At the beginning of the 1950s the British company Armstrong Siddeley designed a turbojet engine, named Viper, for light aircraft and trainers. Its production lasted for over twenty-five years, even when the original manufacturer merged with the Bristol company in 1960 and when the latter was in turn acquired by the Rolls Royce Group in 1966. The Viper was developed in several versions, with continuous technical upgrades and performance improvements, ranging from a thrust of 6.8 kN in first models to over 19 kN in the latest versions. Thanks to its performance and versatility, the engine achieved considerable commercial success, both in military and civil application. The Viper was built under license in various countries, including Italy, where Piaggio produced the mk. 22 model for the Aermacchi MB-326 trainer, while FIAT the more powerful series 600.