Aircraft Accident Report AAR 1/2018 - G-WNSR, 28 December 2016

Sikorsky S-92A (G-WNSR), loss of yaw control on landing at West Franklin Platform, North Sea.


The Air Accidents Investigation Branch (AAIB) became aware of the accident during the morning of 5 January 2017. In exercise of his powers, the Chief Inspector of Air Accidents ordered an investigation into the accident to be carried out in accordance with the provisions of Regulation EU 996/2010 and the UK Civil Aviation (Investigation of Air Accidents and Incidents) Regulations 1996.

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, supported by advisers from the helicopter manufacturer and the Federal Aviation Administration (FAA). The helicopter operator, the European Aviation Safety Agency (EASA) and the UK Civil Aviation Authority (CAA) also assisted the AAIB.

The sole objective of the investigation of an accident or incident under these Regulations is the prevention of accidents and incidents. It shall not be the purpose of such an investigation to apportion blame or liability.


The helicopter was being operated from Aberdeen on a contract on behalf of an offshore oil and gas company. On 27 December 2016, during a flight on the day prior to the accident, the Health and Usage Monitoring System (HUMS) recorded vibration data which contained a series of exceedences related to the tail rotor pitch change shaft (TRPCS) bearing. Routine maintenance was carried out overnight which included a download and preliminary analysis of the HUMS data. Whilst an anomaly for tail rotor gearbox (TGB) bearing energy was detected by the maintenance engineer, the exceedences were not identified, in part, due to the way they were presented in the analysis tool; the helicopter was released to service without further investigation.

On 28 December 2016, during the first sector of the day, the HUMS recorded further exceedences but these were not scheduled to be downloaded and reviewed until the helicopter returned to Aberdeen; there was no method in place for either the flight crew or maintenance personnel to be made aware of these further exceedences until then.

During lift off on the second sector, the helicopter suffered an uncommanded right yaw through 45° and the flight crew re-landed. The helicopter was again lifted into the hover and responded normally to the controls, so the event was attributed to a wind effect and the helicopter departed en route.

The five-minute flight to the West Franklin wellhead platform was uneventful but, in the latter stages of landing, yaw control was lost completely and the helicopter yawed to the right. The crew landed the helicopter expeditiously, but heavily, on the helideck. The helicopter continued to rotate to the right and the crew closed the throttles before it came to rest near the edge of the helideck having turned through approximately 180°. There were no injuries.

The investigation determined that the TRPCS bearing had degraded and failed. As a consequence, the tail rotor pitch change servo was damaged resulting in uncommanded and uncontrolled inputs being made to the tail rotor (TR). The manner in which the servo was damaged had not been previously identified.

The investigation identified the following causal factors to the loss of yaw control:

● The TRPCS bearing failed for an undetermined reason.

● The TRPCS bearing failure precipitated damage to the tail rotor pitch control servo.

The investigation identified the following contributory factors:

● Impending failure of the TRPCS bearing was detected by HUMS but was not identified during routine maintenance due to human performance limitations and the design of the HUMS Ground Station (GS) Human Machine Interface (HMI).

● The HUMS GS software in use at the time had a previously-unidentified and undocumented anomaly in the way that data could be viewed by maintenance personnel. The method for viewing data recommended in the manufacturer’s user guide was not always used by maintenance personnel.

Despite being unable to determine the exact cause of the bearing failure, the helicopter manufacturer has identified and introduced a number of changes intended to reduce the risk of a recurrence including: introducing HUMS software with enhanced diagnostic capabilities and improved user interfaces, tighter control of bearing manufacturing and assembly tolerances, consistency in lubricating grease quality and its application, and in service temperature monitoring.

In this report, the AAIB makes two Safety Recommendations concerning the timeliness of acquiring, accessing, analysing and promulgating Vibration Health Monitoring (VHM) data, to enhance the usefulness of VHM data for the timely detection of an impending failure.

Download report:

High resolution version: Aircraft Accident Report 1/2018, G-WNSR High resolution

Low resolution version: Aircraft Accident Report 1/2018, G-WNSR Low resolution

Special Bulletin

A Special Bulletin AAIB Special Bulletin S1/2017 was published on the 11 January 2017 to provide preliminary information gathered from an initial ground inspection, recorded data, and other sources.

Published 22 March 2018