Update on S-92 gearbox housing foot cracks:
Root cause of S-92 gearbox cracks remains elusive
US and European regulators say a new and improved main gearbox mounting structure for the Sikorsky S-92A continues to exhibit the cracking problems that plagued an earlier 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 to eliminate the cracking problem.
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 new design included a modification to the main gearbox foot pads and the addition of a six-stud attachment for the main gearbox oil filter, a problem area that could be linked to a fatal 2009 ditching of a Cougar Helicopters S-92 en route to an oil platform off the coast of Newfoundland.
In that accident, Canadian investigators found that two of the three titanium filter bowl attachment studs had broken, possibly allowing the main gearbox oil to drain and causing the tail rotor to drive to fail. The Canadian Transportation Safety Board has not yet released its final report on the accident in which 17 passengers and crew died.
The FAA says failure of the mounting feet 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.