The Boulton Paul Balliol and Sea Balliol are monoplane military advanced trainer aircraft built for the Royal Air Force (RAF) and the Royal Navy Fleet Air Arm (FAA) by Boulton Paul Aircraft. Developed in the late 1940s, the Balliol was designed to replace the North American Harvard trainer. It used the Rolls-Royce Merlin engine. The Sea Balliol was a naval version for deck landing training. The Balliol was developed to meet Air Ministry Specification T.7/45 for a three-seat advanced trainer powered by a turboprop engine, competing against the Avro Athena. It was a conventional low-wing monoplane with a retractable main undercarriage and a fixed tailwheel. Pilot and instructor sat side by side ahead of a second student who would watch the instruction given to the pilot. In August 1948, Boulton Paul received an order for four prototypes, to be powered by the Rolls-Royce Dart turboprop, and in August 1946, this was followed by an order for 20 pre-production aircraft, with ten each to be powered by the Dart and the Armstrong Siddeley Mamba turboprop, with delays to development of the Dart meaning that the prototypes would now be fitted by the Mamba. The Mamba was not flight ready when the first prototype was completed, so it was fitted with a 820 hp (611 kW) Bristol Mercury 30 radial engine for initial testing. Thus powered, it first flew on 30 May 1947. The second prototype, powered by the intended Mamba, first flew on 17 May 1948, the world's first single-engined turboprop aircraft to fly. The Air Ministry had meanwhile had second thoughts about its training requirements, and in 1947 issued a new specification, T.14/47, requiring a two-seat trainer, powered by a Rolls-Royce Merlin piston engine, available in large numbers from surplus stocks, rather than the expensive turboprop engines. The Merlin powered Balliol, designated Balliol T.2, first flew on 10 July 1948, and after extensive evaluation, it was chosen over the Athena, with large orders being placed to replace some of the Harvards in RAF service. The observer's seat of the Mk 1 was removed, the side-by-side seats remaining. The Sea Balliol T.21 had folding wings and arrestor hook for deck landings. By 1951, however, the Air Ministry changed its mind about its training requirements yet again and decided to introduce a jet-powered advanced trainer, the de Havilland Vampire T.Mk11. Variants P.108 Balliol T.Mk 1 Prototypes, 3 built, powered by the Armstrong Siddeley Mamba turboprop engine Balliol T.Mk 2 Two-seat advanced training aircraft for the RAF; 196 built, (166 built by Boulton Paul, and 30 built by Blackburn Aircraft). Sea Balliol T.Mk 21 Two-seat advanced training aircraft for FAA. A total of 30 built by Boulton Paul.
