De Havilland: DH.121 Trident

The Trident, model DH121 was a short/medium-range airliner designed by De Havilland in the 1950s, and built by Hawker Siddeley in the 1960s when de Havilland was merged with several other British aviation firms.

Designed specifically to a British European Airways (BEA) requirement, the Trident had limited appeal to other airlines and sold only in small numbers. 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 their fleet with the Boeing 737 and Boeing 757 in the late 1980s. In China the Trident remained in military service as a VIP and troop carrier to the mid-1990s.

In July 1956 BEA offered a contract for a new medium-haul jet aircraft to replace the turbo-prop Vickers Viscount on their 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 Airco DH.121. T

1958 The DH.121 was selected as the winner. De Havilland's associates were Hunting Aircraft and Fairey Aviation Co through the medium of a newly formed consortium called Airco

The DH.121 was the first "tri-jet" design, a configuration which its designers felt offered a good trade-off between cruising economy and take-off safety in case of an engine failure. The plane included a tail design similar to the De Havilland Comet, as opposed to the T-tail it would later use. With the engines clustered at the rear, as in the Sud Aviation 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 very distinctive offset, sidewards retracting front landing gear. It was designed this way was to accommodate the Smiths Automatic Blind Landing System computers. The DH.121 was 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. The Trident was the first commercial aircraft to be fitted with a flight data recorder.

At this point BEA decided that the 111-seat aircraft was too large for their existing routes, and they tried to tailor "their" aircraft to their 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 lb (48 000 kg), a range of 930 miles (1500 km), and seating for 97. This version gained the 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.

1960 Hawker Siddeley Aviation was formed; it started looking for additional customers for the Trident, and entered discussions with American Airlines in 1960. They demanded longer range, somewhat ironic as the original DH.121 design would have filled their requirements almost perfectly. Nevertheless they started design work 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.

Some of these changes were nevertheless added into the original prototype, and it was eventually 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[1], 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. 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, something that had previously caused many problems at London Heathrow and resulted in thousands of cancellations and delays.

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 DH.121, but powered with 7,000 lbf (31 kN) less thrust. The Trident was one of the fastest subsonic commercial airliners, regularly cruising at over 600 mph. 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 and this, combined with the low power, meant takeoffs tended to be very long. The aircraft gained the nickname the "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. 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 their 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.

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 HS.132, 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 HS121 or 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 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 somewhat odd mixture as the Trident 3B. BEA 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.

A notable feature of the Trident was its use in the development of a completely automatic blind landing system. This allowed the plane to land itself in conditions that would cause other planes to have to divert to alternate airports, thereby improving its on-time ratings. The Trident also had the almost unique capability to allow the use of reverse thrust in flight. This was limited to the two "pod" engines, and the normal landing procedure was to close the thrust levers 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. This was an extremely useful facility on wet or slippery runways resulting in a firm but well-controlled touchdown significantly reducing the probability of aquaplaning and producing a very short landing run. This compensated for the rather poor braking characteristics of the double wheel pairs on each axle main gear.

The use of reverse thrust up to 10,000 rpm was also permitted away from the landing flare essentially for emergency descent purposes. At indicated airspeed below 280 knots it was also permissible to extend the main landing gear (but not the nose-wheel) as an emergency air-brake, and when combined with conventional speed-brakes and reverse thrust this produced a phenomenal rate of emergency descent in the region of 10,000 feet per minute. [citation needed]> However use of this facility was restricted after it was realised that this facility was being used by crews far more often than had been envisaged by the manufacturers.

Other unusually advanced features for the era included provision of a cockpit moving map display on the main 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 ground-speed 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 itself for along track.

In 1977, fatigue cracks were discovered in the wings of British Airways' Trident 1's, 2's 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 Trident specification, sold over 1,700.

See Also

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Sources of Information

• [1] Wikipedia