In 1915, Thieffry joined the "Compagnie des Ouvriers et Aérostiers" —the Belgian Army Air Corps— and with some difficulty qualified as a pilot at Étampes. On 1 February 1916 he joined the 3rd Squadron as an observer for artillery, where he was appreciated for his exactitude and courage. He crash-landed so many aircraft that he was promptly assigned to a single-seat fighter squadron, as no one would fly with him! He was rapidly transferred to 5th Squadron (The Comets) under Captain Jules Dony based at De Panne in December 1916.