If Airport X has a runway 30, and Airport Y’s top-numbered runway is 27, logic suggests that Airport A is the larger airport, right? Actually, the number of the runway has nothing to do with the total number of runways at an airport. Instead, it is a navigation aid for pilots, and while the number is painted on concrete, it is not set in stone; runway numbers can change. 

The number is tied to the magnet compass heading a plane follows if it is landing or taking off from that runway. Simply add a zero to the end of the runway designation, and that is the magnetic heading. All runways numbered 27, for example, follow a magnetic heading of 270 degrees — due west. A runway with number 13, on the other hand, leads 130 degrees — a heading of southeast. 

This helps pilots have a uniform way of identifying runways in airports with multiple landing options. It also helps clarify toward which end of a given runway the pilot should aim. Runway 27 goes westward, but depending on flying conditions, it may be necessary to land to the east. That same strip of concrete has another number painted at the other end. In this case that would be 9, indicating air traffic moving due east. 

The numbers are tied to the magnetic heading of the runway, not the true heading. This presents interesting challenges, since earth’s magnetic field is constantly in motion, and the location of its magnetic poles drifts around. 

When the magnetic heading shifts sufficiently in a given area, the FAA will renumber the runways. Oakland, California’s pilots, for example, were long accustomed to landing and departing from runway 27, but in 2016 runway was renumbered to 28, accounting for the drift of the magnetic poles and the compass headings in that area.