Hawker Siddeley Trident
|Trident 1 G-ARPC at the SBAC Farnborough Airshow, 8 September 1962.|
|First flight||9 January 1962|
|Introduction||1 April 1964|
|Status||Out of production, out of service|
|Primary users||British European Airways
The Hawker Siddeley HS 121 Trident (originally the de Havilland D.H.121 and the Airco DH 121) was a British short and medium-range airliner. It was the first T-tail rear-engined three-engined jet airliner designed. It was also the first airliner to make a blind landing in revenue service in 1965.
The Trident emerged in response to a call by the state-owned British European Airways Corporation (BEA) for a jet airliner for its premier West European routes. BEA had been induced by government to issue this call despite its unwillingness to buy a large jet fleet. The airline's requirements fluctuated greatly in the 1950s and a decade later evolved radically away from what the Trident could offer. Adherence to BEA's changing specification was widely seen as limiting the Trident's appeal to other airlines and delaying its service entry.[N 1]
During its gestation, the Trident was also involved in a government drive to rationalise the British aircraft industry. The resulting corporate moves and government interventions contributed to delays causing it to enter service two months after its major competitor, the Boeing 727, losing further potential sales as a result.
By the end of the programme in 1978, 117 Tridents were produced. BEA's successor British Airways withdrew its Tridents by the mid-1980s. Trident services ended in China in the early 1990s.
Original specification 
Following the entry into service of jet airliners in 1952, many airline managers and economists remained sceptical and advocated turboprop aircraft to replace piston-engined aircraft instead. In April 1956, BEA's chief executive Anthony Milward stated that he "would rather do without [jet airliners]." Nevertheless, in July the same year, BEA announced what it called "outline requirements" for a short-haul "second generation jet airliner", to work alongside BEA's large fleet of turboprop airliners. It would carry a payload of some 20,000 lb (9,100 kg) or some 70 passengers over up to 1,000 miles (1,600 kilometres), weigh about 100,000 lb (45,000 kg), use 6,000 ft (1,800 m) runways, cruise at a very high speed of 610 to 620 mph (980 to 1,000 km/h) and have "more than two engines".[N 2] While not 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 also proposed the four-engined Avro 740; Avro eventually shelved its proposal and joined forces with Bristol and Hawker Siddeley. Vickers proposed a four-engined aircraft closely related to the in-development VC10, designated the VC11. The de Havilland company considered three possible contenders for the specification. Two were four-engined developments of the Comet: the D.H.119 and the D.H.120, the latter also intended for offer to BOAC. The third, the D.H.121, was announced in May 1957, had three engines, and was the contender that was pitched to BEA. In February 1958, BEA announced that the D.H.121 had come closest to its requirements and that it would order 24 with options on 12 more. It took a further six months for the British government to approve a formal BEA order for the D.H.121; the government had favoured the Hawker Siddeley Bristol 200 for industrial policy reasons. Reportedly, BEA had a considerable interest in Sud Aviation's Caravelle, however it was politically unacceptable, thus BEA had to favour de Havilliand and the Trident.
The D.H.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 two systems (or both of them) failing. The engines were to be 13,790 lbf (61.34 kN) Rolls-Royce Medways. The D.H.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 miles (3,330 kilometres), 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 Sud Aviation 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 D.H.121 was planned to employ avionics then considered very advanced for the era. Amongst other capabilities, as intended, they offered the capability of automatic approaches and landings within a few short years following the airliner's service entry. The avionics were also to have triplicated components for reliability and to allow "majority 2:1 voting" for aircraft guidance during automatic approaches and landings. The bulky size of the period's avionics required them to be housed in a large hold beneath the flightdeck; its size dictated a distinctive nose undercarriage design: offset by 2 ft (61 cm) to the port side and retracting sideways to stow across the D.H.121's longitudinal axis.
Revised specification 
In 1958, BEA's management concluded that the D.H.121's payload and range capacity was too great for their needs and petitioned de Havilland to reduce the scale of the design to suit their revised preferences. BEA was concerned due to three factors: a short-lived airline recession in the late 1950s; the imminent arrival into service of the turboprop Vickers Vanguard which duplicated the D.H.121's general payload and range area, and the growing trend to higher-density seating. At the time, many reputable airlines were reducing their short/medium range seat pitches from the then-customary minimum of 36 in (91 cm) to 34 in (86 cm) or less. As of 1959, BEA had a considerably large fleet, either currently in operation or on order, and the issue of over-capacity was a critical concern.
Although de Havilland stated that they generally concurred with these BEA views, upper 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 progress. 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 [N 3] project, which was significantly smaller than the D.H.121. At the time Boeing and Douglas were also downsizing their DC-9 and 727 projects and it was felt that the original large D.H.121 would have to compete against the Convair 880 and Boeing 720 some four years after their entry into service, whereas a cut-back design would be more competitive against the then-projected 75–100 seat, two-engined DC-9.
The downsizing of the aircraft involved substantial changes, including the powerplant being changed from the initial Medway engine to 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), range was cut by more than half to 930 miles (1,500 kilometres), 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 17 ft (5.2 m), wing area by 30 percent and overall length by 13 ft (4.0 m). The revised design retained some features of the original larger one, notably the fuselage diameter. It had a smaller flightdeck and single-axis, two-wheel, four-tyre main undercarriage legs in place of the four-wheel bogies of its larger predecessor.
Details of the emerging aircraft, including of the pioneering onboard avionics systems, were announced to the public in early 1960. It was this revised aircraft which BEA ultimately ordered on 24 August 1959, initially in 24 examples with 12 options. The future airliner's name, Trident, was announced at the Farnborough Airshow in September 1960, as a reflection of its then-unique three-jet, triple-hydraulic layout.
Industry consolidation and competition 
While the D.H.121 emerged in the late 1950s, British government policy on the aircraft and aeroengine industries was that that the then-many smaller companies should merge to form a few large groups. Accordingly, de Havilland had consorted with Hunting Aircraft and Fairey Aviation to manufacture and market the D.H.121 under the corporate name of Airco, after Geoffrey de Havilland's defunct First World War employer. The Minister of Supply, however, stated of the Airco consortium that "this is not quite what [he] had in mind". By 1960, de Havilland (along with Blackburn) had been acquired by the Hawker Siddeley group.
Airco executives intensively explored alternatives to the merger, such as the possibility that Boeing may drop its 727 project and instead manufacture Airco D.H.121s in the USA. This involved two Airco visits to Boeing and a return visit by Boeing executives and engineers. British commentators have tended to interpret this episode as involving the acquisition of sensitive proprietary data on the D.H.121 by a direct competitor. After the de Havilland takeover by Hawker Siddeley, 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 D.H.121 as a complementary, smaller member of the same airliner family. Fairey Aviation, partially incorporated into Westland Aircraft, also left the D.H.121 project. [N 4] With the move to Hawker Siddeley Aviation, the designation eventually changed to the HS 121.
The effort to compete with the Boeing 727 was immensely hindered by delays caused by BEA's indecision and industry reorganisation; in one situation alone, Trans Australia Airlines determined the Trident to be superior to the Boeing 727 from an operational standpoint, however it was viewed as commercially risky to choose a different fleet from rival airlines such as Ansett Australia, who had hastily selected the 727.
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 610 mph (980 km/h). At introduction into service its standard cruise Mach Number was 0.88/ 380kts 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 6,000 ft (1,800 m) 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 4,500 ft (1,400 m) field length criteria.
The Trident was routinely able to descend at rates of up to 4500 ft/min (23 m/s) in regular service. In emergency descents it was permissible to use reverse thrust of up to 10,000 rpm. Below 280 kt 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 a very short landing run on wet or slippery runways, while preserving wheel brake efficiency and keeping wheel brake temperatures low.
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 = 200 ft (61 m) 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 minimums, initially with Category 2 (100 ft decision height and 400 metres RVR) and soon being allowed to undertake "zero-zero" (Category 3C) operations. Because the Trident fleet could operate safely to airfields equipped with suitable ILS installations, it could operate as scheduled irrespective 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 derived from 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 
Hawker Siddeley Aviation, by this time, the parent of de Havilland, needed additional customers for the Trident, so entered into discussions with American Airlines (AA) in 1960. They demanded an aircraft with a longer range, which meant that the original DH121 design would have fulfilled American's requirements almost perfectly. To fill AA's needs, design began on a new Trident 1A, powered with uprated Rolls-Royce Spey 510s of 10,700 lbf (47.6 kN) thrust, and a larger wing with more fuel, raising gross weight to 120,000 lb (54,000 kg) and range to 1,800 miles (2,900 kilometres). American Airlines eventually declined the aircraft in favour of the Boeing 727, an aircraft which filled the original DH121 specifications almost exactly.
Some of these changes were nevertheless 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 115,000 lb (52,000 kg) and range to 1,400 miles (2,300 kilometres). The first Trident 1, G-ARPA, made its maiden flight on 9 January 1962 from Hatfield Aerodrome, and 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 use 11,400 lbf (50.7 kN) Spey 511s, have a gross weight of 128,000 lb (58,000 kg), 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 powered 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 PIA (later sold to CAAC), two each for Channel Airways and Northeast Airlines, and one for Air Ceylon. Channel Airways aircraft were operated with cramped, 21" pitch, seven-abreast seating in the forward section.
Trident 2E 
At this point, BEA decided that the Trident was now too short-legged for its ever-expanding routes, and that an even longer-ranged version was needed. Hawker-Siddeley responded with another upgrade as the Trident 1F. It would have the Spey 511 engines, a 2.8 m fuselage stretch, a gross weight of 132,000 lb (60,000 kg) 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 droops with slats, and extended the span with Küchemann-style tips. It had a gross weight of 142,400 lb (64,600 kg) and a 2,000 miles (3,200 kilometres) range. BEA bought 15, two were bought by Cyprus Airways and 33 by CAAC, the Chinese national airline. 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 the BEA fleet and BEA wanted an even larger aircraft. Hawker-Siddeley 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 and led to a modern-looking design very similar to the Boeing 757. 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 role of both the BAC 1–11 and Trident, but this plan was vetoed by the British government.
BEA returned to Hawker Siddeley and instead chose a stretched version of the basic Trident, the Trident 3. A fuselage stretch of 5 m (16 ft 5 in) made room for up to 180 passengers, raised the gross weight to 143,000 lb (65,000 kg), and made modifications to the wing to increase its chord; the engines remaining the same. BEA rejected the design as being unable to perform adequately in "hot and high" conditions, in light of such issues experienced on the Trident 2E. Since the Spey 512 was the last of the Spey line, extra power would be difficult to add. Instead of attempting to replace the three engines with a completely different type, which would have been difficult in regards to the engine buried into the tail, Hawker Siddeley decided to add a fourth engine in the tail, the tiny 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 would only be used when needed. BEA accepted this as the Trident 3B, and ordered 26. In some configurations, BEA (later British Airways) Trident aircraft had a number of rearward-facing passenger seats, an uncommon seating arrangement for civil aircraft. 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 would eventually be sold.
In 1977, fatigue cracks were discovered in the wings of the British Airways Trident fleet. The aircraft were ferried back to the manufacturer, and repaired, then returned to service. In total, 117 Tridents were produced, while the Boeing 727, built to the original airline specification for the Trident, sold 1,832 units.
- Trident 1C
- Production version for British European Airways, 24 built
- Trident 1E
- Increased seating capacity uprated engines and addition of leading edge slats, 15 built
- Trident 2E
- As Trident 1E version with triplex autoland system, 50 built
- Trident 3B
- High-capacity short-medium range version of the 2E with a 16 ft 5 in (5.00 m) stretch, 1 RB.162 booster engine in the tail; 26 built
- Super Trident 3B
- Extended range by 692 (430) miles, two built
Civil operators 
- Air Charter Service of Zaire
Military operators 
Aircraft on display 
Four complete aircraft are preserved in the United Kingdom:
- Trident 1C G-ARPO Save the Trident Group at North East Aircraft Museum.
- Trident 2E G-AVFB at Duxford near Cambridge.
- Trident 3B G-AWZJ at the Dumfries and Galloway Aviation Museum at the former RAF Dumfries, Scotland.
- Trident 3B G-AWZK in Aviation Viewing Park at Manchester Airport, moved from Heathrow in September 2005 (clipped wings).
- Trident 3B G-AWZM at Science Museum's exhibit store at Wroughton in Wiltshire.
Various other Tridents survive as static exhibits in China.
Sections of aircraft in the United Kingdom:
- Trident 2E G-AVFH (nose/cockpit and front cabin sections only) at the de Havilland Aircraft Heritage Centre.
- Trident 2E 5B-DAB at Nicosia International Airport (derelict). As of February 2012, the aircraft was still in existence, having stood for 35 years.
- Trident 3B G-AWZS at International fire training centre Teesside Airport (complete).
Several aircraft or sections in use as fire service training aids and aircraft either preserved or in storage at various locations in China (three airframes, one with a broken back, can be seen at the Beijing Aeroplane 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 that the Trident, formerly a tourist attraction, was limiting business.
- On 3 June 1966, Trident 1C G-ARPY entered into a deep stall whilst on a test flight and crashed at Felthorpe, Norfolk, killing all four crew.
- On 30 June 1966, Trident 1E registration 9K-ACG touched down 3 miles short of the runway at Kuwait International Airport. There were no fatalities but the aircraft was written off.
- On 3 July 1968, Trident 1s G-ARPI and G-ARPT were struck by Airspeed Ambassador G-AMAD, which crashed at London Heathrow Airport due to the failure of a flap operating rod. G-ARPI was severely damaged, but repaired and returned to service, whereas G-ARPT was written off.
- On 13 September 1971, a People's Liberation Army Air Force Trident 1E crashed in Mongolia under mysterious circumstances during an attempt by Lin Biao and his family to defect to the Soviet Union. Official PRC accounts claim that the Trident ran out of fuel.
- On 18 June 1972, British European Airways Flight 548, a Trident 1, G-ARPI, stalled due to pilot error and crashed at Staines shortly after takeoff from Heathrow Airport. All 118 on board were killed and it became known as the "Staines air disaster". As of 2012, it is still the worst aviation accident to have occurred on British soil (Pan Am Flight 103 was a terrorist incident).
- On 10 September 1976, a British Airways Trident 3B, G-AWZT, collided in midair with an Inex Adria McDonnell-Douglas DC-9, YU-AJR, over Yugoslavia, killing everyone on both aircraft. The collision of the two aircraft was attributed to an air traffic control error.
- On 14 March 1979, a CAAC Trident 2E, B-274, crashed into a factory near Beijing, injuring at least 200. The crash was caused by an unqualified pilot who stole and flew the airliner. Total fatalities were all 12 crew, 32 ground, and no passengers.
- On 26 April 1982, CAAC Flight 3303 Trident 2E, B-266 crashed near Yangsuo, China killing all 112 passengers and crew.
- On 31 August 1988, the right outboard flap of a CAAC Trident 2B operating as CAAC Flight 301 hit approach lights of runway 31 of Hong Kong's Kai Tak Airport while landing in rain and fog. The right main landing gear then struck a lip and collapsed, causing the aircraft to run off the runway and slip into the harbour. Seven people were killed.
Specifications (Trident 2E) 
- Crew: Three
- Capacity: 115 passengers
- Length: 114 ft 9 in (35 m)
- Wingspan: 98 ft (28.9 m)
- Height: 27 ft (8.3 m)
- Wing area: 1,462 sq ft (135.82 sq m)
- Empty weight: 73,800 lb (33,475 kg)
- Max. takeoff weight: 142,500 lb (64,636 kg)
- Powerplant: 3 × Rolls-Royce RB.163-25 Spey 512 , 11,930 lbf (53.1 kN) each
- Maximum speed: 590 mph
- Cruise speed: 580 mph
- Range: 2,700 miles (4,345 km)
- Service ceiling: 27,000 to 36,000 ft (8,000 to 11,000 m)
|Trident 2E||Trident 3B|
|Capacity||115 Passengers||180 Passengers|
|Length||114 ft 9 in (34.98 m)||131 ft 2 in (40 m)|
|Wingspan||98 ft (1,200 in)|
|Height||27 ft 0 in (8.23 m)||28 ft 3 in (8.61 m)|
|Max Takeoff Weight||142,500 lb (64,600 kg)||150,000 lb (68,000 kg)|
|Cruise Speed||590 mph (510 kn; 950 km/h)||550 mph (480 kn; 890 km/h)|
|Range||2,700 mi (2,300 nmi; 4,300 km)||2,235 mi (1,942 nmi; 3,597 km)|
See also 
- Aircraft of comparable role, configuration and era
- Related lists
- British aviation writer Bill Gunston was influential in elaborating this discourse.
- "B.E.A.'s feelings on these points were not inflexible. Their specification constituted a basis for discussion ..."
- redesignated the Bristol 205
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- "Accident description: Hawker Siddeley HS-121 Trident 1C, 18 June 1972." aviation-safety.net. Retrieved: 13 November 2010.
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- Winchester, Jim, ed. "Hawker Siddeley Trident." Civil Aircraft. Kent, UK: The Grange, 2004. ISBN 1-84013-642-1.
Further reading 
- "Classic Aircraft (Trident at 50: Tales of the Trident). Classic Aircraft online, March 2012, pp. 51–64, ISSN 2049-2081. Hersham, UK: Ian Allen Publishing.
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