# NTSB CEN11IA156: Sikorsky 92, N920AL, Galliano, LA, January 19, 2011

- Type: NTSB incident investigation
- Classification: Incident · No injuries · Incident investigation · Final report
- Date: January 19, 2011, 11:34 local (17:34 UTC)
- Place: Galliano, LA
- Coordinates: 29.437778, -90.256668 (estimated)
- Nearby events: https://openairdocs.org/map.md?lat=29.4378&lon=-90.2567&mi=10
- Phase of flight: [Approach](https://openairdocs.org/ntsb-phases/500) (OpenAirDocs' grouping)
- Defining event: [Approach: Flight control sys malf/fail](https://openairdocs.org/ntsb-events/333)
- Highest injury: No injuries
- Injuries: 18 uninjured
- Status: Completed, report February 12, 2013
- Report: Approved by Office Director
- NTSB office: Central Region
- Parties: FAA Baton Rouge FSDO, Sikorsky Aircraft Corporation, FAA AVP-100, Bristow US LLC, National Transportation Safety Board
- NTSB case key (mKey): 78197
- NTSB case page: https://web.ntsb.gov/investigations/?ntsbnumber=CEN11IA156
- NTSB report (PDF, downloads): https://data.ntsb.gov/carol-repgen/api/Aviation/ReportMain/GenerateNewestReport/78197/pdf
- NTSB docket: https://data.ntsb.gov/Docket?ProjectID=78197
- Page: https://openairdocs.org/ntsb/CEN11IA156

## Probable cause

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

## Findings

NTSB's coded findings; those marked are part of the probable cause.

- [Tail rotor drive system: tail rotor drive shaft](https://openairdocs.org/ntsb-findings/01046510): Aircraft › Aircraft propeller/rotor · Incorrect service/maintenance (probable cause); cases with it: [Sikorsky S-92A, 1 case](https://openairdocs.org/ntsb-findings/01046510/cases.md?model=8141832), [All aircraft, 25 cases](https://openairdocs.org/ntsb-findings/01046510/cases.md)

## Sequence of events

The events NTSB coded, in order, by phase of flight. The defining event best describes what happened.

- [Prior to flight: Aircraft maintenance event](https://openairdocs.org/ntsb-events/040); cases with it: [Sikorsky S-92A, 1 case](https://openairdocs.org/ntsb-events/040/sequence.md?model=8141832), [All aircraft, 484 cases](https://openairdocs.org/ntsb-events/040/sequence.md)
- [Prior to flight: Aircraft inspection event](https://openairdocs.org/ntsb-events/050); cases with it: [Sikorsky S-92A, 2 cases](https://openairdocs.org/ntsb-events/050/sequence.md?model=8141832), [All aircraft, 229 cases](https://openairdocs.org/ntsb-events/050/sequence.md)
- [Other: Part(s) separation from AC](https://openairdocs.org/ntsb-events/338); cases with it: [Sikorsky S-92A, 1 case](https://openairdocs.org/ntsb-events/338/sequence.md?model=8141832), [All aircraft, 563 cases](https://openairdocs.org/ntsb-events/338/sequence.md)
- [Approach: Flight control sys malf/fail](https://openairdocs.org/ntsb-events/333) (defining event); cases with it: [Sikorsky S-92A, 1 case](https://openairdocs.org/ntsb-events/333/sequence.md?model=8141832), [All aircraft, 410 cases](https://openairdocs.org/ntsb-events/333/sequence.md)
- [Approach-VFR go-around: Flight control sys malf/fail](https://openairdocs.org/ntsb-events/333); cases with it: [Sikorsky S-92A, 1 case](https://openairdocs.org/ntsb-events/333/sequence.md?model=8141832), [All aircraft, 410 cases](https://openairdocs.org/ntsb-events/333/sequence.md)
- [Landing: Off-field or emergency landing](https://openairdocs.org/ntsb-events/440); cases with it: [Sikorsky S-92A, 1 case](https://openairdocs.org/ntsb-events/440/sequence.md?model=8141832), [All aircraft, 3,365 cases](https://openairdocs.org/ntsb-events/440/sequence.md)

## Analysis

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.

## Factual narrative

On January 19, 2011, at 1134 central daylight time, a Sikorsky 92A helicopter, N920AL, experienced a flight control malfunction and sustained minor damage during initial descent for landing to Air Logistics Galliano Heliport (2LS0), Galliano, Louisiana. Visual meteorological conditions prevailed and a company flight plan was filed for the 14 Code of Federal Regulations Part 135 non-scheduled passenger flight. The three crewmembers and 15 passengers were not injured. The flight originated from an off-shore helipad at Green Canyon 858 in the Gulf of Mexico, with 2LS0 as its intended destination. During the initial approach into 2LS0, as airspeed reduced through 70 knots, the helicopter began an uncommanded right yaw of more than 100 degrees that could not be stopped by application of left anti-torque pedal. The crew immediately lowered the nose to regain airspeed and streamline the aircraft. They diverted to South Lafourche Airport (GAO), Galliano, Louisiana, and performed a successful roll on landing, touching down at 72 knots, utilizing differential braking to maintain 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 tail rotor hub and paddles remained attached to the pitch change links and the pitch beam assembly and did not depart the helicopter. Recent maintenance had been performed to remove and replace the pitch change shaft. Following that maintenance the tail rotor pitch change beam retaining nut and pitch beam retaining washer had been reinstalled and the retaining nut had been torqued to the required value. All of the operator’s mechanics said they thought that all the maintenance items had been performed correctly and they did not notice anything odd or out of place. Following this incident the manufacturer conducted tests on an exemplar assembly that showed it was possible for the pitch beam retaining washer to be misaligned on the spline and have it partially engage the shaft teeth as the retention nut was rotated and tightened. With the locking washer misaligned, it turned approximately 90 degrees clockwise as the retention nut was being preloaded. At that point the locking washer partially engaged the spline teeth and remained in the cocked condition until the full amount of torque was achieved. The gaps created by the cocked condition and the thread protrusion of the pitch beam retaining nut were visible and measureable. The post-test condition of the teeth on the locking washer included barely visible evidence of the misalignment against the shaft splines. The manufacturer subsequently issued a change to the aircraft maintenance manual which included additional confirmation of proper alignment of the pitch beam retaining washer and a check for thread protrusion of the pitch change shaft above the tail rotor pitch beam retaining nut.

## Aircraft

- Registration: [N920AL](https://openairdocs.org/deregistered/N920AL-2011-04-08), registration cancelled 2011-04-08, S/N 920088, built 2008 (OpenAirDocs)
- Reported make and model: SIKORSKY 92 (A)
- Registry model: [Sikorsky S-92A](https://openairdocs.org/model/sikorsky-s-92a) (registry code 8141832) (OpenAirDocs)
- Registry engine: [GE CT7-8A](https://openairdocs.org/engine/ge-ct7-8a) (registry code 30136, turbo-shaft) (OpenAirDocs)
- Category: Helicopter
- Serial number: 920088
- Airframe hours: 1248
- Engines: GE CT7-8A 2520 horsepower turbo shaft; GE CT7-8A 2520 horsepower turbo shaft
- Damage: Minor damage
- Flight: Part 135
- Operator: Bristow US LLC
- Registered owner: Bristow US LLC
- Airworthiness certificate: Standard, Transport
- Last inspection: Continuous airworthiness, January 17, 2011, 4 hours before
- Departed: Gulf of Mexico, LA
- Destination: 2LS0, Galliano, LA
- Flight plan: Company VFR
- Airport: GAO, South Lafourche Airport, runway 36, 6,502 × 100 ft, asphalt

## Flight crew

As NTSB recorded them. NTSB leaves out names.

- Pilot; age 63; injuries: none; airline transport, commercial; airplane ratings: single-engine land; instrument ratings: helicopter; medical: class 2, with waivers/limitations; last flight review July 10, 2010; seat: right; flight hours: 33,647 total, 624 in this make and model, 23,485 as pilot in command, 70 in the last 90 days, 18 in the last 30 days, 4 in the last 24 hours
- Co-pilot; age 64; injuries: none; commercial, airline transport; airplane ratings: single-engine land, multi-engine land; instrument ratings: helicopter; medical: class 1, without waivers/limitations; last flight review July 10, 2010; seat: left; flight hours: 21,677 total, 799 in this make and model, 14,040 as pilot in command, 55 in the last 90 days, 14 in the last 30 days, 4 in the last 24 hours

## Weather

- Conditions: Visual (VMC), Day
- Wind: 10° at 11 knots, gusting 15
- Visibility: 10 statute miles
- Temperature: 59 °F, dew point 46 °F
- Altimeter: 30.07 inHg
- Observed at: KGAO

## FAA reports on this event (1)

The FAA's own accident and incident reports, matched to the aircraft by N-number and date (OpenAirDocs).

- [Incident 20110119002029I](https://openairdocs.org/incident/20110119002029I), Jan 19, 2011: Sikorsky S-92A, Galliano, LA, Minor damage. The tail rotor pitch change shaft was replaced 4.6 hours prior to this incident. It was determined that the pitch change beam retaining nut and lock washer…

## Docket (7 files)

The files NTSB released with the investigation on December 11, 2012, on NTSB's docket system: https://data.ntsb.gov/Docket?ProjectID=78197. 5.6 MB in all.

- Summary of FAA Inspector's findings on-scene (item 1, .PDF, 57 KB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=40382806&FileExtension=.PDF&FileName=Summary%20of%20FAA%20Inspector%27s%20findings%20on-scene-Master.PDF
- Pilot/Operator Aircraft Accident Report, NTSB Form 6120.1 (item 2, .PDF, 1.1 MB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=40382558&FileExtension=.PDF&FileName=Pilot%2FOperator%20Aircraft%20Accident%20Report,%20NTSB%20Form%206120.1-Redacted.PDF
- Weather Summary (item 3, .PDF, 47 KB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=40382559&FileExtension=.PDF&FileName=Weather%20Summary-Master.PDF
- Systems 9 - Factual Report of Group Chairman (item 4, .PDF, 6 photos, 949 KB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=40354789&FileExtension=.PDF&FileName=Systems%209%20-%20Factual%20Report%20of%20Group%20Chairman-Master.PDF
- Map - Airport Diagram (item 5, .PDF, 37 KB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=40382560&FileExtension=.PDF&FileName=Map%20-%20Airport%20Diagram-Master.PDF
- Statement of Party Representatives to NTSB Investigation (item 6, .PDF, 2.0 MB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=40382561&FileExtension=.PDF&FileName=Statement%20of%20Party%20Representatives%20to%20NTSB%20Investigation-Redacted.PDF
- Sikorsky's Tail Rotor Componenent Stack-up Tests (item 7, .PDF, 40 photos, 1.4 MB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=40382564&FileExtension=.PDF&FileName=Sikorsky%27s%20Tail%20Rotor%20Componenent%20Stack-up%20Tests-Master.PDF

From the NTSB's public API. Aircraft are matched to registry aircraft by N-number and event date, checked against the reported serial number, make and model; the registry's history has gaps, so older matches can be wrong. Compare the reported make and model.

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