How runway numbers work

The big white number painted at the end of a runway is not a serial number. It is a direction.

The number is a compass heading

A compass has 360 degrees: 90 is east, 180 south, 270 west and 360 north. A runway's number is the direction it points, rounded to the nearest ten degrees, with the last zero dropped. A runway pointing 90 degrees, due east, is runway 09. One pointing 270 degrees, due west, is runway 27.

One strip, two numbers

A runway can be used in both directions, so each end has its own number, and the two always differ by 18 (that is, 180 degrees). The same strip of concrete is runway 09 when you land heading east and runway 27 when you land heading west. That is why it is written 09/27.

Left, right and centre

Big airports have runways side by side, pointing the same way. Letters tell them apart: L for left, R for right and C for centre, as the pilot sees them. Delhi has 11R/29L and 11L/29R among its runways. Note that the letters swap at the other end: the left runway going one way is the right runway coming back.

Why the direction matters

Aircraft take off and land into the wind, because wind over the wings gives extra lift and shortens the run. Airports are built with runways lined up with the usual wind, and the control tower picks the end in use according to the wind of the hour.

Numbers can change

Runway numbers use magnetic north, which is what a compass shows, and magnetic north slowly drifts. Every few decades an airport has to repaint a runway with a new number, although the runway has not moved.

Length and surface

The other things listed for a runway are its length, its width and its surface. Big jets need long, hard runways of asphalt or concrete. Small airfields may have grass or gravel. High airports and hot places need longer runways, because thin or warm air gives less lift. See the longest runways.