What happened
The aircraft was conducting a test flight on behalf of the Telecommunications Research Establishment (TRE) of the Royal Air Force. It departed RAF Defford during daylight hours with a crew of five and six engineers aboard. While en route, engine number four caught fire. The pilot attempted an emergency landing, but at an altitude of 500 feet, a portion of the right wing broke away from the airframe. With the aircraft out of control, it crashed into a field at a speed of 150 mph and was destroyed. All 11 occupants were killed in the accident. The mission involved the test and development of the H2S airborne radar system, which was highly secret; consequently, the death of one of the engineers was not officially announced until three years later.
Findings
The investigative board determined that the crash was caused by an engine fire resulting from mechanical failure. A tappet nut on the starboard outer engine had been improperly tightened by a maintenance fitter approximately three hours before the flight. This error led to excessive valve clearance and a fracture of the valve stem, which pumped ignited fuel outboard of the rocker cover and along the outside of the engine. The resulting fire in the engine nacelle was constantly fueled by the broken intake, causing it to burn rapidly along the wing and fuselage. Eventually, a large section of the wing separated from the fuselage at approximately 350 feet of altitude. The loss of control caused the aircraft to roll inverted before striking the ground.