Cid only has one airship in FF3 unless I'm mistaken. You find it in the desert just west of Kazus. You may be referring to the Enterprise, which is the next ship obtained in the game, from the Vikings and altered by Cid so that it can fly. If so like I said you obtain it from the Vikings after appeasing Nepto the sea serpent. Take it to Cid with the Time Gear that you obtain in Castle Argus later on.