What happened
On 25 September 2021, a Sikorsky S-92A helicopter, registration LN-ONH, was returning to the Ekofisk Lima offshore platform following a search and rescue training mission. While the aircraft was approximately 1.9 nautical miles from the helipad, the crew received an aural alert indicating low oil pressure in the main gearbox.
Data from the flight recorders revealed a rapid decline in pressure, dropping from 59 psi to 32 psi in just three seconds. Within 30 seconds of the initial alert, the pressure had plummeted to approximately 6 psi. The crew immediately altered their course to perform a direct landing at the platform. The aircraft landed safely at Ekofisk Lima roughly one minute and 50 seconds after the initial warning. There were no injuries to the five crew members on board.
The investigation
The Norwegian Safety Investigation Authority (NSIA) examined the aircraft's components following the incident. Maintenance personnel inspected the oil pumps and discovered that the Vespel spline adapter responsible for driving oil pump #1 had been completely worn down. The second spline adapter also showed signs of wear.
Investigators analyzed the flight data, crew interviews, and the physical condition of the parts. The investigation focused on why the component failed despite being within its scheduled service interval. Testing of the failed adapter did not reveal any specific manufacturing defects or material flaws.
Findings
- The loss of oil pressure was caused by the complete wear of the Vulpel spline adapter connecting the oil pump to the bevel gear.
- The failure was likely driven by a combination of factors, including natural variations in production and material, the specific torque load on the pump, and a higher actual service life than what was recorded in the logs.
- The investigation noted that the component's actual service life was significantly higher than the 700 logged hours, potentially reaching 966 hours.
- The crew's experience of the rapid pressure drop differed from the scenarios they had practiced in flight simulators, which caused unexpected uncertainty during the event.