What happened
On 26 August 1998, a Sikorsky S-61N, registration G-ATBJ, was performing a public transport flight returning to its base at Dyce Airport, Aberdeen, after transporting passengers from a North Sea platform. During the final approach to Runway 32, the aircraft was descending normally with a slight crosswind.
As the aircraft slowed during the flare, the handling pilot applied collective pitch. At this moment, the helicopter began to yaw to the right. Despite the pilot applying maximum left pedal input, the yaw rate continued to increase. The crew observed the aircraft rotating rapidly, completing approximately two to three full revolutions. The pilot attempted to use the cyclic and collective controls to maintain stability, but the aircraft eventually drifted off the runway and performed a skidding touchdown on the grass. There were no fatalities and no injuries to the two crew members or the eight passengers on board.
The investigation
Investigators examined the aircraft's flight recorders and the tail rotor control system. The data showed that as the airspeed dropped to 23 knots, the yaw rate accelerated to 59 degrees per second. The investigation focused on the physical condition of the tail rotor control cables.
Technical examination revealed that one of the two cables had separated within the tail cone. Detailed analysis of the fracture showed that while some strands failed due to fatigue, others failed due to overload. Crucially, investigators found evidence that the cable had been misrouted over a guard pin. Wear patterns on the cadmium plating of the pin indicated that the cable had been rubbing against the spring steel component during operation. Re-enactment tests confirmed that routing the cable over the aft guard pin rather than under it allowed the controls to remain functional but caused the cable to rub against the pin until failure.
Findings
- The primary cause of the incident was the separation of a tail rotor control cable due to fatigue and overload.
- The cable failure was caused by the misrouting of the cable over a guard pin during maintenance.
- The misrouting occurred during a 300-hour check performed 44 flying hours prior to the accident.
- The error was not detectable during standard 'full and free' control checks, as the controls still operated correctly despite a slightly different tactile feel and increased noise.
- Unlike similar design updates implemented for the S-76 model, the S-61 had not yet received hardware changes to prevent cable damage in the event of misrouting.