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Single Physical Runways
A runway designator consists of a two-digit number, which is the whole number nearest to one tenth of the magnetic North when viewed from the direction of approach. For example, if the azimuth of the centreline is 153 then the runway designator will be 15. When this rule results in a single digit number then the designator is preceded by a zero (e.g. if the runway cenreline azimuth is 82, then the designator will be 08). North-oriented runways are designated 36 (not 00). In simple words, the runway designator represents the heading used for taking off or landing at the runway.
If the runway is used in both directions, then each of them receives its own designator. This means that although there is one physical surface used for take-offs and landings, it is treated as two runways. The difference between the numbers is 18, e.g. if one of the runways is 12 then the other will be 30.
On parallel runways the number is supplemented with a letter which defines the relative position of each of them. The possible letters are L (left), R (right) and C (centre), in line with the point of view of the observer standing at the runway threshold. For example, two parallel runways (East direction) would be designated as 09L and 09R. If the runways were three, then the designations would be 09L, 09C and 09R. Note that the opposite ends (27 in this case) would also include the observer's view point, which means that the opposite direction of 09L would be 27R and vice versa.
If more than 3 parallel runways exist, then some of the runways are named to the nearest one-tenth magnetic azimuth and the other adjacent runways are designated to the next one-tenth. For example, a group of four parallel runways (facing North) can be designated as e.g. 36L, 36R, 35L and 35R.
In the case of parallel runways, each runway designation number shall be supplemented by a letter as follows, in the order shown from left to right when viewed from the direction of approach: — for two parallel runways: “L” “R”; — for three parallel runways: “L” “C” “R”; — for four parallel runways: “L” “R” “L” “R”; — for five parallel runways: “L” “C” “R” “L” “R” or “L” “R” “L” “C” “R”; and — for six parallel runways: “L” “C” “R” “L” “C” “R”
Runway Designator Marking
A runway designation marking is to be provided at the thresholds of a paved runway and it is recommended that such marking is also provided, as far as practicable, at the thresholds of unpaved runways.
Due to the extreme importance of unambiguous runway identification, the size, shape and spacing between the runway designator numbers and letters are specified in ICAO Annex 14 (Aerodromes). If the numbers are incorporated in the threshold marking, larger dimensions are to be used in order to adequately fill the gap between the stripes of the threshold marking.
For a single runway, the designator is usually placed after the threshold marking but may be incorporated within it.
For parallel runways, the letter (L, R or C) is placed after the threshold marking and the number (e.g. 11) is placed after the letter.
- ICAO Annex 14 - Aerodromes