Oil and gas operators could return their Eurocopter EC225s to service within weeks now that the European Aviation Safety Agency (EASA) has certified fixes to the bevel gear vertical shaft, a critical component in the main gearbox that drives lubrication pumps.

The July 9 clearance by EASA ends a nine-month grounding of more than half the worldwide fleet of EC225 and EC725 helicopters imposed after air accident investigators concluded that two ditchings into the North Sea by U.K. operators — Bond Offshore in May 2012, and CHC in October 2012 — were caused by the failure of the bevel gear vertical shaft.

In its announcement, EASA says Eurocopter’s solution reduces the “likelihood of any shaft crack initiation,” and that while it is satisfied that the changes ensured “safe and airworthy operations of the EC225 type,” it may update its directive based on further investigation.

Eurocopter says the crack can only appear during an “unlikely combination of factors,” and that this “explains why it appeared only after seven years and 300,000 flight hours for the EC225/725 fleet.”

The manufacturer’s four interim fixes, as previously reported by Aviation Week, include a new shaft-cleaning procedure that removes mud generated by the wear of the gearbox splines in the localized humid environment on the shaft. The company says this would “significantly reduce the possibility of active corrosion and the likelihood of crack initiation.”

This measure will be enhanced with the addition of new oil jets.

To help detect cracks, EASA has approved the use of an ultrasonic non-destructive inspection (NDI) as an alternative to the eddy current method now in use. Company documents suggest tests should take place every 8-10 hr. Eurocopter says training on the new method will be provided for free.

The primary interim fix, however, is real-time monitoring of the shaft using an update to the aircraft’s onboard health and usage monitoring system, known an M-ARMS. When fitted, the modification — dubbed Mod 45 — will issue an onboard cockpit amber warning if vibration levels indicate the presence of a crack. In the event of a warning, the aircraft will be able to continue to safely operate for sufficient flight time to permit the pilot to return to base or perform a normal landing.

Eurocopter has tested the system in flight using a pre-cracked gearbox, flying the aircraft to the point of failure as part of the certification process.

The company says that with the detection methods in place, the probability of a bevel gear crack going undetected by M-ARMS or NDI is now lower than 109/FH, or one per billion, which the company says is “more severe than the certification standards.”

Eurocopter says that while shaft ruptures can be eliminated, the risk of finding a crack can not, so it is developing a new bevel gear vertical shaft that will feature improvements in surface, finish, lubrication and geometry to avoid what the manufacturer calls “corrosion accretion.”

The design will also eliminate stress hot spots, which engineers say was one of the factors for the failure of the original design. The new shaft will also be thicker, but it is not clear if the component comes with a weight penalty.

According to Eurocopter, the new shaft will be available for retrofit in the second half of 2014, and will eliminate the need for the interim fixes.

The U.K. Civil Aviation Authority says it will amend its operational restrictions on each aircraft once the requirements of the EASA directives are met.

Operators have welcomed the decision. A spokesman for Bond Offshore Helicopters says, “This is an important step forward in the validation of the safety measures proposed by Eurocopter by the airworthiness authorities. Bond continues to work with Eurocopter, the regulators, our oil and gas industry customers and other operators to achieve a safe return to flying operations for the aircraft.”

A CHC spokeswoman, meanwhile, says, “Today’s action is an important step forward for industry toward the safe return of the EC225 aircraft to fly overwater service. We are continuing with careful and thorough preparations for resuming such flights pending additional regulatory approvals.”