The latest main gearbox (MGB) problem for the Sikorsky S-92A: foot-mount cracking that could lead to the complete separation of the MGB and subsequent loss of helicopter control.
In October the EASA and FAA issued mirror Emergency Airworthiness Directives that mandate visually inspecting the MGB assembly mounting foot pads and foot ribs for cracks and corrosion every 10 flight hours. The ADs follow the issuance of Sikorsky Alert Service Bulletin (ASB) No. 92-63-020 dated Sept. 11, 2009, that called for the inspection. If a crack and corrosion is suspected, then a follow-on fluorescent or dye penetrant inspection must be conducted. If a crack is found, the entire MGB must be replaced before further flight. If corrosion, bubbled paint, or paint discoloration is found, then the MGB must be repaired before further flight. If no crack is found, then corrosion preventive compound must be applied to each foot pad and the rib area.
Operators first reported cracks in the MGB mounting foot pads and foot ribs in January. The problem was initially confined to S-92s flying in the North Sea region, eventually appearing on 25 percent of the 19 aircraft there, approximately one-fifth of the worldwide S-92 fleet. However, in September one case of cracking was so severe that one of the MGB’s four mounting feet completely separated, prompting Sikorsky to issue the ASB. Another crack was found on a replacement gearbox housing with only 70 hours in service. Now the problem apparently has spread beyond operations in the North Sea. Early last month, Cougar Helicopters reported finding a cracked MGB right-hand footing in one of its Halifax, Nova Scotia-based S-92s.
Previous Gearbox Trouble
Cougar was at the center of a previous S-92 MGB controversy. On March 12 this year an S-92 operated by the company crashed into the Atlantic Ocean off St. John’s, killing 17 of 18 aboard. The pilots were attempting to ditch after MGB oil pressure fell to zero. While Canada’s Transportation Safety Board (TSB) has yet to make a final ruling as to the cause of the crash, examination of the wreckage revealed that a titanium main gearbox oil bowl-mounting stud failed, producing a rapid and catastrophic loss of oil.
Sikorsky subsequently recommended the grounding of all S-92s unless their titanium main gearbox oil bowl-mounting studs had already been replaced with steel ones. TSB investigators suspect the broken stud was a “contributing factor” in the crash and recommended that the FAA issue an emergency AD grounding all
S-92s that had not had the part replaced. The FAA did so on March 23. Before the crash, Sikorsky had issued an alert Service Bulletin recommending that the studs be replaced within one year or 1,250 hours, whichever came first. That requirement became mandatory and immediate after the Cougar crash.
The exact cause of the MGB foot cracking remains under investigation, but Sikorsky enumerated several theories in a letter to S-92 operators on October 7. The MGB is attached to the fuselage by four mounting feet, each secured with two bolts.
Sikorsky analysis has shown that the helicopter will fly for a “period of time” with one of the feet completely severed. Some of the cracks occurred in feet after one of the bolts had broken and then been replaced. During the interim period the load had been redistributed to the surviving bolt, increasing stress on the affected foot.
Sikorsky believes that a new requirement to replace the feet-mounting bolts every 500 hours should eliminate this problem. However, some of the cracks appear to have originated at the bolt hole itself, perhaps caused by the press fit of the steel bushing into the magnesium housing, or by corrosion pitting of the magnesium.
Sikorsky said it has “identified a process improvement” to reduce press fit stresses for new transmissions and spare housings used at overhaul and that “appropriate application of sealant” per the maintenance manual around the bolt heads and the perimeter of the foot-to-airframe interface should prevent corrosion.
But the leaders of oil workers unions in Canada and the North Sea, whose members rely on the S-92 for offshore platform transport, expressed deep concern about this latest S-92 revelation, calling it “worrisome” and “astonishing.” Meanwhile, Cougar Helicopters is conducting MGB footing inspections after every flight.