Air units do not move around the map like ground and naval units. They must be "based" somewhere. The City Center can always hold one air unit, but building an Aerodrome, Airstrip, or Aircraft Carrier can increase your available slots. The Aerodrome can have up to 8 slots, the Airstrip 3, and the Aircraft Carrier begins with 2 but can gain more through promotions.
Air Bases
Air units do not move around the map like ground and naval units. They must be "based" somewhere. The City Center can always hold one air unit, but building an Aerodrome, Airstrip, or Aircraft Carrier can increase your available slots. The Aerodrome can have up to 8 slots, the Airstrip 3, and the Aircraft Carrier begins with 2 but can gain more through promotions.