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Stewart’s Plane Said Depressurized

October 25, 1999

WASHINGTON (AP) _ Everything known about the crash of Payne Stewart’s jet pointed to a sudden loss of pressurization in the ``very hostile″ environment of high-altitude flight.

Experienced pilots say such an occurrence is extremely rare for civilian aircraft _ and extremely unforgiving.

It could have been brought on by a blown door or window seal. Experts consider that the most likely because the two pilots and three passengers aboard Stewart’s Lear 35 did not broadcast a mayday before the plane crashed in South Dakota on Monday. All aboard died.

Controllers last talked to the pilots when they confirmed an instruction to climb to 39,000 feet as the plane flew from Orlando, Fla., to Love Field in Dallas.

If pressure was lost at that altitude, everyone aboard would have been incapacitated almost immediately. Autopsies will tell if they died before the crash or from the impact.

Another possibility is smoke or some other form of contamination in the cabin, but experts considered that much less likely because the chance of someone noticing and taking some form of corrective action would have been greater.

``If you are in the cabin of a Learjet, you are in a very small pressure vessel, quite different than a DC-10, 757 or a large passenger jet,″ said John Nance, a veteran airline captain and aviation analyst.

``Almost certainly something blew out. It could have been a window, a door seal or a duct seal. Whatever it was, it doesn’t take much to empty the cabin (of oxygen) of a Lear because it’s a very small cabin,″ he added.

The human body has limited ability to function above 10,000 feet. As the altitude increases, the air thins, and two things happen: There is less oxygen in the air, and there is less pressure to force that oxygen through the lungs and into the blood stream.

Airplane designers compensate by pressurizing the cabin area. Air enters the engines from outside and some of it is drawn off, heated, compressed and fed into the cabin. Valves at the back of the aircraft open and close to maintain a consistent pressure.

Normally, the atmosphere inside a plane never feels any higher than 8,000 feet, despite the actual height of the aircraft.

If a plane loses pressure, a warning light goes on in the cockpit and oxygen masks drop from the ceiling. People use them _ if they are able.

In aviation circles, there is a term known as ``time of useful consciousness.″ It is the measure of the time the body can cope without oxygen, and it diminishes almost quickly with altitude.

At 20,000 feet, the time is 10 minutes. At 26,000 feet, it is two minutes. At 30,000 feet, it is 30 seconds. At 40,000 feet, it is 15 seconds.

FAA officials said Stewart’s plane ultimately climbed as high as 45,000 feet during its wayward flight across the nation’s heartland, most probably on autopilot.

Jim Brennan, a former Navy fighter pilot and a retired Boeing 747 captain, said that if the cabin lost proper pressure, the people inside would have suffered from hypoxia, decreased oxygen in the blood, or anoxia, a lack of oxygen in the blood.

``If there was a major depressurization, the pressure would go down instantly,″ Brennan said. ``Unless you had an oxygen mask handy, you couldn’t do much about it.″

He said even a more gradual leak could have fatal consequences.

``Someone might have noticed that someone was not operating normally, but by that time, they might have not been operating normally themselves,″ said Brennan.

Nance, who was preparing to fly his own private plane on Monday, said most people never realize that they fly in such a precarious balance.

Inside a plane, they can watch a movie, read a book or curl up with a blanket. Just outside a plane’s aluminum skin, the temperature is about 50 degrees below zero and the air too thin to support life.

``It is a very hostile environment and you don’t need to worry about it because everything works fine 99 percent of the time,″ he said. ``When something goes wrong, you have a very short time to do something about it.″

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