How can I determine the runway number from its true bearing and magnetic variation? [duplicate]

Question

Given:

• Runway True Bearing 007°
• Magnetic Variation 4°E

What is the runway number (or designator)?

Answer 1

Runways are named after their magnetic headings.

To convert from True to Magnetic, use "East is least (i.e. subtract), West is best (i.e. add)".

If you are standing at this location looking in the direction 007 T, to find the direction you are looking in magnetic terms, subtract the 4 degrees E variation to get 003 M.

This should not just be a rote rule in your mind, but rather should make intuitive sense as you picture the situation in your head -- for example, if you are standing somewhere on the west coast of the US with a Silva-style hand-held compass with the sighting arrow pointing at your target and the big arrow on the rotating bezel pointing to true north, and then you twist the bezel to change the big arrow on the bezel to line up with with the magnetic needle and point to magnetic north, you'll be twisting the bezel clockwise (because the magnetic north pole is in northeastern Canada), and the number under your sighting arrow will get smaller.

Hence "East is least"-- a line to magnetic north pole is, from your position, aimed to the east of a line to the actual north pole, so you to go from True to Magnetic you twist the bezel clockwise, which subtracts from the number under the sighting arrow.

Now just round to the nearest 10 following normal rounding rules, and drop the last zero. 003 rounds to 000 (or 360) which becomes runway 00 (or 36). Which one of these two choices is it?

Values that round to 360 or 000 -- these are the same thing-- are always designated runway 36, not runway 00.

Sometimes you'll have to go "over the hump" and cross from a heading east of north to one west of north or vice versa-- just add or subtract 360 as needed for the answer to make sense -- for example 10 degrees true minus 20 degrees easterly deviation gives negative 10 degrees but just throw in an extra 360 degrees and you'll come out with 350 degrees which makes sense. Again you should be able to picture in your mind exactly what is going on here. If it doesn't make intuitive sense to you, get a Silva-style hand-held compass and practice; it will help you visualize everything clearly.