The North the Paint Remembers

The number painted on a runway is its compass bearing: runway 27 points roughly 270° magnetic. Magnetic, not true, which means the correct number is not a property of the runway. It is a property of the runway and the date. Earth's field drifts, the paint does not, and every so often an airport closes for a night and repaints its identity. Drag the year below and watch a real runway change its name.

The field model is IGRF-14, running in your browser from the coefficient table the International Association of Geomagnetism and Aeronomy publish. The runway's true bearing is a geodesic azimuth between its two published threshold coordinates on the WGS-84 ellipsoid. Nothing on this page is a stored answer: move the slider and the arithmetic runs again.

The rule is two sentences long

ICAO Annex 14, the standard almost every country's aerodrome regulations descend from, states it in one clause.

On a single runway, dual parallel runways and triple parallel runways the two-digit number shall be the whole number nearest the one-tenth of the magnetic North when viewed from the direction of approach. ICAO Annex 14 Volume I, 8th edition (2018), paragraph 5.2.2.4

The FAA says the same thing in more words and adds the case ICAO leaves open.

For a magnetic azimuth ending in the number “5” such as 185 degrees, the runway designator marking may be either 18 or 19. FAA AC 150/5340-1M Chg 1, paragraph 2.3

That second quotation is why this page throws away of its runways before scoring anything. A runway whose magnetic bearing sits within half a degree of a multiple of five has no wrong answer to give, so counting it as wrong would be the page lying to make a number bigger. The discarded share comes to , against the 10.0 per cent you would expect if a one-degree window in every ten were being cut, which is a small piece of evidence that the bearings are distributed the way an honest sample should be.

How many runways no longer round to their own number

of runways, at . That is runways whose painted number is not the number the rule returns today. Almost all of them are wrong by exactly one: are off by a single designator, meaning the field has carried the bearing just past a five-degree boundary, and by two or more.

This is not the first time the count has been made, and it should not be presented as though it were. NAV CANADA put the same question to an AIRAC dataset in a 2022 briefing on what a global switch to true-referenced navigation would cost:

25732 World-wide hard surface runways analyzed · 8044 would need to renumbered in MAG · 11316 would be left alone switching to TRUE · 14416 would need to be renumbered in TRUE · 5656 are out of MAG alignment today NAV CANADA, “Magnetic North vs True North: Vision to 2030”, 28 February 2022, slide “2030 World Wide Airport Impact, AIRAC cycle 21-11/2020 epoch”

5,656 of 25,732 is 21.98 per cent, at the 2020 epoch, on hard-surface runways. Restricting this page's sample the same way and winding the model back to 2020 gives of . Two samples that share no data pipeline, one industry and one open, landing apart. Their slide gives a count and no method; this page gives the method and the coefficients, which is the part that was missing rather than the number.

The year the world's paint agrees with

A count of today's disagreements is a thin thing. The richer question is which year's magnetic field the world's painted numbers fit best, because the stock of paint was not laid down at any one moment: it accumulated over decades and is repainted piecemeal. Slide the whole model backwards and forwards and score the entire sample at every year, and the agreement rate traces a curve with a peak.

The peak sits at , at agreement, against for the field of today. Every year within one percentage point of the peak lies between and . Read that as what it is: not a claim that runways were repainted in 2002, but a summary statistic. The world's runway numbering is, in aggregate, a photograph of a magnetic field about a quarter of a century old.

Doing the same fit country by country turns the statistic into something closer to a portrait, because the peak year for a national stock of runways is a rough measure of how recently that country's charts were revised. It only means anything where the curve has a sharp peak, which happens where the field has moved fast enough for the rule to discriminate; where declination has barely changed, the fit is flat and says nothing, and those rows are marked.

What the regulator's own books say

Everything above compares a painted number against the live field, and a live field is not what a regulator uses. The FAA numbers runways from a magnetic variation of record, a value it assigns to an airport and revises in steps.

When the difference between the MV of record and the nearest future epoch year value of any NAVAID, or the assigned airport MV of record, will exceed three (3.0) degrees [five (5.0) degrees for VORs and VORTACs], the MV of record must be changed to the nearest future epoch year value and applied to airport reference point (ARP) and all on-airport NAVAIDs. FAA Order 8260.19K, paragraph 2-5-3.b

So a runway can be correctly numbered and still disagree with today's field, by up to about three degrees, entirely legally. That distinction is worth more than the headline, and the FAA makes it checkable, because its 28-day NASR subscription publishes for every airport both the variation of record and the year that record was set. Cycle carries of them.

The median is . Not the median of a long tail: airports, more than half the file, carry a magnetic variation stamped 1985 and never revised since. And the records are not sloppy. Compared against IGRF-14 evaluated at the year each record itself names, the median discrepancy is and are within a degree. They were right when they were written. They simply were not written recently.

Measured against the field of today, the median US airport's variation of record is off by , and of airports, which is , are past the three degrees at which the order above says the record must be changed. The median airport sits within a tenth of a degree of the trigger, which is a coincidence of arithmetic rather than of policy, but a striking place for a national stock to have come to rest.

Two lags, in series

With the FAA's own true alignments and its own variations of record, the question splits cleanly, and neither half needs anything of ours except the model.

The paint is behind the paperwork, and the paperwork is behind the field. Updating an airport's variation of record does not repaint anything: the renumbering is a separate, discretionary, expensive act, which is why it tends to happen when a runway is closed for other work anyway.

Six repaintings that actually happened

A census cannot be wrong in a way the world will correct. This part can. Six airports are on record as having repainted their designators because the field moved, and for each one the old number, the new number and roughly the date are published. The model is told none of that. It is given the runway's geometry and asked in which year the rule stops returning the old number.

All six leads are positive, and that is the result rather than a disappointment. A crossing after its repainting would mean the model was broken. Crossings a few years before are exactly what the variation-of-record mechanism above predicts: the number goes stale first, the record is revised on the epoch cycle, and the paint follows when someone can close the runway. The median lead here is years.

One of the six also published a number rather than only a name. When Cork redesignated 17/35 as 16/34 in April 2018, the regulator's account gave the new headings as 164 and 344 degrees magnetic. The model, from the threshold coordinates and IGRF-14 at that date, returns .

Where the rule is not magnetic at all

Near a magnetic pole a compass is useless and the whole scheme fails. Canada files a formal difference against the ICAO paragraph quoted above.

Runways within Canadian Northern Domestic Airspace are designated with reference to the true azimuth because magnetic compasses are unreliable in the area. AIP Canada (ICAO), GEN, Differences from ICAO Standards, effective 9 July 2026

Those runways are marked with a T, so 17T is not a magnetic designator; the source data carries of them and this page refuses to score any of them, which is the exception removing itself.

The more interesting question is whether the rest of the world's numbering really is magnetic, and that can be measured rather than assumed. Where declination is near zero the two rules agree and tell you nothing. Where it is large they diverge, so score every runway under both and watch which rule wins as the fork widens.

At small declination the two rules split the runways almost evenly, which is what coincidence looks like. By twenty degrees the magnetic rule wins by more than forty to one. The handful of survivors on the true side at high declination are the polar exception showing through, and they are nameable individually.

What was thrown away, and why

Of runway records in the source, survive to be measured. The largest cut by far is simply missing coordinates: rows do not publish both thresholds, and without both there is no measured bearing to compare anything to. The rest of the cuts are judgements and are listed so they can be argued with.

One of those cuts is worth showing rather than asserting. Some records give the two thresholds a byte-identical latitude, or a byte-identical longitude, which asserts that the strip runs exactly due east-west or exactly due north-south. No survey returns that. Somebody took one end and stepped along a cardinal, and the bearing read back out is their assumption wearing the clothes of a measurement. You can see them: they pile up as a spike at zero in the distribution of true bearing modulo ten degrees.