The Hawker Siddeley HS-121 Trident (originally the de Havilland DH.121 and briefly the Airco DH.121) is a British airliner produced by Hawker Siddeley.

In 1957, de Havilland proposed its DH.121 trijet design to a British European Airways (BEA) request.

By 1960, de Havilland had been acquired by Hawker Siddeley.

The Trident's maiden flight happened on 9 January 1962, and it was introduced on 1 April 1964, two months after its main competitor, the Boeing 727.

By the end of the programme in 1978, 117 Tridents had been produced, and the Trident was withdrawn from service in 1995.

The jetliner is powered by three rear-mounted Rolls-Royce Spey low-bypass turbofans, it has a low swept wing and a T-tail.

Advanced avionics allowed it to be the first airliner to make a blind landing in revenue service in 1965.

The initial Trident 1/2 could seat 101-115 passengers over up to 2,350 nmi (4,350 km).

The Trident 3 was stretched by 5 m (16 ft) to seat 180 over 1,940 nmi (3,590 km; 2,230 mi), and had an additional RB.162 booster engine in the tail.