One day after the crash of Helios Airways Flight 522 outside Athens, Greece, aviation experts pointed to a loss of cabin pressure as the likely cause of the disaster that killed all 121 people aboard.

Yet all pilots receive intense training in how to cope with such a problem and have backup oxygen close at hand. Consequently, the airline industry is having a hard time figuring out how things went so terribly wrong aboard the Cypriot 737-300.

"It's a damned odd one," said Bill Waldock, an aviation-safety professor at Embry-Riddle Aeronautical University in Arizona.

Waldock said the big question is why the two pilots didn't put on their own oxygen masks and then take the Boeing jet into a steep dive to

 a safer altitude — the normal procedure to restore proper air pressure in the cabin.

"Decompressions are not all that uncommon, but normally the pilots are able to handle it," he said.

Lost contact at 34,000 ft.

The plane was flying at 34,000 feet when it lost contact with air traffic controllers. If a depressurization occurs at that altitude, most people will lose consciousness in 15 to 60 seconds due to lack of oxygen. Outside air temperatures hover around minus-65 degrees Fahrenheit.

Warning horns blare inside the cockpit if there is a problem with the pressurization system.

"They are pretty loud and annoying," said Tony Salmon, a 737 pilot for Alaska Airlines and a spokesman for the Air Line Pilots Association in Seattle.

"The minute you hear those warnings going off, you're trained to grab the oxygen mask and put it on," Salmon said.

Once their masks are on, pilots go through a quick checklist to see if they can regain proper pressure, Salmon said. If they can't, they go into an emergency descent to 10,000 feet, where the air is breathable, he said. 

Salmon said pilots have access to their own oxygen bottles, which "last a fairly long time depending on the altitude and pressure."

In the cabin, flight attendants have portable oxygen supplies.

Passengers get oxygen from overhead masks, which are supplied by canisters above each half row. When passengers pull on a string to tighten their oxygen masks, they trigger the canisters.

Flew on autopilot

The plane that crashed, en route from the Cypriot city of Larnaca to Athens, flew on autopilot for more than an hour before slamming into a hillside outside Athens.

About a half-hour into the flight, the pilots had reported a problem with the air-conditioning system that cools hot air produced by the engines and mixes it with cabin air to produce the right air pressure and temperature.

Like most modern jetliners, the 737-300 diverts highly compressed air from its two jet engines to maintain pressure, provide clean air for passengers and power a variety of systems onboard.

These "bleed-air" systems take in a constant flow of air from the engines, said Chuck Eastlake, a professor of aerospace engineering at Embry-Riddle.

An outflow valve at the rear of the plane controls the pressure. Open it wide, and pressure drops. Close it down, and pressure rises.

An outflow valve stuck wide open was cited as a possible cause of the depressurization of a Lear jet that killed golfer Payne Stewart in 1999.

"When there is a pressurization problem the outflow valve is always on the list of the things you look at first," Eastlake said.

The plane's data and cockpit-voice recorders were sent to Paris for examination, but a Greek safety official said the voice recorder might be too damaged to provide useful information.

Did bottled oxygen fail?

Jim Hall, former chairman of the National Transportation Safety Board, said it was possible the bottled oxygen in the cockpit of the Cypriot carrier failed. The NTSB in recent years has expressed concern about pilots' ability to get their masks on quickly enough, Hall said.

"The accident did not have to occur," Hall said. "It either has to be a training issue or an equipment issue."

Eastlake also theorized that contamination or failure of the oxygen supply could explain why the Helios pilots did not follow normal depressurization procedures, though he said it would be extremely unusual.

"The concept that the oxygen may have somehow been contaminated is a very low-probability event," he said. "But this is such a weird situation, that's definitely something I would look at if I were investigating."

Salmon said Alaska Airlines pilots receive yearly training on a simulator on what to do during depressurization. He said he has never experienced depressurization in flight.

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.

No one was killed in those cases, though 28 people were injured. Twenty of the incidents occurred on 737-300s.

1996 incident

In 1996, an incident on an American Trans Air flight showed that minor missteps can have serious consequences.

On that flight from Chicago to St. Petersburg, Fla., the cabin lost pressure at 33,000 feet with 105 people aboard, according to FAA records.

The first officer, who was flying the Boeing 727-200, and the flight engineer put on their oxygen masks, but the captain waited to ask the lead flight attendant to check whether passengers' masks had deployed.

After the flight attendant reported the masks were working, the captain tried to put on his mask but failed and lost consciousness, according to the FAA records. The flight attendant also lost consciousness.

The flight engineer left his position and put a mask on the attendant, who regained consciousness, while the first officer put the plane into an emergency dive.

The flight attendant then put masks on the captain and the engineer, who was now unconscious after his mask somehow became dislodged.

After regaining consciousness, the captain landed the plane in Indianapolis with no injuries.

After Stewart's Lear jet crash in 1999, the FAA directed that flight manuals be rewritten to emphasize the importance of pilots and crew putting on oxygen masks at the first sign of depressurization.

Seattle Times staff reporter Justin Mayo and The Associated Press contributed to this story.

David Bowermaster: 206-464-2724 or dbowermaster@seattletimes.com Steve Miletich: 206-464-3302 or smiletich@seattletimes.com