World War I served as a testbed for the use of the airplane as a weapon. Airplanes demonstrated their potential as mobile observation platforms, then proved themselves to be machines of war capable of causing casualties to the enemy. The earliest known aerial victory with a synchronized machine gun-armed fighter aircraft occurred in 1915, by German Luftstreitkräfte Leutnant Kurt Wintgens. Fighter aces appeared; the greatest was Manfred von Richthofen. Most airplanes are constructed by companies with the objective of producing them in quantity for customers. The design and planning process, including safety tests, can last up to four years for small turboprops or longer for larger planes. Supersonic airliner flights, including those of the Concorde, have been limited to over-water flight at supersonic speed because of their sonic boom, which is prohibited over most populated land areas. Isomorphisms of the topological plane are all continuous bijections. The topological plane is the n