The European Aviation Safety Agency (EASA) today mandated that operators of Sikorsky S-92A helicopters replace the twin-engine 19-passenger helicopter’s main gearbox (MGB) oil filter assembly with a new design that is less prone to leaks.

“Investigation of recent incidents with S-92A helicopters has shown that loss of oil from the MGB filter was a factor in the sequence of events,” says EASA in the airworthiness directive (AD).

The most high profile MGB oil loss occurred on a Cougar Helicopters S-92A that crashed off the coast of Newfoundland in March 2009. Seventeen of the 18 passengers and crew onboard died as a result of the crash. Sikorsky earlier this month reached an out-of-court settlement for an undisclosed amount with the survivor and other families to close out a lawsuit filed against the airframer.

 Sikorsky S-92

Preliminary information from Canadian investigators revealed that damaged oil filter mounting studs was a likely cause of the oil leak that ultimately downed the helicopter after the gearing for the tail rotor drive portion of the MGB failed.

Both the FAA and EASA issued several directives related to the MGB in the aftermath, including a mandate to immediately replace the mounting studs, and a December AD to inspect the oil filters for installation damage caused in part by incorrect part numbers in the Sikorsky maintenance manual.

EASA is requiring operators to replace the existing single-piece oil filter with a two-piece design developed by Sikorsky within 100 flight hours or 60 days of 9 February.

The US Federal Aviation Administration (FAA) tomorrow will issue a proposed rulemaking that would require US operators of the helicopter to make the same changes within 60 days once finalized.

“This proposed AD is prompted by tests indicating that an existing MGB filter bowl assembly can fail under certain loading conditions including those associated with a damaged MGB filter or mounting study resulting from high frequency maintenance tasks,” says the FAA.

“Testing of the improved MGB filter bowl assembly demonstrates a significant increase in strength and durability over the existing filter bowl.”

Along with installing the two-piece filter bowl, outlined by an 18 December 2009 Sikorsky service bulletin, operators of 22 US-registered helicopters would also have to inspect oil filters for damage and replace the mounting studs if the directive is finalized as written.