Canada transportation safety officials are calling on the US FAA to remove from helicopter certification standards a clause that allows manufacturers to bypass a traditional 30min transmission run-dry capability for "extremely remote" failures.
The clause is a central feature in the Transportation Safety Board (TSB) of Canada's final report on the 12 March 2009 ditching of a Cougar Helicopters Sikorsky S-92A off of Newfoundland. Seventeen of the 18 on board did not survive.
The final report, issued today, highlights 16 factors in the accident, which began when titanium studs holding the main gearbox (MGB) filter bowl broke, allowing all of the transmission oil to leak out. The Flight 91 ditched 11min later after pilots lost tail rotor control due to stripped transmission gears. An animation provided by the TSB shows how events unfolded.
Recommendations to Canadian regulators include prohibiting certain over-water helicopter operations "when the sea state will not permit safe ditching and successful evacuation", and making available underwater breathing apparatus.
Mark Clitsome, TSB's director of air investigations said during the meeting that the 30 minute run-dry timeframe was originally a requirement for military helicopters, but that the FAA when it developed its Part 29 rotorcraft certification rules, "believed that civilian aircraft could also meet this."
"However, when it came time to perform the run-dry test [for the S-92A], there was a catastrophic failure after just 11min," says Clitsome. "Following the failed test, Sikorsky and the FAA reviewed the rules and decided that a total loss of oil lubricant would only happen if the oil cooler system failed. Any other source of total oil loss was seen as-and I quote-'extremely remote.'"
"For this reason, [Sikorsky] chose to redesign the main gearbox's lubrication system to include a bypass valve for the oil cooler instead of taking steps to redesign the gearbox," Clitsome says. "What they did not consider was a failure in the main gearbox oil filter bowl-or its titanium studs. This is exactly what happened to Flight 91."
He notes that while the titanium studs for the S-92A have been replaced with steel studs via an emergency airworthiness directive, the S-92A gearbox remains the same.
"In the event of a sudden loss of oil, there would still be only 11min before the gearbox fails," says Clitsome. "The S-92 is the only helicopter to be certified using the 'extremely remote' provision," he continues.
"In fact, since that rule came into effect, [EASA] has certified four helicopters as capable of meeting the 30min 'run dry'. The FAA and Transport Canada also certified one each. So, we know it's possible for civilian aircraft to meet this requirement."
Source: Flight International