Sikorsky Upgrades Gearbox for S-92

 - November 1, 2011, 9:00 AM
Legacy of Heros S92

Sikorsky is developing a next-generation main gearbox (MGB) with enhanced lubrication capabilities with built-in redundancy systems such as chip detectors and oil scavenge systems, according to Spencer Elani, Sikorsky’s S-92 program manager. “We are continuing to make a lot of improvements to the current gearbox, and to automate some of the features,” Elani said.

The new MGB is part of a program improvement plan to the oil supply system designed to prevent another catastrophic accident such the March 2009 crash of a Cougar Helicopter S-92 into the Atlantic Ocean. The aircraft crashed following oil loss due to failure of the MGB oil-filter bowl assembly.

A key change has been the replacement of the original filter bowl assembly with a new double oil-filter bowl. Six steel studs now secure the assembly to the MGB in place of three titanium studs in the original design. A higher-capacity filter is also part of the new assembly, which has been retrofitted to all S-92s, and is now standard on all new-production S-92s.

The next-generation gearbox will also feature an automatic oil-cooler bypass switch, unipleat oil filter and improved durability with redundant scavenge and an auxiliary/emergency lubrication system. “The aircraft’s gearbox today already has a lot of redundancy,” Elani said. “We’re adding to that to further improve safety.”

Visual and aural warnings of a drop in oil pressure or a failed oil pump indicator will help pilots and crew in the decision-making process.

The March 2009 crash has created an industry-wide controversy as to certification requirements for large Category A transport helicopters.

Current certification requirements of the FAA, EASA and Transport Canada require that the aircraft be able to continue safe flight for at least 30 minutes after the flight crew has detected lubrication system failure or loss of lubrication.

However, there is a provision within that regulation for all three agencies that adds the caveat “unless such failures are [determined to be] extremely remote.” This was the provision under which the S-92 was certified.

An FAA spokesman stated that Sikorsky had initially tried to show that the main gearbox would continue to produce power for 30 minutes after loss of lubrication was detected by a noticeable decrease in oil pressure. “But it failed to do so,” he said.

Sikorsky said that it identified the external oil cooler system as the one component in the main gearbox lubrication system potentially subject to a leak that was not considered extremely remote. The company said that, “with the consent of the FAA,” it incorporated a bypass valve before the oil cooler designed to maintain some oil reserve in the gearbox should that failure occur.

The FAA said that with activation of the bypass valve after sudden massive leakage was detected, “[the S-92] passed the 30-minute test.”

The 30-minute test has now created the highly ambiguous term “run-dry,” but without a clear definition as to whether “run-dry” means totally without any lubrication, or with only residual lubrication, with an auxiliary or emergency lubrication system, or with an auxiliary cooling system.

The FAA spokesman noted that the term “run-dry” is not in the regulation. Instead it states that “residual oil supply or secondary lubrication systems may be used to show compliance.” It was the complete loss of lubrication that caused the problem with the Cougar aircraft, a problem that Sikorsky and the certifying agencies had stated was “extremely remote.”

However, on July 2, 2008, approximately nine months before the Cougar accident, a CHC S-92 experienced sudden loss of oil pressure, putting the CHC crew in a “land immediately” situation. The aircraft landed safely, and an inspection of the main gearbox showed that two of the three MGB oil filter bowl studs had fractured, causing a total loss of oil.

In December 2009 Sikorsky issued an Alert Service Bulletin calling for replacement of the MGB oil filter bowl with a two-piece oil filter bowl held on by six steel studs. The FAA issued an AD effective June 21, 2010. But it still did not call for a 30-minute “run-dry” capability.

TSB Recommends Rule Changes

The Transportation Safety Board of Canada, which investigated the Cougar accident, has issued two recommendations to Transport Canada, the FAA and the EASA regarding a true 30-minute operating capability for Category A transport helicopters.

The first, A11-01, recommends that the “extremely remote” provision of the loss of lubrication certification requirement be removed “for all newly constructed Category A transport helicopters and, after a phase-in period, for all existing ones.”

The second, A11-02, recommends that the FAA “assess the adequacy of the 30-minute main gearbox run-dry requirement for Category A transport helicopters.”

In its response to these recommendations, the EASA simply stated that it acknowledges receipt of the recommendation and that it is “under consideration and the outcome will be communicated (to the TSBC) in due course.”

The FAA’s response reiterated its requirement that the MGB must be able to run for at least 30 minutes with a loss of lubricating oil, “unless a lubrication failure resulting in a loss of lubrication is determined to be ‘extremely remote.’”

It noted that the S-92A had met the 30-minute “loss of lubrication” requirement by using an oil cooler bypass valve, thereby eliminating the most likely sources of leakage. However, the agency also stated that “events that have occurred during the operational use of the Model S-92A, such as the MGB oil filter bowl failure associated with the S-92 Cougar accident, have shown that certain failures not considered during certification testing are more likely than ‘extremely remote.’ The service history therefore does not support the method of compliance that was originally accepted by the FAA at the time of the Model S-92A type certification.”

The FAA said that it will therefore propose a rule change to either clarify or eliminate the ‘extremely remote’ provision of the regulation.

Having said that, the FAA response then went on to state that the agency does not believe it is practical or necessary to require all existing and newly manufactured transport Category A helicopters to be equipped with MGBs that meet the 30-minute loss of lubrication requirement, based on the millions of cumulative flight hours on those helicopters and service histories that show they are operating at a satisfactory level of safety.

Modifying those helicopters with the new MGBs would have a “significant economic impact on the aviation community, with the cost outweighing any improvement in safety,” said the agency.

While it is working on the rule change to clarify or eliminate the ‘extremely remote’ provision, it is “revising guidance material to ensure consistent interpretation and standardized methods of compliance for the current rule,” the FAA spokesman said. The revised guidance is expected to be released for comment next spring.

In its response, Transport Canada said it has initiated with the FAA and EASA “a coordinated formal review” of the rules regarding the 30-minute requirement and the “extremely remote” provision “to reach an international agreement on what changes may be required to the rules.”

Transport Canada also stated that it is accelerating a review of the guidance material referred to in the recommendations to identify “additional direction or clarification for the Canadian certification of Category A helicopters by this fall.”

Eurocopter System

As a method of meeting the FAA and EASA certification requirement to operate 30 minutes without lubrication for its EC225, Eurocopter developed a system consisting of an 11-liter tank of glycol connected to the MGB. If the aircraft experiences a total loss of oil, the glycol will be slowly sprayed into the MGB to reduce the temperature and allow the MGB to function. Eurocopter said that during certification trials, the system demonstrated 52 minutes of operation before the MGB failed. The EC225 was certified and delivered in 2004.

Eurocopter said that its EC175 also has a 30-minute “dry run” capacity, but that MGB protection is based on specific gears treatment providing very high resistance to hot temperature, so [it] does not encompass an additional glycol tank as back-up.”

Eurocopter also noted that no EC225 has ever experienced a loss of oil pressure that would require it to activate the system.