Is your airport changing its runway numbers? Yes - quite possibly it is - costing $$$$ in paint and signage each time it’s done. The reason - as explained in this 2018 article
The shifting of the (north) magnetic pole - upon which the numbers of airport runways are based.
What happens to airports very near the magnetic north pole? (with the “real” pole and “magnetic” pole within a couple of hundred km of each other currently in uninhabited areas - this question isn’t really relevant at this instant).
But what happens in the southern hemisphere? Is (or can) magnetic “north” be used? Or do (can) they use magnetic “south” as a base for their orientations?
Are the north and south magnetic poles exactly antipodal from each other?
My nearest airport * “…has one runway designated 1/19”* and I doubt Amateur County will renumber until the magnetic poles shift. (See Runway Naming for what the designation means.)
Wikipedia sez, “Since Earth’s magnetic field is not exactly symmetrical, the North and South Magnetic Poles are not antipodal, meaning that a straight line drawn from one to the other does not pass through the geometric center of Earth.”
At any point on the Earth magnetic North and South are exactly 180 degrees apart. But that is just the nature of a smooth field. There is actually nothing special about magnetic North/Northern Hemisphere. Down here in the antipodes our compasses still point North. However the location of Magnetic North and Magnetic South are not antipodal. What matters when navigating is just that you have something that points in a known direction.
It isn’t just a matter of the location of magnetic North moving. That is simply a symptom of a much wider phenomenon. The Earth’s magnetic field is a complex beast, and is three dimensional. Despite what one might imagine, the lines of the field do not simply point to the magnetic North. The field is created by the geodynamo, which is a result of movement of conductive fluid in the Earth. The manner in which this occurs is complex and messy, and not stable over long periods. Nor does it create a uniform field, such as the simplistic bar magnet description of how the Earth’s field works. As the fluid churns it both moves the field around - hence the movement of the magnetic poles, but it isn’t uniform, and anywhere you are on the Earth, you will find that your compass never points quite at the Magnetic North anyway. Traditional maps carry the deviation of Magnetic North from True North, and this is measured locally, it isn’t just a universal correction made to point to the Magnetic North Pole. So if you have a map and a compass, your compass always points Magnetic North as measured to be correct for that map. But each map is different.
There is no special reason why there should even be a single North and South pole. Some planets have more than one of each.
Geophysicists have lots of reasons to be interested in the nature of the magnetic field, and there is a constantly maintained mapping. Current maps can be found here. Those maps show all manner of weirdness. Perhaps the best known being the South Atlantic Anomaly. A dip in field strength so strong it affects LEO satellites, as the Van Allen belts come closer to the Earth.
The Wikipedia page for Magnetic Declination shows an animated picture of the shifting isogonic lines over time from the 16th century through about 1980. Each line from magnetic north pole to magnetic south pole is not a “straight” (geodesic great circle) line from pole to pole, but is a wavy line. Sometimes with local closed loops.
There is an agonic line (where deviation is zero, that is, magnetic north is in the same direction as true north) running through North America roughly along the Mississippi River or a bit west of that (it’s moving too). There appears to be another agonic line running through roughly eastern Europe / western Asia.
Repainting all those runway numbers, replacing all the signage, updating all the aero charts, I suppose is all cheaper than moving all the airports.
For runways and signage, LEDs are our friends. And charts can be reprinted. If the local county airport was moved, planes would plop into the water district’s reservoir. Let it be.
Runway numbers have to be repainted due to fading/wear anyway.
The agonic line used to run through central Wisconsin, it is now closer to Rochester, MN.
When I was instructing about ten years ago my airport changed runway numbers. They did one side, then a week later they did the other. So for a week we were the only airport in the world with runways 36 and 19.
Airports in the high arctic sometimes use true bearings rather than magnetic bearings, which avoids the issue quite nicely. For example, Alert Airport (the northernmost airstrip in Canada numbers its runways 05 and 23, since the true bearings are 48° and 228°. Note that the bearings in the chart I linked have a “T” after them, which (I believe) indicates that they’re true bearings and not magnetic bearings. Magnetic deviation is something like 50° W near Alert, so the numbering would be quite different if they used magnetic bearings.
Other examples: Grise Fiord and Cambridge Bay, both of which number their runways with true bearings.
Not in the US, at least, and (maybe) not yet. Power fails, LEDs fail, computers driving them fail. Mice eat wires, etc… And there’s no need for frequent changing of the message. Paint just sits there, immune to technological failures. And there’s more to it than just changing the big number at the end of the runway - they also have to change the taxiways and crossings. The specifications for all the markings are rather detailed.
The good thing is that the declination changes are predictable so an airport can plan for it. An airport in Alaska may need to re-number every 20 years, but for some quirk of geology, Washington DC might never need to change. Something else that tends to happen on a predictable cycle (the original design expectation at the FAA was 20 years) is needing to repave runways, so if the runways in Alaska last that expected 20 years, it’s trivial to paint new numbers once the concrete has set.
The airport wouldn’t have to be moved to a different place, just rotated a few degrees so that the runways line up with the new magnetic headings. Easy peasy!
Wasn’t someone fooling around with the idea of banked circular runways not long ago? No worries about compass headings, renumbering or moving the airport - just land wherever you want.
Runways are numbered with the closest 10º (Magnetic) so unless the alignment was just on the cusp (about 5 or 6 degrees for the last digit), there is probably no need to change in a hurry. Plus, I don’t think there are too many airports with 2 runways at an angle of 10 degrees or so, meaning the likelihood of confusion is pretty low. “Wait, an I supposed to land on runway 23 or 24?” Plus I’m sure most pilots are aware of the issue and know which runway is intended…
