Why are some airports so close to each other?

I’ve always wondered this. For instance, KBFI and KSEA. They are less than half a mile apart (I think, don’t quote me on that) so aren’t collisions a hazard? Not to mention it’s placed right under where aircraft are approaching. (There are many other examples of things like this but here’s one) so can someone explain to me why some are so close?

They just are, one might have been an older airport and they have now moved over to the new one.

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I feel like most the time that’s the case, so thanks.
But some airports have absolute no relation but are close to each other. Is it the same reason?

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It’s not a hazard if altitude restrictions are mandated by ATC. Airport procedures (SID/STAR) are setup with such restrictions for airports within close proximity.

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Oh I get it. Thanks!

Look at KJFK and LGA. Imagine 2 planes landing at the same time on the certain runways…

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Yep. That would be scary, yet again that’s why LGA has the expressway approach

But still, a plane can be landing runway 4 at LGA, and another at 13R on JFK. How would that play out? 🤔

Most likely whoever is further away from their runway would make a go-around.

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Not necessarily. Usually, the wind conditions in the NY area are very similar so most of the time, LGA and JFK use the same runway configuration.

For this example, if 4 is being used at LGA and 13s and JFK, the normal ILS will be utilized for the former, while the Canarsie visual is used for the latter.

Visual weather conditions permitting of course.

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KBFI is 4.43 miles from the threshold of 16L at KSEA.

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Hence the reason I said “don’t quote me on that” 😂
They just looked half a mile apart when I was on final at KSEA

In Europe airports are normally only that close to each other if one is a military (only) airport. I couldnt give an example of two civil airports that close to each other in Europe.

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Yeah two military airports make sense to me, but I don’t get civil.
I don’t really have a problem with the US placing of airports, I just thought it was kinda weird

I’ve also wondered that about the SF Bay Area, what’s the point of having flights to San Fran, Oakland, And San Jose

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That’s like saying, “what’s the point of LaGuardia, Newark and JFK” or “what’s the point of Heathrow, city and Gatwick”. Just because there’s more than one airport, doesn’t mean it’s unnecessary.

Sometimes smaller airlines can’t afford slots at a major airport, or it’s cheaper to operate out of a different airport. Many cities worldwide have multiple airports, which benefits both those airports and the airlines serving them.

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Yeah an example of this is that southwest doesn’t fly to KDFW, but to KDAL instead since American Airlines is dominant there

Whoops. Meant “KDAL”

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You mean KDAL? KIAD is in the D.C. area.

But yes, that is a good example. :)

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Yeah It was a typo lol