The US Federal Aviation Administration has mandated that operators of 44 US-registered Sikorsky S-92A helicopters inspect the twin-engine heavy-lift helicopter’s main gearbox (MGB) oil filters for damage by 28 December.

"This amendment is prompted by three reports of damaged oil filters or packings resulting from installing the filter assembly with an oversized packing possibly because of incorrect part numbers in the maintenance manual,” says the regulator in an AD, issued today.

Sikorsky S-92 

Rig testing at Sikorsky has shown that installing the filter assembly with an oversized packing, or O-ring, in the oil filter double bypass valve can cause “excessive” fatigue loads in the oil filter bowl or the mounting studs that secure the device to the MGB. Sikorsky has issued a temporary revision to the maintenance manual to correct any errors, says the FAA.

Canadian investigators had determined that two of the three mounting studs had broken off from the MGB oil filter bowl of a Cougar Helicopters S-92A that crashed off the coast of Newfoundland in March, allowing all of the transmission oil to leak out.

 s92 cracked studs
 ©Transportation Safety Board of Canada

Pilots in that crash had reported a loss of all MGB oil pressure before they attempted to return to land. The helicopter impacted the ocean approximately 10 minutes later during an attempted autorotation, about the length of time that the transmission had lasted without oil during certification testing. Such failures were determined by Sikorsky through analysis to be “extremely remote”, paving the way for certification despite not being able to operate for the traditional 30 minutes under FAA Part 29 rules with no oil in the transmission.

The Cougar accident and other incidents, along with several MGB directives, question that initial assertion however.  Along with mandates to replace the original titanium mounting studs with stainless steel, operators were also recently called on to check the mounting feet for cracks. FAA this summer met with Sikorsky to review the potential failure modes of the MGB.

In the new directive, operators must perform a one-time check of the primary and secondary oil filters for damage, replacing any filter, packings and mounting studs before further flight if there is damage, and further, replacing the oil filter bowl within 30 days of replacing a damaged filter. Operators must also perform daily inspections for oil leaks during the 30-day period.