A sudden loss of main gear box (MGB) oil pressure in a Sikorsky S-92A, registration LN-ONH, last year was linked to a worn vespel spline adapter that drives one of the two oil pumps, according to a recent accident investigation report. Council of Norway (AIBN).
The helicopter, operated by Bristow Norway, was conducting search and rescue training on 25 September 2021 when the flight crew noticed the MGB oil pressure drop from 58 to 6 psi in 30 seconds. The crew immediately made a successful landing on the Ekofisk platform in Lima (ENLE) 1.9 nm. Data from the flight data recorder showed that the oil pressure dropped from 59 to 32 psi in three seconds. It stabilized at the 5 to 6 psi mark after 33 seconds.
One minute and 50 seconds after the low oil pressure warning, LN-ONH landed at ENLE. The S-92A MGB holds 10 gallons of oil. It is pumped from the crankcase through a separate external oil cooler in the MGB.
AIBN found that the excessive adapter wear was caused by a combination of factors including “longer than reported component life, natural variations in the manufacturing and materials of the vespel spline adapter, and the individual torque load on the pump. » After the incident, Sikorsky shortened the life of the vespel slot adapter to prevent a recurrence.
This is not the first report of problems with the Vespel Spline adapter on the S-92A. In 2005 the MGB oil pressure in the S-92A suddenly dropped to 5-6 psi due to a worn Vespel spline adapter. After this incident, Sikorsky adjusted the adapter change interval to 50 flight hours, resurfaced the pump drive shaft, and added a check valve to stop oil from flowing back into the pump in the wrong direction.
In another incident in 2008, a Vespel spline adapter moved on the driveshaft so that only half of the adapter was engaged, causing the MGB oil pressure to drop to 15-17 psi. No further incidents were officially reported, and eventually Sikorsky extended the lifespan of the Vespel adapter to 750 flight hours. However, in July 2021 Bristow noticed excessive wear on the Vespel spline adapter when replaced after 750 hours and sent the adapter and pump to Sikorsky for analysis.
https://www.ainonline.com/aviation-news/general-aviation/2022-09-26/accident-board-takes-fresh-issue-s-92-main-gearbox The Accident Board is looking into a new problem with the S-92 main gearbox