An American Trans Air jet was cruising over the Midwest at 33,000 feet on May 12, 1996, when a blaring horn sounded in the cockpit. The Boeing 727 was rapidly losing cabin air pressure.

Within seconds, the captain and the flight engineer lost consciousness, victims of a lack of oxygen. Only the co-pilot, who had donned an oxygen mask at the first sign of trouble, remained conscious and able to fly the plane, according to an official account of the incident by the National Transportation Safety Board.

The plane, carrying 112 passengers and crew, landed safely in Indianapolis after diving to lower altitudes.

The incident demonstrated how quickly a loss of pressure on a high-flying jet can put lives in jeopardy.

Preliminary evidence from Sunday's crash of a 737 in Greece suggests that passengers and crew aboard Helios Airways Flight ZU522 were not as lucky.

Two Greek air force F-16 pilots who flew alongside the Helios jet saw the co-pilot slumped over his controls. A man told Greece's Alpha television that he received a text message from his cousin aboard the jet saying that the pilots were unconscious.

Aviation experts say it's too early to say why the flight from Cyprus to Athens slammed into a mountain, killing all 121 aboard. But investigators will be focusing on whether the jet lost the air pressure that allows people to fly in comfort above 30,000 feet.

John Purvis, a former accident investigator at Boeing, says jets have lost pressure because of pilot errors, improper repairs, corroded metal or bombs.

The puzzling thing for investigators and pilots is that jets are designed to withstand a loss of pressure. All airline jets are equipped with pure oxygen for pilots and passengers to breathe in an emergency. Flight attendants remind passengers of this before every flight.

From the moment they learn to fly at high altitude, pilots are trained how to respond if pressure is lost. A warning horn sounds if cabin pressure falls to dangerous levels. Jets are designed to allow pilots to rapidly descend to safe altitudes in case of a problem.

Pilots repeatedly practice the maneuver.

The NTSB's list of accidents since 1962 does not contain a single case of mass fatalities on an airliner caused by a loss of cabin air pressure alone. In several cases, jets have broken open in flight, but pilots always have been able to descend and land if the plane stayed in one piece.

``If it did decompress, I would not expect it to result in the loss of the airplane,'' says John Cox, a retired 737 pilot who works as an aviation safety consultant.

But events such as the American Trans Air flight in 1996 show how quickly pilots can get into trouble if they don't have emergency oxygen when jets lose pressure.

People rapidly lose good judgment at altitudes above 20,000 feet.

Above 35,000 feet, the air is so thin that people can pass out in seconds.

Retired airline pilot Patrick Clyne recalls training in a simulated high-altitude chamber. At the equivalent of 25,000 feet, he was asked to play tic-tac-toe. ``It's insidious,'' Clyne says. ``You start off real well. In a matter of a very few seconds, you can't even draw the box.''

On the 1996 flight, the NTSB concluded that the captain did not follow the checklist requiring that he don an oxygen mask as soon as the emergency horn sounded. The flight engineer's mask apparently came off his face, and he passed out.

The co-pilot, who had only flown in that model of jet for 10 hours, few them to safety.