An air unit, as its name implies, is a unit that fights in the air. There are only two kinds of air units: Fighters and Bombers. Air units can move over any type of terrain and can only be attacked by other air units. Unlike ground units and naval units, however, they require fuel to operate. They must return to a friendly city to refuel every few turns, or they will crash and be destroyed. Armies of air units are referred to as "wings." Once they are formed, they cannot be split up.