AVIATION experts have warned that a helicopter dubbed the workhorse of the oil and gas industry which has caused the deaths of a number of Scots suffers from a potentially catastrophic failure in its design.

A report currently being circulated among member of the industry claims to have identified a serious problem within the gearbox of the Super Puma aircraft.

The failure of the gearbox has already been blamed for the deaths of 14 passengers, many of the Scots, and two crewmen during the crash of a Super Puma operated by Bond Offshore Helicopters in the sea off Aberdeen in 2009.

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It is also suspected to be the root of another crash near Bergen in Norway, which cost the lives of Scot Iain Stuart, 41, and 11 other people earlier this year.

But the report says that the gearbox malfunctions could be linked to as many as six other crashes dating back to 1980 and suggests that a complete redesign, potentially taking five years to fully complete, is needed before the aircraft is safe.

The document identifies a process known as 'spalling' occurring within in the gearbox, which is a technical term for the cracking and weakening of the device's bearings.

Airbus Helicopters, owners of the Super Puma, are aware of deficiencies within the gearbox and recently announced an 'interim action' involving replacement parts and tighter monitoring.

This moved the European Aviation Safety Agency (EASA) to lift a ban on Super Puma EC225 LP and AS332 L2 models taking to the air since the crash in Norway in April.

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However, the report states: "Although the ‘interim action’ could reduce the frequency of spalling, spalling with the potential for catastrophic failure remains an inherent characteristic of the Puma family for which no long term solution has yet been identified.

"Application of latest certification rules to the 11 tonne H225 gearbox originally designed in the 1960s for the 6.7 tonne SA330 Puma carries a high risk of being unachievable within the space and weight constraints of the current design without significant change.

"Consequently in order to develop a long term solution, more extensive modifications will be required ... Such a solution is estimated to take up to five years to develop and maturity test."

Despite the EASA decision, both the Civil Aviation Authority (CAA) and the Norwegian Civil Aviation Authority have kept bans on Super Pumas flying in their airspace in place.

The two organisations are awaiting the final report into the crash near Bergen, which is expected to be several months away.

A spokesman for the CAA said: "The Super Puma helicopter accident in Norway on Friday 29 April is still under investigation by the Norwegian authorities and we remain in close contact with all offshore helicopter operators to continue to assess the situation.

"We are united in our approach with the Norwegian Civil Aviation Authority. Both agencies now await further information from the accident investigation before considering any future action. The safety of those who travel on offshore helicopter flights is a key priority."

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A spokeswoman for Airbus said that it retained confidence in the Super Puma fleet.

She said: "Airbus Helicopters is not aware of said report. It is too early to draw conclusions ahead of the AIBN's final report into the Norway accident, however, progress on the investigation so far has revealed many important findings have already been made available. Based on these findings, several potential scenarios/initiating events have been established.

"We have implemented protective measures to deal with each of these scenarios, including a design change involving the use of a single 2nd stage planet gear in all main gearboxes across the H225 and AS332 L2 fleet.

"Since the introduction of these measures, EASA has lifted its temporary flight suspension, confirming that the aircraft is safe to fly."