Traffic Collision Avoidance System

The traffic collision avoidance system, or TCAS, also known internationally as the airborne collision avoidance system, is an aircraft mounted system built to reduce the risk of mid air collisions, and it works independently of air traffic control. It interrogates the transponders of nearby aircraft on a frequency of 1030 megahertz and listens for their replies on 1090 megahertz, using the range, altitude and bearing of each reply to build a three dimensional picture of surrounding traffic, from which it issues a traffic advisory to alert the crew or, where a real risk of collision exists, a resolution advisory that tells the crew to climb or descend by a specific amount. Research into airborne collision avoidance dates to the 1950s, but development accelerated after the 1956 Grand Canyon mid air collision spurred the International Civil Aviation Organization and the United States Federal Aviation Administration into action; the FAA decided to implement TCAS in 1981 and certified the TCAS II version in April 1986. TCAS is now mandated internationally on aircraft weighing more than 5,700 kilograms or carrying more than nineteen passengers.