The Trident, model DH121 or HS121, was a short/medium-range airliner designed by de Havilland in the 1950s, and built by the Hawker-Siddeley Group in the 1960s when de Havilland was merged, along with several other British aviation firms. Designed specifically to a British European Airways requirement, it had limited appeal to other airlines and sold only in small numbers. Nevertheless it was an important airliner in Europe but high operational costs doomed it to a short lifespan. British Airways chose to upgrade their fleet with the Boeing 737, and the Trident left service in the 1980s.

In July 1956 BEA offered a contract for a new medium-haul aircraft to replace their Vickers Viscount on their longer European routes with a jet-powered aircraft. The new aircraft would work beside a smaller design for shorter ranges, which would eventually emerge as the BAC 1-11. Several designs were returned for this longer range role, including the Bristol 200, the Avro 740, and de Havilland's Airco DH.121. The DH.121 was selected as the winner in 1958.

The DH.121 was the first "tri-jet" design, a configuration the designers felt offered the ideal trade-off between cruise economy and takeoff safety in case of an engine failure. The plane looked, perhaps unsurprisingly, like a smaller Comet with three engines, including a tail design similar to the Comet, as opposed to the T-tail it would later use. With the engines clustered at the rear as in the Sud Caravelle, the wing was left free from engine mounts and was designed with high-speed cruise in mind, with a speed of over 600 mph being the goal. The DH121 was to be powered by 13,790 lbf (61.34 kN) Rolls-Royce Medway engines, have a gross weight of 150,000 lb (63,000 kg), a range of 2,070 mi (3,330 km), and seat 111 in a two-class layout.

At this point BEA decided that the 111-seat aircraft was too large for their existing routes, and they tried to carefully 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 (1,500 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 held a competition to name it) and placed a contract for 24 on August 12, 1959.

Hawker-Siddeley Trident 1C, built 1962 and destroyed in a fire at London (Heathrow) Airport in 1975Hawker-Siddeley, which had formed by this point, 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 up rated 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 mi (2900 km). American Airlines eventually declined the aircraft in favour of the Boeing 727, an aircraft that filled the original DH.121 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 115,00 lb (52,000 kg) and range to 1,400 mi (2,250 km). The first of the Trident 1, G-ARPA, made its first flight on January 9, 1962, and entered service on April 1, 1964. By 1965 there were 15 Tridents in the fleet and by March 1966 this had risen to 21.

Hawker-Siddeley then proposed an improved 1C as the Trident 1E. This would use 11,400 lbf (50.7 kN) Spey 511's, 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 wing, designed for high-speed, gave limited lift at lower speeds, and combined with the low power the takeoffs tended to be very long — it gained the nickname the "ground gripper" for the way it stuck to the runway, and it was also joked that Tridents only became airborne due to 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 decided 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 1F's, plus an option for 14 aircraft. As work continued on the 1F the changes became so widespread that they decided to rename it the Trident 2E, E for Extended Range. Now powered by the 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 (3,200 km) range. BEA purchased 15, 2 for Cyprus Airways and 33 for CAAC, the Chinese national airline. The first flight of this version was made on July 27, 1967 and it entered service with BEA in April 1968.

By this point the Trident was becoming the backbone of the BEA fleet, and now that it was in widespread service BEA wanted an even larger design. 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 HS.134 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 RB.178. BEA instead decided to purchase 727s and 737s to fill the role of both the 1-11 and Trident, but this plan was later vetoed by the government (the owners of BEA).

BEA returned to Hawker-Siddeley and instead chose a stretched version of the basic Trident, the HS.121 or Trident 3. The 3 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 wings 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 RB.162 turbojet, fed from the same intake as the middle Spey. 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, the first flight was on December 11th, 1969, entering service on April 1st, 1971. A simple change resulted in the Super Trident 3B, two of which were sold to CAAC.


The boost engine is visible in the image but is not being used on this takeoff. An interesting 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. Although it was not known at the time, the somewhat less accurate flares led to the aircraft making rather hard landings, which led to premature fatigue in the spars of many aircraft. When the problem was discovered in the 1970s, many operators simply removed the affected aircraft from service rather than pay for the expensive work to fix the problems.

In total only 117 Tridents were produced. The great irony is that the 727, built to the original Trident specification, sold over 1,700 airframes.

The Trident has a very distinctive offset front landing gear. The reason it was designed this way is if the gear is retracted sideways it occupied significantly less cargo hold space.

The Trident was the first commercial aircraft to be fitted with a Flight Data Recorder.

The Trident performed the first instrument landing using ILS, pioneering the use of flights in low visibility. Good for dealing with fog in the UK.

Incidents
In 1972, a Trident 1, G-ARPI crashed on takeoff from Heathrow Airport, killing all on board after hitting the ground at nearby Staines. At the time this was the worst air crash to have occurred on British soil. See Staines air disaster.

On March 14, 1979, a Trident 2E crashed into a factory near Beijing, killing at least 200. According to another source, this crash was caused by an unqualified pilot who stole and flew the plane. That source mentions total fatalities of all 12 crew, 32 ground, and no passengers.

Models
Trident 1C, 24 built
Trident 1E, 15 built
Trident 2E, 50 built
Trident 3B, 28 built

Specifications
For Trident 2E:

Length: 35 m
Span: 28.9 m
Height: 8.3 m
Max range: 2,400 mi (3,860 km)
Max takeoff weight: 143,500 lb (65,000 kg)
Engines: 3x Rolls-Royce RB.163-25 Spey 512, 11,930 lbf (53.1 kN) thrust
Accommodations: 3 crew + 149 passengers

For Trident 3B:

Length: 40 m
Span: 28.9 m
Height: 8.6 m
Max cruise speed: 573 mph (936 km/h)
Max range: 1,800 miles (3060 km)
Empty weight: 83,104 lb (37,695 kg)
Max takeoff weight: 155,000 lb (70,300 kg)
Engines: 3× Rolls-Royce RB.163-25 Spey 512, 11,930 lbf (53.1 kN) thrust + 1× Rolls-Royce RB.162-86, 5,250 lbf (23.4 kN) thrust
Accommodations: 3 crew +