In the Zone
Cotton’s vision, which he refined over the years, was built around the concept of safety “zones.” Each plane would maintain two electronic surveillance zones: an inner “protected zone” around itself, nestled in a larger “alert zone” spreading out in front. To keep the protected zone inviolate, any overlap of alert zones would send a warning, prompting course corrections and restrictions.
An early step toward implementing the concept of Cotton’s zones was the Traffic Alert and Collision Avoidance System (TCAS), which grew out of collision-avoidance logic developed by Bendix in the 1950s. Required on all U.S. passenger aircraft since 1993, TCAS sends radio signals that, when returned by other planes’ transponders, inform the system of those planes’ approximate bearings and altitudes. It scans these data for prospective collisions and advises the pilot to climb, descend or stay steady.
Subsequently modified to prevent false alarms, TCAS has proved to be a lifesaving backup to traffic control and navigation. It has also been seen as a possible tool for realizing the tantalizing notion of free flight.
Indeed, it was TCAS that brought two of the key players in today’s free-flight debate together-before driving them apart. United pilot R. Michael Baiada had worked on navigation technologies as an engineer at Bendix and on proto-free-flight efforts as a manager at a small airline. In 1987 he signed on at United under Cotton to help test TCAS.