Accident S92 de Couguar

Feds, Sikorsky: Too little too late on Cougar S-92 crash
By John Croft on June 24, 2009 1:44 PM | Permalink |ShareThis
Safety Investigators in Canada have determined that pilots of a Cougar Helicopters Sikorksy S-92 that crashed in the Atlantic Ocean off the coast of Newfoundland on 12 March, resulting in the deaths of 17 of the 18 on board, were likely trying to autorotate the heavy twin helicopter after a tail rotor drive failure, precipitated (my words, not theirs) by a loss of main gear box (MGB)`lubrication fluid. After the helicopter hit the water, the floatation system, for reasons unknown, failed to inflate. Pictured above is the reconstruction of the crash helicopter by the Transportation Safety Board of Canada (TSB).

Though the details of how successful that autorotation maneouver turned out to be are yet to be determined -- initial data shows the helicopter hit the water with a force of 20Gs -- what is clear is that the problem that initiated the deadly chain of events in the first place might have been flagged up before the helicopter was certified by the US Federal Aviation Administration.

Though the analysis is not yet complete, the evidence strongly suggests that two cracked titanium mounting studs used to hold an external oil filter for the helicopter's main gear box (MGB) cracked and allowed the transmission oil to quickly drain out. Picture below, from TSB, shows a one of the missing oil filter studs on the right side...

An emergency airworthiness directive after the fact required the replacement of those studs with steel equivalents throughout the fleet. Sikorsky earlier had given operators a year to do the changeout, a suggestion spawned by similar but non-lethal failures in the fleet.

Once that oil drained, the rotating machinery had no cooling. In picture below, also from TSB, damage to the tail rotor pick-off gear (connected to the MGB to power the tail rotor) is obvious when compared to a new pinion on the left...

In the aftermath of the crash, the Transportation Safety Board of Canada (TSB) says the FAA and Sikorsky "are working to identify all the modes of failure that might lead to Sikorsky S-92A MGB oil loss, determining their probability of occurrence, and developing appropriate mitigation strategies."


Huh? Shouldn't such a failure modes and effects analysis (FMEA) be done BEFORE certification?

Apparently not...

Though under Part 29 certification rules for rotorcraft, failures that result in the loss of lubrication to the drive train, in this case the MGB (the mechanism that takes in power from the S-92's two turboshaft engines to drive the main rotor, tail rotor, power systems, etc), must allow for 30 minutes of flight time after the crew receives an indication of the problem in the cockpit, there's an escape clause of sorts in the regs... If a failure mode is determined to be "extremely remote", the 30 minute rule no longer applies...

It's not clear in the case of the Cougar crash whether pilots, through tribal knowledge, assumed there was 30 minutes to get back to shore despite emergency guidance material for such anomalies. The helicopter ultimately went into the water about 10 minutes after losing its lubrication oil.

And why didn't the "30 minute rule" apply?

"Based on the applicable guidance material at the time of certification, the lubrication failure modes of interest were limited to the failure of external lines, fittings, valves, and coolers," writes the TSB in an 18 June update on the accident.

"This practice was consistent with industry experience, which had found that loss of lubrication tended to be associated with external devices. Therefore, the possibility of a failure at the oil filter was considered to be extremely remote. As a result of the fracture of the filter bowl mounting studs, resulting in the loss of a large quantity of oil, the certification guidance material is being reviewed."

Admin a écrit:Bonjour vector,

Je ne le connaissais pas coup du pain de cire... je pensais que c'était 30 min à sec !
Quelqu'un peut-il confirmer ?

Bonne journée

EASA mandates Sikorsky S-92A gearbox modification

The European Aviation Safety Agency (EASA) today mandated that operators of Sikorsky S-92A helicopters replace the twin-engine 19-passenger helicopter’s main gearbox (MGB) oil filter assembly with a new design that is less prone to leaks.
“Investigation of recent incidents with S-92A helicopters has shown that loss of oil from the MGB filter was a factor in the sequence of events,” says EASA in the airworthiness directive (AD).
The most high profile MGB oil loss occurred on a Cougar Helicopters S-92A that crashed off the coast of Newfoundland in March 2009. Seventeen of the 18 passengers and crew onboard died as a result of the crash. Sikorsky earlier this month reached an out-of-court settlement for an undisclosed amount with the survivor and other families to close out a lawsuit filed against the airframer.

Preliminary information from Canadian investigators revealed that damaged oil filter mounting studs was a likely cause of the oil leak that ultimately downed the helicopter after the gearing for the tail rotor drive portion of the MGB failed.
Both the FAA and EASA issued several directives related to the MGB in the aftermath, including a mandate to immediately replace the mounting studs, and a December AD to inspect the oil filters for installation damage caused in part by incorrect part numbers in the Sikorsky maintenance manual.
EASA is requiring operators to replace the existing single-piece oil filter with a two-piece design developed by Sikorsky within 100 flight hours or 60 days of 9 February.
The US Federal Aviation Administration (FAA) tomorrow will issue a proposed rulemaking that would require US operators of the helicopter to make the same changes within 60 days once finalized.
“This proposed AD is prompted by tests indicating that an existing MGB filter bowl assembly can fail under certain loading conditions including those associated with a damaged MGB filter or mounting study resulting from high frequency maintenance tasks,” says the FAA.
“Testing of the improved MGB filter bowl assembly demonstrates a significant increase in strength and durability over the existing filter bowl.”
Along with installing the two-piece filter bowl, outlined by an 18 December 2009 Sikorsky service bulletin, operators of 22 US-registered helicopters would also have to inspect oil filters for damage and replace the mounting studs if the directive is finalized as written.

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.