Runways are numbered based on compass heading. Why?

Does this help pilots in locating the runway, even in today's GPS age? Or are there some other technical reasons behind this?

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    $\begingroup$ Because there is no other sensible way of doing it? Many aircraft do not have GPS $\endgroup$ – Simon Aug 15 '15 at 10:33
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    $\begingroup$ It's important to note that GPS doesn't have any way of measuring heading. GPS-enabled devices that do show an actual heading (as opposed to a track) are actually just using an internal magnetic compass. $\endgroup$ – reirab Aug 15 '15 at 19:32
  • $\begingroup$ And there is no sensible way to show True North $\endgroup$ – rbp Aug 16 '15 at 20:15

There are two parts to the question:

Why number runways based on their direction?

The runways have to be identified somehow and the identification has to be directional. So the, rounded, heading offers itself and it has advantage that:

  • The pilot can verify they are approaching the intended runway by looking at the compass, which
    • works at night, in bad visibility or early on the approach when the numbers on the ground are not visible yet and
    • serves as cross-check for verifying the instruments are set correctly during instrument approaches.
  • Since the pilot needs to know the runway heading for navigation and for checking the wind anyway, it makes it easier to remember.

Why use magnetic heading instead of true?

In the early days of aviation, magnetic compass was the only tool available for determining direction. It is simple and reliable and well tested in naval navigation. So magnetic headings became the obvious standard.

Later gyrocompass was introduced to overcome the limitation of magnetic compass that it is biased in turns and requires some time to stabilize. However gyrocompass precesses, so it has to be periodically corrected, which was, and is, still done by reference to the magnetic compass.

On the other hand GPS is still a rather recent technology. It is only precise enough to determine heading (or rather tracking) for last 15 years. It requires a complex electronic device that can fail in a myriad of ways and it is affected by many external factors including the state of ionosphere and the selective availability not being turned on again.

And not all aircraft have GPS anyway. So if you wanted to switch the ones that use GPS to true headings, there would still be some that would need to use the magnetic and it would be a mess. On the other hand when you have GPS, you have a computer already and it can contain and use the magnetic declination tables to give you magnetic heading as well.

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    $\begingroup$ Note that movement of the North Magnetic Pole theoretically requires periodic renumbering of runways; the famous St Maarten runway was renumbered from 09/27 to 10/28 in 2008, while remaining fixed at 082/262 True... $\endgroup$ – DJohnM Aug 15 '15 at 16:15
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    $\begingroup$ @DJohnM: Yes, it does. And the VORs have to be occasionally adjusted as well. $\endgroup$ – Jan Hudec Aug 15 '15 at 16:26
  • $\begingroup$ Palo Alto renamed their runway from 12/30 to 13/31 about a decade ago. $\endgroup$ – Edward Falk Aug 16 '15 at 5:56
  • $\begingroup$ ... and LKPR renamed one runway from 13/31 to 12/30 a few years ago; the declination changes differently in different parts of the world. $\endgroup$ – Jan Hudec Aug 16 '15 at 18:53
  • $\begingroup$ In addition, large airport complexes with a lot of runways pointing the same direction often have to cheat a little; DFW's two western runways are 18/36 while its three eastern ones are 17/35, even though all five are parallel and there's nowhere near 10 degrees difference in magnetic declination between them all. Hartsfield-Jackson is similar, using 8/26, 9/27 and 10/28 for its five parallel runways. $\endgroup$ – KeithS Oct 1 '15 at 22:42

The aircraft heading is measured with reference to the magnetic north. If the same convention is used for runways too, it will make it easy for the pilot to identify the runway and land as the aircraft's heading and the runway will be aligned.

For example, if the aircraft is directed to land in the runway 09, the aircraft heading will be 090° while landing.

Use of another system for naming will lead to unnecessary confusion. This convention is being used from long before GPS was available. Also,as already noted, a large number of aircraft flying today have no GPS and pilot should be able to land the aircraft even without it.

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    $\begingroup$ Not only should, but in the UK, you must demonstrate full navigation ability without reference to a GPS or similar in order to pass your skills test. $\endgroup$ – Dan Aug 15 '15 at 12:08
  • $\begingroup$ Runway 9? Do you mean 09? $\endgroup$ – Dan Hulme Aug 15 '15 at 12:46
  • $\begingroup$ Yes 09. Will edit the answer. $\endgroup$ – aeroalias Aug 15 '15 at 12:52
  • $\begingroup$ "to land in the runway 09, the aircraft heading will be 090° while landing." No, but it will be somewhere between 085° and 095°. I think there's an "approximately" missing in your sentence. $\endgroup$ – David Richerby Aug 15 '15 at 17:15
  • $\begingroup$ Yes it will be in that range. Thought I'll be able to put my point across more easily this way. $\endgroup$ – aeroalias Aug 15 '15 at 17:27

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