The survival of everyone aboard the plane that landed in the Hudson River might seem like a miracle.
But planes are designed to survive water landings, and a skilled crew can use those design elements to keep a ditched aircraft afloat and the passengers safe, according to Bill Waldock, a professor of safety science at Embry-Riddle Aeronautical University in Prescott, Ariz.
"You've heard of airworthiness," he said. "Planes are also designed for crashworthiness."
Waldock, who is also a pilot and an aircraft accident investigator, said planes, like ships, will float "as long as you don't let the plane get full of water. It's the buoyancy provided by the air in the plane."
In this case, the plane that went down Thursday was an Airbus 320, which has a low wing. This allowed most of the fuselage to remain above water, contributing to the aircraft's buoyancy, he said.
Waldock said the escape slide that is sometimes used to evacuate passengers from planes on tarmacs can also double as flotation devices for aircraft, as it did Thursday.
Crew's unusual skill
But he emphasized the skill displayed by the crew in that operation, starting with the pilot.
"He put the tail in the water and gradually slowed the airplane down as much as possible," Waldock said. "You're still going to get a jolt when it slows down enough, but if you do it right, and let the tail hit first, the tail will absorb some of the energy of the impact and bleed it out."
He said a water landing is by no means intrinsically soft. "If you've ever done a belly flop off a diving board, you know water is as hard as concrete. If you hit it wrong, it's an incompressible force."
After the pilot eased the plane into the river, Waldock said, "the flight attendant side of things came into play. You cannot open the cabin door. If you do, the airplane will sink quickly because it fills with water. Your procedures after a ditching are to use the overwing exits and evacuate the passengers out on the wing."
He added: "Initially people may have been panicked, but the flight attendants got control of the cabin quickly."
Waldock said he could not think of another situation where everyone aboard a large plane that ended up in the water survived.
Other factors he cited were the relatively calm waters of the river, compared to an ocean landing, and the quick response of ferries and other vessels that arrived to remove the passengers.
"This is a tremendously unusual event," he added. "Normally when you put a large transport plane in the water, most of the time they do not have a good outcome."
Other major accidents in which planes ended up in the water included two US Air flights taking off from LaGuardia Airport that ended up in Flushing Bay, one in 1992 in which 27 people died and wing ice was the cause; and another in 1989 in which two people died when the plane ran off the runway.
Boston was also the site of two water accidents involving airplanes. In 1982, a DC 10 slid off a runway into Logan International Airport, killing two, and in 1960, 62 people died when a plane took off from Logan and crashed into the water after starlings damaged the engines. Ten survived.