Sikorsky plans to introduce a redesigned main gearbox for the S-92A helicopter this month in a bid to overcome cracking that has required operators to inspect mounting pads within every 10 hours of flight.
FAA has amended a 2009 airworthiness directive (AD) to expand the recurrent inspection requirement to include a previous redesign of the gearbox, which Sikorsky calls Phase II, after discovering it is prone to the same mounting foot-pad cracks and corrosion as the original unit.
“The Phase II gearbox was intended as an interim improvement while the Phase III gearbox was being readied for production and fleet introduction,” the manufacturer says. While it has proved to be stronger and less susceptible to cracks, “the Phase II gearbox was not expected to fully eliminate the foot cracking issue,” it adds.
Introduction of the redesigned Phase III gearbox, which is intended to eliminate the foot cracking, is scheduled to begin this month, Sikorsky says. The company has delivered more than 100 S-92As, the majority to offshore oil-support operators, and the FAA says its amended AD will affect 44 aircraft on the U.S. register.
In June, the FAA issued an AD requiring replacement of the main gearbox filter bowl with a two-piece unit. This follows the 2009 ditching of a Cougar Helicopters S-92 in which all 17 passengers a crew died. Failure of filter-bowl attachment studs may have caused the gearbox oil to drain and the tail-rotor drive to fail.
The Phase II gearbox introduced a six-stud attachment for the filter bowl and more edge distance on the left and right foot pads. Sikorsky has not provided details of the design changes to the new Phase III gearbox.
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