Separation: how air traffic control keeps planes apart
The invisible boxes around every aircraft: how far apart planes must stay, sideways and vertically, and why some need more space than others.
The short answer
Controllers keep aircraft apart either vertically, usually by at least 1,000 feet, or sideways, usually by 3 or 5 nautical miles on radar depending on the airspace. Behind big aircraft the gap is bigger, because of the invisible turbulence they leave behind.
From the window, another aircraft can look close. To a controller, it’s in a different box. Here’s how the boxes work.
Two ways to keep apart
Two aircraft are separated if they’re far enough apart vertically or horizontally. One is enough; they don’t need both.
Vertically. The standard is 1,000 feet. Above FL410, where fewer aircraft have the equipment for the tighter rule, it’s 2,000 feet. See flight levels for how everyone agrees on their height.
Horizontally. On radar, the usual minimum is 5 nautical miles (about 9 km) in the en-route airspace between airports, and often 3 nautical miles closer to big airports, where radar updates faster. Over oceans, with no radar, the distances are much bigger.
Bigger aircraft need bigger gaps
Every wing leaves behind two spinning tubes of air called wake vortices. Behind a big, heavy aircraft they’re strong enough to roll a smaller one. So behind an Airbus A380 or a Boeing 777, the gap for a following smaller aircraft is larger, especially on approach and departure.
That’s one reason a runway can take fewer landings an hour when lots of heavy aircraft are arriving.
How we do it
- Levels: give two aircraft different flight levels.
- Headings: turn one of them, so their paths don’t meet.
- Speed: slow the one behind, speed up the one in front.
Controllers plan this minutes ahead, not seconds. Aircraft also carry a collision avoidance system (TCAS) as a last safety net.
Why it causes delays
Every runway, sector and route has a limit set by these gaps. When demand goes over it, someone waits: on the ground with a slot, or in the air in a hold.