S92 Helicopter Suffered Dangerous Loss of Pressure Caused by Loose Washer
Posted 03/11/2023 09:45
On Friday 25 September 2020, a Sikorsky Aircraft Corporation S-92A, operated by Bristow Norway, was returning to Sola after a flight to the West Elara oil installation in the North Sea. During the descent, at an altitude of approx. 4,500 ft and 25-30 NM southwest of the airport, the crew received several warning lights of loss of oil pressure and increasing oil temperature in the helicopter's main gearbox. The crew called the distress message MAYDAY, but were able to continue the flight to Sola and land there. No one was injured in the incident. After the helicopter was parked and the rotor stopped, it became clear that a significant oil leak had occurred on the left side of the main gearbox.
The Norwegian Safety Investigation Authority's investigation has shown that the loss of oil pressure and the increase in oil temperature was a result of a metal stop washer found stuck in the main gearbox's oil system.
The washer was found standing on its edge in the oil passageway and thus reduced the oil flow in the return oil channel. The washer had clear signs that it had previously been in use, and the NSIA believes it is likely that it has entered the gearbox during maintenance when the generator is removed from the flange on the accessory module.
Despite the distress situation, the crew managed to land the aircraft safely, averting any injuries.
The NSIA has not been able to establish when the washer entered the gearbox. The last time the opening was available was 487.35 flight hours before the incident. The NSIA therefore has reason to believe that the washer has been inside the main gearbox for at least 487.35 flight hours.
As a result of the investigation, the NSIA has submitted a safety recommendation to the Norwegian Civil Aviation Authority. There, the Norwegian Civil Aviation Authority is advised to follow up the helicopter operators' procedures and routines related to FOD, and FOD in connection with gearbox maintenance in particular.