Seatac landings
This is bordering on “transit”, but it’s still something I’ve been wondering for a while. Maybe someone out there (James?) knows how to read all the documents and can explain this to me…
I’ve noticed that planes landing at SeaTac airport almost always seem to fly out over the Puget Sound, circle around, and then come in for landing from the north. Even when they’re coming in from almost the direct south. I’ve landed in this direction, I’m almost certain, 100% of the times I’ve arrived in Seattle by plane.
Now, obviously, if the tower is landing planes from the north, they won’t suddenly decide to let a random plane come into the same runway from the south. Duh.
But I guess what I’m wondering is if the decision to land from the north in (almost?) all cases is one of weather, obstruction, convenience, what?
I poked around a bit on the Internet and found some details on this site: http://www.airnav.com/airport/SEA. There I discovered that the northern approach landings are identified as coming in on 16L or 16R runways. Southern approach landings are identified by 34L and 34R.
So I guess my question is better rephrased: “why do they only use 16L and 16R for landings and not 34L and 34R”? Or perhaps even: “under what conditions do they use 34L or 34R for landings?”
I also notice from this website and some of the PDF files I poked through that there is less landing lighting on the 34 runways, and at least 34L has some obstructions.
But is there something else? If anyone knows, or can provide reasonable conjecture, please let me know so I can stop wondering about this. Thanks! 
Jamus said,
January 7, 2006 at 11:57 am
There are a number of factors that determine traffic flow at an airport.
1 – Winds
Pilots prefer to takeoff and land into the wind. It is the speed of air over the wings that generates lift, so the higher the headwind, the higher the lift at a constant groundspeed…i.e., they can land at a lower ground speed with a headwind than with a tailwind, which makes it safer and easier to stop.
As a result, generally airports build runways aligned with the prevailing winds. In most of the U.S., this means east-west aligned runways, with the western runways being the most used. Much of the west coast, however, and some other regions have different prevailing winds, and in Puget Sound, they tend to be north-south.
Also, big airports do not usually like arrival traffic coming from one direction and departure traffic coming from another, as it creates more traffic conflits. It’s smoother to have planes arriving and departing in the same direction, and it’s hard to change over because it interrupts the flow, so once they start using 16 L/R, they probably keep doing so all day [sometimes even if it means operating with a light tailwind].
2 – Obstructions:
Big airplanes generally descend more slowly than they climb. If there were an obstruction to the south, they might prefer to land from the north. I don’t think this is a big issue at SEA, but I don’t have a chart to check. Boeing Field might have something to do with it, too…
3 – Navigational Aids
Planes have arrival procedures based on navigaitonal aids. There are several stations on the ground [and imaginary "intersections" in the air] near SEA that help route traffic. The typical arrival procedures may be to fly to one of these to help sequence traffic.
3 – Noise:
This is the most sensitive one with me, but I’ll try to save the editorializing for later. SEA handles ~1000 flights each day, mostly jets. That leads to a LOT of noise around the airport – especially on the ends of the runways. The airports go to extraordinary lengths to save the residents [who generally chose where to build/buy there houses] from this irritation. SEA actually pays to insulate people’s houses. Also, there are “noise abatement procedures,” one of which requires descending over the water, where there are no houses. You may also notice turns immediately after takeoff to route departing traffic over the water and to a certain altitute before turning east. Departing is generally the more noisy phase of flight – planes are using higher throttle at takeoff, so it could be that at SEA, the noise sensitive neighbors live north of the airport, so they airport might prefer using 16 L/R when winds are below 5 knots, for example.
More info is available at: http://www.seatac.org/community/environment/noiseabatement.shtml
Evan said,
January 10, 2006 at 7:40 am
That makes a bunch of sense, and it’s about what I thought. I hadn’t considered the noise issue too much, but I totally should have! When there was talk last year of opening up Boeing Field for more commercial traffic, it was the neighborhood concerns about noise that eventually served as the official reason to shut down the plan… Thanks James!
Jamus said,
January 10, 2006 at 9:45 am
No worries. I can talk all day about flying. And wow, I promise I know how to spell and count. I guess this is what spell check does to grammatical attention to detail.