How runway numbers work

Every airport page on this site lists runways with names like 09L/27R or 16/34. They look like codes, but there’s no secret to them. A runway’s number is the direction it points.

The number is a compass heading

Take the compass direction of the runway in degrees, round it to the nearest ten and drop the last zero. That’s the number.

• A runway pointing due east (090°) is runway 09.
• Due south (180°) is runway 18.
• Due west (270°) is runway 27.
• Due north (360°) is runway 36. There is no runway 00.

So when a controller says “runway two seven”, an aircraft using it is travelling west. That’s all it means.

Why every runway has two numbers

A strip of tarmac can be used in either direction, and each direction gets its own name. The two always point opposite ways, 180 degrees apart, so the numbers always differ by 18.

That’s why runways are written as pairs: 09/27, 16/34, 05/23. Heathrow’s 09/27 is one piece of concrete. Aircraft landing towards the west are on 27, and aircraft landing towards the east are on 09.

If you know one end, add or subtract 18 to get the other. It’s a handy check when you’re trying to work out which way traffic is flowing on a camera.

Left, right and centre

Big airports have parallel runways, which all point the same way and would all get the same number. A letter separates them: L for left, R for right, and C for centre if there are three. Left and right are from the pilot’s seat, looking down the runway.

That’s where the confusing-looking names come from. Heathrow’s northern runway is 27R if you’re landing westward and 09L if you’re landing eastward. It’s the right-hand runway from one direction and the left-hand one from the other, so the letter flips along with the number.

What happens with four or five parallel runways

There are only three letters, so airports with more parallel runways cheat. They nudge some of the numbers by one, even though the runways point the same way.

Los Angeles has four parallel runways. The northern pair are 24L and 24R, and the southern pair are 25L and 25R. All four point in the same direction. The different numbers are only there to tell the pairs apart, and they’re the reason LAX has two separate live streams, one per pair.

Dallas Fort Worth does the same with five north–south runways: 17L, 17C and 17R on one side of the terminals, 18L and 18R on the other.

Aircraft land and take off into the wind

This is the part that matters when you’re watching. Aircraft want to take off and land facing into the wind, because it gives the wings more lift at a lower ground speed and shortens the distance they need.

So the wind decides which end of the runway is in use. A west wind means runway 27. If it swings round to the east, the airport switches to runway 09, and everything reverses: arrivals now come from the other side, and a camera that had a perfect view of landings may suddenly be showing only departures, or nothing.

When a stream goes quiet for no obvious reason, this is usually why. It’s also why some cameras are only good in certain winds. The Maho Beach cams depend on arrivals using runway 10, which the steady trade winds provide nearly every day. On the rare day the wind turns, the beach gets no low landings at all.

When the wind comes from the side

The wind rarely lines up exactly. When it blows across the runway, pilots point the nose into it on the way down and fly the approach slightly sideways, then straighten up as they touch down. That’s a crosswind landing, and it’s the reason clips from Wellington and Madeira get shared so widely. Both have a single runway, so pilots have to take whatever the wind is doing.

Airports with room build runways in more than one direction to avoid this. Amsterdam Schiphol has six, pointing four different ways, and picks whichever combination suits the day.

Runway numbers change

Compass headings are measured from magnetic north, and magnetic north moves a little every year. Over decades a runway’s heading can drift past a rounding point, and the airport has to rename it and repaint the numbers. A runway that was 18 for fifty years becomes 19 one night. It doesn’t happen often, but when you see an old photo with the wrong number on the tarmac, that’s the explanation.

Using this on a live cam

Three things to try the next time you open a stream:

1. Check the runway list on the airport’s page and work out which way each one points.
2. Note which numbers the stream title or the controller is using today. That tells you the direction of traffic.
3. If arrivals stop, look at the other end of the runway, or try another camera at the same airport.

For the phrases you’ll hear alongside runway numbers, see how to follow ATC.