Canada’s Transportation Safety Board is recommending changes to helicopter gearbox design and certification standards following the fatal March 2009 crash of a Sikorsky S-92A off the coast of Newfoundland.
The board wants certification rules changed to require all S-92As to be able to fly for 30 min. after the loss of main gearbox oil, and for the FAA to assess whether a 30-min. run-dry requirement is adequate for rotorcraft operating in extreme environments.
Cougar Helicopters Flight 91 crashed after titanium studs broke on the main gearbox filter bowl. This led to a total loss of oil, after failure of the gearbox after just 11 min. All but one of the helicopter’s 18 occupants died in the attempted ditching.
During certification of the S-92, Sikorsky was required to demonstrate the gearbox could run without oil for 30 min. following failure of the normal lubrication system. But during the run-dry test there was a catastrophic failure after 11 min, says Mark Clitsome, TSB director of air investigations.
Following the failed test, the oil cooler was redesigned, but Sikorsky and the FAA decided that other any other cause of total oil loss was “extremely remote”, Clitsome says, and the S-92 was certificated without have to meet the 30-min. run-dry requirement.
Since the Cougar crash, Sikorsky has redesigned the gearbox to increase its reliability. An interim, strengthened “Phase 2” gearbox housing was fielded beginning in April last year, and production of the final Phase 3 redesign began in January, says Spencer Elani, Sikorsky’s S-92 program manager.
Elani says Sikorsky is designing a new “full improved durability gearbox” for the next-generation of S-92. This will be common with the gearbox for the Canadian Forces’ CH-148 Cyclone maritime helicopter, a military derivative of the S-92.
The new gearbox is to be available in “a couple of years”, but Sikorsky declines to specify its run-dry capability. “We are looking at the lubrication system in its entirety, and are a not ready to take a position, but the objective is an extremely robust gearbox,” says Brian Young, Sikorsky’s director of commercial programs.