Jazmin Li
Airport runways are sometimes renumbered due to a variety of reasons. The most common reason is to avoid confusion between runways at two airports that are close to each other and have runways at the same angle. Airports with four or more parallel runways may also shift runway identifiers by one to avoid ambiguity. Additionally, runway numbers are based on magnetic headings, and changes may occur due to magnetic shifts. Airports with unpredictable water currents may also use non-standard runway naming conventions to denote the lack of a designated landing direction.
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What You'll Learn
Magnetic shifts
The Earth's magnetic field is in a constant state of flux, moving approximately 37 miles (60 kilometres) every year. This movement is caused by the interaction of the Earth's magnetic field with the solar wind and a stream of charged particles from the sun. The motion of convection currents and the mixture of molten iron and nickel in the Earth's outer core also play a role. As a result of these movements, the magnetic north pole shifts, causing the magnetic declination to change.
Runway numbers are determined based on the runway's magnetic bearing, which indicates its position in relation to the Earth's magnetic pole. The numbers are calculated by taking the magnetic azimuth of the runway's centreline and rounding it to the nearest tenth. This number is then measured clockwise from magnetic north. For example, a runway pointing to 092° is rounded to 090°, resulting in a runway number of "09".
Due to the constant movement of the magnetic north pole, the magnetic bearing of runways can change over time. When this happens, the runway numbers may need to be updated to reflect the new bearing. This is done to ensure that pilots can easily identify the correct runway when approaching an airport. The process of updating runway numbers involves repainting the large numbers at each end of the runway and updating the information in various databases and side panels.
The decision to change runway numbers is based on international regulations and the World Magnetic Model (WMM), which measures the Earth's large-scale magnetic field. When the WMM indicates a declination change of more than 7 degrees, runway numbers are typically updated. Additionally, some runways in areas of large magnetic declination, such as northern Canada and Greenland, use true north headings instead of magnetic headings for their runway numbers.
Overall, the magnetic shifts caused by the Earth's dynamic magnetic field can have a significant impact on aviation. Airports must remain vigilant and adapt their runway numbering systems to ensure safe and efficient operations for pilots and air traffic control.
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Differentiation from nearby runways
Runway numbers are determined by the runway's approach direction and are measured clockwise from the magnetic north. The numbers are based on the runway's magnetic bearing, which indicates its position in relation to the earth's magnetic pole. The earth's magnetic field moves approximately 36 miles (60 kilometers) yearly, and if this significantly affects an airport, international regulations require the runway numbers to be changed.
When there is more than one parallel runway at an airport, letters "L", "C", and "R" are appended to the runway number to indicate left, center, and right, respectively. These are based on the approach direction, so, for example, the runway 35L would be called 17R from the opposite direction.
Some airports may use true north for runway numbers due to high magnetic variation. If there are two airports near each other 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. For instance, in the case of DFW (Dallas-Fort Worth), there are five runways with the same heading. Those on the east side of the airport have the traditional L, R, C designations according to standard runway numbers, while those located to the west have been increased by 10 degrees while utilizing the L and R designations.
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True north headings
The Earth's magnetic field moves approximately 37 miles (60 kilometres) each year, causing the magnetic north to shift. This movement affects the magnetic bearing of airport runways, which is the position of the runway in relation to the Earth's magnetic pole. As a result, runway numbers, which are based on magnetic headings, may need to be updated to reflect the change in bearing.
Runway numbers are determined by the approach direction and are measured clockwise from the magnetic north. They are based on the magnetic heading of the runway, rounded to the nearest 10 degrees, and the final zero is removed. For example, a runway pointing to 092° is rounded to 090°, resulting in a runway number of "09".
The World Magnetic Model (WMM) is a global means of measuring the Earth's large-scale magnetic field. The WMM is reviewed every five years by agencies such as the National Oceanic Atmospheric Administration and the FAA. When the WMM indicates a delineation of more than 7 degrees, the runway numbers are updated.
In some cases, airports may use true north headings for runway numbers due to high magnetic variation. This is more common in northern Canada and Greenland. The use of true north headings ensures that pilots approaching from different directions can follow a consistent logic when communicating with air traffic control and referring to international databases.
The consistent numbering of runways is essential for maintaining order and safety in aviation. By adhering to international regulations and standards, airports worldwide can facilitate seamless operations and ensure a clear understanding among pilots and air traffic control personnel.
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Compass bearings
Runway numbers are based on the runway's magnetic bearing, which indicates its position in relation to the Earth's magnetic pole. The Earth's magnetic field moves approximately 36 miles (60 kilometers) yearly, and if this significantly impacts an airport, international regulations require the runway numbers to be changed. This is because the runway numbers are determined from the approach direction and are measured clockwise from the magnetic north. The runway number is the whole number nearest to one-tenth of the magnetic azimuth and is measured from the centerline of the runway.
The magnetic azimuth is determined through the World Magnetic Model (WMM), a global means of measuring the Earth's large-scale magnetic field. Measurements for the WMM come from satellites and a global network of 120 magnetic observatories. Every five years, agencies such as the National Oceanic Atmospheric Administration and FAA review the WMM for changes. When the WMM indicates a delineation of more than 7 degrees, the runway numbers are updated.
The movement of the magnetic north pole is caused by the motion of convection currents and a mixture of molten iron and nickel in the Earth's outer core, which creates the magnetic field due to the heat escaping from the planet. As the magnetic north pole moves, the runway heading may no longer correspond to the nearest 10 degrees, requiring the runway numbers to be changed.
Runway numbers are taken from the magnetic heading of the runway, which is rounded to the nearest 10 degrees, and the final zero is removed. For example, a runway pointing to 092° is rounded to 090°, resulting in a runway number of "09". The direction the runway faces is determined by several factors, including local prevailing wind conditions, as aircraft always take off or land into the wind.
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Prevailing winds
The direction of a runway is determined by the prevailing wind conditions, among other factors such as terrain. Aircraft always take off or land into the wind, so runway positioning is crucial. As a result, most runways are oriented to take advantage of prevailing winds to assist in takeoffs and landings. This is why most runways have two numbers, with the second number differing by 18 or 180 degrees.
Runways with the same heading may be differentiated by adding or subtracting one from the runway number. For instance, runway 35L would be called 17R from the opposite direction. Airports with two parallel runways going in the same direction are designated as the left or right runway, with an L or R. For example, runway 31 would be called 31R or 31L. If there are three parallel runways, the designation of C is assigned to the runway in the centre, resulting in 31L, 31R, and 31C.
Busy airports with more than three runways parallel to each other may have a one-digit difference in the runway numbers, even though all runways have the same heading. For instance, in the case of DFW (Dallas-Fort Worth), the runways on the east side of the airport have the traditional L, R, and C designations, while those on the west have been increased by 10 degrees with L and R designations.
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Frequently asked questions
Runway numbers are sometimes changed due to magnetic shifts. The numbers are based on magnetic headings and help pilots align with the correct runway.
Changes in runway numbers are uncommon because magnetic drift is slow. However, when a runway designation does change, it is often done at night, especially at major airports, as the taxiway signs need to be changed and the numbers at each end of the runway need to be repainted.
Airports may change the runway number by a factor of 1, even if it's not the correct magnetic heading. For example, Rocky Mountain Metro Airport in Colorado changed its runway numbers from 4/22 to 5/23.
