A loose washer jammed in part of the main gearbox (MGB), likely introduced during routine maintenance, was responsible for a significant oil leak and loss of pressure that forced the emergency landing of a Bristow Norway-operated Sikorsky S-92, Norwegian investigators have disclosed.
In fact, the Norwegian Safety Investigation Authority (NSIA) believes the rogue washer had been present in the main gearbox for at least 487 flight hours based on the helicopter’s overhaul history.
When the incident occurred, the S-92 (LN-ONQ) was returning to its base at Sola airport near Stavanger having departed the West Elara oil platform at 12:59 local on 25 September 2020 with two pilots and three passengers on board. It was the S-92’s fourth flight of the day.
About 1h into its flight, as heavy-twin descended to 4,500ft, the caution alarm for the MGB oil pressure illuminated, indicating pressure had fallen below 45psi. At that point, the S-92 was around 60nm (111km) southwest of Sola.
The oil pressure continued to drop and shortly afterwards an oil temperature warning – showing the oil was above 130°C (266°F) – was triggered for the left engine input module, says the report.
Following their emergency checklist, the crew continued their descent to 500ft, reduced the port-side GE Aerospace CT7 engine to idle and issued a ‘Mayday’ distress call.
However, the oil pressure dropped further, to 35psi, the ‘MGB BYPASS’ caution illuminated, indicating that the oil cooler for the main gearbox had automatically disconnected.
Oil was still being circulated within the MGB, flowing directly from the pressure pumps to the gearbox cogs and bearings, but was no longer being cooled, exacerbating the overheating issue.
The crew then reduced altitude further to 200ft, and the S-92 continued at this level until it landed at Sola around 16min after the first oil pressure warning.
“After the helicopter was parked and the rotor had stopped, it became clear that a significant oil leak had occurred on the left side of the main gearbox,” says the NSIA.
Oil was seen “running down the exterior of the fuselage along the cabin”, the report says, with an oil level check showing around 4 litres (1 USgal) – a little over 10% – of the 38.9 litre total were missing.
A post-landing examination of the left main gearbox input module by the airport’s fire and rescue service recorded temperatures close to 250°C on the outside of the component with smoke also present.
Data from the flight recorder showed that the highest recorded oil temperature was 214°C and the lowest recorded oil pressure was 28 psi, it adds.
“The temperature in the left input module had been so high that some plugs of sealing compound had come loose or been deformed,” the report notes.
Pre-departure inspections had not revealed any issues with the gearbox or oil levels, the report says.
Subsequent disassembly of the left input module and a borescope inspection revealed a metal stop-washer standing on its edge 76mm into the MGB’s scavenge return oil system, “thereby reducing the oil flow in the passageway”, says the NSIA.
It calculates that the cross-section of the oil flow would have been cut by around 60%, which eventually “led to [the] overheating”.
“The NSIA believes it is likely that the washer entered the module in connection with maintenance and more precisely through the openings that are exposed when the generator is disconnected from the flange on the accessory module.”
Washers of the same type are used on a particular maintenance task – replacement of the Vespel spline between the generator and input module – which occurs every 3,000 flight hours.
Replacement of the Vespel splines on the left-hand input module had last taken place on 22 May 2020, maintenance logs for the S-92 disclosed; at that point, the helicopter had accumulated 23,422h – 487h before the incident. Previously, the task had been completed on 4 June 2019, 10 days before a new MGB module was installed.
“The NSIA’s investigation has shown that Bristow Norway had used sets of protective covers when working on the accessory module, but no sets were delivered when work was to be done on the input module,” the report points out.
Although the NSIA says it “has not been able to establish when the washer entered the gearbox”, examination of the part suggested it was not new and had previously been installed.
It says the washer “may have entered the accessory module in connection with the work that was done” on 22 May, but adds that “it cannot be ruled out that the washer entered the accessory module at an earlier time.”
Although the crew had hoped to land at Sola, the pilots also prepared for the possibility of a ditching at sea “if the oil pressure was lost completely”, the report notes.
As a result of the incident, the NSIA recommends that the Norwegian regulator checks operators’ procedures and routines related to foreign object debris (FOD), with particular emphasis on MGB maintenance.
The NSIA notes that Bristow has since introduced several FOD preventative measures to avoid similar incidents in future “including the establishment of a FOD policy and programme adapted to the maintenance organisation”.
In addition, Bristow has established “instructions for consistent practice to cover exposed areas during maintenance in addition to the education and training of personnel,” says the report.
In late October, the International Association of Oil and Gas Producers warned that a shortage of S-92 MGBs, and the “extraordinary actions” taken to keep the fleet flying, presented a potential safety hazard for the industry.