American Eurocopter AS350B3 N3939A

Summary

On 18 August 2012 at about 18:15 local time, a American Eurocopter AS350B3 registered N3939A, operated by Dept of Homeland Security Customs & Border, was involved in an accident near Sunburst, Montana, United States. 2 people were 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

18 August 2012 at 18:15

Classification

Accident

Location

Sunburst, Montana, United States

Coordinates

48.8858, -111.8961

Aircraft

American Eurocopter AS350B3

Registration

N3939A

Category

Helicopter

Engines

1

Operator

Dept of Homeland Security Customs & Border

Operating rule

Public Use

Phase of flight

Not recorded

Route

Sunburst → Sunburst

Aircraft damage

Substantial

Weather

VMC

Light

Day

NTSB number

WPR12TA364

Nobody was injured.

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.

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

Read the full 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.

Quoted verbatim from the NTSB record.