How do air traffic controllers keep planes apart?

Controllers keep every aircraft inside an imaginary protected zone, a minimum distance in height and in horizontal spacing, and they give pilots instructions that preserve it. This safe distance is called separation. The minimums come from regulators and depend on the type of airspace, the flight rules in use and the equipment available. [1] [2]

According to the [faa.gov](https://www.faa.gov/media/6151) material, controllers use vertical separation (different altitudes), longitudinal or in-trail separation (distance between a leading and following aircraft on the same route) and lateral separation (side by side on nearby routes). In the United States, below 41,000 feet aircraft must be at least 1,000 feet apart vertically unless lateral separation is assured. In radar-controlled areas, in-trail separation ranges from 2.5 to 10 nautical miles depending on aircraft class, weight and airspace. Lateral separation is at least three nautical miles in terminal airspace and five in en route airspace. [1]

Australia's standards are similar but not identical. [airservicesaustralia.com](https://www.airservicesaustralia.com/about-us/our-services/how-air-traffic-control-works/separation-standards/) says instrument-flight aircraft up to 29,000 feet need 1,000 feet vertically, rising to 2,000 feet above that except where reduced vertical separation minima apply. Horizontal minimums are 5 nautical miles en route and 3 in terminal areas. [3]

The FAA lists several techniques: vectors (headings), speed control, holding patterns or ground holds before departure, altitude assignments that may make aircraft level off during climb or descent, reroutes, and point-outs, which warn another controller that an aircraft may enter their airspace. As an aircraft travels, control passes from one controller and facility to the next, so the work is done by a team. [1]

Controllers also differ in how they see traffic. Tower controllers watch aircraft from the window or with field glasses. When aircraft are too far away or hidden, radar controllers in a terminal radar approach control room follow them on radarscopes showing position, ground speed and other details. [2]

Aircraft fly under instrument flight rules or visual flight rules. Under visual rules, pilots must see and avoid other aircraft and terrain. Under instrument rules, operators file flight plans and navigate by instruments, and most commercial traffic flies this way. Near major airports, controllers separate visual and instrument flights by standards like those between two instrument flights, though a reduced 500-foot vertical gap may be used in certain circumstances in Australia. Outside controlled airspace, visual pilots rely on scanning, radio calls and give-way rules. Where radar or satellite coverage is absent, controllers use procedures based on time and estimated position. [1] [3]

A loss of separation, also called a mid-air conflict, occurs when two aircraft come within the minimum horizontal and vertical distances at once. Controllers, aided by alarms and support tools, spot likely conflicts and advise climbs, descents, turns or speed changes. If aircraft are already in conflict, further layers such as the onboard collision-avoidance system, TCAS, activate. [4]