An acronym for Traffic Alert and Collision Avoidance System, which keeps planes from colliding in midair. Basically, TCAS works like a miniature version of the transponder receivers used by air traffic control. An airplane equipped with TCAS picks up signals from every other plane within a certain distance (usually about 40 miles), and informs the pilot when they get too close to someone else.

Where ground stations use rotating antennae, TCAS systems use a small array of antennae. The TCAS computer is sensitive enough to use their slight separation for triangulation of transponder signals.

The basic TCAS I system, used on small planes, is pretty much limited to warning the pilot about other planes. TCAS II systems also have the capacity to communicate with each other, allowing them to inform both pilots to climb or dive in opposite directions. All commercial carriers in the U.S. are required to fit their planes with TCAS II equipment. TCAS III, currently under development as an upgrade to TCAS II, will allow planes to correct by turning as well.

In addition to its safety uses, TCAS has practical uses as well. Aircraft can use TCAS to maintain separation from each other at much higher resolutions than controllers can distinguish, making airways and instrument approaches more efficient. In conjunction with GPS systems, TCAS can keep controllers informed with an aircraft's coordinates even when it's outside the range of radar.

Pilots initially disliked TCAS, but now many see the system as a life saver and a tool to be used for more efficient air travel.