<P> Employing the main fuselage section and engine of the de Havilland Vampire mated to a longer fuselage with a single tailfin and swept wings, the de Havilland DH 108 was proposed in 1944 as a test "mule" for the DH 106 Comet which had initially been considered a tailless, swept - wing concept . Despite the Comet design taking on more conventional features, the value of testing the unique configuration to provide basic data for the DH. 110 spurred de Havilland to continue development of the DH 108 . Selecting two airframes from the English Electric Vampire F 1 production line, the new aircraft had unmistakable similarities to its fighter origins, especially in the original forward fuselage which retained the nose, cockpit and other components of the Vampire . The Ministry of Supply named the DH 108 the "Swallow", a name that was never officially adopted by the company . </P> <P> The new metal wing incorporating a 43 _̊ sweepback was approximately 15% greater in area than the standard Vampire wing . Control was based on the conventional rudder in combination with elevons that were part elevator and ailerons, fitted outboard of the split trailing edge flaps . Although the Vampire fuselage was retained, as development continued, a revised nose and streamlined, reinforced canopy were incorporated . </P> <P> The first DH 108 prototype, serial number TG283, utilising the Vampire fuselage and a 43 ° swept wing, flew on 15 May 1946 at RAF Woodbridge . Designed to investigate low - speed handling, it was capable of only 280 mph (450 km / h). The de Havilland Chief Test Pilot Geoffrey de Havilland Jr., son of de Havilland company owner - designer Geoffrey de Havilland, gave a display flight in the DH 108 during the 1946 Society of British Aircraft Constructors (SBAC) airshow at Radlett . In later low - speed testing designed to clear the rear fuselage at high angles of attack, the first prototype was fitted with longer Sea Vampire landing gear . </P> <P> The second, high - speed prototype, TG306, with a 45 ° swept wing incorporating automatic leading - edge Handley Page slats and powered by a de Havilland Goblin 3 turbojet, flew soon after in June 1946 . Modifications to the design included a more streamlined, longer nose and a smaller canopy (framed by a strengthened metal fairing) facilitated by lowering the pilot's seat . While being used to evaluate handling characteristics at high speed, on 27 September 1946 TG306 suffered a catastrophic structural failure that occurred in a dive from 10,000 ft (3,050 m) at Mach 0.9 and crashed in the Thames Estuary . The pilot, Geoffrey de Havilland Jr., was killed in the accident . Early wind tunnel testing had pointed to potentially dangerous flight behaviours, but pitch oscillation at high speed had been unexpected . The subsequent accident investigation centred on a structural failure that occurred as air built up at Mach 0.9, pitching the aircraft into a shock stall that placed tremendous loads on the fuselage and wings . The main spar cracked at the roots causing the wings to immediately fold backwards . </P>

British pilot crashed and was killed while trying to break the sound barrier on september 27th 1946