S92, West Franklin Wellhead Platform North Sea, 2016


On 28 December 2016, yaw control was lost during touchdown of a Sikorsky S92A landing on a North Sea offshore platform and it almost fell into the sea. The Investigation found that the loss of control was attributable to the failure of the Tail Rotor Pitch Change Shaft bearing which precipitated damage to the associated control servo. It was also found that despite HUMS monitoring being in place, it had been ineffective in proactively alerting the operator to the earlier stages of progressive bearing deterioration which could have ensured the helicopter was grounded for rectification before the accident occurred.

Event Details
Event Type: 
Flight Conditions: 


Flight Details
Type of Flight: 
Public Transport (Passenger)
Intended Destination: 
Take-off Commenced: 
Flight Airborne: 
Flight Completed: 
Phase of Flight: 
Location - Airport
Event reporting non compliant, Helicopter Involved, PIC less than 500 hours in Command on Type
Inappropriate crew response (technical fault), Plan Continuation Bias, System/Component HMI
Significant Systems or Systems Control Failure
Inadequate Maintenance Inspection, Component Fault in service
Damage or injury: 
Aircraft damage: 
Non-aircraft damage: 
Non-occupant Casualties: 
Number of Non-occupant Fatalities: 
Number of Occupant Fatalities: 
Off Airport Landing: 
Causal Factor Group(s)
Aircraft Technical
Safety Recommendation(s)
Aircraft Airworthiness
Investigation Type


On 28 December 2016 a Sikorsky S92A (G-WNSR) being operated by CHC Scotia on a contracted North Sea offshore passenger transport flight from the Elgin Process Utilities Quarters (PUQ) platform to the West Franklin wellhead platform failed to complete the final stages of a routine landing at the destination in day VMC when directional stability was suddenly lost. The helicopter came to rest very close to the edge of the helideck and although both the deck and the helicopter sustained minor impact damage, the 11 occupants were uninjured and disembarked onto the unmanned platform.

The helicopter in its final stopping position after the loss of yaw control. [Reproduced from the Official Report]


Although the helicopter operator sent a Mandatory Occurrence Report (MOR) to the UK CAA as Regulatory Authority the same day, the AAIB was unaware of the event until media reports about it appeared during the morning of 5 January 2017 and after obtaining and reviewing a copy of the MOR, a Field Investigation was commenced. It was found that the operator of the Helideck involved was unaware that they were also, along with the helicopter operator, responsible for reporting such an event directly to the AAIB. Relevant recorded data were obtained from the Multi-Purpose Flight Recorder (MPFR) fitted to the helicopter and from the Health and Usage Monitoring System (HUMS) installed to meet the EASA requirements for Vibration Health Monitoring (VHM). CCV recordings from both helidecks were also used to corroborate witness evidence. A ‘Special Bulletin’ was published on 11 January 2017 to provide preliminary information.

The 58 year-old Captain had been employed by the Operator for 11 years and had previously flown AS332 and EC 225 helicopters in the UK Offshore sector of the North Sea. He had accumulated 8,785 total flying hours of which 243 hours were on type. The 37 year-old First Officer had been employed by the Operator for 2½ years and had accumulated 4,490 total flying hours of which 1,400 hours were on type. Both had experience of flying to the Elgin and West Franklin Platforms.

It was established that the accident flight was the second in a four-sector sequence from Aberdeen to the Elgin-Franklin Offshore Field and that the commander had been PF both the first and second sectors. The first flight from Aberdeen to the Elgin PUQ platform was uneventful but as the helicopter initially lifted from the Elgin PUQ helideck for the five minute transit to the West Franklin Wellhead platform, it yawed to the right through 45° without command and in response full left yaw pedal had been used to check the rotation before landing back onto the helideck. The flight crew reported having then discussed the situation encountered and, after concluding that it could be attributed to local turbulence or wind effects created by the platform structures, decided to continue with the flight to the West Franklin situated 3.3 nm to the south. During the subsequent lift off into the hover, normal response to an application of left yaw pedal was reported to have been obtained and after verifying that all control responses were normal, the transit was made at 500 feet. In respect of the unexpected yaw experienced during the first attempt at departure from the Elgin PUQ platform, it was noted anecdotally that similar consequences of low level mechanical turbulence in the vicinity of some offshore platform structures were "not uncommon" in the case of this particular helideck.

The flight was uneventful until it reached the destination but then, about 4 feet above the helideck with touchdown imminent, the helicopter yawed rapidly to the right. The crew responded by landing the helicopter expeditiously, but heavily, on the deck whilst it continued to rotate to the right, reaching a maximum rate of 30 degrees per second and also rolling 20° to the left. The left main landing gear contacted the deck first and the turn to the right then continued on the left main and nose wheels before the right main wheels contacted the surface. The First Officer closed the throttles and the helicopter came to a stop on a north easterly heading having spun through just over 180° from its initial into-wind heading.

The Investigation found that one of the flights made by the same helicopter on the day before the accident had produced recorded vibration data in the Health and Usage Monitoring System (HUMS) which included a series of exceedances associated with the Tail Rotor Pitch Change Shaft (TRPCS) bearing. This was discovered when a routine 50 hour maintenance check and a download and preliminary analysis of the HUMS data was carried out. However, whilst an anomaly relating to Tail Rotor Gearbox (TGB) bearing energy was detected, the related exceedances were not identified partly because of the way they were presented in the analysis tool. The engineer primarily involved reported having intended to draw the attention of his supervisor to the anomaly before completing the release to service but forgot to do this and finished work and went home. The helicopter had therefore been released to service without further investigation.

The technical investigation soon became focused on the tail rotor assembly and once the necessary panels were removed, it was immediately apparent that the tail rotor servo piston was damaged. When this component was removed, it was found that the double row angular contact bearing of the Tail Rotor Pitch Change Shaft (TRPCS) was "in a severely distressed condition" - see the illustration below.

The recovered TRPCS double row angular contact bearing. [Reproduced from the Official Report]

Further inspection found evidence of severe overheating having occurred with extreme wear on both the inner and outer thrust races and the barrel shaped rollers of the bearing and that the roller bearings had seized to the inner member. The outer race roller was found to have enough axial play to allow the tail rotor driveshaft to impart a torsional load to the tail rotor servo and it was concluded that this load had caused the primary piston rod to fracture inside the servo. It was further concluded that because of the failure of this rod, the secondary piston sleeve had separated axially from the primary piston adjacent to the link fitting which led to the complete loss of control of the tail rotor. Forensic analysis at Sikorsky subsequently indicated that the failure of the TRPCS bearing progressed quite quickly over a flight time of only 4½ hours between the first recorded HUMS exceedance the previous day and the point of failure.

It was not possible to identify the initiating cause of the bearing failure because the damage to it destroyed the evidence needed for this. Such evidence may conceivably have been able to show that loss of lubrication to the bearing was the initiating cause since it was clear that the lubrication of the bearing broke down rapidly during the bearing failure sequence. It was also not possible to reproduce the bearing failure which “leads to the conclusion that the failure was because of undetermined factors which occurred within this particular bearing assembly”. The key finding, however, is that had the HUMS evidence that an exceedance had been generated by the TRPCS bearing the previous day been recognised, then the bearing would have been replaced before the helicopter was released to service. This would have been more likely if the engineer tasked with overnight download and review of the HUMS data had not suffered from “prospective memory failure” in forgetting his mental note to report his uncertainty to his supervisor.

More generally, it was considered that there was room for improvement in the VHM process in respect of both the acquisition of data and the timeliness of and ease of access to any alerts it might generate. It was also considered that such access should ideally bring pilots directly into the audience for VHM alerts given that the concept of a pilot-HUMS interface has already been demonstrated on the Airbus Helicopters EC225. It was noted, too, that the HUMS process applied to some UK military helicopter fleets appears to be more proactive than in most civil operations.

Operational Aspects

The Investigation reviewed the uncommanded yaw which the crew had experienced on their initial lift from the Elgin Platform. The Captain’s intuitive response when the first attempt at takeoff was unsuccessful, was to try again so when the helicopter responded normally to his control inputs, both he and the First Officer were reassured and neither pilot considered that there was anything mechanically wrong.

However, it was extremely likely that the already degraded state of the bearing which led to the subsequent failure of the TRPCS was already capable of having an effect on the response to control inputs. Retrospective analysis of yaw pedal positions by Sikorsky “showed that the use of full left yaw pedal could provide an indicator of advanced bearing degradation for flight crews” but since the flight crew did not know this, they tried to find a reasonable explanation for the yaw and concluded that local wind conditions and associated turbulence were responsible.

The Captain’s expectation was that the helicopter could be more awkward to handle in a left crosswind and the First Officer’s expectation was that the Elgin Platform could be “tricky due to the other installations and the position of the helideck and superstructure creating a wind rolling up onto the deck or coming round from behind”. Both expectations may have influenced their acceptance of wind effect as an explanation for the unusual yaw. It was also considered that the crew’s continuing discussion about the yaw event as they departed the second time suggested that they had not yet reached a satisfactory explanation for it and was evidence of ‘plan continuation bias’ - the unconscious tendency to stick with the original plan despite changing conditions - being a factor in their decision to make the transit to West Franklin.

There was no Rotary Flight Manual (RFM) procedure to deal specifically with the consequences of a Tail Rotor caused by the mode of failure encountered, but the Operator’s Emergency Checklist did include a response for both ‘Tail rotor control failure in the hover’ and ‘Tail rotor drive failure’. The former stated that “uncontrolled yaw” would require an immediate landing whilst the latter specifically referenced “uncontrollable yaw to the right” with checklist actions which would also have resulted in an immediate landing and shutdown. However, since the crew did not consider the possibility of a technical malfunction on their first lift from the Elgin Platform, they did not review any emergency checklist procedures. Had they done so, it was considered unlikely that they would have continued with the flight.

However, it was concluded that whilst the crew had subsequently “reacted expeditiously to an uncontrollable yaw” whilst landing at West Franklin, “if the loss of yaw control had occurred at an earlier stage of the flight, the helicopter would most likely have made an uncontrolled descent into the North Sea”.

The Investigation formally identified two Causal factors in respect of the loss of yaw control:

  • The Tail Rotor Pitch Change Shaft (TRPCS) bearing failed for an undetermined reason.
  • The TRPCS bearing failure precipitated damage to the tail rotor pitch control servo.

Two Contributory factors were also identified:

  • Impending failure of the TRPCS bearing was detected by the Health and Usage Monitoring System (HUMS) but was not identified during routine maintenance due to human performance limitations and the design of the HUMS Ground Station Human Machine Interface.
  • The HUMS Ground Station 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.

Safety Action taken as a result of the accident and known to the Investigation was noted as including but not limited to the following:

  • CHC Scotia made a number of changes to improve the chances of detecting impending bearing degradation using HUMS.
  • Sikorsky issued ASB 92-64-011 on 10 January 2017 requiring a once-only inspection of the TRPCS and its bearing assembly for ratcheting, binding, or rough turning to be accomplished prior to the next flight from a maintenance facility with three flight hours allowed in order to return directly to such a facility. Compliance with this ASB was mandated by FAA AD 2017-02-51 issued on 13 January 2017 which added a requirement to carry out a 10-hourly borescope inspection of the subject bearing in situ until further notice.
  • Sikorsky published an amendment to the AMM on 10 January 2017 which required operators to use S-92A HUMS ground station software to review Tail Rotor Gearbox energy analysis Condition Indicators (CIs) for alert conditions on a reduced flight hour interval with CIs in excess of published alert levels requiring inspection of the pitch change shaft and bearing.
  • Sikorsky developed a temperature-sensing plug which could be retrofitted to in-service TRPCSs to establish fleet-wide trends and on 9 March 2017 issued ASB 92-64-012 as installation authority with a scheduled compliance date of 13 April 2017.
  • Sikorsky has worked with the bearing manufacturer to identify and implement a number of improvements to the bearing manufacturing process. An improved end play measuring tool has been introduced, and grease is now drawn from sealed cartridges and injected into the bearing races using a syringe to ensure a more consistent distribution. The bearing is also now weighed before and after grease application.

Two Safety Recommendations were made as a result of the Investigation as follows:

  • that the European Aviation Safety Agency commission research into the development of Vibration Health Monitoring data acquisition and processing, with the aim of reducing the data set capture interval prescribed in the Acceptable Means of Compliance to CS 29.1465 and thereby enhancing the usefulness of VHM data for the timely detection of an impending failure. [2018-006]
  • that the European Aviation Safety Agency amend the regulatory requirements to require that Vibration Health Monitoring data gathered on helicopters is analysed in near real-time, and that the presence of any exceedance detected is made available to the flight crew on the helicopter; as a minimum, this information should be available at least before takeoff and after landing. [2018-007]

The Final Report of the Investigation was published on 22 March 2018.

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