How are runways numbered?

Danny Beckett
  • How are runways numbered? Danny Beckett

    I'm not a professional pilot, just an enthusiast, so this might seem obvious to others; but I noticed that airports don't use sequential numbering for runways, starting from 1.

    Liverpool (for example) has runway 9/27; so how are these numbers assigned? Also, what do Left and Right signify?

  • Runway numbers are determined by their heading, e.g. for Liverpool, runway 9/27 is facing approx. magnetic heading 90° in one direction, and 270° in the opposite direction.

    If an airport has parallel runways, these would then be marked Left, Center and Right, e.g. 9L/9C/9R.

  • The information from the US AIM may be useful here:

    b. Runway Designators. Runway numbers and letters are determined from the approach direction. The runway number is the whole number nearest one‐tenth the magnetic azimuth of the centerline of the runway, measured clockwise from the magnetic north. The letters, differentiate between left (L), right (R), or center (C), parallel runways, as applicable:

    1. For two parallel runways “L” “R.”

    2. For three parallel runways “L” “C” “R.”

    As to why the numbers are 1, 9 or something else, that is determined by the local prevailing wind conditions among other considerations (terrain etc.). Aircraft always take off or land into the wind (as far as possible), so that is a key factor in how runways are positioned. NASA has a very good article on airport layouts that goes into more detail.

  • Runways are usually numbered according to their direction. Consider a plane flying toward the runway on final approach. Divide its magnetic heading by 10 and you'll usually get the runway number. Runway 35 will be a runway used for landings (and takeoffs) to the north, for example.

    Opposite ends of the same runway have different numbers, 18 (which represents 180 degrees) apart. A runway with 35 for landings to the north will have runway 17 for landings to the south. Even though these are the same strip of concrete, they are treated as separate runways by pilots and controllers.

    If there are two airports near one another with runways at the same angle, sometimes one of the airports will add or subtract one from the runway number to help planes differentiate between the airports.

    Occasionally a runway number will change when the magnetic declination angle changes across 5 degrees.

    Some runways in areas of large magnetic declination use true instead of magnetic headings for the runway numbers. This is not unusual in northern Canada and Greenland.

    When there is more than one parallel runway at an airport, L, R, or C may be appended to the runway number for Left, Right, or Center. These are based on the approach direction, so, for example, the runway 35L would be called 17R from the opposite direction.

Related questions and answers
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  • I'm not a professional pilot, just an enthusiast, so this might seem obvious to others; but I noticed that airports don't use sequential numbering for runways, starting from 1. Liverpool (for example) has runway 9/27; so how are these numbers assigned? Also, what do Left and Right signify?

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