Supply chain shortages on the back of a safety-driven retrofit programme have left 20 Airbus Helicopters Dauphin medium-twins temporarily grounded as they wait for the delivery of spare parts.

European regulators issued a safety directive on 5 July covering both the AS365 and EC155 models (as well as the older SA365), requiring that operators replace main gearbox components on certain affected helicopters. This must be performed within 50h of flight time or the rotorcraft can no longer be flown.

Airbus Helicopters says about a quarter of the global Dauphin fleet of nearly 800 aircraft – including both civil and military variants – needs the gearbox upgrade.

So far, about 30 main gearboxes have been retrofitted, the manufacturer says.

However, a shortage of parts from suppliers has slowed the process, leaving some helicopters which have reached the 50h threshold effectively grounded.

About 20 aircraft are currently "not flying awaiting retrofit", says Airbus Helicopters.

Little detail is available on the operators affected, although at least three helicopters are thought to be based in the UK. However, one of the largest operators, the US Coast Guard, has escaped the retrofit requirement on its 98-strong fleet of Dauphin-based MH-65s.

Although the airframer says it is "working to remedy the situation" and has engaged in a ramp-up effort with gearbox supplier Avio Aero, it estimates that the retrofit process could drag on until early 2019.

"We cannot be complacent about safety and together with our suppliers we are implementing a strong quality assurance process to support the ramp up," it says.

"It is important to note that the lead time of production and quality controls of these parts is long and therefore improvements are not quickly visible."

However, one operator notes that by the time the offending parts are replaced according to the current schedule, its Dauphin will have been grounded for a whole year.

The safety directive was triggered by research carried out by the airframer in the wake of a 2016 fatal crash of an H225 in Norway, which was traced to the failure of a second-stage planet gear in the main gearbox.

Airbus Helicopters found that one of the two different gears used was more susceptible to spalling and cracking thanks to the higher bearing race contact pressures.

It applied these findings to all the helicopters in its range and discovered that two of the three types of second-stage planet gear used in the Dauphin had similar contact-pressure characteristics, and, as a safety measure, has mandated their replacement.

However, in an alert safety bulletin issued in June to address the problem, it states: "The additional safety measure is made as a precaution notwithstanding that as of today, the Dauphin fleet has logged around 6.5 million flight hours, with no accident or serious incident related to the main gearbox planet gears."