A serious incident involving a Sikorsky S-92 helicopter in Norway is being investigated.
About four litres of oil leaked from the appeared to have leaked from the main gearbox of the aircraft, according to Accident Investigation Board Norway (AIBN).
The Bristow-operated helicopter managed to land safely at Stavanger airport on Friday, but a ditching at sea was being considered at one point.
AIBN said no explanation had been found for what happened so far.
The main gearbox has been removed and will be sent to Sikorsky for checks.
The incident took place while the S92 was flying from the West Elara platform to Stavanger.
During its decent, at an altitude of about 4,500ft, the crew received a warning that the oil pressure in the gearbox had dropped below 45 psi and, subsequently, below 35 psi, at which point the oil cooler was automatically switched off.
Oil temperatures rose, reaching a highest recorded level of 214 degrees Celsius before landing.
AIBN said: “The crew sent an emergency message and went down to a height of 200 ft.
They were preparing to land on the sea if the oil pressure were to disappear completely.
“The landing on Sola was undramatic and the helicopter was followed to park crews from the fire and rescue department.”
Preliminary checks identified some oil spill at the left engine and the left side of the main gearbox and some oil spill on the outside of the aircraft, especially on the left-hand side of the hull.
The oil seemed to come from the area where the shaft from the left engine enters the gearbox.
There were no signs of leakage in the oil cooler or the hoses of the oil cooler, but four litres of a total amount of 30 litres of oil were “missing on the main gearbox”.
AIBN added: “The indicator that warns of a high pressure difference across the oil filter had come out.
“Initial visual inspection of oil filter showed no signs of particles. The filter will be examined further.”