Hawker Siddeley Trident

Development

Background and original specification

In 1953, as British European Airways (BEA) introduced the world's first turboprop-powered civil airliner – the Vickers Viscount – into passenger service, the operator was already considering what would be required of a potential successor. Following the entry into service of jet airliners in 1952, many airline managers and economists remained sceptical, and advocated turboprop airliners as replacements of piston-engined airliners. In 1953, while several manufacturers across the world were investing in pure jet-powered aircraft, BEA chose to favour turboprops on the basis of their superior economics and produced a specification that called for an aircraft capable of seating 100 passengers and attaining a maximum speed of 370 knots (430 mph; 690 km/h). As a result of the BEA specification, Vickers developed an enlarged derivative of the Viscount for BEA, the Vickers Vanguard, which was ordered by the airline on 20 July 1956. By this point, however, the French-built Sud Aviation Caravelle had conducted its maiden flight during the previous year, and BEA was beginning to recognise that jet aircraft could soon be providing stiff competition.

In April 1956, Anthony Milward, chief executive of BEA, stated that he "would rather do without [jet airliners]". Nevertheless, in December of that same year Lord Douglas of Kirtleside, BEA's chairman, stated that a number of jet-powered short haul aircraft might need to be introduced while retaining turboprop aircraft as the mainstay of the company's inventory for the foreseeable future. In July 1956, BEA had announced what it called "outline requirements" for a short-haul "second-generation jet airliner", to work alongside its turboprop fleet. It would carry a payload of some 20,000 lb (9,100 kg) or some 70 passengers up to 1,000 mi (870 nmi; 1,600 km), weigh about 100,000 lb (45,000 kg), use 6,000-foot (1,800 m) runways, cruise at a very high speed of 610 to 620 mph (530 to 540 kn; 980 to 1,000 km/h), and have "more than two engines". According to aviation author Derek Woods, BEA "wanted something that was faster than the Caravelle which was threatening to be highly competitive". While they were not intended as an express requirement, commentators ever since have taken these figures to constitute a definite call to industry.

Four companies prepared projects to match the BEA outline. Bristol proposed the initially trijet Bristol Type 200. Avro proposed the futuristic Avro 740 trijet before shelving it and joining forces with Bristol and Hawker Siddeley. Vickers proposed the VC11 four-engined airliner, derived from its in-development VC10. The de Havilland company considered three possible contenders for the specification; two of these were four-engined variants of the D.H.118: the D.H.119 and the D.H.120, the latter being also intended to be offered to British Overseas Airways Corporation (BOAC). In July 1957, de Havilland made another submission in the form of the DH.121; this proposal was furnished with three turbojets, Rolls-Royce Avon engines, and greatly resembled the eventual production aircraft. By August 1957, the DH.121 proposal had been revised; differences included the adoption of the in-development turbofan, the Rolls-Royce Medway, and an expansion to accommodate a maximum of 98 passengers.

The DH.121 was to be the world's first trijet airliner. Its designers felt this configuration offered a trade-off between cruising economy and take-off safety in case of an engine failure; moreover, the BEA specification had called for "more than two engines". Each of the three engines would drive its own hydraulic system, offering triple redundancy in case of any of the other systems failing. The engines were to be 13,790 lbf (61.34 kN) Medway engines. The DH.121 was to have a gross weight of 123,000 lb (56,000 kg) or optionally, up to 150,000 pounds (68,000 kg), a range of 2,070 mi (1,800 nmi; 3,330 km), and seating for 111 in a two-class layout (or for over 140 in a high-density, single-class layout as typical from the 1960s onwards on inclusive-tour charter flights). The design initially included a cruciform tail layout similar to that of the Caravelle. The engines were clustered at the rear, with the centre engine situated in the extreme rear of the fuselage fed by air ducted through a large oval intake at the front of the fin, a configuration similar to the later Boeing 727; the design eventually settled on a variable-incidence T-tail.

From the outset, the DH.121 was planned to employ avionics that were very advanced for the period. Among other capabilities, they would offer automatic approach and landing within a few years of service entry. The avionics were also to have triplicated components for reliability and to allow "majority 2:1 voting" for aircraft guidance during automatic approach and landing.