He made two further attempts to reach Congo. The first on 9 March 1928 in an ACAZ C.2 with Joseph Lang and Philippe Quersin, did not get any further than Philippeville. The second on 26 June in a Stampe et Vertongen RSV.22-180, again with Philippe Quersin, also failed, this time ending in a marsh at Clapier, near Vauvert. Thieffry then developed a plan to set up an internal air service in Congo. During his second test flight in Congo on 11 April 1929, flying Aviméta 92, Thieffry, with fellow flyer Gaston Julien, was killed in a crash close to Lake Tanganyika (only a mechanic survived). He was 36 years old. It would be another 10 years before a regular air service was established between Brussels and Kinshasa.