Weather Blog

What do those runway numbers mean?

What do those runway numbers mean?
A Horizon Air jet takes off from Sea-Tac Airport Runway 34L
OK, so not exactly a weather topic today, but the weather is boring, so in perusing some of my past e-mails for inspiration, I found a discussion I had with a former Alaska pilot in talking about airport elevations.  But I also learned some neato stuff about how aviation works in general, and it's topical with the opening of Sea-Tac's new third runway last week.

The first question was about the elevation of Sea-Tac Airport for when I created this list of elevations of cities and landmarks across Western Washington.  I had originally had the elevation at around 330, which is what Google Earth had as the rough elevation. But the pilot says the official elevation of Runway 16L is at 433 ft.

Turns out, the airport slopes a little from one end to the other to where one end of the runway is 90 feet higher than the other. It's a slope of 0.7% over two miles, and the pilot says runways are allowed to slope as much as 2%. But the official airport elevation is listed as the runway at its highest usual point, which makes sense, because you want to err on the side of being too high than being too low.

But a secondary conversation revolved around the names of the runways.  You might have seen big numbers painted on the end of runways, or heard the lingo.  Sea-Tac's main runways are (now) 16L, 16C, and 16R, and then there's 34L, 34C and 34R.

Wait a minute, 6 runway names? But there's only three there? The trick is, each runway has two names, depending on which way you're landing.

The numerical number is assigned based on the compass heading the runway is pointed to, multiplied by ten.  So runway 16 means the runway is pointing 160 degrees, or just east of south. (The number is rounded to the nearest ten degrees, so 164 degrees gets a runway 16 designation, but if it were 165, it would be named 17). 

[Note that these headings are based on magnetic north pole, which slowly drifts over time. My pilot says there are occasions when an airport will have to change runway names if this drift pushes them to the next round number (i.e., from 164 to 165 degrees)]

The "L", "C" and "R" are just simple "left, center and right".  Runway 16L means it's the left runway that points at 160 degrees. 16R means it's the runway on the right that points 160 degrees.

But if the winds change, air traffic controllers will have the planes land from the other direction, and now the same runway has a different name -- in Sea-Tac's case, 34L, 34C and 34R, because from the pilot's perspective the runway is now pointing just west of north.

So to put this all together, 16L is the same runway as 34R, 16C is also 34C and 16R is the same as 34L.

(**Correction from original posting. This image pre-dates the completion of Sea-Tac's third runway. The pavement on the far left is actually a taxi-way. The labels now sit where the third runway is today.)

Now, if you have a really big airport, like Dallas-Fort Worth or LAX, that have runways pointing the same way but on other sides of the airport, they will make one set of runways one number off, say 17R instead of 16R as to not cause confusion. If you're flying into one of these airports, you'll know this ahead of time.

To tie this to weather, you can watch the flight path of planes flying over Seattle as a nice weather vane. With Seattle nestled between the Olympic and Cascade Mountains, the winds are predominately out of the south or north here. 

Planes like to land and take off in a due head-wind, since that gives them extra lift should they need it, and that's why Seattle's airport runways are aligned north-to-south.  (Other cities' airports are also aligned in such a way as to try and take advantage of local prevailing winds.)

That means in a north wind, planes will land from the south on Runway 34 and take off to the north.  When it's a south wind, the planes land from the north on Runway 16 and take off to the south.

North winds are generally an indicator of nice weather, while south winds are typically associated with cloudy and rainy weather. So when you see the planes coming over Downtown to land to the south, it usually means it's cloudy or rainy (if, for some reason, you haven't noticed the sky behind the jets.)

So next time you're landing at Sea-Tac and you see what runway you're heading for, you'll know what weather to expect when you walk off the plane :)