Pilot Debrief

SIKORSKY 92 near Galliano, LA — 2011-01-19

Final reportCEN11IA156
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Date
2011-01-19
Location
Galliano, LA, USA
Airport
GAO
Aircraft
SIKORSKY 92
Registration
N920AL
Category
Helicopter
Highest injury
None
Fatalities
0
Phase of flight
Initial climb

Probable cause

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

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.

Analysis

Primary failure mode
Mechanical failure
First missed decision gate
Proper alignment of the pitch beam retaining washer during maintenance could have prevented failure.

NTSB coding

  • Flight control sys malf/fail · Approach

Evidence available

  • 7 docket documents
View NTSB final reportView NTSB docket

Docket documents7

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