"The new airplane is flying above a big city."

Translation: Az új repülőgép egy nagy város fölött repül.

[basdeleeuw]

Why is fent not possible in this case?

[96314081311257]

Fent/fenn is an adverb, fölött is a postposition. I hope I got your question right.

[RyagonIV]

To elaborate a bit on this: fent is an absolute position, it just means 'above'. If you want to give a relative position (above something), you'll have to use the postposition fölött.