TT737FO From United States of America, joined Jun 2001, 472 posts, RR: 9 Reply 1, posted (12 years 2 months 3 weeks 5 days 5 hours ago) and read 903 times:
It's called the phenomenon of magnetic deviation (degrees variation from true north). It depends on where you live. Each year the deviation may be 0.1degree. Since the asphalt was laid and the numbers painted on your runway, enough deviation has taken place such that the runways must be re-painted.
Sabenapilot From Belgium, joined Feb 2000, 2712 posts, RR: 48 Reply 2, posted (12 years 2 months 3 weeks 5 days 4 hours ago) and read 889 times:
As TT737FO more or less explained, RWY's are referred to by their MAGNETIC heading. If the heading of a RWY is 186°, it'll be called RWY 19.
Due to magnetic variation the magnetic heading of a RWY will change over time. This might result in a RWY heading of for instance 185°, thus causing that runway to be renamed RWY 18.
Did you know that in aereas of big magnetic abnormalities (e.g. close to the poles) we do not refer to a MAGNETIC heading but to a TRUE heading when giving a number to a RWY?
Well, it's actually clever isn't it? Wouldn't it be confusing to rename your RWY every week?
Im not 100% sure about this, but I think these RWYs also have a letter T in their name to make sure the pilot does not get confused. (e.g. RWY 19T)