The Hawker Siddeley HS 121 Trident is a three-engine short-to medium-range narrowbody airliner for max. 180 passengers produced by the British manufacturer Hawker Siddeley Aircraft Ltd., developed by de Havilland as the D.H.121.

The HS 121 Trident was the world's first trijet airliner and the first airliner capable of automatic landings.
The Trident 1C was the first model with seating for 96 passengers, powered by Rolls-Royce Spey 505 engines with a wing span of 27,38m (89ft 10in).
The Trident 1E has a seating of typical 103 and max. 149 passengers, powered by Rolls-Royce Spey 511 engines with a wing span of 28,96m (95ft).
The Trident 2E is a longer range variant with same seating as the Trident 1E, powered by Rolls-Royce Spey Mk.512 engines.
The Trident 3B is a stretched variant with a total length of 39,98m (131ft 2in) for typical 136 and max. 180 passengers. The Trident 3B shares the engines and wings with the Trident 2E but is equipped with an additional Rolls-Royce RB162-83 boost engine.