Great quiz! I think that you should also show the name of the airport for cities that appear more than once. Reguarding Milan, I think that at least "technically" the third one is in Bergamo, although it's so far up because many people fly there to get to Milan (http://en.wikipedia.org/wiki/List_of_the_busiest_airports_in_Italy)
Yeah, it's odd Narita isn't even accepted for the Tokyo airport that is in Narita. I was also prepared to type in Heathrow, Gatwick, Stansted, London City etc. I guess this is easier.