What Runway Numbers Mean: Headings, Letters, Arrows

The large number painted at the end of a runway is its direction on a compass, rounded to the nearest 10 degrees with the last zero dropped. A runway pointing at about 183 degrees is Runway 18, spoken "one-eight". The number tells a pilot which way they will be heading as they land or take off, and it is the reason every runway has two names, one at each end.

How the number is worked out

Runways are named from their magnetic heading, not true north. A magnetic compass points toward the magnetic pole, and pilots navigate with magnetic headings, so the markings match what is on the cockpit instruments. Take the heading from the direction of approach, round it to the nearest 10 degrees and divide by 10. According to Wikipedia, 09 points east, 18 south, 27 west and 36 north.

A runway has two ends, and each end faces the opposite way. The two numbers differ by 18, because 180 degrees is half a turn. Runway 15 at one end is Runway 33 at the other. The AOPA (Aircraft Owners and Pilots Association) notes that the leading zero is dropped for runways 1 to 9 in the United States; Wikipedia adds that airports following the ICAO (International Civil Aviation Organization) style keep it, so the same runway is written 09 there.

Letters for parallel runways

Large airports often build runways side by side. They would have the same number, so a letter is added: L for left, R for right and C for centre, as in 15L, 15C and 15R. Seen from the other end, the sides swap, so Runway 03L is Runway 21R when used in the opposite direction.

With four or more parallel runways, the numbers are shifted by one so that no two runways in the group share a designation. Wikipedia gives Los Angeles as an example, with 6L, 6R, 7L and 7R, and Dallas/Fort Worth with 17L, 17C, 17R, 18L and 18R.

Why runway numbers get repainted

Magnetic north is not fixed, so a runway's magnetic heading slowly changes even though the runway does not move. When the rounded value changes, the runway is renamed. Wikipedia says this happens more often at high latitudes: Fairbanks renames its runways roughly every 24 years, most recently in 2009. A runway at 226 degrees is Runway 23 until its heading drifts to 224 degrees, when it becomes 22.

Renaming means updating charts and documents, so it is usually done at night and only when needed. Northern Canada avoids the issue by using true north for designations.

The other markings you can see

The number is only one of several markings. The AOPA describes the main ones for runways used with instrument approaches:

• Threshold stripes: bold stripes at the very start of the runway. The number of stripes shows the width, from four for a 60-foot runway to sixteen for a 200-foot runway.
• Touchdown zone marks: sets of three short parallel stripes at 500-foot steps on precision runways, which help a pilot judge how much runway has been used.
• Aiming point marks: two wide white rectangles about 1,000 feet from the threshold, where the approach is aimed.
• A large X: the runway is closed.

Displaced thresholds

Sometimes the landing area starts partway along the pavement. This is called a displaced threshold. White arrows along the centreline point toward the point where landing may begin, and a 10-foot-wide bar crosses the runway at that point. The section before it can be used for taxiing, take-off and the roll after landing, but not for touching down. Wikipedia lists obstacles, pavement strength and noise limits as common reasons.

How long are runways?

Runway sizes vary widely. According to Wikipedia, small general aviation strips can be as short as about 245 metres and 8 metres wide, while large international airports can have runways about 5,500 metres long and 80 metres wide. A runway of at least 1,800 metres usually suffices for aircraft under about 100,000 kilograms, and widebody airliners generally need at least 2,400 metres at sea level. The length a given aircraft needs depends on its weight, the weather and the altitude of the airport.

This is general information about markings, not guidance for flying. Pilots rely on official charts and training.

Sources

• Runway, Wikipedia
• Runway markings, AOPA Flight Training magazine