sentences:  The second prototype, powered by the intended Armstrong Siddeley Mamba turboprop, first flew on 17 May 1948, the world's first single-engined turboprop aircraft to fly.The Air Ministry had second thoughts about its training requirements, and issued a new specification, T.14/47, requiring a two-seat trainer, powered by a Rolls-Royce Merlin piston engine.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.