His first confirmed victory was on 15 March 1917, flying a Nieuport 11. His second followed eight days later above Gistel, and his third on 12 May above Houthulst. His fourth was on 14 June—an Albatros D.III above Westende. The 5th Squadron then relocated to Les Moëres, and was equipped with Nieuport 17s. Thieffry gained official status as an "ace" when he shot down two German fighters over Diksmuide on 3 July. In August he received the first SPAD VII fighter in the Belgian Air Force, bought by the Belgian prince. He gained three more victories with it.