I consider airport codes fairly obscure. Maybe knowing them is common in the US; in my experience (I don't have any statistics) it's rare for Europeans, even well-traveled ones, to know them.
To insist, since I see a movement towards standardizing on airport TLAs as tags: TLAs do not look professional, they look jargony. Expecting most visitors to recognize them at first site and understand them out of context (or even in an air travel context) creates an entry barrier, a sense that there is a clique that they're not part of and not welcome to. In fact, we should have a policy that each post always introduces an airport by its usual name before using its TLA (this is a good policy for all kinds of abbreviations).
I suggest appending the IATA code to the city name, e.g. san-francisco-sfo, london-lhr. That makes the purpose of the tag reasonably clear from the name: even if you don't know about airport codes, when you see the tag, you immediately have a general idea of what it's about. More importantly (since tag wikis also say what a tag is about),
san-francisco-sfo is discoverable;
sfo is not. Additionally, it allows people to subscribe to
san-francisco* and cover the airport as well.
That's assuming airport tags are warranted in the first place. How small can a place be and still deserve a tag? Should there be tags for major train stations too? Are there people who will subscribe to paris-cdg but not paris?