Air bases are <l:6,63>tile improvements<e> that can be built outside cities. They are indispensable structures necessary for those who have planes. All planes can only move a limited number of tiles before running out of fuel. When they run out of fuel, if they do not come to rest in an air base or a city, the plane will be destroyed. Certain units can <l:6,44>Paradrop<e> from air bases. Air bases have a vision range of three. The air base costs 1000 PW and takes four turns to build.