Date:	Wednesday 28 December 2016
Time:	08:44 LT
Type:	Sikorsky S-92A
Owner/operator:	CHC Scotia
Registration:	G-WNSR
MSN:	920250
Year of manufacture:	2014
Fatalities:	Fatalities: 0 / Occupants: 11
Aircraft damage:	Substantial
Category:	Accident
Location:	West Franklin Offshore Installation, Central North Sea -   United Kingdom
Phase:	Landing
Nature:	Offshore
Departure airport:	Elgin PUQ Offshore Installation
Destination airport:	West Franklin Offshore Installation
Investigating agency:	AAIB
Confidence Rating:	Accident investigation report completed and information captured

Narrative:
AAIB Special Bulletin S1/2017 on Sikorsky S-92A, G-WNSR: Loss of yaw control on landing at West Franklin Platform, North Sea 28 December 2016. Flight 21N. Loss of tail rotor control while landing. Heavy landing on deck. There were no injuries. The landing resulted in the wheel assembly causing damage to the helideck surface. Aircraft transported back to Aberdeen by ship 30 December 2016. The investigation is focusing on the Tail Rotor Pitch Change Shaft Bearing Assembly. AAIB have classified this occurrence as an accident. Bearing found severely distressed. An Alert Service Bulletin was issued followed by an Emergency Airworthiness Directive on 13 January 2017.

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 exceedances 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 exceedances 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 exceedances 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 exceedances 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.

Nature of Damage to airframe:
Per the AAIB Report "Left outer mainwheel rim distortion, seized tail rotor pitch change shaft bearing, servo piston fracture and minor damage to helideck"

1. https://www.energyvoice.com/oilandgas/north-sea/128044/fresh-pictures-emerge-showing-aftermath-of-north-sea-helicopter-emergency-landing/
2. http://www.bbc.co.uk/news/uk-scotland-north-east-orkney-shetland-38521817
3. CAA: https://siteapps.caa.co.uk/g-info/rk=WNSR
4. https://www.stepchangeinsafety.net/news-events/news/offshore-helicopter-safety-leadership-group-statement
5. http://rgl.faa.gov/Regulatory_and_Guidance_Library/rgad.nsf/AOCADSearch/A8CA3CCE933E47BC862580A7007958F5?OpenDocument
6. http://aerossurance.com/helicopters/s92a-loss-tail-rotor-control/
7. https://www.gov.uk/government/news/aaib-special-bulletin-s12017-sikorsky-s-92a-g-wnsr
8. Aircraft Accident Report AAR 1/2018 - G-WNSR, 28 December 2016: https://assets.publishing.service.gov.uk/media/587640a9ed915d0aeb00013f/AAIB_S1-2017_G-WNSR.pdf

Special Bulletin S1/2017 Sikorsky S-92A G-WNSR loss of yaw control on landing at West Franklin Platform, North Sea https://t.co/i5HRPOH4ZQ pic.twitter.com/CipLsF5XhJ

— AAIB (@aaibgovuk) January 11, 2017

Sikorsky S-92A G-WNSR: Inbound to Sumburgh Airport (LSI/EGPB) (landing on runway 15) Sumburgh, Shetland, September 8 2022

Date/time	Contributor	Updates
06-Jan-2017 08:32	Aerossurance	Added
06-Jan-2017 08:33	Aerossurance	Updated [Narrative]
11-Jan-2017 15:18	Aerossurance	Updated [Time, Source, Narrative]
11-Jan-2017 18:55	Aerossurance	Updated [Total occupants]
13-Jan-2017 18:42	Aerossurance	Updated [Source, Damage]
14-Jan-2017 10:22	Aerossurance	Updated [Source, Narrative]
16-Jan-2017 18:21	Medevac	Updated [Source]
04-Mar-2017 00:02	Dr.John Smith	Updated [Source, Embed code]
04-Mar-2017 00:16	Dr.John Smith	Updated [Source, Embed code]
04-Mar-2017 00:17	Dr.John Smith	Updated [Source]
12-Dec-2017 08:44	Aerossurance	Updated [Source]
22-Mar-2018 18:07	Aerossurance	Updated [Source, Narrative]
23-Mar-2018 10:43	harro	Updated [Narrative]
24-Oct-2018 15:52	harro	Updated [Source, Accident report, ]
09-Apr-2024 14:59	Dr. John Smith	Updated [Source, Embed code, Narrative]