History of the Flight
On 25 September 2021, Sikorsky S-92A helicopter LN-ONH, operated by Bristow Norway AS, conducted a search and rescue training mission offshore. After completing the exercise, the helicopter was returning to the Ekofisk Lima platform. During the right downwind phase of the landing pattern, approximately 1.9 nautical miles from the platform and with the platform 90° off the right side, the main gearbox oil pressure began to drop rapidly. The pressure fell from 58 psi to 6 psi within 30 seconds. The crew immediately altered course toward Ekofisk Lima and executed a successful landing. The helicopter was carrying a crew of five and no passengers. No injuries were reported in the source.
Aircraft Information
The helicopter was a Sikorsky S-92A, registration LN-ONH, owned by VIH Aviation Group Ltd., Canada, and operated by Bristow Norway AS. The incident occurred at approximately 2115 local time on 25 September 2021, at coordinates 56.509552°N 003.222427°E, offshore near Ekofisk Lima in the North Sea.
Investigation Findings
The Norwegian Safety Investigation Authority (NSIA) determined that the loss of oil pressure resulted from the failure of a Vespel spline adapter in one of the main gearbox oil pumps. The adapter's failure prevented the pump from supplying the required oil pressure. The NSIA considered it likely that a combination of factors contributed: a higher service life on the component than logged, natural variation in the Vespel spline adapter's production and materials, and the individual torque load on the pump. The loss of oil pressure did not pose an immediate risk of main gearbox failure.
Crew and Operational Context
The crew consisted of five individuals. The flight was a routine training mission. No anomalies were reported prior to the oil pressure loss. The crew's response to the caution and warning systems allowed them to land safely. The incident was initially classified as serious but later reclassified as an aviation incident after consultation with Sikorsky, as it was not close to becoming an accident.
Component Examination
The failed Vespel spline adapter was examined as part of the investigation. The wear patterns and material characteristics were consistent with factors including higher-than-logged service life, production variability, and operational torque loads. The NSIA did not identify any immediate airworthiness concerns beyond the specific failure mode.