Estimated reading time 5 minutes, 22 seconds.
A failed bearing led to the recent loss of tail rotor control in a Sikorsky S-92 helicopter operating in the North Sea, according to the U.K. Air Accidents Investigation Branch (AAIB).
In a special bulletin published on Jan. 11, the AAIB reported that the S-92’s tail rotor pitch change shaft (TRPCS) double row angular contact bearing was found in a “severely distressed condition” with signs of overheating and extreme wear.
Investigators found that the barrel-shaped rollers of the bearing had seized to the inner member, while the outer race roller had excessive play, sufficient to impart a torsional load to the tail rotor servo.
This torsional load caused the primary piston rod to fracture inside the servo, leading to the separation of the secondary piston sleeve and subsequent total loss of tail rotor control, the AAIB stated.
No one was injured in the incident, which occurred as the S-92 operated by CHC was landing at the West Franklin wellhead platform in the North Sea on Dec. 28, 2016.
As described in the AAIB bulletin, as the aircraft was descending to land on the West Franklin helideck, it yawed rapidly to the right while approximately four feet above the surface. At the same time it rolled left, causing the left main landing gear to contact the helideck as the aircraft continued to yaw.
The S-92 rotated through more than 180 degrees before the crew was able to land it on the helideck and shut it down. It was subsequently removed from the helideck by crane and transported on a ship back to its base in Aberdeen, Scotland.
Initial analysis of the helicopter’s health and usage monitoring system (HUMS) data indicates that the failure of the bearing was rapid, with only 4.5 hours of elapsed flight time from the first exceedance of the relevant bearing condition indicator to complete failure.
The failing bearing may have caused an uncommanded yaw earlier in the accident flight. The AAIB bulletin notes that the S-92’s commander and co-pilot experienced about 45 degrees of unanticipated right yaw while lifting off from another offshore helideck nearby.
The crew landed the aircraft, but decided that local turbulence was the likely cause of the yaw. The commander lifted off for the second time without incident and continued to the West Franklin platform, around three nautical miles to the south, where the complete loss of tail rotor control occurred.