High Quality Content by WIKIPEDIA articles! The Hawker Siddeley HS 121 Trident (alternatively the D.H.121) was a British short/medium-range three-engined jet airliner designed by de Havilland and built by Hawker Siddeley in the 1960s and 1970s. The Trident is notable for its pioneering avionics which enabled it to become the first airliner to make a fully automatic approach and landing in revenue service in 1965 and to be the sole airliner capable of automatic landings in regular service from 1966 until versions of the Lockheed TriStar were also cleared to perform them in the mid-1970s.