Sheboygan Airport Runway Gets New Numbers

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Magnetic compass.What does a giant ocean of hot molten metal have to do with airport runways?

It causes notices to airmen (NOTAMs) like this:
SBM RWY 3/21 NOW RWY 4/22 WEF 1206262200

Translation: Sheboygan County Memorial Airport’s runway 3/21 is now runway 4/22, as of June 26, 2012 at 5 p.m. local time.

Why? Did the runway move? No. The earth’s magnetic north pole moved.

Earth's Core
The earth’s outer core, made of liquid iron, is responsible for changes in the earth’s magnetic field.

Liquid iron in the earth’s core influences the earth’s magnetic field, and it’s always shifting. The earth’s magnetic north pole moves around. This is where a compass points — not to be confused with the geographic north pole.

Since pilots rely on a magnetic compass, airport runways are numbered according to their magnetic heading. The magnetic heading gets rounded to the nearest ten degrees, then the last digit is removed to create the runway number.

Each end of a runway gets a different number. The two numbers are reciprocals of each other (180 degrees opposite).

Aerial photo of an airport runway
A runway numbered “28” would have a magnetic heading between 275 and 284 degrees. (stock photo)

For example, a runway that lines up not quite east-west, say 98 and 278 degrees, would be numbered 10 on one end and 28 on the other. Pilots would refer to “runway 10″ when taking off or landing to the east, and to “runway 28″ when taking off or landing to the west. The whole piece of pavement could be called runway 10/28.

Runway digits are pronounced separately, such as “two-eight” for 28.

If the magnetic north pole moves enough, a runway’s heading might need to be rounded differently, causing a change in numbering.

What used to be runway 3/21 at Sheboygan actually points about 37 and 217 degrees these days. Since those headings round to 40 and 220 degrees, the runway needs to be called 4/22.

The other runway at Sheboygan, runway 13/31, points 132 and 312 degrees. It’s safe from a numbering change for now. The FAA remeasures runway headings every five years.

Official FAA publications like terminal procedures (approach charts), airport diagrams, and the airport/facility directory (A/FD) don’t necessarily update as soon as the runway’s new paint is dry. Pilots need to check NOTAMs to learn about mid-cycle changes.

Current terminal procedures and the A/FD don’t reflect Sheboygan’s changes as of this writing. These publications are effective until July 26th. Until the publications catch up, the runway markings and signs won’t match what’s in the books.

Most things affecting aviation happen above ground, but it’s an underground phenomenon causing runway numbers to change. More runways will need to be renamed as the earth’s magnetic north pole continues to move over time.

The FAA just added a new runway safety section to the Pilot’s Handbook of Aeronautical Knowledge. You can download the new Runway Incursion Avoidance appendix as a PDF file. Updates to the printed books are on the way.

To learn more about airport signs, markings, runway incursion avoidance, check out the AOPA Air Safety Institute’s free online Runway Safety course.

AOPA Runway Safety Course banner
Click to take AOPA’s Runway Safety online course (requires Flash).

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