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I was talking to a contractor at a major U.S. airport, a Project Manager who told that they were moving the runways due to the shifting of 'Magnetic North'. I have heard that the poles have shifted even after the Chilean quake that registered at >8.0. I have no knowledge of planes, flight or the reason for needing to shift runways.

How much have the poles shifted and can anyone knowledgeable in aeronautics either confirm or deny a need to do this?

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The story is slightly less dramatic than it sounds.

The runways themselves are not being "shifted" (i.e. construction crews getting out and laying tarmac), but just renamed and signposted.

From a Livescience report

Runways are designated according to the points on a compass, and the drifting magnetic north means that they, periodically, need to be renamed.

"Recently, the drift has caused our runways' orientations to be closer to the next increment on the magnetic compass," Tampa International Airport spokeswoman Brenda Geoghagan told LiveScience in an e-mail.

For example, the west parallel runway, which was named 36Left —18Right to designate compass points of 360 degrees and 180 degrees, is being renamed to 1Left — 19Right, to indicate 10 degrees and 190 degrees, since the runway designations are separated into 10-degree increments.

Adjustments to runways like this and to navigational aids are ongoing, according to Kathleen Bergen, a spokeswoman for the Federal Aviation Administration's Southern Region. A third airport is scheduled to rename its runways in October; however, the changes aren't necessary for all airports in the Tampa Bay area, she said.

A similar change was required in Stansted Airport in England in 2009.

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    all of these answers and comments of examples renumbering runways fall in line with the information I got received from the PM as it was an electrical contractor not a demo or concrete company. – Charlie Brown Sep 30 '13 at 8:25
  • and several years ago several runways at Amsterdam were also renamed. It's just a matter of printing new charts and driving a truck out to the ends of them to replace the painted numbers there. – jwenting Sep 30 '13 at 13:46
  • @jwenting: The story of the Brussels Airport renumbering tells us the synchronization is the hardest part: "hundreds if not thousands of official documents and plans", "huge impact for air traffic control management". – Jan Fabry Sep 30 '13 at 17:42
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    @JanFabry yes, it's a lot of work, but not difficult work. And in case of Amsterdam, it was done at the same time a new runway was constructed so all that had to be done anyway. My guess is such renumbering is usually synchronised with other work that would see the charts and procedures updated for that very reason. – jwenting Sep 30 '13 at 18:58
  • This is also explained at Wikipedia, Runway–Orientation and dimensions. Quote: Runway designations change over time because the magnetic poles slowly drift on the Earth's surface and the magnetic bearing will change. [...] – Jeppe Stig Nielsen Sep 30 '13 at 19:29
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I've long thought this graphic gives a good idea of the issue

The problem at an airport is variation in local magnetic declination, not the variation in location of the magnetic north pole. Due to local magnetic anomalies, magnetic compasses rarely point at any magnetic north pole.


As others have noted, Runway naming is mostly based on local magnetic north (not on the direction of any north pole).

Runways are named by a number between 01 and 36, which is generally one tenth of the magnetic azimuth of the runway's heading in degrees: a runway numbered 09 points east (90°), runway 18 is south (180°), runway 27 points west (270°) and runway 36 points to the north (360° rather than 0°).[1] When taking off from or landing on runway 09, a plane would be heading 90° (east). However, runways in North America that lie within the Northern Domestic Airspace of Canada are numbered relative to true north because proximity to the magnetic North Pole makes the magnetic declination large.

My emphasis. Ref (via Wikipedia)


A sidenote on declination:

Magnetic declination is the angle between [magnetic] compass north (the direction the north end of a [magnetic] compass needle points) and true north (the direction along the earth's surface towards the geographic North Pole[1]). The declination is positive when the magnetic north is east of true north. The term magnetic variation is a synonym, and is more often used in navigation. Isogonic lines are where the declination has the same value, and the lines where the declination is zero are called agonic lines.

Note this definition deliberately does not mention the magnetic north pole - it's location and wanderings are irrelevant. It's the direction a nearby magnetised needle points that is important (and it mostly doesn't point towards any magnetic north pole - of which the earth sometimes has several).

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    Wow. Also I was puzzled until I understood that the map doesn't show magnetic contours: "declination" is a measure of how much the contours are wrong; so the map shows that the amount of wrongness varies plus or minus 30 degrees over most of the inhabited/navigable globe. – ChrisW Sep 30 '13 at 14:19
  • Interesting graph and don't quite understand it, despite ChrisW's attempt. Couldn't help but notice the quick snap over the Bermuda Triangle which brings me to another question could this make compasses go haywire. – Charlie Brown Oct 1 '13 at 6:25
  • @CharlieBrown: These changes are happening gradually over tens of years, in polar regions it may be as much as one degree in three years. These slow changes won't cause any sudden movements in magnetic compasses. – RedGrittyBrick Oct 1 '13 at 8:34
  • During my flight instruction, it was made clear that this is all due to the history and reliable simplicity of magnetic compasses, so all runways, VOR navigation beacons, and airways are specified in magnetic directions, so every several years they have to be readjusted. It's like QWERTY keyboards, an artifact of history. – Mike Dunlavey Oct 1 '13 at 14:49
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    @CharlieBrown Interesting graph and don't quite understand it, despite ChrisW's attempt Looking at the Bermuda Triangle, for example, it shows that in the 16th century the declination there was approximately 0, i.e. compasses pointed directly towards the North Pole; but in the 20th century, the declination reached 20 degrees. The "snap" is where two contour lines of exactly 20 degrees declination briefly joined; before the snap they weren't joined, because the declination there was less than 20 (e.g. it was 19 degrees). – ChrisW Oct 1 '13 at 16:10
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At all major U.S. airports, the runways are labelled based on magnetic north, not true north. Only in the Northern Domestic Airspace of Canada are runways numbered based on True North (Transport Canada Aeronautical Information Manual, at p. 189).

Transport Canada says the reason that they use True North in Northern Domestic Airspace is because "the Magnetic North Pole is located near the centre of the Northern Domestic Airspace, therefore magnetic compass indications may be erratic". (Transport Canada AIM, at p. 189)

So, most runways are numbered based on their magnetic heading (rounded to the nearest 10), and magnetic north does shift.

enter image description here

When magnetic north shifts far enough, runways need to be renumbered. For example (http://www.yyc.com/News/tabid/91/articleType/ArticleView/articleId/129/language/en-US/Runways-at-YYC-and-YBW-will-be-renumbered-in-2013.aspx):

For instance, Runway 34 at YYC is numbered 34 because it lines up with 340 degrees magnetic and because the magnetic north pole is slowly moving (this movement is part of the earth's natural process), every 60 or 70 years runways have to be given new designations.

True north also moves (http://en.wikipedia.org/wiki/Polar_motion). It has shifted about 20m since 1900. This is not enough to trigger airport runway renumbering, even in Canada's Northern Domestic Airspace.

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