Sikorsky 92 N920AL

Summary

On 19 January 2011 at about 17:34 local time, a Sikorsky 92 registered N920AL, operated by Bristow US LLC, was involved in an incident near Galliano, Louisiana, United States. 18 people were on board and nobody was injured. The aircraft was slightly damaged. The NTSB has published a probable cause for this accident; it is quoted in full below.

Date

19 January 2011 at 17:34

Classification

Incident

Location

Galliano, Louisiana, United States

Nearest airport

South Lafourche Airport (GAO)

Coordinates

29.4378, -90.2567

Aircraft

Sikorsky 92

Registration

N920AL

Category

Helicopter

Engines

2

Operator

Bristow US LLC

Operating rule

Part 135: Air Taxi & Commuter

Phase of flight

Not recorded

Route

Gulf of Mexico → Galliano

Aircraft damage

Minor

Weather

VMC

Light

Day

NTSB number

CEN11IA156

Nobody was injured.

The misalignment of the retaining washer during routine maintenance, resulting in the subsequent separation of the washer and nut.

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

Read the full NTSB narrative

During the initial approach, as the airspeed decreased through about 70 knots, the helicopter began an uncommanded right yaw of more than 100 degrees, and the yaw could not be stopped by the application of left pedal. The crew diverted to a nearby airport and performed a successful roll-on landing, touching down at 72 knots using differential braking to maintain the runway heading. An after-landing inspection showed that the tail rotor pitch change beam retaining nut, pitch beam retaining washer, and associated screws and washers were missing. The remainder of the tail rotor assembly remained attached and did not depart the helicopter. Recent maintenance had been performed to replace the pitch change beam retaining nut and pitch beam retaining washer. According to maintenance personnel, the maintenance had been performed per the existing maintenance procedures, and the mechanics did not notice anything odd or out of place. Evidence of witness marks along with plastic deformation on the end faces of the pitch change shaft splines is consistent with the retaining washer installed with its indexing spline slightly off, resulting in a cocking condition and incomplete engagement of the spline teeth. Accordingly, there was likely an improper alignment of the components during reassembly, which would have prevented the nut from locking in place. Subsequent tests on an exemplar assembly showed that it was possible for the pitch beam retaining washer to be misaligned with the teeth on the shaft splines, which then showed barely visible evidence of the misalignment.

Quoted verbatim from the NTSB record.