The Bristol 175 had just completed a test flight of 1 and 40 minutes. Tests included a strain-gauge measurements on the non-standard propeller of the no. 2 engine, and high speed upset manoeuvre recovery tests in connection with the US certification. Returning to Filton, the aircraft entered a circuit and partial gear extensions occurred for unknown reasons. Attempts may have been made to complete undercarriage free fall tests as these had failed the previous day; such test were not on the programme however. At 1500 feet a left turn to base leg was initiated. The right wing suddenly dropped and the aircraft went into a very steeply banked right hand turn. The Britannia briefly recovered but banked steeply again and struck the ground in a wood near a residential area. PROBABLE CAUSE: “The accident was the result of the aircraft developing a very steep descending turn to the right which the pilot was unable to control. The reason for this could not be determined, but the possibility that it occurred as the result of malfunctioning of the autopilot cannot be dismissed.”