Self-Control in the Skies; January 2000; Scientific American Magazine; by Scott; 2 Page(s)

It shouldn't have happened. This past summer over China, two Boeing 747s were flying toward each another along the same airway but separated vertically by a safe and wholesome 2,000 feet. As the jets drew within spitting distance, however, the Traffic Alert and Collision Avoidance System (TCAS) installed in the lower jet began its auditory warning to the pilot, "Climb! Climb!" And so the pilot did-to within just a few hundred feet of the oncoming airliner's belly. Only by that narrow margin did 422 passengers and crew miss going down as the worst midair collision in history.

Some 3,000 TCAS units are on passenger and cargo aircraft. Developed in the 1980s in response to several midair collisions, TCAS works by interrogating the altitude-reporting radar transponder of other aircraft, displaying the altitude of traffic nearby and issuing an auditory warning. There's no doubt that it works: TCAS has reduced the incidence of near-collisions in the U.S. from 20 annually to four, and so far engineers believe the erroneous climb warning in the China incident to be a rare anomaly. Still, common versions of TCAS are most accurate within 14 miles-less than a minute's notice for airliners on a head-on trajectory.