An Aug. 16 article and an accompanying graphic on the crash of a Helios Airways flight near Athens incorrectly identified Helios as a Greek airline. It is based in Cyprus. (Published 8/17/2005)

The loss of cabin pressure -- an apparent factor in the crash of a Helios Airways flight that killed 121 near Athens on Sunday -- is a rare event in commercial planes and requires a quick response, aviation experts said yesterday.

Airline crews are trained to don masks immediately when they hear the alarm that alerts the crew to a sudden loss of cabin pressure. Passengers and crew have more time to don their masks if the plane is flying at lower altitudes when decompression occurs.

Investigators in Athens said there were several signs a loss of cabin pressure could have caused the accident. Greek authorities said they do not believe the tragedy was a terrorist act, but they likely won't know the actual cause of the crash for months or longer.

Fighter pilots reported seeing the co-pilot slumped over his seat, and oxygen masks deployed in the cabin, according to news reports. The wreckage showed that oxygen masks had been deployed. The flight crew did not respond to repeated calls from air traffic control and fighter pilots who watched it descend from 34,000 feet until it hit the ground, a possible sign they were unconscious.

Developments yesterday hampered the inquiry, as the plane's cockpit voice recorder was reported to be in poor condition from the impact.

A coroner reported that a preliminary examination of six bodies indicated that the passengers were breathing and their blood was circulating before impact, according to a Reuters report. If cabin decompression was a factor, passengers and crew aboard the Boeing 737 did not have much time -- most likely, seconds -- to don oxygen masks, aviation experts said.

For example, passengers have 20 to 30 minutes to get the masks on if the plane is flying at 18,000 feet, while those on a plane flying at 35,000 feet would have 30 seconds to a minute because oxygen is more scarce at higher altitudes, according to Federal Aviation Administration research. After that, passengers could lose consciousness or have very little cognitive ability, according to FAA expert Rogers V. Shaw II.

A loss of oxygen, known as hypoxia, slows a person's physical responses and ability to think. Vision becomes impaired. Some people feel euphoric while others feel fatigue and dizziness. Some become nauseated and others hyperventilate.

Shaw said passengers can recover quickly if oxygen is restored soon enough. "If you get a couple breaths of oxygen, the recovery rate is instant," he said.

A loss of cabin pressure was determined to be the cause of a Learjet crash in 1999 carrying golfer Payne Stewart. In that case, the plane's crew did not respond to air traffic controllers shortly after being cleared to climb to 39,000 feet. The plane flew for nearly four hours before it crashed near Aberdeen, S.D., killing all aboard. The National Transportation Safety Board could not determine what caused the loss of cabin pressure.

The FAA requires one member of a flight crew to wear an oxygen mask when flying at 25,000 feet or above if the other crew member leaves the cockpit for any reason. It was unclear yesterday whether Greek officials have the same requirement.

Many passengers do not realize that the instructions they receive about how to put on the mask and tug on the line to get the flow of oxygen going should be carried out in a matter of seconds, experts said.

"That's why we say put on your own mask first and then assist a child," said Candace K. Kolander, coordinator of air safety, health and security for the Association of Flight Attendants. "If you don your child's first, then you could pass out and then you're good to no one. If for some reason the oxygen masks drop, it really is serious."

Kolander said that if a plane loses cabin pressure, passengers also should expect a drop in temperature and a quick descent as the flight crew tries to get the plane to a lower altitude, where there is more oxygen. The plane's emergency system should provide passengers with about 10 minutes of oxygen.