The Federal Aviation Administration (FAA) has issued an emergency airworthiness directive (EAD) calling on Sikorsky S-92 operators to repetitively inspect the helicopter's main gearbox mounting feet for cracks, defects that could lead to gearbox detachment and loss of control of the helicopter.

The FAA mandate follows a similar AD issued by the European Aviation Safety Agency on 23 October after reports that cracks had been found in the feet, pads and foot ribs on main gearboxes during inspections.

"In one case, the mounting foot was completely severed from the gearbox," says EASA. "Other cases include cracks at the fore and aft mounting bolt location on the right-hand side of the mounting foot."

Regulators began scrutinizng S-92 main gearboxes after the fatal crash of a Cougar Helicopters S-92 off the coast of Newfoundland in March. Early findings from that investigation, which is ongoing, revealed that mounting studs for the main gearbox oil filter had potentially broken off in flight, leading to the loss of all lubricant. Authorities later issued ADs to replace the original factory titanium mounting studs with steel versions.

The FAA began developing the EAD in September but was unable to issue it until Sikorsky submitted a failure analysis for the main gearbox mounting feet and other information.

The FAA says the cracking phenomenon, while seen in the North Sea, has not been experienced by operators in the USA and Gulf of Mexico.

"This EAD is prompted by reports of cracks in the main gearbox (MGB) mounting foot pads and foot ribs. The manufacturer is investigating the root cause of these cracks. Contributing factors may include corrosion and the bushing press fit in the mounting foot bolt hole. This condition, if not detected, could result in loss of the MGB and subsequent loss of control of the helicopter," the EAD states.

The FAA reviewed Sikorsky Alert Service Bulletin No. 92-63-020, dated September 11, 2009 (ASB), which describes procedures for visually inspecting the MGB assembly mounting foot pads and foot ribs for a crack and corrosion. The ASB specifies a 10-hour recurring visual inspection.

"If you suspect a crack, the ASB specifies a fluorescent penetrant inspection (FPI) or a dye penetrant inspection (DPI). If you find a crack, the ASB specifies replacing the MGB before further flight. Also, the ASB specifies treating any corrosion. This unsafe condition is likely to exist or develop on other helicopters of the same type design," the EAD continued.

The EAD requires within 10 hours time-in-service (TIS), unless accomplished previously, and thereafter at intervals not to exceed 10 hours TIS, cleaning and inspecting each MGB assembly mounting foot pad and rib for a crack and corrosion. If no crack is found, the AD also requires applying a corrosion preventive compound. If S-92 operators find a crack, the AD also requires replacing the MGB before further flight. If one find corrosion, bubbled paint, or paint discoloration, the AD also requires repair the MGB before further flight.

"The actions specified in this AD are interim actions," the FAA EAD noted.