Hawker Siddeley Trident

The 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 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By the end of the programme in 1978, 117 Tridents had been produced, and the Trident was withdrawn from service in 1995.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 .
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. 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. 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 it might be necessary for a number of jet-powered short haul aircraft 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". 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-four-engined Bristol Type 200. Avro proposed the futuristic Avro 740 trijet before shelving it and joining forces with Bristol and . 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 developments of the early Comet, the world's first jet-powered airliner: 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 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). 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 which 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. The physical dimensions of most avionics of the period required them to be housed in a large compartment beneath the Trident's flightdeck; 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, and thus 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.
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. 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 airline 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 tri-jet configuration. Boeing had begun its studies into this sector of the market in 1956, and elected to launch its own tri-jet 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, Boeing revealed few details of their plans for the 727, while virtually all information on the DH.121 had been shared with Boeing, 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. 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. It took a further six months for the government to approve a formal BEA order for the DH.121; the government had favoured the Bristol 200 for industrial policy reasons. Reportedly, BEA had a considerable interest in the Caravelle itself, however this would have been a politically unacceptable choice. BEA also favoured , and therefore the Trident submission, due to the firm's established experience with jet airlines 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. 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.
In March 1959, BEA, which had become concerned by a recent decline in passenger growth, concluded that the DH.121's payload-range capacity could be too great for their needs and petitioned de Havilland to reduce the scale of the design to suit their revised projections. Fearing that the proposed scale of the Trident was too large, the airline had elected to effectively tear up the programme for its redesigning for their immediate situation. In 1959, BEA had a large fleet in operation and on order, and the issue of overcapacity was a critical concern. The airline's concerns reflected three factors: a short-lived airline recession in the late 1950s; the imminent arrival into service of a large fleet of turboprop Vickers Vanguards which duplicated the DH.121's general payload-range area, and the growing trend to higher-density seating.
Although de Havilland stated that they generally concurred with BEA, its management also stated that they had worked "under terms more onerous than anything D.H. had previously undertaken". Industry observers at the time felt that the British aircraft industry had again stumbled "into the pitfall of having designed exclusively for one customer an aeroplane that has potentially a much wider scope": a sentiment which would be echoed throughout the Trident's subsequent history. The de Havilland board elected to submit to BEA's demand, over-riding input from its own sales and market research departments which indicated that other airlines sought the larger model instead. It was, however, noted that de Havilland had not yet secured a formal and final BEA order and that its competitor Bristol was actively promoting their 200 project, which was significantly smaller than the DH.121. At the time Boeing and Douglas were also downsizing their DC-9 and 727 projects. It was felt the original large DH.121 would have to compete against the Convair 880 and Boeing 720 some four years after their service entries, whereas a cut-back design would be more competitive against the then-projected 75–100 seat, twin-engined DC-9.
Downsizing the Trident involved substantial changes to the design being made, including a powerplant change from the Medway to a scaled-down derivative, the 40 percent less powerful 9,850 lbf (43.8 kN) Rolls-Royce Spey 505. The gross weight was cut by about a third to 105,000 pounds (48,000 kilograms), while the range was cut by more than half to , and mixed-class seating was cut by about a quarter to 75 or 80 (97 in a single-class layout). Wing span was reduced by approximately , wing area by 30 percent and overall length by . The revised design retained some features of the original one, notably its fuselage diameter. It had a smaller flightdeck and single-axis, two-wheel, four-tyre main undercarriage legs in place of four-wheel bogies. Woods summarised the BEA-mandated redesign as: "At one blow the 121 was emasculated in terms of size, power and range".
Six months following BEA's request, de Havilland and the airline came to an agreement on the downsized DH.121. Details of the emerging aircraft, including its pioneering avionics, were announced to the public in early 1960. It was this revised aircraft that BEA ultimately ordered on 24 August 1959, initially in 24 examples with 12 options. In September 1960, the future airliner's name, Trident, was announced at the Farnborough Airshow; this name had been chosen as a reflection of its then-unique three-jet, triple-hydraulic configuration.
Further development and proposals.
By 1960, de Havilland had been acquired by the Hawker Siddeley group. After the de Havilland takeover, Airco was disbanded. Hunting was marshalled into the competing newly formed British Aircraft Corporation (BAC); their departure removed any putative possibility of the Hunting 107 (later the BAC One-Eleven) being marketed alongside the DH.121 as a complementary, smaller member of the same airliner family. Fairey Aviation, partially incorporated into Westland Aircraft, also left the DH.121 project. With the move to Hawker Siddeley Aviation, the designation was eventually revised to the HS 121. The reorganisation of the industry had compounded upon the delays caused by BEA's changes to the specification, which had in turn harmed the Trident's competitiveness against the Boeing 727.
The rival Boeing 727 had quickly established a lead over the Trident. The 727's early lead only strengthened it in subsequent competitions; one such example is Trans Australia Airlines, which had determined the Trident to be superior to the Boeing 727 from an operational standpoint, however it was also viewed as having been commercially risky to choose a different fleet from rival airlines such as Ansett Australia, who had already selected the 727. By 1975, only 117 Tridents had been sold against over 1,000 727s.
According to Woods, a significant opportunity that may have enabled the Trident to catch up with the 727 was lost during the 1960s in the form of two competitions for a maritime patrol aircraft; a NATO design competition to replace the Lockheed P-2 Neptune, and Air Staff Requirement 381, which sought a replacement for the Royal Air Force's piston-engined Avro Shackleton. Amongst the various submissions that had been produced in response was a bid by Avro, part of the Hawker Siddeley Group, which was designated as the Avro 776. The proposed Avro 776 mated the Trident's fuselage with a redesigned and enlarged wing along with more powerful Rolls-Royce RB178 engines capable of 16,300 lb of thrust. In addition to the maritime patrol requirement, Avro envisioned that the aircraft could be utilised in various military roles, including as a 103-seat troop transport as well as being armed with up to four GAM-87 Skybolt air-launched ballistic missile (ALBM) as a nuclear-armed bomber. In addition to Avro's proposals, Armstrong Whitworth had also proposed their own military variants of the Trident.
Later revisions of the Avro 776 substituted the RB.178 engine for the newer turbofan, the Rolls-Royce RB211, the development of the latter being supported by the 776's procurement if selected. Rolls-Royce Limited, having shelved development of the Medway following the Trident's redesign, was keen to develop an engine to slot between the 10,000 lb Spey engine and the 20,000 lb Rolls-Royce Conway engine; if such an engine had been produced, it could have equipped new versions of the civil Trident as well. Furnished with a more capable engine that could provide more thrust than the Spey was capable of, an extended fuselage could also have been adopted and existing landing restrictions could have been discarded; overall, the Trident would have been a far closer match to the 727. Wood summarised the importance of this prospective development as: "For the Trident programme, the RB.177 would have been a God-send".
At one point, the Avro 776 looked set to win the competition to be selected as the RAF's new maritime patrol aircraft. However, due to a desire to cut costs, the RAF decided to issue an entirely new Operational Requirement, under which the demands for speed, endurance, and capacity had all been diminished. As a result of the changed requirement, the design team was recalled and the Avro 776 was entirely sidelined for a new proposal. This new proposal, based upon the de Havilland Comet's fuselage, had little to do with the Trident save for the use of its existing Spey engines; this would go on to be selected and procured as the Hawker Siddeley Nimrod. As a result of this loss, prospects for an enlarged, higher-power Trident effectively evaporated.
Hawker Siddeley Aviation, which had absorbed de Havilland, needed additional customers for the Trident, so entered into discussions with American Airlines (AA) in 1960. American requested additional range than what the aircraft initially had, which meant that the original DH121 design would have fulfilled its requirements almost perfectly. In response, design began on a new Trident 1A, powered with up-rated Rolls-Royce Spey 510 engines of 10,700 lbf (47.6 kN) thrust, and a larger wing with more fuel, raising gross weight to and range to . However, American eventually declined the aircraft in favour of the Boeing 727.
Some of these changes were added into the original prototype, and it was renamed the Trident 1C. The main difference was a larger fuel tank in the centre section of the wing, raising weights to and range to . The first Trident 1, "G-ARPA", made its maiden flight on 9 January 1962 from Hatfield Aerodrome.
Design.
Overview.
The Trident was a jet airliner of all-metal construction with a T-tail and a low-mounted wing with a quarter-chord sweepback of 35 degrees. It had three rear-mounted engines: two in side-fuselage pods, and the third in the fuselage tailcone, aspirating through an S-shaped duct. One version, the 3B, had a fourth "boost" engine aspirated through a separate intake duct above the main S-duct. All versions were powered by versions of the Rolls-Royce Spey, while the boost engine was also by Rolls-Royce: the RB.162, originally intended as a lift engine for VTOL applications.
The Trident was one of the fastest subsonic commercial airliners, regularly cruising at over . At introduction into service its standard cruise Mach Number was 0.88/ 380 kn IAS, probably the highest of any of its contemporaries. Designed for high speed, with a critical Mach number of 0.93, the wing produced relatively limited lift at lower speeds. This, and the aircraft's low power-to-weight ratio, called for prolonged takeoff runs. Nevertheless, the Trident fulfilled BEA's field length criterion and its relatively staid airfield performance was deemed adequate before the arrival into service of the Boeing 727 and later jet airliners built to field length criteria. The aerodynamics and wing was developed by a team led by Richard Clarkson, who would later take the Trident's wing design for the wing of the Airbus A300; for the Trident he won the Mullard Award in 1969.
The Trident was routinely able to descend at rates of up to 4500 ft/min (23 m/s) in regular service. In emergency descents of up to 10,000 ft/min, it was permissible to use reverse thrust. Below 280 kn IAS, it was also possible to extend the main landing gear for use as an airbrake. The Trident's first version, Trident 1C, had the unusual capability of using reverse thrust prior to touchdown. The throttles could be closed in the flare and reverse idle set to open the reverser buckets. At pilot discretion, up to full reverse thrust could then be used prior to touchdown. This was helpful to reduce hydroplaning and give very short landing runs on wet or slippery runways while preserving wheel brake efficiency and keeping wheel brake temperatures low. Brakes were fitted with the Dunlop Maxaret anti-skid system.
Avionics.
The Trident had a complex, sophisticated and comprehensive avionics fit which was successful in service. This comprised a completely automatic blind landing system developed by Hawker Siddeley and Smiths Aircraft Instruments. It was capable of guiding the aircraft automatically during airfield approach, flare, touchdown and even roll-out from the landing runway. The system was intended to offer autoland by 1970. In the event, it enabled the Trident to perform the first automatic landing by a civil airliner in scheduled passenger service on 10 June 1965 and the first genuinely "blind" landing in scheduled passenger service on 4 November 1966.
The ability to land in fog solved a major problem at London Heathrow and other British airports. Delays were commonplace when Category 1 (Cat 1 = decision height and 600 metre runway visual range RVR) instrument landing system (ILS) was in use. The Trident's autoland system pioneered the use of lower landing minima, initially with Category 2 (100 ft decision height and 400 metres RVR) and soon after "zero-zero" (Category 3C) conditions. Since Tridents could operate safely to airfields equipped with suitable ILS installations, they could operate schedules regardless of weather, while other aircraft were forced to divert.
The Trident's advanced avionics displayed the aircraft's momentary position relative to the ground on a moving map display on the centre instrument panel. This electro-mechanical device also recorded the aircraft's track using a stylus plotting on a motor-driven paper map. Positional information was given by a Doppler navigation system which read groundspeed and drift data which, alongside heading data, drove the stylus.
The Trident was the first airliner fitted with a quick access flight data recorder. This sampled 13 variables, converted them into a digital format, and stored them on magnetic tape for ground analysis.
Operational history.
Introduction.
The first Trident entered service on 1 April 1964. By 1965, there were 15 Tridents in BEA's fleet and by March 1966, the fleet had increased to 21.
Hawker Siddeley then proposed an improved 1C, the Trident 1E. This would be powered by 11,400 lbf (50.7 kN) Spey 511s, have a gross weight of , an increased wing area by extending the chord, and the same fuselage but with up to 140 seats in a six-abreast configuration. This specification took the 1C closer to the larger concept of the original DH121, but with 7,000 lbf (31 kN) less thrust. There were only a few sales of the new design: three each for Kuwait Airways and Iraqi Airways, four for Pakistan International Airlines (later sold to CAAC), two each for Channel Airways and Northeast Airlines, and one for Air Ceylon. Channel Airways' aircraft were equipped with cramped, 21" pitch, seven-abreast seating in the forward section, seating 149 passengers.
Trident 2E.
At this point, BEA decided that the Trident was too short-legged for its ever-expanding routes, and that an even longer-ranged version was needed. responded with another upgrade designated Trident 1F. It would have the Spey 511 engines, a 2.8 m fuselage stretch, a gross weight of and up to 128 seats in the original five-abreast configuration. BEA planned to buy 10 1Fs, plus an option for 14 further aircraft.
As work continued on the 1F the changes became so widespread that it was renamed the Trident 2E, E for "Extended Range". Now powered by newer Spey 512s with 11,930 lbf (53.1 kN) thrust, it also replaced wing leading-edge droop flaps with slats, and extended the span with Küchemann-style tips. It had a gross weight of and a range.
BEA bought 15, while two were bought by Cyprus Airways. CAAC, the Chinese national airline, bought 33. The first flight of this version was made on 27 July 1967 and it entered service with BEA in April 1968.
Trident 3B.
Subsequently, the Trident was becoming the backbone of BEA's fleet and BEA wanted an even larger aircraft. offered two new designs in 1965: a larger 158-seat two-engine aircraft otherwise similar to the Trident known as the HS132; and the 185-seat HS134, which moved the engines under the wings, a design very similar to the Boeing 737. Both were to be powered by a new high-bypass engine under development at the time, the Rolls-Royce RB178. BEA instead opted for Boeing 727s and 737s to fill the roles of both the BAC 1–11 and Trident, but this plan was vetoed by the British government.
BEA returned to Hawker Siddeley and chose a stretched version of the basic Trident, the Trident 3. A fuselage stretch of made room for up to 180 passengers; raised the gross weight to and made modifications to the wing to increase its chord; the engines remained the same. BEA rejected the design as being unable to perform adequately in "hot and high" conditions, in light of such issues experienced with the Trident 2E. Since the Spey 512 was the last of the Spey line, extra thrust would be difficult to obtain. Instead of attempting to replace the three engines with a completely different type, which would have been difficult with one engine buried in the tail, 's engineers decided to add a fourth engine in the tail, the tiny Rolls-Royce RB162 turbojet, fed from its own intake behind a pair of movable doors. The engine added 15% more thrust for takeoff, while adding only 5% more weight, and it would only be used when needed. BEA accepted this design as the Trident 3B, and ordered 26. The first flight was on 11 December 1969 and the aircraft entered service on 1 April 1971. Addition of extra fuel capacity resulted in the Super Trident 3B.
The Trident experienced some key export sales, particularly to China. Following a thawing of relations between Britain and the People's Republic of China, China completed several purchase deals and more than 35 Tridents were eventually sold.
In 1977, fatigue cracks were discovered in the wings of British Airways' Tridents. The aircraft were ferried back to the manufacturer and repaired, then returned to service.
The beginning of the Trident's end came in the early 1980s, since ICAO began drafting noise regulations that would require first- and second-generation jet airliners to fit hush kits to the engines. These regulations would go into effect on 1 January 1986. British Airways, the type's main operator, saw the required refits as not viable and instead chose to phase the Trident out of their fleet in 1985. The Trident's services in China ended in 1995, marking its permanent retirement from service.
Only 117 Tridents were produced, and are no longer in service. In contrast, 1832 Boeing 727 aircraft (designed to the Trident's original specification) were built and the last continued in passenger service until 2019.
Aircraft on display.
Several aircraft are either preserved or in storage at various locations in China. Three airframes, one with a broken back, can be seen at the Chinese Aviation Museum at Datangshan, north of Beijing. In 2008, the personal aircraft of Mao Zedong was offered for sale after a decision by merchants at a market in Zhuhai, China that the Trident, formerly a tourist attraction, was limiting business.