August 16, 2005
Tuesday

When a jetliner suddenly loses cabin pressure at high altitude, those aboard have only seconds to put on oxygen masks before losing consciousness.

But flight crews are trained to respond to such emergencies quickly, and disasters such as the Cypriot airliner crash are exceedingly rare, leading some experts to wonder what went wrong Sunday that led to the deaths of 121 people.

Authorities believe Helios Airways Flight ZU522 decompressed at 34,000 feet, sending oxygen levels and temperatures plummeting. But the Boeing 737's crash puzzles aviation experts, who say that warning systems alert flight crews to such emergencies and that airliners carry enough oxygen to keep passengers alive until the plane can descend to safety.

"This is not the first time an airliner has had a pressurization problem, " said John Cox, a former airline pilot who logged 8,000 hours flying 737s and now leads Safety Operating Systems, an aviation safety consulting firm in Washington, D.C. "What is unusual is it resulted in the loss of the aircraft."

Although professional golfer Payne Stewart and five other people died when their Learjet decompressed and crashed in South Dakota almost six years ago, such crashes are all but unheard of among commercial jets.

National Transportation Safety Board records of commercial air crashes since 1962 show there has never been an accident on U.S. soil that was solely the result of cabin depressurization. Although jets have torn open in flight - a 24-foot section of Aloha Airlines Flight 24 opened at 24,000 feet in 1988, killing one person - pilots have always landed if the aircraft remained intact.

Airplanes that fly higher than 10,000 feet are pressurized because air at that altitude is too thin to breathe. Air is drawn into the engines and compressed, then released into the cabin to maintain the atmospheric pressure found at 8,000 feet.

The pressure inside the cabin is called "cabin altitude." Should cabin altitude rise to 10,000 feet, an alarm warns the pilots, who put on oxygen masks and dive to 10,000 feet - where passengers can breathe without oxygen masks.

When cabin altitude rises to 14,000 feet, oxygen masks above each passenger are deployed. Passengers receive about 10 minutes of oxygen created by generators; pilots can descend to safety from a cruising altitude of 36,000 feet within 2 1/2 minutes, experts said.

"That's enough oxygen to get you down safely from anywhere," said Rogers Shaw of the Federal Aviation Administration's Civil Aerospace Medical Institute in Oklahoma City.

Time is of the essence when the cabin depressurizes at cruising altitude. The effects of oxygen deprivation can come on in as little as 30 seconds, causing loss of coordination and judgment, blurred vision and, ultimately, unconsciousness and death.

Such symptoms come on more slowly at lower altitudes or if the plane depressurizes slowly because of, say, a poor seal in a cabin door, experts said.

Hypothermia also is a risk, as the air temperature at 36,000 feet can reach 60 degrees below zero.

Cabin depressurization can result from a catastrophic structural problem, such as a hole in the fuselage or the loss of a window or cabin door. Or it can follow a malfunction in the pressurization equipment, although such systems have built-in redundancies to prevent such problems.

What happened aboard Flight ZU522 remains a mystery, with clues that aviation experts said were both tantalizing and puzzling:

- The pilot reported a problem with the airplane's air conditioner about 30 minutes after takeoff, suggesting something may have been amiss with the pressurization system. The pilot later reported that the problem had been fixed, in what was to be the plane's last communication with air traffic controllers.

- F-16 pilots sent up to check out the plane did not see any ice on the windows, which would have suggested sudden loss of pressure.

- The fighter pilots also saw the passengers' oxygen masks dangling unused. There were reports the fighter pilots also saw two people struggling to control the doomed aircraft.

- Firefighters on the ground said many of the bodies were frozen, and none wore an oxygen mask. The chief coroner said at least six people had been alive - but perhaps unconscious - when the plane crashed.

"It's odd," Terry McVenes, executive air safety chairman for the Air Line Pilots Association, told the Associated Press. "It's a very rare event to even have a pressurization problem, and in general crews are very well trained to deal with it."

Perhaps most puzzling is why the pilot was not in his seat and his co- pilot was slumped over on his controls. Airliner flight decks have their own supply of oxygen, and the masks force air into the lungs and are designed to be strapped on with one hand in less than five seconds, aviation authorities said.

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