Above 40,000 feet in the air, where Payne Stewart's Learjet was flying before it crashed mysteriously last week, even a top athlete would lose consciousness in about 8 seconds and slip into a frozen death, experts say.
Investigators suspect that Stewart's plane lost pressure at high altitude, incapacitating the crew and setting off a four-hour, autopilot ghost flight that ended in a nose-dive crash in South Dakota.
Whatever the cause, aviation experts say, the crash should remind passengers -- who tend to ignore flight attendants' preflight safety demonstrations -- that modern jets are little more than tubes of simulated sea level, flying through a sky so inhospitable that it would kill most humans before they took a second breath.
"They didn't suffer. You just go out so quickly," said Bernice Haydu, a former flight instructor and aircraft dealer from Riviera Beach, Fla., who went through decompression training with the Women Airforce Service Pilots of World War II.
"It would have been instantaneous" at that altitude, she said. "If there's such a thing as a merciful death, that's what it was."
What looks like a bright, sunny day from behind an airplane window is an environment so hostile to the human body that it might as well be at the bottom of the sea. Oxygen is sparse, and the temperature can be 70 below zero.
Airplanes never dared go into such harsh conditions 60 years ago because humans need to stay within about 10,000 feet of sea level to avoid passing out. But since Boeing Co.'s introduction in 1940 of the Stratoliner, the first aircraft with a pressurized cabin, planes have used sealed compartments and forced air to duplicate conditions at 8,000 feet, taking oxygen along for emergencies.
Commercial aircraft typically fly at 35,000 feet, where the air is thinner, friction is diminished and high speeds are possible.
Business jets can reach 50,000 feet or more.
The risks of traveling in such simulated comfort are low. A search through 15 years of accident investigations by the National Transportation Safety Board revealed fewer than a dozen cases in which cabin pressurization contributed to a crash or other accident.
Some longtime pilots think the crash of Stewart's plane will prompt more commercial pilots to seek training in chambers that simulate conditions at high altitude.
Accidents are rare; but when they happen, the danger is immediate, extreme and often deadly.
If a cabin decompresses rapidly because of a blown door or ruptured fuselage, air inside the cabin expands and rushes through the opening, sucking loose objects or people along with it, said Rogers Shaw, who runs pressure-chamber simulations for the Federal Aviation Administration's Civil Aeromedical Institute in Oklahoma City.
Nine passengers were blown out of a United Airlines flight near Hawaii in 1989 when a cargo door blew off and ripped into the passenger compartment. The plane lost pressure at 22,000 feet.
The risk isn't over for those who remain on the plane, aeromedical experts say. Air also expands inside passengers' lungs, blowing out of their mouths and leaving them feeling as if they have just exhaled vigorously. Depending on the altitude, they could have little or no chance of getting oxygen back in.
Humans can't breathe normally at high altitude. The air has the same 20 percent oxygen content it has on the ground, but the molecules are spread farther apart. And because of the reduced gravity and atmospheric pressure, the oxygen has a hard time bonding to the blood's hemoglobin for the trip to the brain.
Because blood cycles from the heart, through the lungs, to the brain and back in about eight seconds, people exposed to conditions above 35,000 feet could be unconscious before they realize what happened.
"All humans go unconscious almost instantly at 32,000 feet, even Olympic athletes," said Dr. Stanley R. Mohler, director of aerospace medicine at the Wright State University School of Medicine in Dayton, Ohio.
"Mountain climbers have made it up to about 29,000 feet, the top of Mount Everest, but they've done it gradually, and if they weren't using supplemental oxygen, then they weren't thinking clearly up there."
A gradual decompression is equally dangerous because it can go unnoticed.
Euphoria is a common effect of oxygen deprivation, so people might ignore other symptoms before it's too late.
Only military pilots are required to train in altitude chambers that let them experience what oxygen starvation, or hypoxia, feels like.
Symptoms include dizziness, nausea, hot and cold flashes, and a tingling sensation that feels like ants crawling on the skin, said Shaw.
"Some people get euphoric; some people get belligerent," he said. "It's like a free six-pack."
Brian Batty experienced hypoxia about five years ago during three training sessions that simulated cabin decompression at 22,000 feet. Within two to four minutes without an oxygen mask, his vision narrowed, he got hot and cold flashes, and his world stopped making sense.
Batty, a technical assistant at Aerosafe Inc., a consulting company near Columbus, Ohio, that specializes in plane accidents, was given small shapes that children play with and told to fit them in the proper boxes. One minute everything was fine; the next minute he was trying to fit a circle into a triangular hole.
"All of a sudden I couldn't find the shape I was looking for, and I knew I hadn't dropped it," he said. "I knew no one took it from me. I found myself trying every single piece in this hole."
Haydu said she, too, lost cognitive abilities in the altitude chamber.
"I apparently blacked out very quickly, probably in 10 to 15 seconds," she said. "But I kept functioning because the gals all said I was very funny in the things I was doing. I apparently was argumentative with the instructor."
Without understanding the symptoms, Batty said, someone suffering hypoxia "could look at your plane heading for a mountain and say: 'Look, here's the mountain. I wonder what's going to happen?' That's the state of mind you're in."
Pilots are taught to recognize symptoms of hypoxia long before they get to this stage and to watch for them as soon as they reach 12,500 feet.
Mohler is among those who think the Federal Aviation Administration should require commercial pilots to train in altitude chambers, not just receive classroom instruction.
"It's sort of like reading a book on swimming," he said. "Until they throw you in, you're not going to know what you're doing."
Jerry Allison, a corporate pilot for 20 years, said the crash of Stewart's plane will probably draw more pilots into altitude chamber training.
"There are a lot of people like me who have intended to go through the chamber over the years and never had the time to do it," he said. "I'm sure the lines will be long for doing the chamber ride."
Cabin-pressure problems are rare in commercial aircraft, and the FAA requires airlines to provide emergency oxygen in case they occur.
Flight attendants show passengers how to use the oxygen masks before every flight. Few people have seen them used.
The systems work, experts say. Once the masks are deployed, hypoxia can be eliminated with four or five deep breaths of oxygen. Pilots are trained to quickly descend below 10,000 feet if a cabin loses pressure.
It is an experience that few passengers would want to endure, Shaw said.
Because the foremost concern is that planes land safely after a decompression, the FAA's guidelines are designed mostly to help the pilots. In extreme circumstances, federal guidelines say, passengers may be exposed to conditions at 25,000 feet for as long as two minutes, nearly the limit of a person's "time of useful consciousness."
In extreme circumstances, such as when a cabin depressurizes, the terrain is mountainous and oxygen reserves are low, airlines may legally fly passengers unpressurized at 15,000 feet. That won't cause medical damage, Mohler said, but half of the passengers might pass out.
"If you're [exposed] above 10,000 feet, you're losing your cognitive abilities, and you probably don't know it," said Shaw.
"Jets are safe," he added. "Cabin compression is safe. But they're creating an environment that is artificial. It can fail."