UK investigators believe the “rapid” failure of a bearing in the tail rotor assembly of a Sikorsky S-92 helicopter was behind a potentially disasterous incident in the North Sea that saw the type temporarily pulled from service for checks.
The CHC Scotia-operated S-92 (G-WNSR) performed a series of uncommanded movements as it came in to land on the West Franklin oil platform, following “total loss of control of the tail rotor”.
At a height of 4ft, the twin-engined helicopter “yawed rapidly to the right, reaching a maximum rate of 30° per second”.
“At the same time it rolled 20° to the left, at which point the left main landing gear contacted the helideck,” the Air Accidents Investigation Branch says in a special bulletin on the 28 December incident.
The S-92 continued to yaw on its left main and nose wheels before the right-side gear contacted the surface, the AAIB says; the helicopter had rotated through 187° during the incident.
A less severe incident had occurred on an earlier flight that day, it notes, but the crew had attributed this to “local turbulence or wind effects”.
However, subsequent examination of the tail rotor assembly by the AAIB discovered damage to the tail rotor servo piston.
This, it says, was caused by the failure of the tail rotor pitch change shaft (TRPCS) bearing, which was in an “extremely distressed condition” with “signs of severe overheating” and “extreme wear” on sections of the component.
Analysis suggest that the failure of the bearing was “rapid”, coming just 4h 30min after a warning was triggered in the helicopter’s health and usage monitoring system (HUMS).
In fact, the AAIB says that post-incident analysis of the HUMS data showed that limits for the bearing were exceeded on 27 December. Although a "routine" download from the HUMS was performed that day, the aircraft was released to service.
The AAIB report says there have been two previous incidents involving the same bearing. However, it adds: “At this early stage of the investigation [Sikorsky] is not clear whether this bearing degradation is the result of a new root cause, or a previously unidentified failure mode.”
On 10 January Sikorsky issued an alert service bulletin to all S-92 operators worldwide requiring immediate inspection of the TRPCS bearing, causing many aircraft to be withdrawn from service.
The impact in the North Sea region was particularly acute thanks to the continuing grounding of the Airbus Helicopters H225 by the Norwegian and UK authorities.
An 18 November emergency airworthiness directive issued by the US Federal Aviation Administration, covering “binding” of the TPRCS bearing, mandated inspection of the assembly if it had been in service for less than 80h.