When pilots land an aircraft, they see large numbers at the end of the runway. But what do these numbers mean, and why do airports occasionally change them? The answer lies in the Earth’s constantly shifting magnetic field, which moves approximately 37 miles (60 kilometers) each year. Because of this movement, international aviation regulations require airports to update runway numbers to ensure accuracy in navigation.
How Runway Numbers Are Determined
At every airport, large numbers are painted at each end of the runway. These numbers are crucial for pilots, air traffic control, and global navigation databases. They represent the runway’s magnetic heading—its direction in relation to the Earth’s magnetic north.
Understanding Magnetic North and Its Movement
In 1831, British explorer Sir James Clark Ross discovered the Earth’s magnetic pole in northern Canada, about 1,000 miles (1,609 kilometers) south of the geographic North Pole. The Earth’s magnetic field is generated by the movement of molten iron and nickel in the outer core. This motion creates convection currents, which, in turn, cause the magnetic north to shift gradually.
As the magnetic north moves by around 37 miles (60 kilometers) per year, the alignment of runways changes slightly. If the shift exceeds a certain threshold (typically seven degrees), aviation authorities require the runway numbers to be updated.
Why Do Airports Change Runway Numbers?
Runway numbers are based on the nearest 10-degree increment of the magnetic azimuth. For example, if a runway points toward a magnetic heading of 172 degrees, its number would be rounded to 17. If it faces the opposite direction, it would be labeled 35.
Because the Earth’s magnetic field is constantly shifting, runways must be renamed periodically to ensure their numbering remains accurate. For example, when Geneva International Airport updated its runway numbers, it required repainting over 100 sign panels using more than 330 pounds (150 kilograms) of paint.
The Role of the FAA in Runway Numbering
The Federal Aviation Administration (FAA) plays a critical role in updating runway numbers in the United States. The FAA follows a standardized system in which runway numbers range from 01 to 36, based on the compass heading. The last digit of the heading is dropped.
For instance:
- A runway pointing north (360°) is labeled 36.
- A runway pointing south (180°) is labeled 18.
- If there are parallel runways, they are distinguished using L (Left), R (Right), or C (Center).
Airports near the Earth’s magnetic poles, such as Fairbanks International Airport (FAI) in Alaska, experience faster magnetic shifts, requiring more frequent renumbering—approximately every 24 years.
Runway Configurations and Design Considerations
When designing an airport, engineers consider several factors, including wind direction, terrain, and surrounding infrastructure. Runways are typically aligned to maximize prevailing wind conditions, ensuring safer takeoffs and landings.
Special Cases: Airports with More Than Two Runways
Some of the world’s busiest airports have more than two parallel runways. When an airport already uses L (Left) and R (Right) designations, an additional runway in the same direction is labeled C (Center).
Example: Dallas/Fort Worth International Airport (DFW)
DFW, one of the busiest airports in the world, has seven runways, with different directions and designators:
| Runway Direction | Length (feet) | Length (meters) | Surface |
|---|---|---|---|
| 13L/31R | 9,000 | 2,743 | Concrete |
| 13R/31L | 9,300 | 2,835 | Concrete |
| 17L/35R | 8,500 | 2,591 | Concrete |
| 17C/35C | 13,400 | 4,084 | Asphalt |
| 17R/35L | 13,400 | 4,084 | Concrete |
| 18L/36R | 13,401 | 4,085 | Concrete |
| 18R/36L | 13,400 | 4,085 | Asphalt |
Similarly, Amsterdam Airport Schiphol (AMS) has six runways, each named based on nearby villages or landmarks:
| Runway Direction | Length (feet) | Length (meters) | Name |
|---|---|---|---|
| 18R/36L | 12,467 | 3,800 | Polderbaan (reclaimed land) |
| 06/24 | 11,483 | 3,500 | Kaagbaan (village of Kaag) |
| 09/27 | 11,329 | 3,453 | Buitenveldertbaan (Buitenveldert village) |
| 18L/36R | 11,155 | 3,400 | Aalsmeerbaan (Aalsmeer town) |
| 18C/36C | 10,827 | 3,300 | Zwanenburgbaan (Zwanenburg village) |
| 04/22 | 6,608 | 2,014 | Oostbaan (Eastern runway) |
Final Thoughts
The numbering of airport runways is more than just a simple labeling system—it is a crucial aspect of global aviation safety. Since pilots rely on these numbers for navigation, changes in the Earth’s magnetic field require periodic updates to maintain accuracy. With technological advancements, organizations like the FAA and other aviation authorities continue to monitor these shifts, ensuring that runways remain correctly labeled for safe takeoffs and landings worldwide.
