The Airbus A-320 that made an emergency landing in the Hudson River on Thursday was equipped with a ditch switch on the aircraft's overhead panel (see the orange arrow above and to the right). In the event of a water landing, the pilot can manually activate the switch, which closes the outflow valve and avionic ventilation ports, which are the openings below the aircraft's float line. All commercial airliners are required to have a ditch switch according to Federal Aviation Administration regulations. It isn't yet known whether Chesley Sullenberger, the captain of U.S. Air flight 1549, or his first officer, Jeffrey Skiles, activated the ditch switch during yesterday's emergency landing.

In preparation for a ditch landing over water, a cockpit crew briefs the flight attendants and passengers to brace for impact. Then, the pilot shuts down the engines just before touching down. In a water landing, the pilot needs to keep the tail down to keep the plane from cartwheeling if the wings touch the water. It's not clear exactly which of these procedures were followed during Thursday's emergency landing. While flight crews review the procedures for ditch landings frequently, in practice, they're extremely rare. —Allen St. John

Correction: This article originally said, that in preparation for a ditch landing over water, the pilot raises flaps up to slow the plane. Spoilers are typically used to slow aircraft on landing while while flaps are placed in the downward position during takeoff and landing to increase the amount of lift produced by the wing so that the airplane does not stall at slower speeds.