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. The Trident was withdrawn from service in 1995.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 . The Trident 3 was stretched by to seat 180 over , and had an additional RB.162 booster engine in the tail.
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.Wood 1975, p. 99. 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.Staniland 2003, p. 149. 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 .Wood 1975, pp. 99–100. 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.Wood 1975, p. 100.
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 or some 70 passengers up to , weigh about , use runways, cruise at a very high speed of , and have "more than two engines".Munson 1967, pp. 153–154.Wood 1975, pp. 100–101."Thoughts on the D.H.121". Flight International 28 February 1958, p. 267. According to aviation author Derek Woods, BEA "wanted something that was faster than the Caravelle which was threatening to be highly competitive".Wood 1975, p. 101. 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 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.Wood 1975, p. 102. 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 or optionally, up to , a range of , 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).Wood 1975, pp. 97, 102. 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."Deep-Stall Avoidance". Concept to Reality. Retrieved: 13 November 2010.
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."Touchdown by Computer". Time, 18 June 1965. Retrieved: 17 July 2009. The physical dimensions of most avionics of the period required them to be housed in a large compartment beneath the Trident's flight deck; the compartment's size was among the factors dictating a distinctive nose undercarriage design, with the nose landing gear offset by to the port side and retracting sideways to stow across the DH.121's longitudinal axis.
Industry consolidation and selection BEA soon selected the DH.121 as the basis for detailed negotiations, but these talks were protracted due to multiple factors, including wider policy decisions of the British government and indecision within BEA itself. During the time that the DH.121 had emerged in the late 1950s, the British government came to view the airframe and aeroengine industries as too fragmented into small companies; accordingly, a policy favouring mergers into a few large groups was adopted. De Havilland was keen to retain their independence and leadership of the DH.121, so approached the government with a proposal to form a consortium under which de Havilland would produce the fuselage, Bristol would manufacture the wings, and various other companies, including Hunting Aircraft and Fairey Aviation, would be responsible for other elements; however, Bristol strongly opposed this arrangement and chose to work with Hawker Siddeley in competition against de Havilland.Wood 1975, pp. 102–103.
Companies vigorously competed to be selected by BEA due to the lure of its £30 million contract (), as well as the likelihood of lucrative overseas export sales.Wood 1975, p. 103. On 4 February 1958, de Havilland, along with Hunting and Fairey, announced that they had agreed to form a partnership for the purpose of manufacturing and marketing the DH.121; the consortium adopted the corporate name of the defunct Airco company, which had been Geoffrey de Havilland's employer during the First World War. The Minister of Supply stated of the Airco consortium that "this is not quite what [he] had in mind". Nevertheless, both Airco and the rival Bristol-Hawker Siddeley team proceeded to conduct their own approaches to various overseas airlines; sufficiently interested, American Pan American World Airways invited both teams to present their proposed airliners in January 1958. Sir Matthew Slattery, chairman of Bristol and Short Brothers, appealed for BEA to delay any decision until after one of the competing firms had already secured an export order for their airliner. In response, Lord Douglas stated that BEA wished to order the DH.121 and was awaiting approval from the government; Douglas's reply has been viewed as the death knell for the rival Type 200 proposal.
Meanwhile, a rival airliner emerged, this time from Boeing in the United States, in the form of the 727, which also had a trijet configuration.Wood 1975, p. 104. Boeing had begun its studies into this sector of the market in 1956, and elected to launch its own trijet programme in 1959. Airco executives, who were at the time intensely exploring various alternatives and further partnerships with other aircraft companies, considered the possibility that Boeing might choose to drop the 727 project and instead co-manufacture the DH.121 in the USA; Lord Douglas was one of the proponents of this initiative. As a result, Airco invited a team of Boeing engineers and executives to Hatfield. Boeing later permitted a return visit by de Havilland representatives to Seattle, however the company revealed few details of their plans for the 727, while virtually all information on the DH.121 had been shared during the Hatfield visit, an openness that had allegedly "amazed" them. British commentators have tended to interpret this episode as involving the acquisition of sensitive proprietary data on the DH.121 by a direct competitor.Bacon, Roger. "Straight & Level". Flight International. 16 December 1960, p. 953. Woods remarked that "de Havilland solemnly handed all its research over to its rivals ... the crowning piece of stupidity".
On 12 February 1958, the British government authorised BEA to commence contractual negotiations along with the issuing of a letter of intent for 24 aircraft. Accordingly, that same month, BEA announced that the DH.121 had come closest to its requirements and that it would proceed to order 24 with options on 12 more."Airco D.H.121". Flight International, 25 July 1958. A further six months were needed for the government to approve a formal BEA order for the DH.121; the government had favoured the Bristol 200 for industrial policy reasons."Bristol 200 – a Preliminary Appraisal". Flight International, 24 January 1958, p. 109."B.E.A.'s Jet: Cards on the Table". Flight International, February 1958, p. 167. Reportedly, BEA had a considerable interest in the Caravelle itself, but this would have been a politically unacceptable choice.Staniland 2003, pp. 149–150. BEA also favoured , and therefore the Trident submission, due to the firm's established experience with jet airliners with its prior development of the Comet.
In April 1958, de Havilland firmed the general configuration of the DH.121 and established a development timetable, including a projected date for the type's maiden flight to be conducted during mid-1961. The company's market research department was forecasting that as many as 550 airliners in its category would be sold by 1965. Noting that a greater preference for the seating dimensions of what would become economy class was emerging amongst airlines, design alterations were made to adopt a slightly larger diameter fuselage to accommodate six-abreast seating, providing for a maximum configuration of 111 seats.Wood 1975, pp. 103–104. According to Woods, this enlarged version of the DH.121 was "on the verge of building the right aeroplane for the market and the success of the Viscount looked like being repeated".
Revised specification