Report on serious aviation incident during approach to Stavanger Airport Sola on 25 September 2020 involving a Sikorsky S-92A, LN-ONQ operated by Bristow Norway AS
Aviation report 2023/08
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.
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.
The washer as it was found in the scavenge oil passageway after the input module had been disassembled. The unit is photographed lying on its side. When the module is installed in the helicopter, the passageway is at the lower end of the unit. Photo: NTSB/NSIA
Published 03.11.2023
Safety recommendation
Safety recommendation no. 2023/05T
On 25 September 2020, a Sikorsky S-92A helicopter made an emergency landing at Stavanger Airport, Sola after a loss of oil pressure and overheating of the main gearbox. A metal washer had entered the gearbox and reduced the oil flow in the return oil passageway. A helicopter gearbox is a safety-critical component and any possibility of foreign objects entering the gearbox must be avoided.
The NSIA advises the Norwegian Civil Aviation Authority (CAA-N) to follow up the helicopter operators' procedures and routines related to FOD, and FOD related to gearbox maintenance in particular.
Facts
| Location | 25-30 NM soutwest of Sola (ENZV), Norway |
| Occurrence date | 25.09.2020 |
| ICAO Location indicator | ENZV |
| Aircraft | Sikorsky øvrige |
| Operator | Bristow Norway AS |
| Registration | LN-ONQ |
| Meteorological conditions | VMC |
| County | Rogaland |
| Type of occurrence | Serious incident |
| Type of operation | Offshore |
| Category of operation | Heavy, helicopter (> 2 250kg) |
| Aircraft category | Helicopter, Multi-engine, Turboprop/turboshaft |
| FIR/AOR | ENSV (Stavanger ATCC) |