Why Runways Are Numbered the Way They Are

The Magnetic Compass and Runway Numbering

Every runway on earth is identified by a number derived from its magnetic heading — the direction the runway points relative to magnetic north, measured in degrees. To create the runway number, simply take the magnetic heading, round it to the nearest 10 degrees, and drop the final zero. A runway pointing 270 degrees (due west) becomes Runway 27. A runway pointing 360 degrees (due north) becomes Runway 36. A runway pointing 090 degrees (due east) is Runway 09 — the leading zero is always included to make the designation a two-digit number.

Every runway is actually two runways in one, used in opposite directions. Because aircraft almost always land and take off into the wind, the active direction reverses depending on which way the wind is blowing. The opposite end of any runway has a number exactly 18 higher or lower (representing 180 degrees difference in heading). Runway 27, used when the wind comes from the west, becomes Runway 09 from the other end — used when the wind comes from the east. Runway 14 becomes Runway 32 from the other direction. This elegant system means pilots and controllers always know both ends of a runway just from one number.

The numbering system was standardized by ICAO (International Civil Aviation Organization) and is used identically at every commercial airport in the world. Whether you're at a remote Alaskan airstrip or a major international hub, Runway 27 always points roughly westward. This universality is intentional — aviation demands that conventions be globally consistent so that pilots transitioning between countries, aircraft types, and airports operate within the same mental framework.

Magnetic vs. True North: Why Runways Occasionally Change Numbers

The earth's magnetic north pole is not the same as geographic (true) north, and more importantly, it moves. Magnetic north is currently located near Ellesmere Island in northern Canada, significantly displaced from the true North Pole, and it drifts at a rate of roughly 50 kilometers per year as changes in the earth's liquid outer core alter the planet's magnetic field. This drift means that the magnetic heading of a fixed runway changes over time — slowly, but measurably.

When a runway's magnetic heading shifts enough that rounding to the nearest 10 degrees produces a different number, the airport repaints the runway and updates all charts, publications, and systems accordingly. This is not a frequent event — it typically happens every few decades at mid-latitude airports — but it represents a significant operational undertaking. In 2011, Tampa International Airport (TPA) in Florida had to briefly close its main runway to repaint the numbers after the magnetic heading shifted from 18/36 to 19/01. The old numbers were still accurate enough for safety, but ICAO standards require renaming when the rounding changes.

Airports near the magnetic poles face more frequent renaming needs because magnetic variation changes more rapidly there. Airports in Canada, Greenland, and parts of Russia have updated runway numbers several times over the decades. Some very high-latitude airports use a modified system or reference true north instead of magnetic north to avoid constant renaming — these exceptions are documented in the Aeronautical Information Publication (AIP) for each country.

Parallel Runways: L, C, and R Suffixes

Many major airports have multiple parallel runways pointing in the same direction. Since they all share the same magnetic heading, they would all have the same number — an obvious problem when controllers need to specify which runway an aircraft should use. The solution is a letter suffix: L for Left, R for Right, and C for Center, assigned from the perspective of a pilot approaching to land. An airport with three parallel runways might have 28L, 28C, and 28R, each receiving or dispatching aircraft simultaneously.

Dallas/Fort Worth International Airport (DFW) has seven runways, including three on the east side and four on the west side. The east runways are designated 17L, 17C, and 17R (and their reciprocals 35L, 35C, 35R). The FAA limits the use of L/C/R to three parallel runways per direction. When airports like O'Hare (ORD) with its planned expansion or Dallas with its four-runway west side need more than three parallels in one direction, the standard solution is to adjust the magnetic heading by one degree — rounding one runway's number differently — so that two pairs can both use the L/R system without conflicting.

Chicago O'Hare uses this approach on its west side. Two pairs of close parallels are numbered 9L/9R and 10L/10R, even though all four runways point in nearly the same direction. By rounding slightly differently, O'Hare achieves four distinct designations without violating the L/C/R convention. This slight numerical fiction is perfectly legal under ICAO rules and is thoroughly documented in charts so pilots always know exactly which physical surface they are using.

Runway Markings: Reading the Surface

Runway numbers are painted in enormous white digits at the threshold — the beginning — of each runway direction. The numbers are 18 meters tall on most commercial runways, large enough to be read clearly from several hundred feet in the air on final approach. Accompanying the number is a centerline stripe running the entire length of the runway, plus threshold markings (parallel stripes indicating the beginning of the usable runway surface), aiming point markings (two rectangular bars 300 meters from the threshold showing where aircraft should touch down), and touchdown zone markings (additional stripes bracketing the first 900 meters of runway).

Displaced thresholds are an important runway marking variation. Sometimes the physical beginning of the paved surface cannot be used for landing — perhaps due to obstacles on the approach path, structural limits on the pavement, or noise abatement requirements. In these cases, the threshold is displaced further down the runway, marked by a row of white arrows pointing toward the actual threshold and painted chevrons on the pavement before it. Aircraft may roll through a displaced threshold area for takeoff or taxi but must land beyond it.

Runway condition codes (RwyCC) are communicated through NOTAMs (Notices to Air Missions) and updated frequently in winter operations. A runway covered in packed snow has a very different friction coefficient — and therefore different stopping distance — than a dry runway. Codes from 6 (dry) down to 0 (near frictionless) allow flight crews to calculate whether their aircraft can stop within the available runway length under current conditions. Some airports use automated friction-measuring vehicles called MUs (Mu meters) that continuously patrol runways in icing conditions and broadcast updated runway condition reports.

How Pilots Use Runway Numbers in Practice

For pilots, runway numbers are fundamental to situational awareness and cockpit communication. Before every flight, pilots receive an ATIS (Automatic Terminal Information Service) broadcast — a recorded weather and airport information update that includes the active runway. A typical ATIS message might state: "Expect ILS Runway 28 Left approach. Departure Runway 28 Right." Pilots read back runway assignments to controllers and confirm the number before entering any runway surface.

Runway confusion — entering the wrong runway — is a recognized safety hazard with dedicated training and mitigation procedures. Cockpit-mounted moving map displays now show runway designations in real time, and many large airports use stop bar lights (red lights across taxiway intersections with runways, controllable by the tower) to prevent inadvertent runway entry. Some airports have implemented Runway Status Lights (RWSL), an FAA-developed automatic system that illuminates red lights at runway entry points whenever the runway is active, without requiring any controller action.

The relationship between runway number and wind is intuitive once understood. If the wind is from 250 degrees and Runway 25 is available, that runway is almost perfectly aligned with the wind and will be the preferred active runway. A crosswind component — the portion of wind perpendicular to the runway — increases with the angle between wind direction and runway heading. Aircraft have published crosswind limits: a Cessna 172 is certified for crosswinds up to 15 knots, while a Boeing 747 can handle up to 33 knots of crosswind. When crosswinds exceed limits, controllers switch to a different runway even if that means accepting a tailwind component.