CFI Brief: Runway Designation

“N123AS cleared to land runway one zero right”. Have you ever wondered how a runway gets its number and letter designation, like 10R (One Zero Right)? The answer is actually simple and serves as a safety feature for departing and landing traffic. The runway number is the whole number nearest one-tenth the magnetic azimuth of the centerline of the runway, measured clockwise from magnetic north (AIM 2-3-3).

That statement can be a bit confusing so let’s break it down into simpler terms. If you were to stand at the beginning of the runway and point a compass directly down the runway centerline you’re going to come up with a magnetic direction, for example 242°.This magnetic direction is rounded to the nearest one-tenth (240°) and the zero omitted (24). So in this particular example, a runway with a magnetic direction of 242° would be assigned the runway numbers 24. The same is done if standing on the opposite end of the runway, facing the other direction. Or you could simply find the reciprocal, since most runways are straight: just subtract or add 180.

240° – 180° = 60° (24 – 18 = 6)

Two runways will often share the same pavement so in the case of our example we would have Runway 6 in one direction and Runway 24 in the other direction (6 – 24). Looking at the figure below you can see Runway 18 aligned north to south and at the other end Runway 36 aligned south to north. If you were to make a straight out departure from runway 36 you would maintain a magnetic heading of 360° or due north.


Larger airports will often have parallel runways, sometimes up to three runways, all aligned with the same magnetic azimuth resulting in the same runway number (i.e. 35). For situations like this we use Left (L), Right (R), and Center (C) to differentiate between each runway. In the case of two parallel runways you would have 35L and 35R. For three parallel runways you would have runways 35L, 35C, and 35R. This is shown in the figure below depicting two of three parallel runways; note that the actual letter will always be painted beneath the runway number.
Parellel Runways

From the figure below what would be your best guess as to the runway designation painted on the opposite end of the runway? Think about it for a minute and don’t answer to quick! Answer posted in the comments section.


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