In the early stages of The Great War, different air services evolved different methods of assigning credit for aerial victories. None of these scoring systems were ironclad and they were all subject to flux. The actual number of air victories required to officially qualify as an "ace" varied.
The Germans, who did not use the term "ace" but referred to German pilots who had shot down 10 aircraft as Überkanone (big gun), gave credit to a single pilot with each downing of a particular enemy aircraft, and then only after visual verification of the wreckage of the fallen craft or its occupant(s). The French Armee de l'Air system also only counted aircraft destroyed, but granted full credit to every pilot or aerial gunner participating in a victory, which could sometimes be several individuals. Most other nations (including the United States) adopted the French system. British aircraft often fought over German territory, and could not follow the German practice of visual verification of the wreckage of the fallen craft or its occupant(s).Early fighting often only resulted in an opponent being forced to retreat or land ("driven down") within its own territory. Therefore, British practice awarded credit for combats endorsed as "decisive" by the commanding officer of his squadron; this could include German planes "driven down" or aircraft last seen "out of control" but not verified to have crashed. Russian scores sometimes reflected defeats without destruction of the foe. The term "ace" was never used officially by the British.
Albert Ball Charles Nungesser "Billy" Bishop Oswald Boelcke