Whilst learning the Russian language I stumbled upon this sentence:
Хорошо, когда будете в аэропорту в Лондоне, позвони ещё раз.
Good, when you are at the airport in London, call me again.
And I'm wondering why it is аэропорту (dativ / masculine)? During my classes I learned that the preposition в 99% with the prepositional case. So what do I miss here at this point? I would appreciate any kind of hint, thank you!
There are several dozen nouns, 2nd-declension masculine and overwhelmingly monosyllabic (аэропорт is on this list because it mimics the declension of порт), that have two propositional forms; a regular -е prepositional used with о, при, and figurative instances of в and на (Что особенного в этом аэропорте? Вполне обыкновенный на вид), and an -у (or -ю) prepositional used with в and на when they literally refer to location, as in your example. On a side note, the -у/-ю ending is always stressed, making it inherently different from dative in many cases. Dative, in fact, has nothing to do with this whatsoever.
This is all you need to know in order to use these forms correctly (apart from the actual word list; that link in the first comment is comprehensive enough, though one always forgets a few when one tries to list them).
And now I'll ramble on a little more about the theoretical stuff.
You might hear the expression "locative case" thrown around, and there's some potential confusion there. What's known in Russian as the "prepositional case" is, historically, the locative case, and it goes by that name in several other Slavic languages.
In Russian, however, we have a non-standard case ending that refers to location, hence it gets called the "locative case". But like I said, there was a time when "prepositional" was locative, and then Russian has a limited-use "locative" case on top of its locative-renamed-prepositional. The tricky thing here is that while forms like в аэропорту are archaic, the distinction between в аэропорту and в аэропорте is innovative. In Proto-Slavic, these nouns would either always have an -u locative or always have a -ě locative (or always have something else entirely), depending on their paradigm, which was much more complicated than the Russian declension we have now. In Russian, the -ě (-е) ending supplanted all others, but a few leftover -u forms remained, and eventually were grammaticalised in a new role as "the locative locative", as opposed to the "general" locative now called prepositional.
Hard to believe, but this modern word is used with archaic местный (locative) case. It is supposed to not exist nowadays but it actually does. Another example, could be в лесу. Locative case mostly assimilates (uses same inflection) with prepositional, but for some words - with dative.
I learned that the preposition **в** 99% with the prepositional caseNot really. 50% prepositional; 50% accusative.