Runway Heading
Why the Number at the End of the Runway Is More Useful Than It Looks
There may be no larger numbers routinely painted on the ground than the ones at the ends of airport runways. They are designed to be read from the air, which explains their size, but not what they mean. Runway 27 is not the twenty-seventh runway at the airport, and Runway 9 did not necessarily come before it. The numbers are not addresses, construction sequences, or identifiers assigned from a list; they are magnetic directions, rounded to the nearest ten degrees and with the final zero removed.
A runway aligned approximately 270 degrees magnetic is therefore Runway 27, while one aligned approximately 90 degrees is Runway 9. Those two numbers appear at opposite ends of the same strip of pavement.
The Federal Aviation Administration formalizes the system by assigning the whole number nearest one-tenth of the magnetic azimuth of the runway centerline. Because the two ends of a runway point in opposite directions, their designations differ by 18: Runway 9 becomes Runway 27 when approached from the opposite direction, just as Runway 18 becomes Runway 36. An airplane lined up on Runway 18 should therefore be pointing approximately 180 degrees magnetic; on Runway 36, approximately 360 degrees. If the magnetic azimuth falls exactly between two designations—185 degrees, for example—the FAA allows either one. When an airport has two or three parallel runways with the same magnetic designation, letters distinguish them from one another: L for left, C for center, and R for right. Parallel runways aligned to magnetic heading 270, for instance, would therefore be designated 27L and 27R—or, with three parallel runways, 27L, 27C, and 27R.
Once you know the system, it becomes surprisingly difficult not to notice when someone gets it wrong. Near the end of Catch Me If You Can, Leonardo DiCaprio’s character Frank Abagnale looks out the window as he is being returned to the United States and identifies LaGuardia: “That’s LaGuardia right there. Runway 4-4.”
Runway 44, however, cannot exist. The compass contains 360 degrees, so the runway numbering system reaches its upper limit at 36. Keep turning beyond 360 degrees and you have not discovered Runway 44; you have simply started around the compass again.
A Compass Written in Concrete
The convention emerged as aviation moved from open flying fields, where an airplane could often take off or land in whichever direction the wind favored, toward airports organized around defined runways. Defining particular landing directions created a problem that an open field did not have: the wind could change even though the available runway alignment could not. Airport designers responded in part by providing multiple runways on different alignments. Triangular layouts, for example, gave pilots a choice of directions and could place a runway within 30 degrees of the wind.³ As runways became permanent, identifiable parts of an airport rather than simply directions across an open field, distinguishing one from another became increasingly important. The magnetic numbering system ultimately provided an especially useful solution because the runway’s designation could also describe the direction it pointed.
The numbering system also connected the airport outside the airplane with the instruments inside it. A pilot lining up on Runway 22 expects the airplane’s magnetic heading to be somewhere around 220 degrees. The designation therefore does more than identify the runway; it provides another piece of information against which a pilot can check the airplane’s position and orientation.
The correspondence between runway number and cockpit heading provides a simple but important safety cross-check. In 2006, the consequences of failing to recognize when the two did not agree became tragically clear.
On the morning of August 27, 2006, the crew of Comair Flight 5191 was cleared to depart Blue Grass Airport in Lexington, Kentucky, from Runway 22. Instead, the CRJ-100 taxied onto Runway 26. Runway 22 was 7,003 feet long; Runway 26, a general-aviation runway not intended for an airliner departure of this kind, was only 3,501 feet long. The aircraft ran beyond its end and crashed, killing 49 of the 50 people aboard.
The numbers matter here in a way that goes beyond nomenclature. The crew had set their heading bugs to 227 degrees, corresponding to Runway 22, but as takeoff power was being applied, the airplane was actually pointing approximately 266 degrees—the heading of Runway 26. The NTSB noted that the resulting forty-degree disagreement provided a conspicuous cue that the aircraft was not lined up on the intended runway.
The Safety Board ultimately determined that the crew failed to use the available cues and aids to identify their position and failed to cross-check that they were on the correct runway before takeoff. The report also discussed the longstanding safety value of comparing an aircraft’s heading indication with runway heading when lining up. The clearance identifies the runway. Signs and pavement markings identify it. The airport diagram shows where it is. And once the airplane turns onto the runway, its heading provides another independent opportunity to verify that the aircraft is pointing where the crew believes it is.
Is Magnetic True?
There is, however, a complication hidden inside this otherwise logical system: although the runway remains fixed in place, the magnetic reference from which its orientation is measured changes over time.
True north is geographical, defined by Earth’s rotational axis and the geographic North Pole. A runway constructed on a fixed geographic alignment therefore maintains essentially the same relationship to true north.
Magnetic north is different. A compass responds to the local horizontal component of Earth’s magnetic field, most of which is generated by the movement of electrically conducting material—primarily liquid iron and nickel—within the planet’s outer core. That field does not align perfectly with Earth’s rotational axis, and it changes over time. The angular difference between true north and magnetic north at any particular location is called magnetic variation. Variation differs from place to place and gradually changes with time.
Aviation has practical reasons for using both references. A report of wind from roughly 220 degrees and an assignment of Runway 22 immediately make sense together: both are referenced to magnetic north, and the runway points approximately into the reported wind. Not all aviation weather information, however, uses the same reference. Winds in a METAR are reported relative to true north, while the local winds pilots hear from the tower or ATIS are referenced to magnetic north. The distinction is purposeful. True north provides a fixed geographic reference that is useful for weather reporting and long-range navigation and flight planning, while magnetic north provides the local directional reference pilots use when operating an airplane. Aeronautical charts make the relationship visible: their geography is fixed to true north, while isogonic lines show the magnetic variation needed to translate between the two. At the airport, that translation has an immediate practical consequence—the heading inside the cockpit should agree approximately with the number painted on the runway outside it.
That means the magnetic direction associated with a fixed runway can change even though the runway itself has not. While the concrete has not rotated, the magnetic directional reference used to number it has changed.
When Runway 18 Becomes Runway 19
Because runway designations are based on ten-degree increments, the system has some tolerance: a runway does not need to be renamed whenever magnetic variation changes by a fraction of a degree. Nor is there a universal schedule requiring airports to renumber their runways every ten, twenty, or thirty years. How quickly a redesignation becomes necessary depends on the runway’s alignment, its location, and the rate and direction at which magnetic variation changes there. Magnetic variation does not change at the same rate or in the same direction everywhere, so one runway may retain its designation for decades while another reaches the threshold for renumbering sooner.
Over time, airports around the world therefore find themselves renumbering runways that have not physically moved at all. The change may be only a single digit—Runway 18 becoming Runway 19, for example—but implementing it involves considerably more than painting a new number on the pavement. Signs, charts, airport diagrams, navigation data, and associated instrument procedures all have to reflect the runway’s new designation. The FAA therefore coordinates runway-number changes well in advance so that the paint on the pavement, the information in the cockpit, and the procedures pilots fly all change together.
Scientists help make that possible by continually measuring and modeling Earth’s magnetic field. NOAA and its partners maintain the World Magnetic Model, used for navigation and updated as the magnetic field evolves. The same processes deep within Earth that interest geophysicists therefore have a remarkably practical consequence at an airport: even a runway that never moves cannot necessarily keep the same number forever.
The Number Is a Direction
Modern airplanes can determine their position with extraordinary precision. Satellite navigation, inertial systems, flight-management computers, moving airport maps, and runway-awareness technologies have transformed the cockpit since directional runway numbers first appeared, yet the enormous white numbers remain.
Their usefulness comes partly from their simplicity. They connect a physical object outside the windshield to a directional reference inside the airplane. Runway 27 does not merely tell a pilot what the runway is called. It makes a proposition that can be checked: if this is Runway 27, the airplane ought to be pointing approximately west.
The number therefore does not describe the pavement alone. It describes the magnetic direction in which an airplane should be pointing when it is aligned with that pavement. As Earth’s magnetic field changes, magnetic variation changes with it, and over time the compass heading associated with a fixed runway can change enough that its designation no longer provides the directional correspondence it was intended to provide.
That is when an airport has to change the number. The runway has not moved, and neither has true north. What has changed is the runway’s relationship to the local magnetic reference—and therefore the magnetic heading a pilot should see when lining up on it. Keeping the designation accurate ultimately means replacing one of the most permanent-looking markings in aviation. The concrete stays where it is, but the number has to keep up with the compass. And eventually, somebody has to get out the paint.