The sudden loss of main gearbox (MGB) oil pressure in a Sikorsky S-92A, registration LN-ONH, last year has been tied to a worn vespel spline adapter driving one of two oil pumps, according to a recent report from the Accident Investigation Board of Norway (AIBN).
Operated by Bristow Norway, the helicopter was conducting search and rescue training on Sept. 25, 2021, when the flight crew noted a drop in MGB oil pressure from 58 to 6 psi in 30 seconds. The crew immediately executed a successful landing at Ekofisk's Lima platform (ENLE) 1.9 nm away. Data from the flight data recorder showed that the oil pressure dropped from 59 to 32 psi in three seconds. It stabilized at around 5 to 6 psi after 33 seconds.
One minute 50 seconds after the low oil pressure alert, LN-ONH landed at ENLE. The S-92A MGB holds 10 gallons of oil. It is pumped from the sump via the separate, external oil cooler and into the MGB.
The AIBN found that excessive wear on the adapter was caused by a combination of factors, including “a higher service life on the component than logged, natural variation in production and materials of the vespel spline adapter, and the individual torque load on the pump.” Following the incident, Sikorsky reduced the service life of the vespel spline adapter to prevent reoccurrence.
This is not the first report of problems with the vespel spline adapter on the S-92A. In 2005, the MGB oil pressure of a S-92A suddenly dropped to 5 to 6 psi due to a worn vespel spline adapter. After that incident, Sikorsky readjusted the replacement interval of the adapter to 50 flight hours, changed the surface finish of the pump driveshaft, and added a check valve to stop oil flowing back into the pump in the wrong direction.
In another incident in 2008, a vespel spline adapter had moved on the drive shaft, such that only half the adapter was engaged, leading to MGB oil pressure dropping to 15 to 17 psi. No further incidents were officially reported and Sikorsky eventually extended the life limit of the vespel spline adapter to 750 flight hours. However, in July 2021 Bristow noted excessive wear on a vespel spline adapter when changed out at 750 hours and sent both the adapter and the pump to Sikorsky for analysis.