"When is the flight?"
Translation:Wann ist der Flug?
Atzend and quis_lib_duo are both correct. If you rearrange the question as a declarative sentence it's easy to sort out the cases: Der Flug ist um sieben Uhr. Now Der Flug is in the first position where you expect to see it, and it's clearly nominative. It works with more complicated questions too
Meine (N) Tante schenke meinem (D) Sohn den (A) Fußball. Wem (D) schenke meine (N) Tante den (A) Fußball?
Also, sein is one of a handful of verbs that takes an object in nominative case, others are heißen, bleiben, werden, and scheinen.