The body leading the probe into the Super Puma crash which killed four people off Shetland last week has undermined official claims that the helicopters are safe to fly.
Accident investigators have questioned the air safety regulator's stance in ruling out technical issues as the cause of the accident.
The Civil Aviation Authority (CAA), which regulates aviation in the UK, issued a statement on Friday saying it did not believe that the accident was caused by an "airworthiness or technical problem", based on the information available.
But the Air Accidents Investigation Branch (AAIB), which is leading the probe into the crash, told the Sunday Herald it "can't figure out" why it arrived at that verdict.
Super Pumas were grounded immediately after the crash. On Friday, the CAA dramatically altered its position on whether they could fly again and backed the recommendation by the oil industry's Helicopter Safety Steering Group (HSSG) that they go back into service.
The AAIB, which has in the past given early indications of the causes of crashes, insisted after its initial investigations it was too early to draw conclusions on what caused the Super Puma AS332 L2 to crash.
Asked about what the UK safety regulator had said, the AAIB spokeswoman added: "We didn't make that [conclusion]. The CAA have said what they have said, that is not what the AIB or the Department for Transport has said. I can't figure out why they have said what they have said. Our line is absolutely that it is an ongoing investigation and a cause has not yet been identified."
On Thursday night, after the HSSG recommendation, the CAA warned no Super Pumas would be sanctioned to fly again until it had given approval.
A CAA spokesman said at the time: "We need to be sure passenger operations on the Super Puma variants are safe and we are working with operators and HSSG to achieve that. They cannot begin passenger operations until we are absolutely sure that it is safe to do so.
But next day the CAA backed the lifting of the ban, saying it drew its conclusions after reviewing the evidence available including that "already published by the Air Accidents Investigation Branch".
It said: "We do not believe that the accident was caused by an airworthiness or technical problem, and consider that the decision by the operators to resume Super Puma flights is appropriate. We would not allow a return to service unless we were satisfied that it was safe."
The revelation that the AAIB cannot back the CAA conclusion has prompted some union officials to fear the oil industry is pressing to get the helicopters in the air too soon, before an investigation has an idea what caused the fatal crash.
One GMB official said of the development: "There is pressure to have the helicopters operational again, but it should not be at the expense of safety. We must have full confidence in the fleet. That was very, very shaky even before this horrendous Shetland crash."
Last night, a spokesman for the CAA said: "In our discussions with the AAIB we have been given certain indications as to the cause of the accident. But obviously the AAIB need to be given time to conclude their investigation which is very much ongoing and might take several months … We have talked to the AAIB about what they already know and that is what we are basing our position on."
Asked if the conclusion that the accident was not based on airworthiness or technical hitches could change, the spokesman called the situation "complicated".
He added: "The situation could change when we have more available information. The flight data recorders will throw up a lot of information that can be useful for an investigation." He added: "We wouldn't be taking this position if we weren't absolutely sure that they [Super Pumas] were safe for passenger operations."
In April 2009, 16 people died when an AS332 L2 Super Puma - the type involved in the latest Shetland accident - fell into the sea after gearbox failure while returning to Aberdeen from an oil platform.
Six weeks before the April 2009 crash, BP suspended flights involving EC225s after a Super Puma with 18 on board came down near a company rig off Aberdeen, and Bond moved to ground other EC225s.
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