Aircraft are flying units produced through a Helipad or an Airfield. Aircraft units have finite ammo. When they run out of ammo, they head back to the nearest unoccupied Helipad (if Allied) or Airfield (If Soviet) and reload.
Note that aircraft travel in a straight line to the base when wanting to reload. This is not always desired as it can sometimes put them directly above enemy anti air units. It is sometimes better to manually fly them back, perhaps using a command queue.
Aircraft can only be attacked by air-attacking units and defenses.