NTSB Identification: CEN11IA156
Nonscheduled 14 CFR Part 135: Air Taxi & Commuter
Incident occurred Wednesday, January 19, 2011 in Galliano, LA
Probable Cause Approval Date: 02/12/2013
Aircraft: SIKORSKY 92, registration: N920AL
Injuries: 18 Uninjured.
NTSB investigators used data provided by various sources and may have traveled in support of this investigation to prepare this aircraft incident report.
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.
The National Transportation Safety Board determines the probable cause(s) of this incident to be: The misalignment of the retaining washer during routine maintenance, resulting in the subsequent separation of the washer and nut. Full narrative available
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