In 1914, following the
German invasion of Belgium, Coppens transferred to
the Motor Machine Gun Corps. On 6 September 1915, he signed up for flight training in the Compagnie des Aviateurs. As a result of shortcomings in Belgium's training, he needed eight weeks of leave to learn to fly. Coppens and 39 other Belgians learned to fly on their own expense in Great Britain. He received his pilot's certificate on 9 December 1915. After finishing in Britain, Coppens underwent further training at the
Farman School in
Étampes, France, and joined the
Sixieme Escradrille as a
sergent 1st class (Sergeant First Class) on 8 April 1917, flying BE-2c two seaters. Later that month, he was assigned to
Quatrieme Escadrille, flying a Farman pusher. On 1 May, he received a
Sopwith 1½ Strutter two seater, which he flew during his first aerial combat. In mid July, he transferred to the single seater fighter unit
1ère Escadrille de Chasse (1st Pursuit Squadron). He received the last remaining
Nieuport 16 in the squadron; everyone else had upgraded to
Nieuport 17s. When
Hanriot HD.1s were offered to the squadron, he was the only pilot to initially accept one. His enthusiasm for the aircraft type prompted other pilots to also move over to Hanriots. . On 19 August Coppens was promoted to
Adjudant. He continued his nervy but unsuccessful combat career against enemy aircraft until 17 March 1918. On that day he carried out his first attack on German observation balloons, as an aid to a ground assault by the Belgian Army. Though handicapped by lack of incendiary ammunition he punctured two balloons, causing the observers to bail out and the balloons to collapse to the ground. Finally, on 25 April Coppens scored his first victory by downing a Rumpler two seater. On 8 May he finally found his metier, when he shot two balloons down in flames. A week later, using his usual tactics of close range fire, Coppens cut a balloon loose from its ties. It bounced up beneath him and momentarily carried his Hanriot skyward. After his aircraft fell off the balloon, he restarted its engine and flew back to base. The balloon sagged into an explosion. From then on, Coppens' record was spectacular. Between April and October 1918 he was credited with destroying 34 German
observation balloons and three airplanes, nearly as many victories as Belgium's other five aces combined. Unlike most fighter pilots of World War I, who used .303 caliber or 7.92 mm guns, Coppens used a larger bore 11 mm
Vickers machine gun, having upgraded his weaponry prior to June 1918. In June, he was promoted to
sous lieutenant, thus becoming an officer. His royal blue plane with its insignia of a thistle sprig wearing a top hat became so well known that the Germans went to special pains to try to kill him. On 3 August he shot down a balloon booby-trapped with explosives that when detonated from the ground narrowly missed killing him. The flaming wreckage of the balloon "fell swift as doom on the watching [German] staff officers, killing many and injuring the rest". On his last mission, 14 October, Coppens downed a balloon over
Praatbos and was attacking one over
Torhout when he was severely wounded by an incendiary bullet, smashing the tibia of his left leg and severing the artery. Coppens crash landed near
Diksmuide and was taken to hospital, where his leg was amputated. Coppens achieved all his victories flying a
Hanriot HD.1 fighter. ==After the war==