Wingspan: Did you know?

by "ttyymmnn" (ttyymmnn)
Published 12/14/2017 at 12:35

Tags: wingspan ; Planelopnik
STARS: 23

Whenever you go flying, your aircraft takes off from and lands on a runway that has a two-digit number, and maybe a letter, painted at each end in characters that are 27 feet tall. But what do those numbers and letters mean?

In the earliest days of aviation, planes took off and landed from an airfield, so named because it was nothing more than a big grass field that could be used for takeoffs or landings in any direction. When a pilot wanted to take off, he simply checked the windsock and pointed his plane into the wind, since a headwind helps generate lift and allows for shorter take off runs. Likewise, landing into the wind permits lower landing speeds and shorter landing distances. As airfields became formalized, it was common to see three runways laid out in a triangle, so crosswind landings and takeoffs could be avoided. But, as airplanes became larger and heavier, paved runways became necessary, and there just wasn’t enough room to fit all those different-direction runways into the same place and handle the greater amount of traffic. Parallel runways became necessary, so two or more runways could be used at the same time.

Since taking off and landing into the wind is preferred, airport designers study the weather and the prevailing winds at the location where they want to build an airport. Then they collect the data into what is called a wind rose , similar to a compass rose , to determine the prevailing wind direction and speed. The runways are then oriented to take advantage of the wind for most months of the year. Below is an example of a wind rose for Austin, Texas, with data collected over an 8-year span.

From this data, we can see that the wind blows out of the south or south-southeast for a majority of the year, mostly during the warmer months. When cold weather fronts blow through Austin, the wind usually shifts out of the north or northeast, and this is shown on the wind rose. Today, Austin-Bergstrom International Airport (KAUS) sits on the location of Bergstrom Air Force Base , which opened in 1942. Like other airfields of its era, Bergstrom had a traditional triangle of runways . But when it became a Strategic Air Command base in 1948, long runways were built to accommodate the new generation of heavy jet bombers. Based on the prevailing winds in Austin, those two long runways were built almost due north-south. Below is what KAUS looks like today, with its two parallel runways.

The giant numbers painted on the ends of the runways at KAUS are 17L (seventeen left) and 17R (seventeen right) if you are taking off or landing to the south, and 35L and 35R if you are taking off or landing to the north. These numbers, and the numbers on all runways that follow guidelines set up by the International Civil Aviation Organization (ICAO), are determined by the magnetic compass heading of the runway in the direction of travel. Since degrees of a circle go up to 360, the third digit is dropped, and all headings are rounded to the nearest 10. So, runway 17 has a bearing of 170 degrees, while the reverse runway, 35, has a bearing of 350 degrees. (Notice that the difference between 17 and 35 is 18, or 180 degrees.) A runway of less than 100 degrees would be labeled 9 for 90 degrees, or 8 for 80 degrees, and so on (other counties, like the UK, use two digits, such as 09 or 08). The designation of L or R for left or right is determined by the relative position of the runways to each other, based on the direction of travel. A single runway will have only the compass heading painted on it, since its not to the left or the right of anything.

But what about airports that have more than two runways? For airports with three runways, they are numbered as usual then designated L, C and R for left, center and right, based on the direction of travel. And, if an airport has four (or more) parallel runways, one set of runways will be designated one digit (ten degrees) off from the other to avoid confusion. Let’s take a look at this runway map of Dallas-Fort Worth International Airport (KDFW).

We can see that the four main runways run north-south based on the prevailing winds in the area. The two runways on the west side of the airport are designated 18L/18R and 36L/36R (truly North-South at 180 and 360 degrees), while the three runways on the east side of the airport are designated 17L/17C/17R and 35L/35C/35R (10 degrees off, though they are actually parallel to the runways on the other side of the terminal). The two shorter runways running NW/SE are designated 13L/13R and 31L/31R (130 degrees and 310 degrees).

So, runways are numbered based on their magnetic compass reading, but what about the fact that magnetic north (as opposed to true north or geographic north) is always moving? As magnetic north wanders under the influence of the Earth’s core, runway designations may need to change with it. Since the runway directions are rounded to the nearest 10, it would take a significant change in magnetic north to affect the runway numbering. For example, a runway that lies at 228 degrees will be designated 23 (for 230 degrees), and even if the magnetic pole moves to a point where the runway is lying at 232 degrees, it will still be rounded to 230 and remain runway 23.

But sometimes, enough change in magnetic north occurs that runways must be renumbered. Though such occurrences are rare, they do happen. In 2009, the single runway at London Stansted Airport (EGSS) changed from 05/23 to 04/22 due to a shift in magnetic north. But these changes, even if necessary, aren’t popular. Any time a runway designation changes, the runway must be closed to have the numbers repainted, directional signage for taxiing aircraft must be changed, and navigational charts must be reprinted to reflect the new heading. Fortunately for London Stansted, it is estimated that it will be at least 48 years before another change in runway heading may be required.

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If you enjoyed this post, please join in the conversation and let me know. For more posts about airplanes, aviation history and aircraft oddities, set your course for   Wingspan .

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Replies (32)

"For Sweden" (rallybeetle)
12/14/2017 at 12:40, STARS: 5

And if you fly for Air Canada, taxiways are also runways.

"ttyymmnn" (ttyymmnn)
12/14/2017 at 12:42, STARS: 3

Harrison Ford Airways, too.

"TheRealBicycleBuck" (therealbicyclebuck)
12/14/2017 at 12:43, STARS: 6

Fantastic post! Single-handedly, y ou are dragg ing the rest of Oppo into the world of aviation!

"Highlander-Datsuns are Forever" (jamesbowland)
12/14/2017 at 12:47, STARS: 0

This is good stuff. I feel smarter already.

"ttyymmnn" (ttyymmnn)
12/14/2017 at 12:48, STARS: 1

Thanks! It makes me feel good to know that people are learning from these things. Once a teacher, always a teacher. Thanks for reading.

"ttyymmnn" (ttyymmnn)
12/14/2017 at 12:49, STARS: 3

Well, I’m trying. Not really dragging, though, just using you all as a captive audience to share my obsession with aviation. Thanks for the kind words, and thanks for reading.

"For Sweden" (rallybeetle)
12/14/2017 at 12:54, STARS: 2

There will be no dragging until we get more tail dragger #content

"ttyymmnn" (ttyymmnn)
12/14/2017 at 12:59, STARS: 3

"Out, but with a W - has found the answer" (belg)
12/14/2017 at 12:59, STARS: 1

EBBR switched from 02/20 to 01/19 in a single night in 2013, which does seem impressive considering the logistics involved.

Do you have an idea of what goes into deciding the number of parallel runways on an airport? Looking at the top 5 airports for passenger traffic, the number varies a lot (KATL 5, ZBAA 3, OMDB 2, KLAX 4, RJTT 2).

"ttyymmnn" (ttyymmnn)
12/14/2017 at 13:00, STARS: 1

I have no idea. I would guess that it’s all based on growth, and whether or not there is room to expand.

"DAWRX - The Herb Strikes Back" (karsonkinja)
12/14/2017 at 13:03, STARS: 0

This was really interesting. It’s great to see actual quality content here on oppo. I appreciate it, keep it up!

"WilliamsSW" (williamssw)
12/14/2017 at 13:05, STARS: 0

Nice...

"ttyymmnn" (ttyymmnn)
12/14/2017 at 13:07, STARS: 3

Thanks! Interesting you mention “quality content.” Back when Kinja was first rolled out, it was new and shiny (and sort of worked), with big pictures and a lot of encouragement to create quality material. For a while, it worked brilliantly. People got away from shitposts and one liners and spent time writing. But as Kinja got pared back, it returned to almost exactly what it looked like in pre-Kinja days, and the quality of work dropped as well.

The other problem is that if you spend lots and lots of time writing something good and lengthy, people just won’t read it. It’s been a challenge finding interesting material while also keeping it to a readable length.

But I’m having lots of fun writing these things, and learning along the way. It’s become an enjoyable hobby, so even though I don’t always get lots of replies, I’m gonna keep on doing it because I like it.

Thanks for the kind words, and thanks for reading.

"ttyymmnn" (ttyymmnn)
12/14/2017 at 13:07, STARS: 2

I wondered if I wanted that in my serious post, but I just couldn’t help myself.

"WilliamsSW" (williamssw)
12/14/2017 at 13:10, STARS: 2

It was what came to my mind too when I read For Sweden’s comment, so I can’t blame you!

"user314" (user314)
12/14/2017 at 13:13, STARS: 0

Ah, now this takes me back to 11th grade and Air Transportation Systems. Learning this kind of stuff was almost as interesting as time on the simulators. Are you going to do another article on tower and runway lights next?

Bonus material: here’s one of the machines they use to clean those runways (and also remove markings if need be)

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"ttyymmnn" (ttyymmnn)
12/14/2017 at 13:18, STARS: 3

I’m trying to figure out what to write next, but a piece about runway and taxiway markings would be an obvious segue.

"ttyymmnn" (ttyymmnn)
12/14/2017 at 13:19, STARS: 2

What is that saying? Great minds think alike? Or was it warped minds think alike?

"WilliamsSW" (williamssw)
12/14/2017 at 13:24, STARS: 1

Great minds. Definitely. Let’s go with that - and hope no one notices.

"Mercedes Streeter" (smart)
12/14/2017 at 13:27, STARS: 0

I would say he’s piloting the rest of Oppo into aviation. I mean, my gaming PC exists almost entirely for avgeeking purposes...ttyymmnn is part of the reason for that!

"Mercedes Streeter" (smart)
12/14/2017 at 13:30, STARS: 1

How about the airline industry of the former Soviet Union? It’s interesting reading about how those airliners are designed compared to their western counterparts. Soviet airliners are a marvel. Yak-42s to Tu-144s, to An-124s, oh my!

"Mercedes Streeter" (smart)
12/14/2017 at 13:32, STARS: 0

ttyymmnn, with every post you make me want to become a pilot even more. Also, you’re pretty much the inspiration to me building my “DC-10-50TI” sim rig. :3

Thank you for this! I was always confused about why runways are numbered the way they are. Now I know! :D

"ttyymmnn" (ttyymmnn)
12/14/2017 at 13:32, STARS: 0

That might be opening up a can of worms. I’m trying to keep these things at an easily readable length. The other problem is that I know next to nothing about Soviet aviation. While it would be good to learn, it’s always easier to write about something you already know a bit about. Otherwise, it becomes a research paper, and I’ve written enough of those in my lifetime!

That said, if you have any ideas for this “Did you know?” series, I’d love to hear them.

"ttyymmnn" (ttyymmnn)
12/14/2017 at 13:43, STARS: 1

Time for an interesting story (well, I think it’s interesting). Right after I completed my doctorate, I began applying for jobs teaching college. I got three final round interviews (thankfully, I never got the jobs, since working in academia would suck), and, in each one, they asked, “Why do you want this job?” Well, duh, I need the money. But I always answered, “I can look back through my life as a musician and say, ‘If it weren’t for so-and-so, I wouldn’t be here today.’ If I could be in a position for just one person to say that about me, then I would feel like my work had been a success.” Though it may sound like BS, it’s an honest answer.

A few years later, I recounted that story to my old high school Latin teacher. And I told her that she had been just that sort of a person for me, a very positive and steadying influence at a very important time in my life. And she said, “You know, you might just end up being that person for somebody else, and you’d never know it. You might play, and there might be somebody in the audience, and you will inspire them to become a musician, but you won’t know it.” I thought that was a very powerful thing, and the thought inspires me every time I play.

So, if my work here serves as an (intended) inspiration, that makes me feel really good about the hours I spend writing, and I appreciate your sharing that. I don’t really do it to be an inspiration; I’m merely sharing my passion with a somewhat captive audience, and I hope I can teach people a thing or two along the way.

Thanks for reading, and keep me posted. Anybody who flies is an inspiration to me. Even though I write all about aviation, I don’t think I could ever become a pilot. It’s hard enough playing trumpet!

"PartyPooper2012" (PartyPooper2012)
12/14/2017 at 15:15, STARS: 1

Through my own curiosity, I took few lessons in piloting a private plane. I learned this there, but this is still an interesting post and I learned a couple of things here as well.

Thanks.

Post more stuff. Debunk some of the myths in popular movies - Die hard / Sully / Snakes on a plane :)

"ttyymmnn" (ttyymmnn)
12/14/2017 at 15:24, STARS: 1

Glad you enjoyed it. And I like your debunking idea, though I have never seen Sully or Snakes on a Plane . Two of my favorite BS airplane scenes are in Air Force One, where the passengers parachute out of a nonexistent rear ramp, and Executive Decision, where the commandos gain access to a 747 by flying underneath it in an F-117, connecting through an extending tube, and climbing into the cargo hold. What a load of crap.

"PartyPooper2012" (PartyPooper2012)
12/14/2017 at 15:35, STARS: 1

I have never seen snakes on a plane. I was just looking for a funny sounding movie with planes in it.

Sully is actually pretty good. My wife is terrified of flying so she cries and wants me to turn it off / change the channel every time it comes on. Wuss. :P If ever I was on a plane and that was the case, I wouldn’t want anyone BUT him to be the pilot. No offense.

It’s worth a watch.

"ttyymmnn" (ttyymmnn)
12/14/2017 at 16:01, STARS: 0

I would watch it for the actual flying and landing sequences, but all the manufactured drama would turn me off.

"TheRealBicycleBuck" (therealbicyclebuck)
12/14/2017 at 18:36, STARS: 1

Bad dog!

"MM54" (mm54mk2)
12/14/2017 at 19:14, STARS: 0

I fly a ton for work, and on Sunday I was taxiing to the gate and it struck me “why the hell are these numbered so nonsensically?”

It makes sense now.

My suggestion for the next one is what all the also-nonsensical signs on the taxiways means

"ttyymmnn" (ttyymmnn)
12/14/2017 at 19:35, STARS: 1

I’m already working on the next one, which will be about the markings on the runways. I might follow that up with one about taxiway markings and lighting.

" The Compromiser" (charger)
12/14/2017 at 22:28, STARS: 1

I did not know that. Neat!