HARLINGEN — Nothing really stays the same, including the compass point to due north, which is on the move.
Airports don’t randomly generate names for runways like 17R/35L (8,309 feet) and 17L/35R (5,949 feet) at Valley International Airport. Instead, those are shorthand which gives pilots what could be a critical compass heading in bad weather or if other navigational aids fail.
Earth’s magnetic field is in constant flux because its inner core is solid but the planet’s outer core is liquid, composed primarily of molten iron and nickel, which floats atop the inner core.
Although major changes in the magnetic field like pole reversals happen only every few thousand years, smaller changes over time necessitate renaming airport runways to align them with the proper compass heading.
“The magnetic compass heading has changed, so we’re actually re-imaging the runways,” said Bryan Wren, assistant director of aviation at VIA. “So instead of 17R and 17L, we’re going to 18R and 18L, and we’re going to have to change all the signs and markings and everything.”
Valley International Airport’s runway 17R/35L’s point on the compass is 176 degrees magnetic and 181 degrees true north, and that discrepancy is called declination. Most places on the planet don’t line up with true north on a compass, and the deviation is called declination, or magnetic declination.
The continuing shift of magnetic north means the 176-degree angle is increasing, and now needs to be listed as 18R/36L as it nudges toward 180 degrees. It’s the same for runway 17L/35R, which will become 18L/36R.
Over the past few decades the Earth’s North magnetic pole has been shifting at an irregular pace. To monitor these shifts, a network of satellites and 120 magnetic observatories around the world compile data for the World Magnetic Model.
The data crunched by scientists working on the WMM can pretty much forecast when an airport will have to rename a runway to match the proper compass heading.
“The magnetic pole actually moves and with that it changes runway headings,” Wren said. “It’s about every 40 years. I thought we were going to do 13 (Runway 13/31, 7,257 feet) as well, but it’s not really going to flip over until 2044.”
