This article has the airport saying “don’t worry about it” vs pilots expessing concern regarding a runway visibility/layout issue at SeaTac. How serious a problem is this, really?
Your linked article says that the taxiway is very lightly used. IANA pilot, but it seems to me that by not fixing the problem, they’re just asking to have a commercial airliner land on a small plane using the taxiway at some time in the future.
Google Maps, and Google Earth for those who have it installed, show the layout and color of the Sea-Tac runways and taxiway very nicely. You can clearly see the “X” that was put on the hillside just north of Taxiway Tango to mark its non-runway status. Google Maps link here.
[BTW, the runways at Sea-Tac seem to run almost exactly North-South, which I thought meant that they should be designated 18L and 18R landing from the North, and 36L and 36R landing from the South. Anyone know why they’re designated 16L/34R and 16R/34L? They’re clearly nowhere near 20 degrees off from a North/South alignment. I’ve seen this at other airports too.]
Just a WAG. Could it be the differance from magnetic north? What is the declanation(sp) at SeaTac?
In conditions of poor visibility, airplanes landing on taxiways happens other places, too. In fact, in other times and places airplanes have occassionally missed the airport entirely and landed on a road next to the airport. Or even the wrong airport. Fortunately, in the overwhelming majority of incidents no one gets hurt (just severely embarassed).
In one sense, landing on a taxiway is no big deal - both the taxi pavement and the airplane can handle it. In some circumstances (something horrible happening on a runway) air traffic control or pilots may elect to delibrately land on taxiways as the safest option in an emergency.
The problem, of course, in unintentional landings, and possible collisions as a result. It’s certainly possible. Not to probable in any given episode, but yes, it would be a Bad Thing.
For whatever reason, Taxiway Tango is visually confusing to pilots. Steps taken to remedy the situation - which have been used successfully in other places - appear to be not entirely effective here. Why? Some combination of fequent local weather phenomena and physical attributes of that particular location.
Solutions:
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Remove taxiway. Not likely, but it would solve the problem.
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Make the taxiway less distinctive. I’d suggest painting it to match the ground surrounding it, since the current problem is that it’s more visible than the runways.
As a helicopter pilot (and when I was current), I landed on the taxiway all the time! 
Just a WAG. Could it be the differance from magnetic north? What is the declanation(sp) at SeaTac? Roughly 20[sup]o[/sup] east sounds right.
Here is a chart of isogonic lines. At the time this chart was made Seattle was somewhat more than 20[sup]o[/sup] east. The runway number is assigned as the magnetic heading (nearest 10[sup]0[/sup]) at the time the runway is first build. I’m pretty sure it isn’t changed with updating of the magnetic deviation charts.
Apropos of nothing:
When I was a kid my dad was in the FSS at the Barstow-Daggett Airport. Winds can be high in the Mojave Desert, and sand storms are not uncommon. A young couple were air-camping in their yellow (natch!) Piper J-3 Cub. J-3s are so slow that given enough wind they fly backwards. (True for any aircraft, only it took less for that to happen to a Cub than, say, a Skyhawk.) It would have been dangerous for them to continue their flight, so they were stuck. Dad let them stay in the spare room in his house at the airport.
When the wind died down, the guy gave me a ride in the Cub. How cool is that? 
What surprised me though, was that he didn’t bother to use the runway. He took off from the ramp. I remember the Chevron sign passing by the port wing. DAG is an uncontrolled airport, and there was no traffic that day. That was my only flight in the airplane that has become, to the non-flying public, the generic name for a light aircraft. I’ll propbably never fly or ride in a Cub again. But I’ll always remember taking off on that warm desert morning in the yellow fabric-covered classic.
I took advanced multi-engine flight training at Fort Sumner, NM. We flew Cessna AT-17’s. You could slow fly then at about 35 mph indicated in calm air.
Anyway there were some fierce winds at Ft. Sumner and on occasion the wind would come up strong after the planes had taken off, usually for the afternoon flights. The drill then was for guys to line up on each side of the runway and when a plane would land, nearly straight down like an elevator, they would run out and grab the wingtips and walk the plane to the tiedown points to help prevent upsets. I’m quite sure a person couldn’t hold the thing down in a really strong gust, but we didn’t have any flipovers.
On an instrument flight I had with an instructor he was taxiing fast to get to the takeoff runway in a hurry and suddenly we were airborne off the taxiway. He landed and continued taxiing but a little slower.
They do change from time to time, mostly when convenient. I used to fly out of LOT in Romeoville, IL on occasion. It originally had a single small east-west runway designated 8-26. Eventually, volume necessitated a larger runway, so a new one was built parallel to the old runway, and the old runway became the taxiway. From the time the old runway was built to the time the new one was built, the isogonic lines had moved enough that the new one was designated runway 9-27. You can even still make out the 8 that was painted on the old runway.
Sorry, you have to zoom that map all the way in. How do you control the zoom of the map in a link?
After looking at the Sea-Tac airport on Google Earth, I can see how a small percentage of pilots can mistake the taxiways for runways. The Taxiways are the same color as the runways, and that color is concrete gray. The concrete gray color presents numerous problems, because as people we train ourselves to ignore this color. With four, north and south strips of pavement, and only two of which are actual runways, all of them are concrete gray, a small percentage of pilots would tend to ignore the background and concern themselves with more pressing issues of landing the plane. Putting the giant “X” on the taxiways helps, but if they painted the taxiway one color, and the runway another color, I bet they would get better results.
Airline pilot here …
The article the OP linked to did a surprisingly good job of outlining the problem for a non-specialist publication.
There are many places around the world that are “set-ups to screw up” in the vernacular. Taxiway T at KSEA is just one.
Ideally they’d fix it by repaving 16L/34R in concrete & some other changes. As a practical matter they aren’t going to. So it’ll remain an opportunity to screw up. The wary pilot knows where most of these traps are and makes a deliberate point of not falling for them. Not the ideal fix from a systems safety perspective, but that’s the way it is.
We could improve statistical safety across the board by finding and eliminating the top 10 traps in the US airspace system. But don’t hold your breath; closing LGA & DCA to jets isn’t going to happen.
To directly answer the OPs question, in avaiation every minor error contains the seeds of a many-fatality-disaster. So in the sense of “What’s the worst that could happen becasue of Twy T’s confusing appearance?” the answer is “Something pretty horrific.”
But in the sense of “how much hazard does Twy T’s confusing appearance add to a typical arrival into SEA?”, the best aswer is “Not very much.”
While we’re fixing SEA, the fact that all 4 runway ends are at the edge of bluffs, the fact there’s significant mountains within 30 miles, and the fact that there are signifiant tall buildings within a few miles are all unhelpful features that would improve safety if eliminated. Not gonna happen because the marginal improvement in safety is nowhere near the incredible increment in cost to fix.
Nothing in safety is black or white. Nothing is "safe"or “unsafe”. Some things are more or less safe than other things, and for every proposed change there’s a marginal cost per unit of safety gained. In most respects, aviation is already well out on the bleeding edge, where we’re actually more safe than mere cost-effectivity would dictate. As long as the public has an irrational fear of airplane accidents that will continue.
And as somebody who hopes to retire rather than get killed at work, I’m just as glad we’re over-spending on safety compared to what a completely rational analysis might dictate. My complaint, and that of most of my fellows, is when gobs of cash get diverted from safety initiatives that will really help and instead are put into either feel-good measures or over-preventing the next occurence of the most recent spectacular, but ultimately very unlikely to be repeated, event.
Thanks y’all.
I got that general impression from the article, that while there’s the possibility of something really bad happening the specifics make it somewhat unlikely, but since I’m generally wary of specialist and technical topics getting appropriate handling in the mainstream media, I figured I’d let some real experts take a look at it.
You guys do not disappoint.
Looking at the Seattle Airnav Page and the airport diagram contained therein, I can very much see how T could be mistaken for the runway.
On the other hand, every single publication relating to 16L/34R has “DO NOT MISTAKE TWY T FOR LNDG SFC.” or something similar marked on it, so it would have to be a very difficult situation to get yourself in to, but not unheard of, especially if everything’s wet.
And yeah, runways are marked using magnetic heading, and Seattle having a variation of 19.4 degrees means that a N-S runway would be marked 16/34. It’s only in (what Canadians call) the Northern Domestic Airspace that runways are marked according to true north. (A small but informative Wikipedia article on the NDA and other Canadian airspace, which is extremely similar to US airspace is availible here.)
For a while, that Google photo was like a “Where’s Waldo” thing as I was trying to find that tiny little “x” Such a cute little thing, once I found it! When you’re busy trying to land, that tiny would be easy to miss.
Why don’t they just grab some paint and put a couple big Xes at either end? Just don’t do a Mayor Daley, OK?
Johnny LA said: " J-3s are so slow that given enough wind they fly backwards. "
I used to have photos of my father, who was a Navy test pilot in WWII, demonstrating just that off the deck of, I think, a destroyer. They built a little wooden square, like a small helicopter pad (only they didn’t have helicopters in the 1942 USN) on the bow of the ship, faced into about a fifteen knot wind, and set the ship at about fifteen knots. A bunch of swabbies hung on to the struts to hold the Cub down, my dad ran the little sixtyfive horsepower Continental up to full power, and straight up into the air he went. Landed the same way. Don’t know why the Navy never showed any interest n the idea.
Because that’s the symbol for a closed runway or taxiway… Usually put there because it’s unsafe.
Thanks, LSLGuy, for an excellent post.