(Updated 10 December 2010 with Sikorsky comments)

US and European regulators say an interim upgraded main gearbox mounting structure for the Sikorsky S-92A continues to exhibit the cracking problems that plagued by the original design.

In a new amendment to a 2009 airworthiness directive that requires operators of S-92A heavylift twin-engined helicopters to inspect the mounting feet every 10h, both the Federal Aviation Administration and European Aviation Safety Agency have added the inspection requirement to a modified main gearbox housing that was designed as an interim solution to the cracking problem. Called the Phase II gearbox, Sikorsky says the hardware was not intended as a final solution, but was an interim solution while the Phase III gearbox, which is designed to eliminate the foot cracking, was being produced and readied for distribution starting in December 2010.

EASA has mandated that the checks take place before the first flight of each day or at 10h intervals, whichever is first.

"This new housing configuration is added to the applicability of this AD because it is prone to the same cracks as the [main gearbox] listed in the current AD," says the FAA. Sikorsky is still investigating the root cause of these cracks, it adds. "Contributing factors may include corrosion and the bushing press fit in the mounting foot bolt hole," the FAA says.

The Phase II design included a modification to the main gearbox foot pads as well as the addition of a six-stud attachment for the main gearbox oil filter. A three-stud attachment design in the original gearbox could have played role in a fatal 2009 ditching of a Cougar Helicopters S-92 en route to an oil platform off the coast of Newfoundland, though the investigation by the Transportation Safety Board of Canada is not yet complete. 

The FAA says failure of the mounting feet on the gearbox could cause loss of the main gearbox and "subsequent loss of control of the helicopter".

The agency says the actions in the AD are "interim" until the root cause of the cracking is determined. "After that determination, we anticipate further rulemaking," the FAA says.

Source: Flight International