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How Do Planes Avoid Colliding?There's a little box that yells "Traffic! Traffic!"

By Daniel Engber
Posted Wednesday, Oct. 4, 2006, at 5:40 PM ET

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The Brazilian courts ordered the detention of two American pilots on Monday, a few days after the jet they were flying crashed into a passenger liner over the Amazon jungle. Interviews with the pilots may help explain how the accident occurred, even though both planes were outfitted with automatic collision-avoidance systems. What's a collision-avoidance system?

Cockpit hardware that warns pilots about approaching planes and tells them how to get out of the way. In the United States, every plane with more than 10 seats has to have a Traffic Alert and Collision Avoidance System, or TCAS. The system calculates how long it will take for another plane to get so close that it can't be avoided. A warning sounds when time starts to run out.

The TCAS displays a map in the cockpit of nearby planes. (Some systems use a dedicated monitor; some piggyback on other displays, like the one used for weather radar.) The pilots' own plane shows up on the display as something that looks a bit like an Eastern Orthodox cross, and unfilled diamonds represent nearby air traffic that poses no threat. When a plane is within 1,200 feet of altitude and 6 nautical miles of distance, it appears as a filled diamond, and if it gets much closer than that, it appears as a filled amber circle and sets off an audible alert: An automated voice says, "Traffic, traffic" over a speaker or headset.

(Other cockpit warning systems also deliver audible alerts. Since a dangerous situation might trigger an unintelligible cacophony of alerts, each system is assigned a priority. For example, the TCAS would get second-billing to a message from the Ground Proximity Warning System—the pilots would hear, "Terrain, terrain, pull up!" before, "Traffic, traffic.")

The system works by using equipment that most planes already have onboard. Any plane that communicates with the air traffic control system on the ground does so by means of transponders, which come in several varieties. Mode A transponders pick up broadcast signals from ground radar and send back a code that identifies the plane they're in. Mode C transponders send back more specific information, like the altitude of the plane. The fanciest equipment—Mode S—can set up dedicated, one-on-one links with the radar beacons and respond to specific requests for information.

If a plane has a TCAS installed, it can communicate with other planes just as the ground-based radar system does. The TCAS pings the other plane's transponder and gets information on its location and altitude. If both planes have TCASs and Mode S transponders, the systems can even coordinate plans for how to avoid a collision. The TCAS in one plane might say, "Climb, climb," while the one in the other says, "Descend, descend." (All American planes with more than 30 seats must be outfitted with this advanced system.)

Most planes didn't have any form of collision avoidance until the early 1990s. Researchers had started working on the problem in 1956, after a major in-air collision over the Grand Canyon. But the early versions were too expensive and required that new equipment be installed on every plane. Two more major collisions—in 1978 and 1986—led the government to develop the TCAS and mandate its use. Its major advantage over previous systems was that you didn't have to have one on every plane. As long as all the other planes had standard transponders, the system would work.

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Daniel Engber is an associate editor at Slate. He can be reached at .
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