Airplane! is a very direct parody of Zero Hour!, down to the plot points, dialogue, character names, and completely unnecessary exclamation point in the title.
And though Zero Hour! holds up pretty well as an unintentional comedy given its melodrama, ZAZ really ramped things up in their own interpretation. Thus the title pays homage to the film's humble origins: a terrible 1950s B-movie that otherwise never would have been remembered.