Questions Remain on S-92 Gearbox Requirements

 - December 2, 2011, 1:50 AM
Canada’s TSB is calling for enforcement of the existing 30-minute “run dry” requirement for the Sikorsky S-92, but the FAA is still thinking about it.

Although Sikorsky is developing a new main gearbox to solve the problems that have plagued the S-92 over the course of its service life, the new transmission will not be able to run for 30 minutes after a total loss of lubrication. The Canadian Transportation Safety Board (TSB) has pressured for this capability to be added to the S-92 and is currently assessing the FAA’s assertion that such a retrofit is not necessary on the S-92 and that the safety statistics of other Category A helicopters are good enough.

During the investigation into the 2009 fatal crash of a Cougar-operated S-92 off Newfoundland, the TSB determined that the main gearbox failed after loss of oil. As a result, the Canadian agency called for enforcement of the existing 30-minute “run dry” requirement on all main gearboxes (MGBs) for new helicopters. In July, the TSB asked the FAA to clarify its original response and is now studying the FAA’s September answer. Sikorsky got a pass on the regulation by asserting to the FAA that the chances of total main-gearbox oil loss were extremely remote; events have proved otherwise.

In the document that the TSB wanted clarifying, the FAA wrote that it “does not believe it is practical or necessary to require that all […] Category A helicopters be equipped with MGBs that meet the 30-minute ‘loss of lubrication’ requirement [because] their service history supports they are operating at a satisfactory level of safety.” Does the same hold true for the S-92 specifically?

The FAA acknowledges that its certification of the S-92 in 2002 has proved flawed. “Certain failures not considered during certification testing are more likely than ‘extremely remote,’ it wrote. So why not insist on 30-minute loss-of-lubrication capability for the S-92, in view of its service history? Although it did answer the TSB, the FAA has yet to decide how to proceed, saying only that it is “in the final stage of reviewing analysis/test results from Sikorsky to see if there is a need for other mandatory action.” These tests focus on the likelihood of oil leaks. Also, “we are now preparing/reviewing revisions to the Part 29 rules and guidance associated with gearbox oil-loss testing,” an FAA spokesman told AIN.

Under current FAA, EASA and Transport Canada certification requirements aircraft must be able to continue safe flight for at least 30 minutes after the crew has detected a lubrication-system failure or loss of lubrication. However, each agency’s regulation contains a provision that adds the caveat “unless such failures are [determined to be] extremely remote.”

TSB: Rule Change Needed

Under the FAA definition, “extremely remote” means “happening at a frequency below once every 10 million flight hours.” In 2009, the worldwide fleet of S-92s had logged fewer than 200,000 hours of total flight time.

As the TSB has pointed out, the S-92 is the only helicopter to be certified using the “extremely remote” provision. In fact, since that rule took effect in 1988, the European regulator has certified four helicopters as able to meet the 30-minute “run-dry” requirement. The FAA and Transport Canada also certified one each.

“It is not an engineering feat to design such a gearbox,” Jean-Pierre Dubreuil, Eurocopter retiree and helicopter expert with the French Air & Space Academy, told AIN. Dubreuil noted that military operators have much more stringent requirements for making some systems literally bullet-proof.

As TSB chairwoman Wendy Tadros stated earlier this year, “The problem is with the rule” in that the “extremely remote” provision negates the 30-minute requirement. “Therefore, it needs to go,” Tadros said. She went on, “It’s as simple as that: We recommend that all Category A helicopters, including the S-92, should be able to fly for at least 30 minutes following a massive loss of main gearbox oil.”

The EASA certified the S-92 in June 2004 after EASA predecessor the Joint Aviation Authorities (JAA) validated the FAA’s certification the previous month.

At least one “certification review item” (CRI) in the JAA’s project illustrates the subtleties of the validation process. The JAA stated, at some point, that it did not believe “that Extremely Remote, as defined in JAR25.1309 [which defines failure likeliness tolerance for critical aircraft components], has necessarily been established. However, it does consider that ‘extremely remote,’ as intended in the context of this requirement, has been met.” The CRI about oil-depleted gearbox testing was thus closed.

Will a 30-minute run-dry capability be required for all helicopters one day? The FAA, EASA and Transport Canada are currently reviewing certification requirements in this area. “It is likely that this review will indeed result in changes that will affect certification testing required to demonstrate capability for continued operation after loss of oil,” said an EASA spokeswoman.