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More than two months after a crash that killed 17 people, Sikorsky S-92 helicopters operated by a Newfoundland charter company are set to fly again, but with new altitude restrictions prompted by concerns with the main gearbox, a critical component that can stop the rotor blades. According to a memo distributed to offshore oil workers, S-92s operated by Cougar Helicopters will not be allowed to fly above 7,000 feet, reducing the time required to make an emergency landing in case of gearbox problems.

The altitude limit is the latest development in an investigative saga that has engulfed the Sikorsky S-92 since March 12, when a Cougar S-92 went down off the Newfoundland coast. Gearbox oil loss has emerged as the likely cause. A Globe and Mail investigation has revealed a troubling history of S-92 gearbox leaks, and raised questions about the way the helicopter met an advanced safety standard.

According to records obtained by The Globe, the Newfoundland crash is the fourth known emergency involving lubrication loss in an S-92 gearbox. Helicopter experts consider this record particularly troubling given the design's limited track record - Sikorsky has sold just 100 S-92s since the helicopter first came to the market in 2004.

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"Those aren't good numbers," says Shawn Coyle, a professional test pilot and author of The Art and Science of Flying Helicopters.

Sikorsky officials defend the S-92's safety record, noting that it is the only helicopter in the world certified to a tough standard known as FAR Part 29. But that very standard is now at the heart of an ongoing debate over the S-92's capabilities. FAR 29 calls for a machine to be capable of running for 30 minutes after losing the oil in its main gearbox, allowing extra time to make a safe landing. But aviation authorities, including the U.S. Federal Aviation Administration (FAA) and Transport Canada, certified the S-92 in spite of its failure to pass such a "run dry" test because the manufacturer demonstrated that the chance of complete oil loss is "extremely remote."

Aircraft certification engineers say the term "extremely remote" is based on the probability that a component will fail approximately once every 10 million flying hours. According to Sikorsky, the S-92 fleet has accumulated approximately 145,000 hours. Four incidents would indicate a failure rate of more than once per 37,000 hours, or about 267 times the "extremely remote" chance of once in 10 million.

"This is not what you would call an extremely remote chance of failure," says one helicopter engineer.

The Newfoundland crash sparked intense discussion among pilots and engineers about the S-92's ability to survive oil loss. In a mayday call made just minutes before their S-92 plummeted into the Atlantic, the pilots in the Newfoundland crash reported that the gearbox oil pressure had dropped to zero. After recovering the wreckage, investigators with the Transportation Safety Board of Canada found broken studs on the S-92's gearbox oil filter assembly which would have allowed the oil to leak out.

Main gearbox failure is one of the most serious problems a helicopter crew can face, and lubrication of the heavily loaded components is key. If the gearbox seizes, the main rotor stops, dropping the helicopter out of the sky with no chance of recovery.

"There's nothing worse that can happen," Mr. Coyle says.

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Sikorsky spokesman Paul Jackson says the helicopter's gearbox issues have been exaggerated, and that maintenance problems appear to be involved in at least two of the known cases.

"Sikorsky's S-92A helicopter is fully compliant with the lubrication system failure requirement of FAR 29.927(c)," Mr. Jackson said in a statement. "... While the investigation into the Cougar S-92A accident continues, we cannot disclose any details that may be pertinent to the investigation. Sikorsky, however, takes exception to the characterization that the helicopter failed to meet any FAR 29 requirement."

Others take a more critical view. Elfan Ap Rees, an expert pilot and publisher of U.K.-based Helicopter International magazine, says the S-92's gearbox problems have raised serious concerns. Mr. Ap Rees's publication has suggested that the S-92's problems may be related to excessive vibration levels - last month, it reported that Sikorsky was experimenting with a vibration-suppression system after a series of complaints from pilots and passengers.

Those criticisms were echoed in a 2008 study by Norway's Flymedisinsk Institute, which found that S-92's vibration levels were 42 per cent higher than the Eurocopter Super Puma, a competing helicopter widely used in the offshore oil industry. The Flymedisinsk study recommended flight-time limits to protect S-92 flight crew and passengers.

Mr. Jackson of Sikorsky said the study was based on a flawed test. "The Institute tested an S-92 that did not have our latest technology, which substantially reduces vibration." Mr. Jackson acknowledged that Sikorsky is experimenting with vibration-suppression systems, but said the technology is aimed at military helicopters, not the commercial version of the S-92.

The first documented case involving S-92 main gearbox lubrication problems occurred just a year after the helicopter went on the market. In April of 2005, an S-92 operated by Norway's Norsk Helikopter made an emergency landing on a North Sea oil rig after pilots received cockpit warnings about lubrication troubles in the main gearbox. Two months later, the FAA issued an Airworthiness Directive (AD) that ordered the replacement of gearbox components called vespel splines.

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The AD warned that failure of the vespel splines could lead to a lubrication breakdown that could stop the rotor blades. "The actions specified in this AD are intended to prevent loss of lubrication to the Main Gear Box ... resulting in loss of power to the rotor system and subsequent loss of control of the helicopter."

In January of 2008, an S-92 operated by Shell Brunei also made an emergency landing after suffering gearbox lubrication problems. Six months later, in June of 2008, another S-92 was forced down off the coast of Australia after losing its gearbox oil when titanium oil-filter studs broke.

That same scenario has emerged as the most likely cause of the Newfoundland crash. On March 23, just 11 days after the accident, the FAA issued an emergency airworthiness directive that grounded the S-92 fleet until the titanium retaining studs were replaced with an improved design.

Mr. Jackson of Sikorsky says it is "factually wrong and irresponsible" to suggest that the S-92's gearbox does not meet safety requirements: "FAR 29 requires that the aircraft be able to fly safely for 30 minutes should any potential main gearbox oil problem - not deemed as a remote possibility for occurrence - lead to loss of oil. Sikorsky designed and installed a system to address the non-remote failure possibilities in compliance with the FAA requirement."

***

POOR ODDS

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1 in 10 million

The expected chance of gearbox oil loss allowed by the FAR 29.927 helicopter certification standard.

1 in 37,000

The failure rate of the Sikorsky S-92 helicopter gearbox (based on four oil-loss occurrences in 145,000 flying hours).

***

WERE PILOTS MISLED?

March's crash of a Cougar Helicopters Sikorsky S-92 has sparked intense debate among pilots, who say they were misled about the Sikorsky S-92's ability to keep flying after losing the oil in its main gearbox. Because it is certified under a tough new standard known as FAR Part 29, many believed it could "run dry" for 30 minutes.

Although the helicopter meets the letter of the regulations, the reality has left some pilots confused. Many have weighed in on online forums such as Rotorheads, where one wrote: "Judging from the posts on here, most pilots seem to have understood there was a run dry capability when there was virtually none."

Although the S-92's flight manual instructs pilots to land as quickly as possible if main gearbox oil pressure is lost, the actions of the Cougar crew suggest they may have believed the S-92 had extended run-dry capability. According to radar data, the crew spent at least 12 minutes descending from cruise altitude, and may have levelled off about 800 feet above the Atlantic to assess their landing options. Although the investigation is still ongoing, it appears that the S-92's gearbox may have seized, stopping the rotor blades.

Experienced pilots say the crew could have made it from 9,000 feet to the water in less than half the time they took. "To me," says a former offshore helicopter pilot, "the descent rate says they thought they had a longer window."

Peter Cheney

***

Deadly descent

The crew of the Sikorsky S-92 helicopter that crashed off Newfoundland in March killing 17 may have thought they had a longer window to descend.

9:21 a.m.: 2,600 ft.

9:32 a.m.: 9,000 ft.

9:44 a.m.: 9,000 ft.

Instruments warn pilot that main gear box oil pressure has fallen to zero.

9:51 a.m.: 900 ft.

9:55 a.m.: 300 ft.

WHAT WENT WRONG

Two titanium studs broke, causing rapid loss of oil

Main gear box filled with oil

NINIAN CARTER/THE GLOBE AND MAIL // SOURCE: FLIGHTAWARE.COM

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