TWO WINGS AND A PRAYER: Flotation device helped

Planes can float, but the US Airways Airbus A320 that

crashed into the Hudson River yesterday had a better chance than most.

That's because it was equipped with a special device unique to Airbus

planes that increased the likelihood it would stay on top of the water.

The device, called a "ditching switch," effectively seals the plane by

closing valves and ventilation ports, a spokesman for the airline said.

Industry experts said the ditching switch is rarely invoked, as "it's not

as if anyone expects to ditch these planes," said Robert W. Mann, who owns a

Port Washington-based aviation consulting company.

With the valves and ports shut, "a float line" is created, Airbus

spokeswoman Mary Anne Greczyn said in an e-mail.

"The ditching switch closes all of the open ports in the bottom of the

fuselage," said Frank Ayers, chairman of the flight training department at

Embry Riddle Aeronautical University in Daytona Beach, Fla.

Mann said opening an emergency exit - which happened yesterday when the

plane was evacuated - is not a problem as long as the door is above the

waterline.

Peter R. Leffe, an aviation accident investigator in Malibu, Calif., noted

that because airplanes in general are pressurized, they have some ability to

float, providing they don't break up when they hit water.

"The airplane is a pressure vessel," he said. "It can float as long as

water doesn't leak in."

But airplanes can't float forever.

Water will eventually find a way to seep in, and the airplane's weight will

drive it to the bottom.

"They're not amphibious," Mann said. "But the general rule of thumb is,

when they're full of fuel, which is lighter than water, they will float, as

long as their structural integrity is intact."

The Airbus A320, along with the Boeing 737-800, is one of the two most

"ubiquitous aircraft" in American air travel, said Mann. It has an aluminum

hull and is "as garden variety as airplanes get," he said.

There are about 2,000 A320 Airbuses worldwide, 425 of which are in North

America, Greczyn said.

The planes, which debuted in 1988, typically seat 150 passengers and are

the most popular Airbuses, she said.