A trijet is a jet aircraft powered by three jet engines. In general, passenger airline trijets are considered to be second-generation jet airliners, due to their innovative engine locations, in addition to the advancement of turbofan technology. Trijets are more efficient than quadjets, but not as efficient as twinjets, which replaced trijets as larger and more reliable turbofan engines became available.
The Dassault Falcon 7X/8X and the Dassault Falcon 900 are the only trijets still in production.[ citation needed ]
One consideration with trijets is positioning the central engine. This is usually accomplished by placing the engine along the centerline, but still poses difficulties. The most common configuration is having the central engine located in the rear fuselage and supplied with air by an S-shaped duct; this is used on the Hawker Siddeley Trident, Boeing 727, Tupolev Tu-154, Lockheed L-1011 TriStar, and, more recently, the Dassault Falcon 7X. The S-duct has low drag, and since the third engine is mounted closer to the centerline, the aircraft will normally be easy to handle in the event of an engine failure. However, S-duct designs are more complex and costlier, particularly for an airliner. Furthermore, the central engine bay would require structural changes in the event of a major re-engining (remodeling of the engine). For example, the 727's central bay was only wide enough to fit a low-bypass turbofan and not the newer high-bypass turbofans which were quieter and more powerful. Boeing decided that a redesign was too expensive and ended its production instead of pursuing further development. The Lockheed Tristar's tail section was too short to fit an existing two-spool engine as it was designed only to accommodate the new three-spool Rolls-Royce RB211 engine, and delays in the RB211's development, in turn, pushed back the TriStar's entry into service which affected sales. [1]
The McDonnell Douglas DC-10 and related MD-11 use an alternative "straight-through" central engine layout, which allows for easier installation, modification, and access. It also has the additional benefit of being much easier to re-engine. However, this sacrifices aerodynamics compared to the S-duct. Also, as the engine is located much higher up than the wing-mounted engines, engine failure will produce a greater pitching moment, making it more difficult to control.
The placement of the remaining two engines varies. Most smaller aircraft, such as the Hawker Siddeley Trident and the Boeing 727, as well as the intermediate-sized Tupolev Tu-154, have two side-mount engine pylons in a T-tail configuration. The larger widebody Lockheed TriStar and DC-10/MD-11 mount an engine underneath each wing. Preliminary studies were done on the TriStar to reuse the fuselage and wing for a twinjet design though these never materialized due to Lockheed's lack of funds. Additionally in the late-1990s Boeing, which had taken over McDonnell Douglas, considered removing the tail engine from the MD-11 to make it a twinjet but instead cancelled MD-11 production altogether. [1]
Trijets are more efficient and cheaper than four-engine aircraft, as the engines are the most expensive part of the plane and having more engines consumes more fuel, particularly if quadjets and trijets share engines of similar power. For widebody aircraft this makes the trijet configuration more suited to a mid-size airliner compared to the quadjet layout for jumbo jets (i.e. the DC-10 versus the quadjet Boeing 747). However the difficulty and complexity of mounting the third engine through the tail will somewhat negate the cost/efficiency advantage. Nonetheless, this was worth the trade-off in the 1960s to the 1990s when widebody trijets and twinjets shared engines of similar output, such as when the DC-10, MD-11, Boeing's 767, and Airbus's A300, A310, and A330 were all powered by the General Electric CF6, and the additional power from the third engine gave the DC-10/MD-11 advantages in longer range and/or heavier payload over the A300/A310/A330 twinjet. Since the 1990s, with further advancements in high-bypass turbofan technology, large twinjets have been equipped with purpose-designed engines such as the Boeing 777's General Electric GE90, allowing twinjets to perform the same tasks as most trijets and even many quadjets but more efficiently.
Due to their added thrust, trijets will have slightly improved takeoff performance compared to twinjets if an engine fails. Because takeoff performance for aircraft is usually calculated to include an extra margin to account for a possible engine failure, trijets are better able to take off from hot and high airports or those where terrain clearance near the runway is an issue.
Unlike twinjets, trijets are not required to land immediately at the nearest suitable airport if one engine fails. (This advantage is also shared with quadjets.) [2] This is advantageous if the aircraft is not near one of the operator's maintenance bases, as the pilots may then continue the flight and land at an airport where it is more suitable to perform repairs. Additionally, for trijets on the ground with one engine inoperative, approval can be granted to perform two-engine ferry flights. Prior to the introduction of ETOPS, only trijets and quadjets were able to perform long international flights over areas without any diversion airports. However, this advantage has largely disappeared in recent years as ETOPS-certified twin-engined aircraft are able to do so as well.
Another major advantage of the trijet design is that the wings can be located further aft on the fuselage, compared to twinjets and quadjets with all wing-mounted engines, allowing main cabin exit and entry doors to be more centrally located for quicker boarding and deplaning, ensuring shorter turnaround times. The rear-mounted engine and wings shift the aircraft's center of gravity rearwards, improving fuel efficiency, although this will also make the plane slightly less stable and more complex to handle during takeoff and landing. (The McDonnell Douglas DC-9 twinjet and its derivatives, whose engines are mounted on pylons near the rear empennage, have similar advantages/disadvantages of the trijet design, such as the wings located further aft and a more rearward center of gravity.)[ citation needed ]
The first trijet design to fly was the Tupolev Tu-73 bomber prototype, first flown in 1947. [3] The first commercial trijets were the Hawker Siddeley Trident (1962) and the Boeing 727 (1963). Both were compromises to meet airline requirements; in the case of the Trident, it was to meet BEA's changing needs, while the 727 had to be acceptable for three different airlines. Although collaboration between the manufacturers was considered, it did not come about. [4]
Early American twinjet designs were limited by the FAA's 60-minute rule, whereby the flight path of twin-engine jetliners was restricted to within 60 minutes' flying time from a suitable airport, in case of engine failure. In 1964, this rule was lifted for trijet designs, as they had a greater safety margin.
For second-generation jet airliners, with the innovations of the high-bypass turbofan for greater efficiency and reduced noise, and the wide-body (twin-aisle) for greater passenger/cargo capacity, the trijet design was seen as the optimal configuration for the medium wide-body jet airliner, sitting in terms of size, range, and cost between quadjets (four-engine aircraft) and twinjets, and this led to a flurry of trijet designs. The four-engine Boeing 747 was popular for transoceanic flights due to its long-range and large size, but it was expensive and not all routes were able to fill its seating capacity, while the original models of the Airbus A300 twinjet were limited to short- to medium-range distances. During this period, different jet airliners shared engines of similar output, such as when the McDonnell Douglas DC-10, Airbus A300, and Boeing 767 were powered by the General Electric CF6, the additional power from the third engine gave the DC-10 advantages in longer range and/or heavier payload over the A300 and 767 twinjets. Thus trijet designs such as the DC-10 and L-1011 TriStar represented the best compromise with medium- to long-range and medium size that US airlines sought for their domestic and transatlantic routes. As a result of these trijet wide-bodies, as well as the popularity of the Boeing 727, in their heyday of the 1980s trijets made up a majority of all such US jet airliners.
From 1985 to 2003 the number of such planes in service had sunk from 1488 to 602. The number of twinjets, on the other hand, had more than quadrupled in the same period. [5] Both Lockheed and McDonnell Douglas were financially weakened competing in the widebody market, which led to Lockheed ending production of the L-1011 in 1984 after producing only half the units needed to break even, while a number of fatal DC-10 crashes also slowed its sales. [1] In 1984 Boeing ended production of the 727, as its central engine bay would require an extremely expensive redesign to accommodate quieter high-bypass turbofans, and it was soon supplanted by Airbus with their A320 and Boeing with their 737 and 757. Further advancements in high-bypass turbofan technology and subsequent relaxation in airline safety rules made the trijet and even the quadjet nearly obsolete for passenger services, as their range and payload could be covered more efficiently with large twinjets powered with purpose-designed engines such as the 777's General Electric GE90.
During the 1980s, McDonnell Douglas was the only Western manufacturer to continue development of the trijet design with an update to the DC-10, the MD-11, which initially held a range and payload advantage over its closest medium wide-body competitors which were twinjets, the in-production Boeing 767 and upcoming Airbus A330. McDonnell Douglas had planned a new trijet called the MD-XX, which were lengthened versions of the MD-11. The MD-XX Long Range aircraft would have been capable of traveling distances up to 8,320 nautical miles (15,410 km) and had a wingspan of 65 metres (213 ft). The project was canceled in 1996, one year before McDonnell Douglas was acquired by Boeing. Boeing ended production of the MD-11 after filling remaining customer orders since the MD-11 would have competed with the 767 and 777. A study to remove the MD-11's tail-mounted engine (which would have made it a twinjet) never came to fruition as it would have been very expensive, and the MD-11 had very little in common in terms of design or type rating with other Boeing airliners. [6] In contrast to McDonnell Douglas sticking with their existing trijet configuration, Airbus (which never produced a trijet aircraft) and Boeing worked on new widebody twinjet designs that would become the A330 and 777, respectively. The MD-11's long-range advantage was brief as it soon was threatened by the A330's four-engine derivative, the A340, and the 777. The only other notable trijet development during the 1980s was in the Soviet Union, where the Tupolev Tu-154 was re-engined with the Soloviev D-30 engine as well as a new wing design and entered serial production from 1984 as the Tu-154M.
With the exceptions of the Dassault Falcon 7X, Falcon 8x, and Falcon 900 business jets, no manufacturer currently produces three-engine airliners.
Modern engines have extremely low failure rates and can generate much higher shaft power and thrust than early types. This makes twinjets more suitable than they were before for long-haul trans-oceanic operations, resulting in eased ETOPS restrictions; modern wide-body twin-engine jets usually have an ETOPS 180 or (in the case of the Boeing 777 and 787) ETOPS 330 rating, and even ETOPS 370 for the Airbus A350. [7] As such, having more than two engines is no longer considered necessary, except for very large or heavy aircraft such as the Boeing 747, Airbus A380 (over 400 seats in a mixed-class configuration), Antonov An-124, and An-225, or for flights through the Southern Hemisphere, primarily to and from Australia (which has not yet adopted the ETOPS 330 standard), where the most direct route for some flights is over Antarctica. [8]
Since 2000, both narrow-body and wide-body trijet production has ceased for almost all commercial aircraft, being replaced by twinjets. As of 2016, the Falcon 7X, 8X, and 900 business jets, all of which use S-ducts, are the only trijets in production. Trijets that are no longer in production, such as the 727, Tu-154, DC-10, and MD-11, have found second careers as cargo aircraft, as well as in limited charter, governmental, and military service. However, because of the average age of trijets and the COVID-19 pandemic, most non-private operators have chosen to retire quadjets and trijets and replace them with more efficient and cost-saving twinjets. Today, the only widely used trijet is the MD-11, mostly operated by UPS Airlines and FedEx Express in cargo service.
For smaller private and corporate operators, where maximum fuel efficiency is often less important than for scheduled-route operating airlines, trijets may still be of interest due to their immunity from ETOPS and the ability to take off from shorter runways and therefore have access to more airports. As a result, a sizeable number of trijets, such as the newly built Dassault Falcons, are in use by private operators and corporate flight departments.
Airbus filed a patent in 2008 for a new, twin-tail trijet design, whose tail engine appears to use a "straight" layout similar to the MD-11, but it is unknown if and when this will be developed or produced. [9] However, the proposed Boeing X-48 blended wing body design, Lockheed's N+2 design study, and Aerion AS2 supersonic business jet were also supposed to have three engines. [10] [11] The AS2 programme was cancelled in May 2021 when Aerion Corporation shut down. [12] [13]
Boom Technology's planned Overture supersonic transport (SST) airliner was originally planned to use three engines, with the third engine installed in the tail with a Y-shaped duct and air intakes on both sides of the rear. However, a revised design with four engines located under a delta wing was unveiled at the Farnborough Airshow on July 19, 2022. [14]
The Airbus A300 is Airbus' first production aircraft and the world's first twin-engine, double-aisle (wide-body) airliner. It was developed by Airbus Industrie GIE, now merged into Airbus SE, and manufactured from 1971 to 2007.
The Boeing 767 is an American wide-body airliner developed and manufactured by Boeing Commercial Airplanes. The aircraft was launched as the 7X7 program on July 14, 1978, the prototype first flew on September 26, 1981, and it was certified on July 30, 1982. The initial 767-200 variant entered service on September 8, 1982, with United Airlines, and the extended-range 767-200ER in 1984. It was stretched into the 767-300 in October 1986, followed by the extended-range 767-300ER in 1988, the most popular variant. The 767-300F, a production freighter version, debuted in October 1995. It was stretched again into the 767-400ER from September 2000.
An airliner is a type of airplane for transporting passengers and air cargo. Such aircraft are most often operated by airlines. The modern and most common variant of the airliner is a long, tube shaped, and jet powered aircraft. The largest of them are wide-body jets which are also called twin-aisle because they generally have two separate aisles running from the front to the back of the passenger cabin. These are usually used for long-haul flights between airline hubs and major cities. A smaller, more common class of airliners is the narrow-body or single-aisle. These are generally used for short to medium-distance flights with fewer passengers than their wide-body counterparts.
The Lockheed L-1011 TriStar is an American medium-to-long-range, wide-body trijet airliner built by the Lockheed Corporation. It was the third wide-body airliner to enter commercial operations, after the Boeing 747 and the McDonnell Douglas DC-10. The airliner has a seating capacity of up to 400 passengers and a range of over 4,000 nautical miles. Its trijet configuration has three Rolls-Royce RB211 engines with one engine under each wing, along with a third engine center-mounted with an S-duct air inlet embedded in the tail and the upper fuselage. The aircraft has an autoland capability, an automated descent control system, and available lower deck galley and lounge facilities.
The McDonnell Douglas DC-10 is an American trijet wide-body aircraft manufactured by McDonnell Douglas. The DC-10 was intended to succeed the DC-8 for long-range flights. It first flew on August 29, 1970; it was introduced on August 5, 1971, by American Airlines.
The Airbus A340 is a long-range, wide-body passenger airliner that was developed and produced by Airbus. In the mid-1970s, Airbus conceived several derivatives of the A300, its first airliner, and developed the A340 quadjet in parallel with the A330 twinjet. In June 1987, Airbus launched both designs with their first orders and the A340-300 took its maiden flight on 25 October 1991. It was certified along with the A340-200 on 22 December 1992 and both versions entered service in March 1993 with launch customers Lufthansa and Air France. The larger A340-500/600 were launched on 8 December 1997; the A340-600 flew for the first time on 23 April 2001 and entered service on 1 August 2002.
The Airbus A330 is a wide-body aircraft developed and produced by Airbus. Airbus began developing larger A300 derivatives in the mid-1970s, giving rise to the A330 twinjet as well as the Airbus A340 quadjet, and launched both designs along with their first orders in June 1987. The A330-300, the first variant, took its maiden flight in November 1992 and entered service with Air Inter in January 1994. The A330-200, a shortened longer-range variant, followed in 1998 with Canada 3000 as the launch operator.
A wide-body aircraft, also known as a twin-aisle aircraft and in the largest cases as a jumbo jet, is an airliner with a fuselage wide enough to accommodate two passenger aisles with seven or more seats abreast. The typical fuselage diameter is 5 to 6 m. In the typical wide-body economy cabin, passengers are seated seven to ten abreast, allowing a total capacity of 200 to 850 passengers. Seven-abreast aircraft typically seat 160 to 260 passengers, eight-abreast 250 to 380, nine- and ten-abreast 350 to 480. The largest wide-body aircraft are over 6 m (20 ft) wide, and can accommodate up to eleven passengers abreast in high-density configurations.
A jet airliner or jetliner is an airliner powered by jet engines. Airliners usually have two or four jet engines; three-engined designs were popular in the 1970s but are less common today. Airliners are commonly classified as either the large wide-body aircraft, medium narrow-body aircraft and smaller regional jet.
The McDonnell Douglas MD-11 is an American tri-jet wide-body airliner manufactured by American manufacturer McDonnell Douglas (MDC) and later by Boeing. Following DC-10 development studies, the MD-11 program was launched on December 30, 1986. Assembly of the first prototype began on March 9, 1988. Its maiden flight occurred on January 10, 1990, and it achieved Federal Aviation Administration (FAA) certification on November 8. The first delivery was to Finnair on December 7 and it entered service on December 20, 1990.
The Extended-range Twin-engine Operations Performance Standards (ETOPS) are safety standards set by the International Civil Aviation Organization (ICAO) for twin-engine commercial passenger aircraft operations. They are a safety measure intended to ensure that in the event of a single engine failure, an aircraft will still be able to reach a diversion airport using the remaining operational engine. This may be at a reduced speed and/or height, and usually applies to flights over water or remote lands, typically routes previously restricted to three- and four-engine aircraft.
The Pratt & Whitney JT9D engine was the first high bypass ratio jet engine to power a wide-body airliner. Its initial application was the Boeing 747-100, the original "Jumbo Jet". It was Pratt & Whitney's first high-bypass-ratio turbofan.
The McDonnell Douglas MD-12 was a large wide-body airliner concept planned by the McDonnell Douglas company in the 1990s. It was first conceived as a trijet larger than the MD-11, then stretched to a quadjet airliner. It was to be similar in size to the Boeing 747, but with greater passenger capacity through two full-length passenger decks. However, the MD-12 received no orders and was canceled. McDonnell Douglas then studied larger MD-11 derivatives named MD-XX without proceeding.
The Boeing NLA, or New Large Airplane, was a 1990s concept for an all-new quadjet airliner in the 500+ seat market. With a proposed size somewhat larger than the 747, it was a similar concept to the McDonnell Douglas MD-12 and later Airbus A380. In 1993, Boeing chose not to pursue development of this concept, focusing instead on the Boeing 747-500X and -600X, and then on the 747X and 747X Stretch, and subsequently on the Boeing 747-8. The project names for this proposal were NLA and Boeing 763-246C.
The General Electric CF6, US military designations F103 and F138, is a family of high-bypass turbofan engines produced by GE Aviation. Based on the TF39, the first high-power high-bypass jet engine, the CF6 powers a wide variety of civilian airliners. The basic engine core also powers the LM2500 and LM6000 marine and power generation turboshafts. It is gradually being replaced by the newer GEnx family.
A twinjet or twin-engine jet is a jet aircraft powered by two engines. A twinjet is able to fly well enough to land with a single working engine, making it safer than a single-engine aircraft in the event of failure of an engine. Fuel efficiency of a twinjet is better than that of aircraft with more engines. These considerations have led to the widespread use of aircraft of all types with twin engines, including airliners, fixed-wing military aircraft, and others.
An S-duct is a type of jet engine intake duct used in several types of trijet aircraft. In this configuration, the intake is in the upper rear center of the aircraft, above or below the stabilizer, while the exhaust and engine is at the rear of the aircraft. The S-duct is located in the tail, or empennage, of the aircraft. The shape of the S-duct is distinctive and easily recognized, and was used in several aircraft, beginning in 1962 with the Hawker Siddeley Trident. The Dassault Falcon 8X and Dassault Falcon 900 business jets are the only aircraft in production that use the S-duct design.
A four-engined jet, sometimes called a quadjet, is a jet aircraft powered by four engines. The presence of four engines offers increased power and redundancy, allowing such aircraft to be used as airliners, freighters, and military aircraft. Many of the first purpose-built jet airliners had four engines, among which stands the De Havilland Comet, the world's first commercial jetliner. In the decades following their introduction, their use has gradually declined due to a variety of factors, including the approval of twin-engine jets to fly farther from diversion airports as reliability increased, and an increased emphasis on fuel efficiency.
The McDonnell Douglas DC-10 Twin was a proposed version of the DC-10, a wide-body trijet airliner, except with only two engines instead of three.
| |||||||||||||||||||||||||||||||||||||
| |||||||||||||||||||||||||||||||||||||
