AAIB Special Bulletin on S-92 Helicopter Incident Elgin- Franklin Platform

AAIB Air Accidents Investigation Branch


AAIB Bulletin S1/2017


Farnborough House

Berkshire Copse Road

Aldershot, Hants GU11 2HH

Tel: 01252 510300

Fax: 01252 376999


This Special Bulletin contains facts which have been determined up to the time of issue. It is published to inform the

aviation industry and the public of the general circumstances of accidents and serious incidents and should be regarded as

tentative and subject to alteration or correction if additional evidence becomes available.

© Crown copyright 2017


Aircraft Type and Registration: Sikorsky S-92A, G-WNSR

No & Type of Engines: 2 General Electric Co CT7-8A turboshaft engines

Year of Manufacture: 2014 (Serial no: 920250)

Location: West Franklin wellhead platform, North Sea

Date & Time (UTC): 28 December 2016 at 0844 hrs

Type of Flight: Commercial Air Transport (Passenger)

Persons on Board: Crew – 2 Passengers – 9

Injuries: Crew – None Passengers – None

Nature of Damage: Left outer mainwheel rim distortion, seized tail rotor

pitch change shaft bearing, servo piston fracture and

minor damage to helideck

Commander’s Licence: Airline Transport Pilot’s Licence (H)

Commander’s Flying Experience: To be confirmed

Information Source: AAIB Field Investigation

AAIB Air Accidents Investigation Branch

AAIB Special Bulletin: S1/2017 G-WNSR EW/C2016/12/04

© Crown copyright 2017 2

The investigation

The accident occurred on 28 December 2016; the operator raised a Mandatory Occurrence

Report and transmitted it to the UK Civil Aviation Authority (CAA) the same day.

The AAIB became aware of the accident1 during the morning of 5 January 2017 and initiated

a Field Investigation. This Special Bulletin is published to provide preliminary information

gathered from an initial ground inspection, recorded data, and other sources.

In accordance with established international arrangements, the National Transportation

Safety Board (NTSB) of the USA, representing the State of Design and Manufacture of

the helicopter, appointed an Accredited Representative to participate in the investigation.

He is supported by advisers from the helicopter manufacturer and the Federal Aviation

Administration (FAA). The European Aviation Safety Agency (EASA), the UK CAA and

the helicopter operator also assisted the AAIB.

History of the flight

The flight was the second sector of a four-sector rotation from Aberdeen to the Elgin-Franklin

Offshore Field in the North Sea.

The helicopter commander was the handling pilot for both sectors. The first sector from

Aberdeen to Elgin Process Utilities Quarters (PUQ) was uneventful. As the helicopter,

on a heading of 270°, with nine passengers on board, lifted from the Elgin PUQ helideck

it yawed unexpectedly to the right through 45°. The commander applied full left yaw

pedal, checked the rotation and landed back onto the deck. The flight crew discussed

the likely cause, which they thought to have been the result of local turbulence or wind

effects created by the platform structures which, anecdotally, is not uncommon for this

helideck. They decided to continue and during the subsequent lift off into the hover the

commander applied left yaw pedal, the helicopter responded and turned to the left; all

control responses appeared normal. The commander then climbed to 500 ft for the brief

transit to the West Franklin wellhead platform, 3.3 nm to the south.

The helicopter made a normal approach and deceleration to the West Franklin and crossed

over the helideck. During the descent to land, at approximately 4 ft above the helideck,

it yawed rapidly to the right, reaching a maximum rate of 30 degrees per second. At the

same time it rolled 20° to the left, at which point the left main landing gear contacted the

helideck. It continued to yaw to the right on its left mainwheels and nosewheels before

the right mainwheels contacted the surface. The helicopter came to rest on a heading of

041° having rotated through 187°.

The helicopter was shut down and the crew and passengers disembarked; there were

no injuries. The helicopter was subsequently craned from the helideck onto a ship and

recovered to Aberdeen.


1 The AAIB have classified this event as an accident; this is consistent with the International Civil Aviation

Organisation definition as this helicopter sustained damage which adversely affected its performance and

flight characteristics, and required replacement of the affected components.


AAIB Special Bulletin: S1/2017 G-WNSR EW/C2016/12/04

© Crown copyright 2017


The meteorological observation from the Elgin PUQ at 0608 hrs was: surface wind from

220° at 17 kt, visibility 10 km or greater, overcast cloud at 2,000 ft and temperature 8°C,

dewpoint 3°C and pressure 1038 hPa. No lightning activity was recorded in the area.

Initial investigation

The technical investigation focussed on the tail rotor and associated components. Once

the panels were removed it was immediately apparent that the tail rotor servo piston was

damaged. The servo was removed and revealed that the tail rotor pitch change shaft (TRPCS)

double row angular contact bearing was in a severely distressed condition (Figure 1).

Figure 1

TRPCS double row angular contact bearing from G-WNSR

Further disassembly and examination of the components found signs of severe overheating

with extreme wear on the inner and outer thrust races and barrel shaped rollers of the

bearing. It was found that the roller bearings seized to the inner member. The outer

race roller had excessive axial play (0.5 in), such that the tail rotor driveshaft imparted

a torsional load to the tail rotor servo. This torsional load caused the primary piston

rod to fracture inside the servo. Due to the failure of the primary piston, the secondary

piston sleeve separated axially from the primary piston adjacent to the link fitting, with the

consequential total loss of control of the tail rotor.

The components were shipped to the helicopter manufacturer for forensic analysis. Initial

findings indicate that the failure of this specific bearing was rapid; a period of 4.5 hours

had elapsed from the first exceedance of the relevant bearing condition indicator recorded

on the operator’s Health and Usage Monitoring System (HUMS) to the point of failure.


AAIB Special Bulletin: S1/2017 G-WNSR EW/C2016/12/04

© Crown copyright 2017

Health and Usage Monitoring System

The HUMS used by the operator for this helicopter was the Integrated Mechanical Diagnostic

HUMS (IMD-HUMS)2. A routine download of the HUMS was performed on the evening of

27 December 2016 and the helicopter was released to service. A detailed analysis of the

data, conducted after the accident, showed that the Tail Gearbox Bearing Energy Analysis

limit had been exceeded on 27 December 2016.

Previous events relating to the TRPCS bearing

There have been two previous events, the first being in 2007, where a degradation of the

TRPCS bearing has occurred, leading to reduced tail rotor control in flight. These events

were identified by the flight crews and resulted in immediate landings. The underlying

causes were identified and a number of safety measures were introduced. At this early

stage of the investigation the helicopter manufacturer is not clear whether this bearing

degradation is the result of a new root cause, or a previously unidentified failure mode.

Safety actions

The initial findings suggest that the damage to the servo in this case is such that it could

have imparted extreme or erratic inputs to the tail rotor at any time after the failure of the

primary piston. Evidence suggests that the yaw which occurred on departure from the

Elgin PUQ was uncommanded and may be related to the condition of the TRPCS bearing.

The AAIB considers that this failure mode would seriously affect the ability of flight crews to

maintain control of the helicopter.

The operator

The operator has subsequently introduced a number of measures to further

strengthen the ability to detect impending bearing degradation. These include: a

review of all HUMS data to ensure no anomalies, fleet-wide borescope inspections,

a requirement for HUMS to be serviceable before flight and the time between

HUMS download/analysis reduced to a maximum of 5 hours. The operator has

also reviewed their HUMS processes and analytical procedures and introduced a

requirement to carry out an additional assurance check.

The helicopter manufacturer

On 31 December 2016 the helicopter manufacturer issued to all operators an ‘All

Operators Letter’ (AOL), CCS-92-AOL-16-0019, which described the event. It

emphasises the use of the HUMS Tail Gearbox Bearing Energy Tool, provided

on the ground station, which will detect a TRPCS bearing that is experiencing

degradation, and recommends that this Tool should be utilised as often as

reasonably possible.


2 The IMD-HUMS includes the use of additional stand-alone mechanical diagnostic software tools for the HUMS

Ground Station (GS) that help assess the condition of a number of specific components, one of which is the Tail

Gearbox Bearing Energy Analysis software tool; however, these require the user to visually inspect the data and

search for exceedances. The helicopter manufacturer now offers an alternative GS analysis system which offers

a number of enhancements to IMD-HUMS, including more advanced algorithms and the automatic alerting of all

exceedances on receipt of new HUMS data.


AAIB Special Bulletin: S1/2017 G-WNSR EW/C2016/12/04

© Crown copyright 2017

Published 11 January 2017

AAIB investigations are conducted in accordance with Annex 13 to the ICAO Convention on International Civil Aviation,

EU Regulation No 996/2010 and The Civil Aviation (Investigation of Air Accidents and Incidents) Regulations 1996.

The sole objective of the investigation of an accident or incident under these Regulations is the prevention of future

accidents and incidents. It is not the purpose of such an investigation to apportion blame or liability.

Accordingly, it is inappropriate that AAIB reports should be used to assign fault or blame or determine liability, since

neither the investigation nor the reporting process has been undertaken for that purpose.

Extracts may be published without specific permission providing that the source is duly acknowledged, the material

is reproduced accurately and is not used in a derogatory manner or in a misleading context.

This was followed by an Alert Service Bulletin (ASB) issued by the manufacturer

on 10 January 2017. ASB 92-64-011 introduces a one-time inspection of the

TRPCS and bearing assembly for ratcheting, binding, or rough turning. The

manufacturer has recommended that compliance is essential and is to be

accomplished prior to the next flight from a maintenance facility; three flight

hours are allowed in order to return directly to a maintenance facility. Concurrent

with the release of ASB 92-64-011, the manufacturer published Temporary

Revision 45-03 to require operators to use S-92 HUMS ground station software

to review Tail Rotor Gearbox energy analysis Condition Indicators for alert

conditions on a reduced flight hour interval. Records in excess of published

alert levels require inspection of the pitch change shaft and bearing.

Ongoing investigation

The AAIB investigation will continue to examine all the operational aspects of this accident

and conduct a detailed engineering investigation of the relevant helicopter components.

The AAIB will report any significant developments as the investigation progresses.




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