Eurocopter has released further details of its planned fix for the problems bedevilling the EC225, which led to an estimated two-thirds of the global Super Puma fleet having being grounded since last October.
The Marseille-based manufacturer released the information to its technical publications website on 24 May, describing it as "proposed improved safety measures and EC225 roadmap for the complete return to service".
It says the proposals - if accepted by EASA - should allow the 11t helicopter to "safely return to flight" in June or July.
The effective grounding of the Super Puma fleet took place following two North Sea ditchings in May and October last year caused by the failure of the helicopter's main gearbox bevel gear vertical shaft.
Although the airframer has proposed a number of short-term fixes to enable the EC225 to fly again, it says further action will be required. "In the longer term, all affected shafts in the EC225/EC725 fleet will be replaced by new shafts. These new shafts will have a modified design to eliminate any risk of crack initiation," it says.
Its investigations have pinpointed the route cause of the cracking to three factors, it says, listing corrosion, manufacturing stresses and "reduced fatigue strength due to stress 'hot spots'".
It proposes a two-step approach - still to be certificated by EASA - of modifying the area around the shaft and updating its maintenance regime to reduce the possibility of corrosion occurring, allied to a regime of increased monitoring. This replaces eddy-current inspections with ultrasonic inspections, it says, and will allow operators to fly for around eight to 10 hours between checks.
Additionally, Eurocopter plans to install a system in the aircraft to enable in-flight detection of the presence of a crack, with an amber warning light in the cockpit to alert pilots.
"Although subject to approval by the authorities, we anticipate that a flight time of two hours will be allowed following the initiation of an in-flight warning," it says.
The new procedures are expected to be introduced early this month, with training commencing on 10 June. Certification of the in-flight warning system is additionally anticipated for early June, it says, with modification of the fleet following.
Meanwhile, Eurocopter expects approval of the modified shaft in 2014, with retrofits available from the third quarter. "All newly delivered aircraft will be fitted with the new shaft," it says.