Just As I Thought

Cleared for landing

Today’s column by John Kelly in the Washington Post has some fascinating facts. You learn something every day!

When I first moved here, Reagan (National) Airport’s primary runway was designated 18/36, meaning directly north/south. A few years ago, it suddenly became runway 1/19. Had the 18/36 ID been erroneous all those years, or did they redo the runway and rotate it 10 degrees in the process?

Paul Gatza, Alexandria

The easily bored might turn to something more scintillating right now — the stock tables, for example, or a George Will op-ed. The rest of us are going to have a quick lesson in runway nomenclature.

Picture a runway, which is basically a straight line on the ground. Now imagine you are standing on that runway holding a compass. If your runway goes from north to south, the compass’s needle will be in perfect alignment with it, with the red end pointing to 360 degrees. The other end will point 180 degrees away — to 180 degrees, as it turns out. This hypothetical runway would be known as 18/36.

Why? Well, to get a runway’s number, look at its compass heading, round it up or down to the nearest 10 degrees — i.e., 174 becomes 170 — then lop off the last digit. The second number is the “reciprocal” of the first, that is, it is 180 degrees away. The lowest number always goes first.

“It makes it easy for pilots to orient themselves,” the FAA’s William Shumann said.

Okay. So why did National’s runway change names?

“There are two flip answers,” William said. One is that the airport rotated slightly. “The other is continental drift at work.”

The real reason is no less weird. The Earth’s magnetic field is not fixed and permanent. The molten iron of the Earth’s core — the place where the magnetic field is generated — boils around like a pot of tomato soup on the stove.

“The magnetic field lines that leave the core and extend out are kind of frozen into the liquid core,” said John Hopkins University’s Peter Olson. “As the fluid circulates, those magnetic field lines are dragged around. That dragging motion of the field lines produces the temporal changes that we measure at the Earth’s surface.”

In other words, the magnetic field keeps shifting slightly.

“We got to the point a few years ago where the runway went from something like 184.5 [degrees] to 185.5,” said the FAA’s William Shumann.

They had to round up instead of down and Runway 18/36 became Runway 1/19.

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