The Hawker Siddeley Trident, model DH121 or HS121, was a British short/medium-range three-engined airliner designed by de Havilland in the 1950s, and built by Hawker Siddeley Aviation in the 1960s, after the former became part of that group in 1960. Designed to a British European Airways (BEA) requirement, it had limited appeal to other airlines and sold in small numbers, with only 117 produced. It was an important airliner in Europe but high operational costs doomed it to a short lifespan. BEA's successor, British Airways chose to replace its fleet with the Boeing 737 and Boeing 757 in the early 1980s. In China the Trident remained active in Air China's service until the mid-1990s. The Trident is notable for being the first commercial airliner to make a fully automatic approach and landing in revenue paying service.
In July 1956 BEA offered a contract for a new medium-haul jet aircraft to replace the turboprop Vickers Viscount on its longer European routes. The new aircraft would work beside a smaller design for shorter ranges, which would eventually emerge as the BAC One-Eleven. Several designs were submitted for the longer-range role, including the Bristol 200, the Avro 740, the Vickers VC11 and de Havilland's DH121. The DH121 was selected as the winner in 1958.
The DH121 was the first trijet design, a configuration which its designers felt offered a trade-off between cruising economy and take-off safety in case of an engine failure. The plane included a cruciform tail design similar to the Sud Aviation Caravelle. With the engines clustered at the rear, also like the Caravelle, the wing was left free from engine mounts and was designed with high-speed cruising in mind, a speed of over 600 mph being the goal. The Trident has a distinctive offset, sidewards retracting front landing gear. The DH121 was to be powered by 13,790 lbf (61.34 kN) Rolls-Royce Medway engines, with a gross weight of 150,000 lb (63 000 kg), a range of 2,070 miles (3330 km), and to seat 111 in a two-class layout.
At this point BEA decided that the 111-seat aircraft was too large for its routes so it tried to tailor its aircraft to its exact needs. The result was a downsizing of the Trident, powered by much smaller 9,850 lbf (43.8 kN) Rolls-Royce Spey 505 engines, with a gross weight of 105,000 pounds (48 000 kilograms), a range of 930 miles (1500 kilometres), and seating for 97. This version gained the variable-incidence T-tail it would have from then on, as well as a new nose design, both of which made it look very different from the Comet-like original version. BEA was happier with this smaller design (now known as the Trident 1 after BEA had held a competition to name it) and placed a contract for 24 on 12 August 1959.
The Trident is a jet airliner of all-metal construction with a T-tail and low-mounted swept wing. It has three rear-mounted engines: two in side-fuselage pods, with the third in the fuselage, fed intake air through an S-shaped duct. Some versions featured a fourth engine fed from a separate duct on top of the normal one. All versions were powered by versions of the Rolls-Royce Spey, the fourth engine being another Rolls-Royce design, the small RB162.
The Trident was one of the fastest subsonic commercial airliners, regularly cruising at over 600 mph (965 km/h). On initial introduction the standard cruise Mach Number was 0.88/ 380kts IAS, probably the highest of any of its contemporaries. The wing, designed for high speed, gave limited lift at lower speeds. This, and its low power, required long takeoff rolls. The aircraft gained the nickname "ground gripper" for the way it stuck to the runway; and it was also joked that Tridents only became airborne because of the curvature of the Earth.
The Trident was the first commercial aircraft to be fitted with a Quick access flight data recorder. It sampled 13 variables, converted them to a digital format, storing them on tape for ground analysis.
The Trident performed the first automatic blind landing by a civil airliner in fog on 4 November 1966, pioneering the ability to land in fog, a major problem at London Heathrow. These delays were relatively common during the period when Category 1 (Cat 1 = 200 ft decision height and 600 metre runway visual range RVR) instrument landing system (ILS) was in use. The Trident, its autoland system and three pilots pioneered the use of lower landing limits starting with Cat 2 (100 ft decision height and 400 metres RVR). Because the Trident fleet could make successful approaches and landings to airfields equipped with suitable Cat 2 ILS installations it frequently operated to its intended destination, while other aircraft were forced to divert.
The Trident also had the unusual capability to use reverse thrust in flight. This was limited to the two "pod" engines, and the normal landing procedure was to close the throttles in the flare, and immediately open the reverser buckets by selecting reverse idle. At the pilot's discretion, up to full power could then be selected in reverse prior to touchdown. Useful on wet or slippery runways, it produced a firm but well-controlled touchdown, and reduced hydroplaning, giving a very short landing. This compensated for its rather poor braking characteristics.
The use of reverse thrust up to 10,000 rpm was also permitted for emergency descent purposes. At indicated airspeeds below 280 kts it was also permissible to extend the main landing gear (but not the nosewheel) as an emergency airbrake, and when combined with conventional speedbrakes and reverse thrust this produced a phenomenal rate of emergency descent in the region of 12,000 feet per minute..
Another advanced feature for the era was a moving map display on the centre instrument panel. This was an electro-mechanical system with a stylus indicating the aircraft position over a motor-driven paper roll. Position was derived from a doppler navigation system which produced groundspeed and drift data, which combined with heading data could drive motors moving the stylus from side to side for lateral position, and the paper roll for the track.
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 (2900 km). American Airlines eventually declined the aircraft in favour of the Boeing 727, an aircraft which filled the original DH121 specifications almost exactly. In fact, de Havilland had invited a group of engineers from Boeing to see the DH121 design and development program in its early stages, partly because Boeing was looking to develop a similar medium-range version of its 707 design.
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 (2250 km). 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 this had risen 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.
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 Kuchemann-style tips. It had a gross weight of 142,400 lb (65 000 kg) and a 2,000 mile (3200 km) 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.
By this point 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 currently under development, 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 later vetoed by the British government (the owners of BEA).
BEA returned to Hawker-Siddeley and instead chose a stretched version of the basic Trident, the Trident 3. This included a fuselage stretch of 5 m 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. However the engines remained the same, and BEA rejected the design as being unable to get off the ground in "hot and high" conditions, given that the 2E was having so many problems already. Since the Spey 512 was the last of the Spey line, extra power would be difficult to add. Instead of attempting to fit a new engine, which would be difficult given that one was buried in 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 moveable 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 somewhat odd mixture 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, two of which were sold to CAAC.
In 1977, fatigue cracks were discovered in the wings of British Airways' Trident1's 2s and 3s. The aircraft were ferried back to the manufacturer, where repairs were made, and the aircraft returned to service.
In total, only 117 Tridents were produced, while the Boeing 727, built to the original airline specification for the Trident, sold over 1,700.
During the type's entire operational history, no Trident ever crashed due to a design flaw or mechanical failure.
Aircraft on display
Three complete aircraft are preserved in the United Kingdom:
Several aircraft or sections in use as fire service training aids and aircraft either preserved or in store in 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 plane of Mao Zedong was offered for sale after a decision by merchants at a market in Zhuhai that the plane, formerly a tourist attraction, was limiting business.
Published - July 2009
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