certificate: flight instructor, commercial pilot, private
- Ratings: multi-engine land; single-engine land; single-engine sea; instructor: airplane single-engine; instructor: helicopter; instructor: instrument airplane; instructor: instrument helicopter; instrument: airplane; instrument: helicopter; rotorcraft: helicopter
- Flight time: 3,000 hours in all; 300 in this make and model; 45 in the last 90 days; 25 in the last 30 days; 2,890 as pilot in command; 1,700 on instruments
- Last flight review: February 15, 2012
- Medical certificate: Class 2 (without waivers/limitations)
- Seat: rgt
- Injury: no injuries
The aircraft
- Airframe total time: 889 hours
- Last inspection: annual inspection, March 30, 2012; 75 hours since
- Maximum gross weight: 5,225 lb
- Seats: 6
- Landing gear: fixed
- Engine: Turbomeca ARRIEL 2B1 (turboshaft); 0 hours total
- Operator: Dept Of Homeland Security Customs & Border
The flight
- Departed from: Sunburst MT at 6:15 pm
- Destination: Sunburst MT
- Flight plan: none
Weather at the time
- Light: daylight
- Visibility: 10 statute miles
- Sky: clear
- Temperature: 81°F (27°C), dew point 25°F (-4°C)
- Altimeter: 30.14 inHg
Injuries
| Fatal | Serious | Minor | None | |
|---|---|---|---|---|
| Flight crew | 2 |
Documents from the investigation the NTSB's docket: the evidence folder behind the report
The docket, the folder of records gathered during an investigation (maintenance records, photographs, witness statements, examinations), has not been read from the NTSB for this case yet; the list appears here once it has. Open the docket at the NTSB.
Everything on this page comes from the NTSB's public records. The narrative, probable cause and findings are the NTSB's own words; the coded tables behind the report are written out in plain English, with pilots' ages, home towns and medical details left out. The documents and photographs are the NTSB's docket, shown as the NTSB released them. This site's own text never names anyone involved. A preliminary report can change; the final report usually follows one to two years later, and the page is refreshed when it does.