Magnetic North and True North, what’s the difference? Why does it matter? Well, just ask those folks in charge of airports and they will tell you that they are not the same. Normally, we would not even blink an eye about the difference between these two, but when it causes a whole airport to shut down, (even for a small amount of time,) then the level of inconvenience makes the headlines.
Magnetic North is dependent on the fluid core of the earth. This fluid core generates magnetic fields and these are aligned to give us Magnetic North and South. The fluid nature of the core has resulted in the migration of Magnetic North between 10 degrees east and 25 degrees west over the past 400 to 500 years. As of 2005, Magnetic North sat at about 82.7° North and 114.4° West in Canada, but it was moving towards Russia at an approximate speed of 40 kilometers per year.
With this information you may be asking what the relevance for airport runways is. The designation for a runway gives information about the approximate angle a plane approaches the runway from either from a northern or southern approach. Consider Stansted airport (on the outskirts of London) which until 2009 had a runway designation of 230/50. This means that planes approaching from the North would approach on a heading of 230°, and those from the South on a heading of 050°. Due to the migration of the magnetic pole this designation had to be changed to 240/40. So now an airplane approaching from the north takes a heading of 240° instead of 230°. The management of Stansted Airport are not unduly perturbed by this occurrence and indeed are already preparing for a further change in just over 50 years because of this continuing drift of Magnetic North.
In Tampa, Florida recently the International Airport had to be closed because of a similar designation change. The runway in question changed from 180R/360L to 190R/10L. The closure of the runway occurred because of the requirement to not only paint the new designation on the runway, but also the need to replace all signs for the runway, as well. Other Tampa runways will be changed in the near future.
Changes like this require a shift in the local magnetic alignment of more than 3°, and so many airports are fortunate that they do not need to consider re-designating a runway. In these rare cases where it happens, the need for safety and aircraft heading in the right magnetic direction is of paramount importance, and so for the short time it takes to modify the airport signage and runway lettering, it makes good sense. The only people who may get upset by this are those who play with flight simulators, because they will need to update their software to include these runway designation changes!