The US Federal Aviation Administration (FAA) today grounded the Sikorsky S-92 fleet so that operators can replace certain original equipment that could have precipitated the 12 March crash of a Cougar Helicopters S-92 off the coast of Newfoundland, killing 17 of 18 on board. The agency says investigators on the crash had found that two of the three main gearbox (MGB) filter bowl assembly mounting studs had broken.

“Prior to the accident, the manufacturer was investigating a July 2008 incident that also involved broken studs,” says the FAA in today's emergency airworthiness directive (AD). “In both cases, the broken studs resulted in rapid loss of oil. The failures have been tied to fretting and galling of the original titanium studs; therefore, we are requiring the removal of all titanium studs and replacement with steel studs. We are issuing this Emergency AD to prevent failure of a stud which could result in rapid loss of oil, failure of the main gearbox, and subsequent loss of control of the helicopter.

Sikorsky yesterday said that most of the 91 S-92 helicopters in service would have retrofitted steel main gearbox (MGB) oil filter bowl studs by week’s end, closing out a January manufacturer’s air safety bulletin that called for replacing three titanium studs with steel within 1,250h or one year from the release date. Sikorsky said 50 helicopters had already been updated. The airframer, a participant in the crash investigation, says it is unclear whether the components failed before the crash or as a result of the crash

Pilots of Cougar Flight 491 issued a “mayday” all with air traffic control enroute to two oil platforms that morning, reporting MGB oil pressure problems. Seventeen of the 18 on board perished after the twin-engine heavy helicopter ditched or crashed into the sea, 47nm (87km) off the coast.