A runway’s number comes from its magnetic heading, rounded to the nearest ten degrees and divided by ten. When Earth’s magnetic field drifts far enough that the rounded heading changes — a shift of more than five degrees in operational practice — the painted number no longer matches the compass.
That trigger is real at airports, but it’s not stated as a hard numeric rule in any FAA order. What the FAA does codify is that location identifiers are permanent, changeable only with strong, documented safety justification.
In 2009, the asphalt at Fairbanks International didn’t move an inch, but the numbers painted at each end of the main runway still changed — 1L/19R became 2L/20R. Pilots, approach charts, and flight management databases all had to catch up. What shifted wasn’t the concrete; it was the planet’s magnetic field.
Runway numbers are navigational shorthand, not addresses: a pilot lining up on runway 19 expects a compass to read near 190 degrees. As magnetic north drifts, that reading slowly changes while the paint doesn’t. When the gap between the painted number and the actual heading grows wide enough, airports rip up signage, rewrite charts, and push updates to flight management databases.
Most coverage treats this as a quirky curiosity — the airport equivalent of a street-name change. The more useful story is that renumbering is a threshold-driven safety decision landing on a schedule nobody publishes. And the threshold itself is thinner on paper than the industry likes to admit.
How a runway number is born
Take a runway aligned to a magnetic heading of 184 degrees. Round to the nearest ten — 180 — and drop the last digit: runway 18. Same runway, same alignment, but magnetic north has since drifted.
The compass now reads 186 degrees. Round that, and you get 190 — runway 19. The painted 18 no longer means what the pilot’s instruments say.
Every runway gets two numbers, one for each direction of approach, separated by 180 degrees: an 18 at one end means a 36 at the other, and the rule behind it is simple arithmetic. Round the magnetic heading to the nearest ten degrees and drop the final zero; north is 36, not zero, because 360 rounds to 36.
That rounding is where the so-called five-degree rule comes from. A designator stays valid as long as the actual magnetic heading lands within a few degrees of the number painted on the threshold. Drift beyond that pushes the heading into a different ten-degree bucket, and the number stops matching what the compass reads.
The five-degree rule that isn’t written down
Here’s where it gets less tidy. Ask most aviation writers why runways renumber, and they’ll cite the five-degree deviation threshold. The FAA’s own documents tell a more careful story.
As the Federal Aviation Administration puts it in the JO 7350.9GG location identifier order: “Location identifiers are considered permanent and will not be changed without strong and documented justification, primarily concerning air safety or a significant change of landing facility status.” The same order fixes no numeric heading tolerance for when a runway designator must change.
That figure circulates as practical trade guidance, grounded in rounding arithmetic rather than a citable regulation, while the FAA and NOAA aim for a narrower target: navigation data should track the World Magnetic Model’s computed variation to within roughly one degree. The model is precise; the threshold is convention.
High latitudes renumber first
Near the Arctic, magnetic variation changes more quickly than it does in Florida, so high-latitude airports feel the drift sooner. Fairbanks International is the clearest U.S. case: its renumbering came once magnetic variation had moved the rounded bearing into a different ten-degree segment of the compass.
None of the FAA or NOAA records reviewed here gives drift rates for a specific airport, or a computed interval between renumberings, for cities such as Anchorage, Edmonton, Helsinki, Reykjavik, or Honolulu. The general pattern — high-latitude airports renumber more often — is well established; the verified interval data is missing.
Northern Canada and Greenland show the exception already working: airports there may use true bearings when magnetic variation is extreme or unstable. The global default is magnetic. The exceptions are already flying.
Wichita shows what changes now
Wichita’s Dwight D. Eisenhower National Airport is the latest U.S. data point. A rehabilitation project currently underway renumbers runways 1L/19R to 2L/20R and 1R/19L to 2R/20L, and crosswind runway 14/32 to 15/33. The published budget runs to $21,877,422, but the notice doesn’t separate the cost of repainting and signage from the broader runway rehabilitation work.
When a designator changes, the cascade is predictable — ground signage, instrument approach procedures, aeronautical charts, and flight management system databases all require updates. What’s less predictable is efficiency: the Wichita documents don’t break out the renumbering cost, sign count, or timeline from the rehabilitation work. The absence of that breakdown is itself part of the story.
True north is already in the database
Under current ICAO rules, runway direction indicators use magnetic degrees — the global default. But ICAO’s TRUE-AG working group is developing a framework and concept of operations for a transition to true north as the reference for headings and tracking in air operations — with no global mandate in place.
The technical groundwork already exists. Behind the scenes, the AIXM runway-direction class and ICAO’s AIP data model already carry both trueBearing and magneticBearing fields. Every runway direction must include a true bearing.
A full switch would demand dual fields stored for each runway globally, synchronized AIRAC transition planning, avionics compatibility work, and charting changes worldwide. It would also require coordination with every state whose procedures assume magnetic references.
The gap between “in the model” and “on the signs” is the unresolved debate. The exceptions prove the concept works locally, but whether the rest of the world follows remains open.
Why the pole moves at all
Earth’s magnetic field comes from the planet’s molten core, and that churn pushes magnetic north unevenly, in fits and starts. A runway’s number is fixed on the ground, but the reference it points to never stays still.
The FAA relies on the World Magnetic Model to track that moving field and keep navigational products aligned. A runway’s number therefore inherits the motion of a planet-scale dynamo that never stops moving — the sign is already outdated the day crews bolt it in place.
Questions? Answers.
How do airports determine when they should change their runway numbers?
A runway’s designator comes from its magnetic heading, rounded to the nearest ten degrees and divided by ten. When magnetic variation drifts far enough that the rounded heading no longer matches the painted number — as happened at Fairbanks in 2009 when 1L/19R became 2L/20R — the airport renumbers. FAA and NOAA guidance aim to keep the magnetic variation used in navigation aligned within about one degree of the World Magnetic Model’s computed values.
What does 9 and 27 mean on a runway?
Runway numbers represent the runway’s magnetic heading to the nearest ten degrees, with the last digit dropped. A runway numbered 9 points to roughly 90 degrees magnetic, east, and a runway numbered 27 points to roughly 270 degrees magnetic, west.
Why are runways numbered 27?
A runway numbered 27 aligns within a few degrees of magnetic heading 270 degrees, which is due west. The number is the runway’s magnetic bearing rounded to the nearest ten degrees and divided by ten.
Why does runway 36 face north?
In the runway numbering system, 36 stands for 360 degrees — magnetic north. The number comes from rounding the magnetic heading to the nearest ten degrees and dropping the final zero.
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