Why Do Runways Get Renumbered?

Every five years, such decisions are based on how much the Earth’s magnetic field changes.

Runway numbers are determined from the approach direction and are measured clockwise from the magnetic north. [Adobe Stock]

Question: For decades the runway of the airport I fly out of has had Runway 16-34. In a few weeks the runway will be closed for a month so that the runway can be resurfaced and renumbered 17-35. I am wondering how the FAA knows when it is time to shift?

Answer: Per the Aeronautical Information Manual, runway numbers are determined from the approach direction. The runway number is the whole number nearest one‐tenth the magnetic azimuth of the centerline of the runway, measured clockwise from the magnetic north. Magnetic azimuth is determined through the World Magnetic Model (WMM), a global means of measuring the Earth's large-scale magnetic field.

The magnetic field is created by the movement of iron and nickel beneath the Earth's surface around its core. The movement creates shifts in the magnetic field, known as declination. The closer the site is to the poles, the quicker declination occurs. Measurements for the WMM come from many sources, including satellites and a global network of 120 magnetic observatories.

According to the National Centers for Environmental Information, every five years agencies, such as the National Oceanic Atmospheric Administration and FAA, review the WMM for changes. When the WMM indicates delineation of more than 7 degrees has occurred, the runway numbers are updated.

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Meg Godlewski has been an aviation journalist for more than 24 years and a CFI for more than 20 years. If she is not flying or teaching aviation, she is writing about it. Meg is a founding member of the Pilot Proficiency Center at EAA AirVenture and excels at the application of simulation technology to flatten the learning curve. Follow Meg on Twitter @2Lewski.

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