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ASN Wikibase Occurrence # 183672
Last updated: 26 May 2020
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Time:11:34 LT
Type:Silhouette image of generic S92 model; specific model in this crash may look slightly different
Sikorsky S-92A
Owner/operator:Bristow US LLC
Registration: N920AL
C/n / msn: 920088
Fatalities:Fatalities: 0 / Occupants: 18
Other fatalities:0
Aircraft damage: Minor
Location:Galliano, LA -   United States of America
Phase: Approach
Nature:Non Scheduled Passenger
Departure airport:Offshore Installation, Gulf of Mexico
Destination airport:Galliano, LA (2LS0)
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.
Probable Cause: The misalignment of the retaining washer during routine maintenance, resulting in the subsequent separation of the washer and nut.



Revision history:

17-Jan-2016 23:59 Aerossurance Added
21-Dec-2016 19:30 ASN Update Bot Updated [Time, Damage, Category, Investigating agency]
27-Nov-2017 18:06 ASN Update Bot Updated [Cn, Operator, Other fatalities, Departure airport, Destination airport, Source, Narrative]
12-Dec-2017 08:32 Aerossurance Updated [Time, Operator, Other fatalities, Departure airport]

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