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Shifts in Earth's magnetic field affect JIA runways
By Jeff Brumley
The Florida Times-Union
December 16, 2011
What may seem like a straightforward process — painting those big, white numbers on airport runways — is actually a complicated task thanks to the Earth’s core and the Federal Aviation Administration.
Work crews at Jacksonville International Airport learned that firsthand this week when they had to change the 60-foot long, 20-foot wide numerals on each end of two runways.
The $20,000, three-day project, completed Thursday, was necessary because of FAA requirements that runway numbers match their magnetic compass headings. And the slow but constant shift in the planet’s magnetic field, said Joe Meert, a geologist at the University of Florida, means its a job that must be done every few years.
Meert, an expert on the Earth’s magnetic field, said the location of magnetic north is constantly shifting — currently westward — because the planet’s outer, liquid core is always moving around its inner, solid core.
So magnetic north is different than true north, which is a geographic reference also known as the North Pole.
“If you followed the magnetic field to the North Pole, you’d never get there,” Meert said. “You’d end up somewhere between Canada and Russia.”
As magnetic north shifts — about 1 degree every decade or so — airports must occasionally renumber their runways when the direction varies 5 or more degrees from the previous heading, said Meert, who’s also a licensed pilot.
That’s why the end of one of JIA’s runways was just changed from a compass heading of 310 degrees to 320 degrees.
Runways get a two-number designation at each end. JIA’s shorter runway is now numbered 32, for 320 degrees on one end and the inverse heading, 14 for 140 degrees, on the other. The longer runway is now numbered 8 and 26.
Aviators sometimes use the runway numbers to confirm which airports they’re landing at or flying past, so the government’s requirements aren’t just a formality, said Terry Dlugos, supervisor of airport operations at JIA.
“When pilots land, they’re looking at their magnetic heading,” Dlugos said. “True North is where Santa lives.”
Further complicating the issue for airports is that some parts of the Earth experience less magnetic shift than others, requiring some airfields to change their numbers sooner than others, and still others not at all, Dlugos said.
It’s partly why JIA is changing its numbers now and Tampa International changed its runway numbers in January. Jacksonville Naval Air Station did so in 2010 and the airport in Gainesville about a decade go.
Mayport Naval Station’s runway numbers have yet to change, and the magnetic shift that impacted JIA has not affected other airports in Jacksonville, officials said.
Dlugos said when a runway was built and what numbers were used can also prolong or hasten the need for new runway numbers.
FAA rules don’t make it any easier. They dictate the height, width and proportion of the numbers, the kind of paint used and even the type of reflective glass beads that must be spread over them.
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