Airport runways: What do those big numbers mean?
September 10, 2015
September 10, 2015
An airfield design expert explains runway markings—and why the compass is important
I’ve been asked many times the meaning of the big painted numbers at the end of each airport runway. What do they mean? It's an interesting question, and one that's obviously important for pilots. First, a few factoids before I explain:
Every public use and military airfield on the face of the earth has (or should have) runway designations permanently painted at each runway end.
English is the universal language of aviation, and runway designations are displayed via the same numeric system (Arabic) as used in the United States and Canada.
Runways are aligned predominantly to take advantage of prevailing winds. Therefore, many large airports have a wide range of runway configurations to accommodate changes in seasonal wind directions.
Runway markings, including the runway designations, are always white in color.
Naval aircraft carriers do not have runway designations painted on the flight deck. The orientation of taking off and landing is specific to wind direction, which is constantly changing on the open seas, so the ship is turned into the wind.
So how are runways numbered? Sequentially, based on the number of runways at the airport? No. All runways are numbered based on the magnetic azimuth (compass bearing) in which a runway is oriented. There are 360 degrees on a compass rose. Runway numbers are determined by rounding the compass bearing of one runway end to the nearest 10 degrees and truncating the last digit, meaning runways are numbered from 1 to 36—as per the diagram below.
The opposite end of the runway always differs by 180 degrees, so it’s numbered 18 higher or lower. For example, Runway 9-27 is oriented east-west.
You might be thinking that the numbers on this diagram are backwards. On a handheld compass, south is 180 degrees (so 18 in runway terms) and west is 270 (27). But the “W” is numbered 9 because the runway number is connected to the direction the plane is traveling. So if you’re on runway 9, then you’re heading east (90 degrees on a compass). Runway 36 means you’re moving north, and so on.
Most runways can typically be used in either direction, depending on prevailing winds. Furthermore, each runway end is identified separately. Therefore, an aircraft taking off easterly on Runway 9-27 would be considered to be utilizing Runway 9 for departure.
Many large airports have parallel runways, which requires further designation of each runway. For example, Boston, Massachusetts’ Logan International Airport (pictured) has two pairs of parallel runways. One is Runway 4L-22R and the other 4R-22L. The ”L” and ”R” designate the relative position (left or right) of each runway respectively when approaching/facing its direction. A small number of airports have three parallel runways—the runway in the middle gets a “C” for center.
During airport operations, runway number designations are pronounced individually. For example, Runway 4L-22R would be pronounced by air traffic control as “Flight 123, you are cleared to land on Runway Four Left” or alternatively “…Two Two Right,” if cleared to land the opposite direction. This level of enunciation ensures clear communication and enhances safety.
So the next time you travel by air plane and the pilot announces that you’ll be taking off on runway 27, you'll know that you’re traveling due west.
Enjoy your flight!