What isn't mentioned in all this, is that the runways were slightly east of true north in the first place. Please see the FAA diagram on wikipedia... http://upload.wikimedia.org/wikipedia/en/8/8e/KTPA-FAA.pdf

If probably didn't take that much more magnetic north change to prompt the need to relabel the runways.

The runways are labelled rounded to the next 10deg so if a runway is 154.9 and the magnetic pole moves 0.2 deg you go from runway 15L to runway 16L

Alternately you could move the runway a bit

I've heard, although I can't find confirmation of it at the moment, that this has happened on occasion at airports in Alaska where, being much closer to the pole, the heading to magnetic north is changing more rapidly.

The rate of change of magnetic north varies wildly around the world, not based on latitude. See this fabulous .gif on Wikipedia

http://en.wikipedia.org/wiki/File:Earth_Magnetic_Field_Decli...

That's fascinating - I always thought it was based on latitude - basically the difference in angle between the north pole and the magnetic north pole.

Still though, even in that animation, Tampa seems pretty stable over time.

Couldn't they label the runways relative to geographic north rather than magnetic north?

Imagine you're flying the landing pattern in a cloud. You just flew through a thermal boundary so your battery jostled and lost voltage for a split second, resetting your electronics that require a >5 minute reboot and config time. How do you determine geographic north?

And whatever you do, don't forget your manual compass deviation rules: accelerate north, decelerate south

Trivia note: I believe the standard compass is called a "whiskey compass". Not sure why. Probably because if that's all the nav gear you got working you'll probably be using copious quantities of whiskey after landing. (Actually if memory serves I believe the early compasses floated in whiskey. No doubt this practice did not last very long. I think they use kerosene now)

The explanation I have read is that it is a "wet compass" (a ball floating in liquid), which was abbreviated "W compass", which is pronounced "whiskey compass" in the NATO phonetic alphabet that pilots use. I doubt whiskey was ever used to fill them, since even water would have worked better in terms of cost, transparency, evaporation, or boiling point.

I suspect it would have been freezing point that was most important, and whiskey was a readily available source of low freezing point liquid.

I've read that as well.

Just trying to share a little FBO-lore. It's common to tell the whiskey compass story in the fashion I have. http://iflyasa.com/2009/10/11/whiskey-compass/

I was aiming for tone, not accuracy. There are all sorts of topics that pilots discuss with great vim and vigor. I was trying to convey that feeling. Looks like I missed the boat.

"We wouldn't be lost if Bob here hadn't drank the compass."

I doubt any pilot would forget their grid north / magnetic north conversions. I hope not, anyway. That stuff got drilled into us in the cadets.

Isn't that the same problem though? Easier from them to do one calculation on the ground in advance rather than pilots having to worry about a conversion in the air.

Particularly since the pilot would still have to look up the local conversion, since it varies from place to place.

ICAO VFR maps contain the magnetic deviation as information - you just have to apply it to all your directions and be able to remember in what direction you have to correct ;) Not so trivial under stress. Anyway, on US and EU latitudes, the difference is not much bigger than 2 or 3 degrees, right?

No. The magnetic declination can be quite significant even in the continental US, and it isn't determined by latitude alone:

http://en.wikipedia.org/wiki/File:IGRF_2000_magnetic_declina...

Thanks the correction! I've only used european maps where the deviation is usually small. Local bias...

Thanks for the map; I always thought the declination was pretty tiny. Kind of shocking to realize it's 20° in my house.

Trick question: the correct answer is you eject immediately.

that just means there is a market for ultra-low-power battery-assisted magnetic-to-true-north converters!

yellowbkpk explained one reason: we fly with magnetic compasses, so we need the runways to be magnetic as well.

Even if we could come up with a safe and reliable way to always have true heading displayed in the cockpit, there remains a very pragmatic reason for keeping runway numbers magnetic rather than true: under the current system, we have to change a few runways every once in a great while; if we switched systems, we would have to change every runway in the whole world all at once. It's not just a simple matter of re-painting the runways: publications have to be changed, notices have to be issued, etc. Changes would cascade out into literally every aspect of the aviation industry.

This is all fine and dandy until the next geomagnetic reversal, at which point we suddenly find everything 180 degrees out of phase.

Thankfully, runways are numbered with _both_ numbers. So 18/36 or 9/27. Total reversal would leave the numbers the same...

No, they'd still have to be swapped (under the current scheme of runway numbering). If you're landing on runway 18, you know your compass better be reading ~180° (after correction for deviation). Similarly, when taking off from runway 18, your compass should read the same.

You can see just how much the magnetic variation changes around the world in this graphic: http://en.wikipedia.org/wiki/File:Earth_Magnetic_Field_Decli...

Each number labels one direction/end, so they'd still have to be swapped round.

Probably has something to do with magnetic navigation. Are there any aviators who can explain things in more detail?

The worldwide ICAO regulation defines runway directions with the magnetic north. Makes sense as GPS is not considered safe enough for main navigation, it's only here for additional help (in theory at least). Also traditional radio navigation (VOR, NDB) is done with magnetic directions. Changing runways directions happens sometimes, it's not such a rare event, as the north is moving on many parts of the globe (not everywhere the same though). I know of a few places here that have changed runway direction.

Anyway, it's not that much of a burden, because there is a very defined and safe process for updating airport and airspace data. When that type of change happens, udates are shipped over many channels that pilots should follow.

If you could come up with a system that could always determine where geographic north is.

Oh, and you can't use any external power source.

Then they wouldn't match a (real) compass.

The lesson here is to use names that don't carry content/meaning. A name is a pointer.

Except that the essential description of a runway is its direction. It would be a lot more painful if you had to look up the direction of every runway.

Exactly.

Two conversations: #1 "Which runway should I land on?" "Runway Charlie." "Which one is that?" "The one on heading 180 deg."

#2 "Which runway should I land on?" "Runway 18."

It's not a "name". That number is the meaning. The number represents the "points on the compass" as it were. Since the "compass" is changing, the numbers have to change.

I find it interesting that they don't use true north though.

Annnd - I need to continue reading. Description why they don't use true north in comments elsewhere in this thread.