Commercial

Program Insider: S-92 Main Gearbox Under Scrutiny in Cougar Crash

By James Careless | April 1, 2009

Offshore

Did the main gear box do it? That’s what investigators are asking after the fatal ditching of a Cougar Helicopters’ Sikorsky S-92A (C-GZCH) on March 12. Here’s what’s known.

When Cougar’s pilots made their mayday call on the morning of March 12, they told air traffic control that they had no oil pressure in their main rotor gear box (MGB). The helicopter, named "Jeanne D’Arc Breeze," turned back to St. John’s Newfoundland, but ending up ditching 40 miles east in the Atlantic Ocean 10 minutes later.

The condition of the submerged wreckage, with its tail boom broken off and lying beside the main fuselage, "is indicative indeed [that] this was not a controlled ditching in the water," said former TSB investigator Nick Stoss in an interview with CBC News.

Sources in the industry said that the S-92A’s MGB was not known for its "robustness." But what does that mean? Again, facts are sketchy, but here’s what we have.

Some of the S-92A’s technology is derived from the Sikorsky UH-60 Black Hawk. In a December 17, 2006 interview with Sikorsky president Jeff Pino, the Connecticut Post reported that the Pentagon had sent Sikorsky a "Level III: Corrective Action Requests" letter after "…the Army discovered the walls were too thin on main gear box housings on Black Hawk helicopters…." Sikorsky declined to specifically comment on the Pentagon letter, but Pino defended the company’s commitment safety.

Sikorsky spokesperson Paul Jackson denied that the UH-60’s 2006 problems revealed any fundamental flaws in its MGB housing. "What really occurred is the local DCMA [Defense Contract Management Agency] did spot-checking and found some units not in conformance," he said. "We took corrective action, and no aircraft were grounded. This was not a design or engineering issue, but a manufacturing escape (sic) with some units." Note: The accident aircraft, serial number 920048, was built in 2006.

Doubts about the UH-60’s MGB are just that; doubts. But according to the FAA, there is a problem with the S-92A’s MGB housing. The proof: On July 26, 2006, the FAA issued AD 2006-15-19, which applies specifically to "Model S-92A helicopter, with main gearbox (MGB) upper main housing assembly (housing assembly), part number 92351-15110-042, installed, certificated in any category."

AD 2006-15-19 reads: "To prevent fatigue failure of the MGB housing, loss of MGB lube oil, loss of main and tail rotor drive, and subsequent loss of control of the helicopter, do the following: (a) Before further flight, replace each MGB housing with 2,700 or more hours time-in-service (TIS) with an airworthy MGB housing with less than 2,700 hours TIS. (b) This AD revises the Airworthiness Limitations section of the maintenance manual by establishing a new retirement life for the MGB housing assembly of 2,700 hours TIS." Australia’s Civil Air Safety Authority (CASA) issued a similar directive on December 13, 2006.

The actual cause of the Cougar crash remains unknown. But given the mayday call and the FAA’s own advisory, the main gear box will likely be a prime suspect.

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