Airbus Helicopters has provided further details on the changes it made to a key gearbox component on the EC225, expected to be delivered to operators for retrofit in July or August.
The majority of the 11t type’s global fleet was grounded for around 10 months following the second of two North Sea ditchings in October 2012. The cause of the two incidents – which led to a ban on overwater flights – was later traced to failure of the bevel gear vertical shaft in the helicopter’s main gearbox.
EASA approved the modified component on 22 April, which will now be rolled out to operators.
Jean-Brice Dumont, executive vice-president for engineering at Airbus Helicopters, says the alterations to the shaft deal with three issues – corrosion, residual stress and stress hot spots.
The shaft has been reshaped, says Dumont, giving it a smoother, less angular profile. This has reduced the stress points on the component.
Its interior is now flush, removing a channel that allowed the accumulation of material and prevented proper lubrication of the shaft.
“There is no more groove. The inner surface is flush so it can be properly lubricated, there is no area where material can be trapped,” says Dumont.
Additional holes have been drilled in the shaft to allow better oil penetration, with these now arranged at 90˚ intervals.
Tolerances governing the smoothness of the surface finish have also been increased. Previously these were “not that high”, as it was not viewed as a “critical part”.
“We have strengthened and reworked the manufacturing process and raised the bar in terms of roughness,” says Dumont.
Although the surface finish did not contribute to cracks forming, it did allow them to propagate, he says, describing this as “an aggravating factor”.
He adds: “Should for any reason a crack recur, which in my opinion could not happen, it would not propagate.”
Serial production of the redesigned shaft is already under way, and Airbus Helicopters will raise output to around 20-30 per month – up from a previous rate of 10.
“We have worked so much on this part’s industrialisation that we are quite confident about a significant increase of production rate,” says Dumont.
He estimates that the global fleet of EC225s and military EC725s will take around two to three years to fully retrofit, depending on operator usage rates.
A series of temporary safety fixes implemented to enable the lifting of the grounding order – including a cockpit warning lamp linked to the Mod-45 alarm on the EC225’s M’ARMS health and usage monitoring system – will be withdrawn once the new shaft is fitted, Dumont adds.
Source: Flight International
