It depends on what aircraft you're flying. In the F-15 you only have to select an air-to-air mode, then fire away. In the A-10, you need to select an air-to-air mode or air-to-ground mode and then fire away. In any of the russian built aircraft you first have to aquire and lock-on to an aircraft and meet the missile launch parameters before you can fire any air-to-air weapons, or you can select a ground mode and fire whenever you want. Or, you could just switch to cannons and blast whatever you want.
