Eurocopter has finally admitted it cannot meet its self-imposed end-February deadline to return the global EC225 fleet to unrestricted service as it has been unable to identify the root cause of two North Sea ditchings in 2012.

The airframer's chief executive Lutz Bertling made the promise to an oil and gas industry forum in November. However, the manufacturer has now revealed its tests on the main gearbox bevel gear vertical shaft - the component that cracked in both incidents - have failed to produce any concrete answers.

Although it says it has been trying to identify a "most probable root cause" for the issue, it admits "this Eurocopter assumption has not been proven by tests".

A third test campaign will be launched at the end of January, it says, which will be "more sophisticated" than previous efforts.

"Bench test results are expected at the end of February," it says.

"In addition, evaluations will continue on three alternative scenarios for the failures' cause and all results will be shared with the UK [Air Accidents Investigation Branch]."

Eurocopter says it is working with USA-based engineering consultancy Shainin Engineering on the issue. Shainin was previously employed by sister company Airbus to solve the cracking of wing components on the A380.

"The investigation remains the highest priority for the Eurocopter teams and the group is fully committed to complete testing and agreeing on corrective measures with the authorities and the various industry stakeholders," the airframer says.

Once the cause is found, Eurocopter still needs to identify a solution, certificate it and roll it out to EC225s across the world.

Civil aviation regulators in Norway and the UK have banned overwater flights of the type, leading to the grounding of 25 airframes across the North Sea region, as revealed by Flightglobal's Ascend Online database. The remainder of the global fleet can perform overwater flights, but with restrictions in place.

Source: Flight International