Like Larry the Flying Guy on Facebook!

Thursday, December 26, 2013

Why do runway symbols change at 8,069 feet?

Like Larry the Flying Guy on Facebook:

Follow on Twitter, too:

A popular question during the oral portion of a checkride is why does one airport have a runway symbol that looks like a circle but another one has a symbol that looks like an actual runway and no circle?

Sectional excerpt from Click image to embiggen.
The one in the upper left is a runway in a circle, while the one at the bottom right has three runways. Why?

Well, this excerpt from the FAA's Aeronautical Chart User's Guide has this in the legend, as does the back of every paper sectional you've ever bought:

OK, that means the airport at the top has a runway of less than 8,069 feet. If we look closely at its data block, the 80 in the bottom right means its runway is 8,000 feet, which is indeed less than 8,069.

But you're still wondering why the crazy number 8,069 instead of something less random-seeming. Well, first you have to understand that the symbols are drawn to scale. That means the bigger the runway, the more of the symbol it is going to take up. For example, compare the symbol above to the little Topton airport from the bottom of the first excerpt:
See how its symbol is smaller and the runway takes up less of it? That's because the 32 at the bottom right means that it's only 3,200 feet long. Smaller runway, smaller symbol. Bigger runway, bigger symbol. That's what "drawn to scale" means, after all.

Now that you understand that, you're still wondering what's so magical about 8,069. And I'm only going to tease you just one tiny bit more with this excerpt from page 9 of the Chart User's Guide:

"Runway length is shown to the nearest 100’, using 70 as the division point; a runway 8070’ in length is charted as 81, while a runway 8069’ in length is charted as 80."

So that explains the 69: that's the last number before the chartmakers round up.

The only thing left to explain is the 80. For that, remember how I just explained that runways are drawn to scale? At 81, they get too big to fit inside the circle anymore. That means they just draw the runway itself instead of a circle.

Fortunately, you won't need to get into that much detail on a checkride. In fact, if you were to go into that much detail, you might end up teaching your examiner something they didn't know. All you need to remember is runways stop being circled when they're over 8,069 feet long.

EXCEPT... for what we'll get into in the second part of this post, which is titled "How can two things be the same and different at the same time?" Look for the answer to that question on Saturday! (Hint: if you looked closely at the first chart excerpt of this post, you may have noticed an unusual coincidence.)

No comments:

Post a Comment