Eurocopter is facing a race against time to return the global EC225 fleet to unrestricted service before a self-imposed end-February deadline.

Lutz Bertling, the airframer's chief executive, made the commitment to an industry forum in November. However, Eurocopter has still to discover the root cause of the cracks in a gearbox component that led to North Sea ditchings of the type in May and October. Once that is identified, it must then come up with a solution, test and certificate the fix, and then roll it out to the EC225 fleet.

However, this is unlikely to be achievable in the seven weeks available, given the trials that remain to be concluded.

Eurocopter issued a statement on 21 December that stressed it was still unable to determine the root cause of the failure. "Ground and flight tests have allowed [us] to observe potential contributing phenomena, but it has not been possible so far to record a sufficient stress level to explain a possible crack initiation," it said.

However, it would not be drawn on whether the February deadline would be achieved. It says: "We are still investigating and have been working hard over the past weeks. It is a commitment we are fulfilling."

According to the latest information issued by the UK's Air Accidents Investigation Branch (AAIB), test activities on the main gearbox and bevel gear vertical shaft - the component that failed in both incidents - "[are] anticipated to extend into 2013".

Aside from analysis of the material the shaft is manufactured from, the AAIB says Eurocopter will conduct further flight tests to "establish if there is an area in the flight envelope where the bevel gear vertical shift might operate at one of its natural frequencies".

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, according to Flightglobal's Ascend Online database. The remainder of the global fleet can perform overwater flights, but with a number of restrictions in place.

Source: Flight International