XX765 was withdrawn from RAF service to demonstrate the feasibility of Active Control Technology (ACT), under development by British Aerospace (BAe). The aircraft’s normal control rods were replaced with a ‘fly-by-wire’ (FBW) control system, which used four independent computer-controlled electrical channels to relay instructions to the flight surfaces. The aircraft was further modified by fitting large leading edge strakes (wing extensions) to move the centre of lift forward and adding ballast to the rear fuselage to move the centre of gravity aft. This enhanced lift and drag characteristics and made the tail plane more efficient thereby allowing smaller lighter engines with greater fuel efficiency to be used, decreasing overall weight by 15%.
Flight trials began in October 1981. Test pilots were impressed by the crisp control responses and smooth flight. The aerodynamic instability of the aircraft enhanced manoeuvrability, but the computer-controlled flight commands provided split-second corrections to compensate for the unstable configuration.
