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New mechanical problems have been discovered with the Sikorsky S-92 helicopter, the model involved in a fatal crash off Newfoundland earlier this year. According to a Sikorsky internal document, 25 per cent of the S-92s operated in Europe's North Sea have suffered cracked transmission mounts.

Transmission failure has emerged as the leading theory in the crash of a Cougar Helicopters S-92 last March while it was heading for an oil platform off the Newfoundland coast, killing all but one of the 18 people aboard.

Sikorsky spokesman Paul Jackson said the company has sent a team of engineers to Europe to analyze the problem, and defended the S-92's safety record: "We are working very closely with customers to identify the cause and eliminate it," he said. "Safety is the number one priority...."

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The newly revealed cracking issues are the latest in a litany of problems with the high-tech S-92, which was once billed as the safest helicopter in the world.

Just last week, European aviation officials ordered emergency inspections after discovering a maintenance manual mix-up that could lead to transmission failure. Last winter, the entire S-92 fleet was grounded until flawed oil filter components were replaced.

Broken oil filter components that allowed lubricant to leak from the main transmission have been identified in two S-92 incidents, including the fatal Newfoundland crash. A Globe and Mail investigation showed that the S-92 failed a "run-dry" test that determines a helicopter's ability to keep flying without oil.

The S-92 was certified after Sikorsky demonstrated that the chances of complete oil loss were "extremely remote," only to suffer a series of oil loss incidents after the helicopter entered service.

The S-92's gearbox problems have also raised questions about Sikorsky's $5-billion Maritime Helicopter Program (MHP) contract with the Canadian government. An S-92 variant called the CH-148 Cyclone was selected as a replacement for Canada's aging Sea Kings.

Mr. Jackson said the CH-148 transmission differs from that of the S-92, but would not specify the changes. "The MHP gearbox, including the mounts, will be different than the current S-92 transmission. The MHP requirements are significantly different. We are in the final MHP gearbox design stages and will proceed to testing immediately thereafter."

Offshore oil workers have been sharply critical about the cracks discovered in the North Sea S-92s after they learned that the problem was identified 10 months ago. One told a British paper he was "disgusted" by the lack of communication about the fault.

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According to Sikorsky, the problem involves the four large metal mounting points (known as "feet") that connect the main gearbox to the S-92's fuselage.

Earlier this month, cracks were discovered in an S-92 that had been repaired only a week before. The company said the failure of a single mounting point will not bring down an S-92: "Analysis shows the aircraft can operate safely for a minimum of 10 hours even assuming a full mount foot fracture and resulting worst-case stresses," Mr. Jackson said. "Currently all operators are flying."

Shawn Coyle, a professional test pilot and author of The Art and Science of Flying Helicopters, said the current problems with the S-92 reveal the complexity of helicopter design and manufacture. Although many of the S-92's components are based on a well-known Sikorsky military helicopter called the Blackhawk, the S-92 has suffered several troubling glitches.

"The engineering challenge of a helicopter defies the imagination," Mr. Coyle said. "It's as tough as it gets. Look at cars - they make a lot more of them, and they've been doing it for longer, and they still end up making recalls."

Even a minor change can have unexpected consequences, Mr. Coyle said, and few components are as crucial as a helicopter's main gearbox, which transmits the power of the engines to the rotor blades. A transmission that performs perfectly in one helicopter may be plagued by failures in another model - weight may be applied at different points, the rotors may develop more thrust, etc.

"A lot's going on in a helicopter transmission," he said. "When it comes to calculating the loads, there's nothing simple about it."

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Next week, an inquiry launched after the fatal Cougar Helicopters crash will begin public hearings as the industry looks for ways to minimize risk for the 1,200 people who commute by helicopter to the oil rigs off Newfoundland.

Robert Wells, former justice of the Supreme Court of Newfoundland and Labrador and national president of the Canadian Bar Association, is heading the inquiry. Standing has been given to a broad range of stakeholders, including the families of the deceased, oil companies, unions, Sikorsky, Cougar and the federal Department of Transport.

With a report from Oliver Moore

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