U.S. and European civil aviation authorities have issued new airworthiness directives (ADs) for the inspection and possible repair or replacement of the Sikorsky S-92A’s main gearbox. The updated ADs build on 2009 directives and mandate action on a new gearbox design that was supposed to solve the problem, at least temporarily. Those directives were prompted by the fatal crash of a Cougar Helicopters S-92A off the coast of Canada in March 2009. Although Canada’s Transportation Safety Board (TSB) has not released its final report yet, investigators consider fatigue cracking a likely contributor in the crash.
The amendment, which took effect on December 6 last year, was prompted by “the need to expand the applicability to include another main gearbox assembly and housing that is prone to the same cracks and corrosion,” the FAA said. The revised AD requires cleaning and inspecting assembly mounting foot pads and ribs for cracks and corrosion. The directive is intended to prevent “the loss of the main gearbox and subsequent loss of control of the helicopter.”
If the inspection reveals a crack, the AD requires replacing the gearbox before the next flight. If the operator finds “corrosion, bubbled paint or paint discoloration,” it should repair the gearbox before the next flight. The operator should apply a corrosion-preventive compound if it finds no crack or corrosion.
The interim gearbox design (Phase II) introduced a six-stud attachment for the oil filter bowl and more edge distance on the right and left foot pads. The original design had only three studs and one of them failed. Corrosion and the bushing press fit in the mounting foot bolt hole might have contributed to the failure.
Sikorsky is still investigating the root cause of these cracks, the FAA said. It may be close to reaching a final fix, as “we expect the Phase III gearbox will begin to be deployed [this month],” a spokesman told AIN.
So far, the required inspection interval is 10 flight hours. The cost of complying with the new ADs is estimated at $590,000 per helicopter. The FAA anticipates further rulemaking.
The EASA added that cracks have been found in the main gearbox assembly mounting feet, pad and foot ribs during regular inspections. In one case, the mounting foot was completely severed from the gearbox. The European agency requires inspections before the first flight of the day or at intervals not to exceed 10 flight hours, whichever occurs first.
The accident helicopter, which ditched 30 nm east of St. John’s, Newfoundland and Labrador, was carrying 18 people, and only one of them survived. According to a TSB investigation update published in June 2009, the tail rotor drive gears had been severely damaged. Further evidence hints at a tail rotor drive failure.
Metallurgical examination of the titanium oil filter attachment studs revealed fatigue cracking. This led to the loss of a large quantity of oil. Also found was evidence of thread damage.
