At the end of the war Thieffry returned to Brussels by way of Switzerland, arriving home on 6 December 1918. He returned to his practice as a lawyer, but remained active in aviation, being one of the founders of Sabena in 1923. He then devised a plan to inaugurate an air link between Belgium and the Belgian Congo. At the start of 1925 he obtained permission from the government for this hazardous flight. Sabena supplied a Handley Page W8f, which Thieffry named "Princesse Marie-José", after getting the support of his friend King Albert.