Because the Earth’s magnetic field has shifted over the decades, the runway numbers at Boeing Field have just been changed, requiring pilots to pay attention and change their charts.

A total solar eclipse next week will highlight for all the wondrousness of the solar system. Yet you likely have not noticed another scientific wonder already upon us: the Earth’s magnetic field has shifted beneath our feet.

And though this slow change is invisible, and detectable only by a compass, navigators who depend upon magnetic bearings better pay attention.

As result of the shift, the runway numbers at Boeing Field have just been changed for the first time in more than six decades.

Workers this week repainted the runway markings and changed all the taxiway signs.
Starting Thursday, pilots communicating with the airport control tower will hear the new numbers during the instructions to land or take off. The airport issued a “Notice to Airmen” on Tuesday, advising of the change.

Pilots, you’ll need to adjust your charts.

Some pilots of small airplanes who still use paper charts can scribble the new runway designation on their maps.

Most will use electronic charts, but those are simply replicas of paper charts and need to be updated.

Bob Bogash, a retired veteran Boeing executive and active pilot of his own self-built kit plane, said the chart updates may lag the change. In the meantime, pilots must be alert to the Notice to Airmen.

If an electronic chart isn’t up to date, many have a scratch pad application that allows a pilot to enter changes before take-off, he said.

Magnetic field changes the number
The main runway at the airfield, officially King County International Airport, was previously designated 13R (or “13 Right”) landing to the southeast and 31L (or 31 Left) landing to the northwest.

No longer. The runway is now designated 14R or 32L. The designation of a secondary runway has been correspondingly changed, too.

That tweak in the figures is important to pilots — it tells them the direction in which they must fly to line up with the runway.

To understand why, know that runway numbers are tied directly to a runway’s location in relation to the Earth’s magnetic poles.

The figures 13 and 31 in the old runway numbers are the shortened forms of the magnetic bearing — or direction — the pilot needs to steer to line up with the runway.

Since at least the 1950s, Boeing Field’s main runway has been approximately 130 degrees in in relation to magnetic north when approached from one direction, rounded to the nearest 10 degrees, and 310 degrees the other way.

But the planet’s magnetic field, which is generated by electric currents within the earth’s core, is constantly shifting. And when the magnetic poles shift more than five degrees, those rounded numbers need to be changed.

The main runway bearing is now closer to 140 degrees from magnetic north landing to the southeast, and 320 degrees landing to the northwest. Hence the new runway designations.

It’s part of a slow progression over the decades. Historical photos taken in the 1940s and 1950s show a Boeing Field runway labeled 12/30.

A few other local airports have recently made similar runway numbering changes.

Seattle-Tacoma International Airport hasn’t yet, but will do so next year, said Port of Seattle spokesman Perry Cooper.

“It’s a pretty big change for all of the signage and notifications for charts,” Cooper said. “So, there’s no one specific time all airports are doing it.”

Sea-Tac’s runways are currently designated 16 and 34, as are the runways at dozens of regional airports similarly aligned to take advantage of the prevailing winds locally. All will likely change their runway numbering around the same time.

For the Boeing Field change, one safety side-benefit is mentioned in the Notice to Airmen sent out this week: In the past, the coincidence of the runway numbers being 13 and 31 has led to some confusion when pilots transposed the figures.

With the new designation being 14 and 32, there’s no such danger.