From the manual of the game:
It's a dogfight... you against the enemy! You're the pilot, gunner and navigator all in one!!! Enemy planes are buzzing you. Anti-aircraft ground forces are shelling you!
Get an enemy plane in your sights and FIRE! Try to shoot down as many as you can! If ground fire (flak) explodes in your sight... you're hit... you quickly lose altitude. Climb back up! Stay clear of the flak!
You must shoot down at least ten planes before you can land to refuel, then take off again. The game ends when you run out of ammo completely, or when you crash. Press RESET and start again. Try to get a better score each time. Challenge a friend!