A severely distressed tail rotor caused the Sikorsky S-92 helicopter to lose total control in the 28 December incident at Total’s West Franklin platform offshore the UK, according to the UK’s Air Accidents Investigation Branch (AAIB).
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TRPCS double row angular contact bearing from G-WNSR. Image from AIBB. |
The AAIB’s initial investigation into the incident, which disrupted operations in the North Sea yesterday (10 January), revealed that following the removal of the servo, the tail rotor pitch change shaft (TRPCS) double row angular contact bearing was in a severely distressed condition (see photo to the right).
“The technical investigation focused 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 AAIB said in its report.
Further 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 also found that the roller bearings seized to the inner member.
“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,” AIBB stated. “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.”
The 28 December incident prompted short-term disruptions in the North Sea after Sikorsky issued an alert service bulletin, requiring inspection for all S-92 aircraft.
The flight
On 28 December, the flight in question was the second sector of a four-sector rotation from Aberdeen to the Elgin-Franklin offshore field in the North Sea, with the same pilot.
According to the AIBB, 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.
The crew 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 500ft for the brief transit to the West Franklin wellhead platform, 3.3nm 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 4ft above the helideck, it 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. It continued to yaw to the right on its left mainwheels and nose wheels 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.
The 28 December flight is under investigation by several aviation authorities including the AIBB, UK Civil Aviation Authority (CAA), the US National Transportation Safety Board (NTSB), US Federal Aviation Administration (FAA), and the European Aviation Safety Agency (EASA).
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