Changing magnetic poles mean new runway numbers at BFI

Boeing Field runway and Mount Rainier

Earth’s magnetic field is-a-changing, and as a result – for the first time in about 50 years – so are the runway numbers at King County International Airport/Boeing Field.

On Thursday, Aug. 17, the Federal Aviation Administration-assigned numbers for BFI’s main and utility runways – which help direct take-offs and landings – will officially change:

  • The Main Runway (previously 13R/31L) is changing to 14R/32L
  • The Utility Runway (previously 13L/31R) is changing to 14L/32R



So why are the numbers changing?

Boeing Field’s runway numbers are tied directly to our location in relation to the Earth’s magnetic poles. For those of us non-geologists, here’s a quick science lesson. The magnetic field is generated via electric currents within the planet’s core and is constantly shifting. That magnetic field is what directs compasses and other navigational devices. The direction an aircraft is pointing with respect to the north end of the magnetic field is called its magnetic heading.

The magnetic field helps the FAA assign runway numbers to Airports. For instance, the magnetic field around Boeing Field’s Main Runway has been approximately 130 degrees in one direction and 310 degrees the other way in relation to the magnetic field. Those numbers are then rounded to the nearest 10 degrees and assigned to the runway, hence the former runway number of 13R/31L.

Historical photos taken in the 1940’s and 1950’s show a BFI runway labeled 12/30.

Now, BFI’s location relative to the magnetic fields poles have shifted to the point where its runway numbers must be rounded up to accurately reflect our position.

Pretty cool stuff, huh?

To learn more about runway numbers in relation to the Earth’s magnetic field, check out this article from Aviation Safety Magazine.