What happened
On 28 December 2016, a Sikorsky S-92A, registration G-WNSR, was performing a commercial passenger flight in the North Sea. The aircraft was operating the second leg of a four-sector rotation, traveling from the Elgin Process Utilities Quarters (PUQ) to the West Franklin wellhead platform.
During the initial departure from the Elgin PUQ helideck, the helicopter experienced an unexpected rightward yaw of 45 degrees. The pilot managed to correct the rotation and landed back on the deck. Believing the movement was due to local turbulence or wind effects from the platform structures, the crew decided to proceed to the next destination.
As the helicopter descended toward the West Franklin helideck, it was approximately 4 feet above the surface when it suddenly yawed to the right at a rate of 30 degrees per second. This was accompanied by a 20-degree roll to the left. The left main landing gear struck the helideck first, followed by the nosewheels and then the right mainwheels. The aircraft eventually came to rest after rotating 187 degrees from its original heading. There were no injuries to the two crew members or the nine passengers on board.
The investigation
The AAIB investigation focused on the tail rotor and its associated mechanical components. Upon removing the aircraft panels, investigators discovered a fractured primary piston rod within the tail rotor servo.
Further inspection of the tail rotor pitch change shaft (TRPCS) revealed a severely distressed double row angular contact bearing. The bearing showed evidence of extreme overheating, with significant wear on the thrust races and barrel-shaped rollers. The investigation established that the roller bearings had seized to the inner member, creating excessive axial play. This allowed the tail rotor driveshaft to impart a torsional load to the servo, which ultimately caused the piston rod to fracture and resulted in a total loss of tail rotor control.
Findings
- The tail rotor pitch change shaft bearing failure was the primary cause of the loss of control.
- The failure of the primary piston in the servo led to the separation of the secondary piston sleeve, causing the loss of tail rotor control.
- The bearing had experienced severe overheating and wear, leading to the seizure of the rollers.
- Data from the aircraft's Health and Usage Monitoring System (HUMS) showed that a bearing condition indicator limit had been exceeded approximately 4.5 hours before the failure occurred.
- The uncommanded yaw during the initial departure from Elgin PUQ was likely related to the deteriorating condition of the bearing.