How do air traffic controllers keep thousands of planes apart?

Air traffic controllers keep planes apart by watching them, talking to their pilots, and making sure each aircraft keeps a minimum amount of empty space around it. Air traffic control is a ground-based service that directs aircraft on the ground and through controlled airspace, mainly to prevent collisions and keep traffic flowing. Controllers monitor aircraft using radar and communicate with pilots by radio, and they enforce traffic separation rules.

The job is split by place. The airport tower handles the airport itself, from a tall windowed structure on the airport grounds: aircraft and vehicles on the taxiways and runways, and aircraft in the air nearby, generally within 5 to 10 nautical miles. Away from the airport, area control centres handle aircraft at high altitude, each covering a region of sky, and some also monitor flights over the ocean. In the sky, a standard pressure setting for altimeters gives every aircraft the same reference, so vertical clearance can be kept in cruise.

It began in 1920, when Croydon Airport near London became the first in the world to use air traffic control, from a wooden hut 15 feet high. In the United States, the 1956 Grand Canyon mid-air collision, which killed all 128 on board, led to the FAA taking responsibility for the air-traffic system.

Human limits matter. Pilots must obey controllers' instructions, but the pilot in command always retains final authority for the aircraft's safety. And errors tend to occur after intense activity, when controllers relax and overlook traffic. The deadliest accident in aviation history, on a runway at Tenerife in 1977, taught the industry the vital importance of using standard phraseology in radio communications.