More than 44 million airplane flights through 19,500 U.S. airports will take place this year. At peak times, 5,000 planes are aloft in American airspace. Yet accidents are few, largely because of the air traffic control system and its alert controllers.

Those controllers confer by radio with pilots about which predefined paths to follow for departure, ascent, descent and landing. While cruising, pilots proceed along each leg of a prefiled navigation plan, advancing along established air routes. Controllers ensure that aircraft are safely spaced. Minimum separations near airports are three miles ahead and 1,000 feet vertically; during high-altitude cruising, the minimums are five miles and 2,000 feet. (This may change to 1,000 feet in 2005.) Controllers hand off supervision of a plane as it advances from one control region to the next and may permit pilots to temporarily go "off route" to avoid bad weather.

This article was originally published with the title Crowded Skies.

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