Initial investigations into a mid-air engine failure on a North Sea helicopter suggest air in its fuel system was to blame.
However, experts looking into the issue with the CHC Scotia-operated Sikorsky S92 last week are still trying to pinpoint an exact cause.
The aircraft was returning to Aberdeen from the Beryl Bravo platform in the North Sea with 16 passengers and two crew on board when the drama unfolded on Thursday morning.
A full emergency response was mounted after one of the two engines cut out 40 nautical miles, or about 46 miles, from the Buchan coast.
It landed safely at Aberdeen International Airport shortly before noon after struggling for around 30 minutes on one engine.
A Mandatory Occurrence Report (MOR) submitted to the Civil Aviation Authority shows that a fuel problem is at the centre of the investigations.
It states: “In the cruse at 2,500ft, number two engine fuel pressure illuminated followed immediately by engine number two failure.
“an engineering investigation is in progress with the assistance of the original equipment manufacturer.
“initial thoughts were that the fuel pump was faulty causing fuel starvation but this was not the case. Additionally the fuel samples taken were also clear and the technical investigation continues.”
One of the passengers who was returning from the Apache-owned rig said he and others on board had been terrified during their ordeal.
“There was this almighty whine, and then the chopper just dropped like a stone,” he said.
“We were all terrified.”
Staff from CHC Scotia and Sikorsky are involved in trying to figure out what caused the failure.
Last night a CHC spokeswoman said that all the data taken from the helicopter showed its descent into Aberdeen to be “normal” despite the loss of an engine.
She said: “Preliminary investigations have shown that a small amount of air may have got into the fuel system.
“However the engine did not cut out, rather it powered down over a few seconds and smoothly transferred its power to the remaining engine.
“Technical data has shown that the descent began at the usual height and the pilot levelled off as expected.
“The aircraft remains out of service and investigations are ongoing.”
A spokeswoman for the Sikorsky Aircraft Corporation said last night: “The investigation is continuing and we believe we have identified the root cause.
“We are conducting tests to confirm so. Pending that confirmation, disclosing further details would be inappropriate.”
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