- The Washington Times - Tuesday, August 16, 2005

Loss of cabin pressure remains a probable cause in the Helios Airways crash that killed all 121 persons aboard outside Athens on Sunday, but decompression rarely causes an aviation disaster, said Chuck Eastlake, professor of aerospace engineering at Embry-Riddle Aeronautical University in Daytona Beach, Fla.

“Problems with pressurization happen infrequently enough that you can call them rare, but they happen. They usually are not a big deal,” Mr. Eastlake said.

Since 1990, there have been 172 reported incidents of loss of cabin pressure on commercial aircraft in the United States, according to Federal Aviation Administration records.

Loss of cabin pressure caused the 1999 crash of golfer Payne Stewart’s private plane, killing four passengers and two crew members.

But reports of similar failures are rare, according to National Transportation Safety Board (NTSB) records.

A so-called outflow valve at the rear of the plane controls the cabin pressure, Mr. Eastlake said. When it is wide open, pressure drops. When the valve is closed, cabin pressure rises.

A Federal Express cargo plane had an outflow valve malfunction on a 1998 flight, according to an NTSB report.

Loss of cabin pressure is more commonly a symptom of another problem, like a malfunctioning door that saps a cabin of oxygen.

Toxic fumes from a faulty air conditioner also could have overwhelmed those aboard the Helios Airways flight.

The Helios jet, a Boeing 737, diverted compressed air from its two jet engines to maintain cabin pressure and funnel air to passengers.

The pilots of two F-16 jets that intercepted the Helios plane reported seeing its co-pilot slumped over and seemingly unconscious. They also said he had his oxygen mask on. The pilot was not in his seat.

That also raises the possibility that the co-pilot’s canister of oxygen was empty, Mr. Eastlake said.

“The fact that he had his mask on and was unconscious is really puzzling,” he said.

Had the oxygen tanks worked properly, the pilot and co-pilot likely would have descended the plane rapidly to a lower altitude, where oxygen is more plentiful.

The Cypriot airline admitted yesterday that the aircraft had prior cabin pressure problems, but insisted it met safety standards.

“In Helios’s six-year history, we have only experienced one incident of decompression on our aircraft,” Helios Airways stated.

“We can confirm that the aircraft that experienced the decompression problem was the aircraft that was involved in this accident,” it said.

• This article is based in part on wire service reports.

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