"Few airplanes fly above."
Translation:Fent kevés repülőgép repül.
fent stands on its own; fölött/felett are postpositions that go with a noun. So "above the tree" would be a fa fölött, for example; you can't say a fa fent; nor can you (I think) say fölött without a noun in front of it.
English has quite a few words that can be either adverb or preposition, which Hungarian separates.
But compare e.g. "next to", which is only a preposition - you can say "He is standing next to the house" but not "He is standing next to" (it would have to be something like "He is standing at the side").
As for fölött and felett, I believe they're completely interchangeable.