European safety agency EASA has approved Eurocopter's proposed interim fixes for its troubled EC225 Super Puma, which has seen large portions of the global fleet effectively grounded since October.
The Emergency Airworthiness Directive issued on 9 July should pave the way for a relatively speedy return to service for affected operators later this month.
Two ditchings in the North Sea in May and October 2012 - caused by circumferential cracks in the type's bevel gear vertical shaft - led to the ban on overwater flights by civil aviation regulators in the UK, Norway and Denmark, impacting a large proportion of the fleet used for offshore transportation for the oil and gas industry.
Eurocopter's root cause investigation later determined that corrosion, fatigue and residual manufacturing stresses were responsible for the formation of the fractures in the shaft, which originated in a hole drilled into the metal.
The manufacturer's interim fixes include both preventative and precautionary measures. Falling into the former camp are mandated manual cleaning of the shaft - between 150 or 400 flight hours depending on the serial number of the shaft and previous maintenance regime - to prevent corrosion, a redesign of an oil jet and a modification to a plug fitting into the weld hole on the shaft that was found to trap moisture.
Precautionary measures include regular ultrasonic non-destructive inspection - after every 11.5h of flight - to enable detection of the initiation of a crack at an early stage. Allied to this is a modification to the type's health monitoring system (HUMS) linked to an amber warning lamp installed in the cockpit, which will alert the crew in flight to a propagating crack.
"In this situation, the helicopter will continue to safely operate for a sufficient time to allow the crew to return to base or perform a normal landing in a safe area," says Eurocopter.
It says HUMS data from the two ditched EC225s - G-REDW and G-CHCN - indicated a rising trend of vibrations from the relevant sensor.
"During the crack propagation campaign, nine test bench and one flight test, that all went up to the rupture, confirmed the previously observed raising trend, and also confirmed the HUMS as a relevant surveillance means able to provide an early warning."
Approvals from national regulators will still be required to allow a full return to service.
In the longer term, Eurocopter will introduce a redesigned shaft to be retrofitted to the affected helicopters.