Test flights began on 18 April 1941, with the Me 262 V1 example, bearing its "Stammkennzeichen" radio code letters of PC+UA, but since its intended BMW 003 turbojets were not ready for fitting, a conventional Junkers Jumo 210 engine was mounted in the V1 prototype's nose, driving a propeller, to test the Me 262 V1 airframe. When the BMW 003 engines were installed, the Jumo was retained for safety, which proved wise as both 003s failed during the first flight and the pilot had to land using the nose-mounted engine alone. The V1 through V4 prototype airframes all possessed what would become an uncharacteristic feature for most later jet aircraft designs, a fully retracting conventional gear setup with a retracting tailwheel—indeed, the very first prospective German "jet fighter" airframe design ever flown, the Heinkel He 280, used a retractable tricycle landing gear from its beginnings, and flying on jet power alone as early as the end of March 1941.