How Do You Find a Missing Plane?

With lots more planes.

An airbus jet

The Brazilian air force announced Tuesday that it may have found wreckage from Air France Flight 447, which disappeared en route from Rio de Janeiro to Paris this weekend. When a plane vanishes at sea, how do you go about finding it?

Send more planes. If air traffic controllers lose contact with a commercial jet and can’t re-establish a connection promptly, it’s time for an aerial search-and-rescue mission. Who, exactly, organizes that mission depends on which country has jurisdiction over the area where the plane is thought to have crashed. In this case, the missing plane probably went down in international waters, so both the French and Brazilian air forces have been pitching in.

Searching for an aircraft in the ocean is especially challenging because location gadgets that work on land may not work under water. For example, all modern commercial aircrafts come equipped with emergency locator transmitters, which send a radio signal to an international satellite system called Cospas-Sarsat. This device comes in handy if a plane crashes in a mountain range or in shallow water, but if one of these distress beacons sinks to the bottom of the ocean, it won’t work. Flight recorders do have sonar devices attached to them, which can transmit signals from roughly 14,000 feet underwater. But the devices can be detected only if the searchers are already in close proximity.

Without a trackable signal, the rescuers must first try to narrow the search area using whatever data are available—like the plane’s last known location, its speed at the time of the crash, its destination, and so forth. Depending on wind patterns and currents, the search area could be about 60 miles on each side of the relevant portion of the flight path. The rescuers then send aircraft (flying from about 1,000 to 2,000 feet above the water) and ships to look for oil slicks and any floating objects like seat cushions and jackets.

The details of a search vary somewhat based on circumstances. If a plane crashes close to the shore, the rescuers may deploy small, fast ships and helicopters. If a plane appears to have crashed out in the middle of nowhere, large ocean liners and fixed-wing planes that can travel long distances are used instead. A large service ship may function as the on-scene command center, and the captain onboard will coordinate the actions of smaller ships and planes, making sure they don’t survey the same regions. One common flight pattern for such missions is called the parallel search, which from a bird’s-eye view would look like a plow field—with multiple planes flying in parallel lines inside a rectangle-shaped area.

Since the plane went down out of radar range and GPS won’t work at 9,000 or 10,000 feet underwater, the searchers are heavily reliant on the fly-over-and-look technique—that is, staring out at the surface of the ocean and trying to find debris with the assistance of night-vision goggles and strobe lights when it’s dark out.

Ship owners often help out with search-and-rescue missions. Through a program called Amver, sponsored by the U.S. Coast Guard, merchant vessels regularly report their whereabouts, which are then logged into a computer. If a ship or plane sends a distress signal, search-and-rescue authorities can tap into the system to see whether there are any vessels in the vicinity that might be able to help out.

Although it’s rare for a commercial plane to go missing, this is hardly the first lengthy investigation. After Egypt Air Flight 990 crashed near Massachusetts in 250-foot deep water, it took searchers five months to pull the plane’s second engine from the ocean. And when Swissair Flight 111 crashed close to Nova Scotia in 180-foot deep water, it took nine days to find the cockpit voice recorder.

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Explainer thanksLes Dorr of the Federal Aviation Administration, Lt. Col. Jed Hudson of the Air Force Rescue Coordination Center, Ted Lopatkiewicz of the National Transportation Safety Board, Richard Schaefer of the Coast Guard Office of Search and Rescue, and Bill Waldock of Embry-Riddle Aeronautical University.