Rules are very simple... shoot down your enemies or they will shoot down you.
Move the mouse on the screen and your biplane will follow. Left click to fire but mind your amount of ammunition! Ammunition indicator is shown on upper left screen border. To re-charge machine gun you must collect ammo bonuses.
On the upper right screen border there is damage indicator, to repair your airplane collect wrench bonuses.
On lower left screen border there is fuel indicator, to re-fuel the airplane collect fuel bonuses.
Lives are shown on lower right screen border. Collect yellow plane bonuses to win extra lives.