I know that during the course of years the magnetic poles of the earth will change, but I was wondering how much the poles have to move before the runways are not lined up to the headings that they are lined up to. Or another way to ask is how often do they repaint the runways to line up to the correct bearings?
Since the runway headings are rounded to the nearest 10 degrees, it takes 5 degrees of drift (on average) before they need to be repainted. The rate of change depends on where you are. Wikipedia has a nice animation of the last few centuries.
Runways at TPA changed this year to magnetic deviation drift:
And, as the article points out, what would really be “bad” is a “pole reversal”, where the north and south magnetic poles flip-flop…which happened only 780,000 years ago:
Another wikipedia discussion of Runways have a little more…
Runway designations change over time because the magnetic poles slowly drift on the Earth’s surface and the magnetic bearing will change. Depending on the airport location and how much drift takes place, it may be necessary over time to change the runway designation. As runways are designated with headings rounded to the nearest 10 degrees, this will affect some runways more than others. For example, if the magnetic heading of a runway is 233 degrees, it would be designated Runway 23. If the magnetic heading changed downwards by 5 degrees to 228, the Runway would still be Runway 23. If on the other hand the original magnetic heading was 226 (Runway 23), and the heading decreased by only 2 degrees to 224, the runway should become Runway 22. Because the drift itself is quite slow, runway designation changes are uncommon, and not welcomed, as they require an accompanying change in aeronautical charts and descriptive documents. When runway designations do change, especially at major airports, it is often changed at night as taxiway signs need to be changed and the huge numbers at each end of the runway need to be repainted to the new runway designators. In July 2009 for example, London Stansted Airport in the United Kingdom changed its runway designations from 05/23 to 04/22 after dark.
An example illustrating the above article is Burbank. Years ago they had runways 7/25 and 15/33, today they have 8/26 and 15/33.
Thanks that really did clear things up. The biggest pain in the butt would be changing all the charts and maps. Hopefully that would not have to happen too often.