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.
One issue with trijets is positioning the central engine. On most trijets this is placed at the tail along the centerline, producing some technical 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 Tristar and, more recently, the Dassault Falcon 7X. The S-duct design has the advantage of reduced drag, additionally since the engine outlet is located lower down, the aircraft will be easier to handle in the event of an engine failure. However, S-ducts are complicated and costly designs. Furthermore, the central engine bay would require structural changes in order to accommodate future larger-diameter engines in the case of the Boeing 727 and Lockheed TriStar. The Boeing 727's central bay was only wide enough to fit a low-bypass turbofan and not the newer high-bypass turbofans which had greater output as well as being quiet enough to meet new noise regulations of the 1980s, but such a redesign was prohibitively expensive and so Boeing ended production of the 727 instead of pursuing further development. The Lockheed Tristar's S-duct cross section was too small to fit an existing double spool engine as it was designed only to accommodate the new triple spool RB211 engine, and delays in RB211 development in turn pushed back the TriStar's entry into service which affected sales. The DC-10 and related MD-11 use an alternative "straight" layout, which allows for easier engine installation, modification, and access. However, this has inferior aerodynamic properties when compared to the S-duct designs. Also, as the engine outlet is located much higher up than the wing-mounted engines, an engine failure will produce a greater moment about the aircraft's lateral axis, making it more difficult to control.
Placement of the remaining two engines varies. Most smaller aircraft, like the Hawker Siddeley Trident, the Boeing 727 and the 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, while in the late 1990s Boeing which had taken over McDonnell Douglas had considered removing the tail engine from the MD-11 to make it a twinjet but ending up instead cancelled MD-11 production altogether.
Advantages and Drawbacks
One major advantage of the trijet design is that the wings can be located further aft on the fuselage, compared to twinjets and quad-jets 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 difficult 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 some advantages of the trijet design, such as the wings located further aft and a more rearward center of gravity.)
Trijets are more efficient and cheaper than quadjets (four-engine aircraft, whose engines are usually under the wing), 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, making the trijet configuration more suited to a mid-range medium size airliner compared to long-range large quadjets. However, the difficulty and complexity of mounting the center engine through the tail in a trijet design will somewhat negate this advantage.
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 takeoff from Hot and high airports or those where terrain clearance near the runway is an issue.
Unlike twinjets, trijets are not required to immediately land at the nearest suitable airport if one engine fails (this advantage is also shared with quad-jets). 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 quad-jets were able to perform long international flights over areas without any diversion airports. However, this advantage has been largely negated in recent years as more twin-engined aircraft have been certified to ETOPS standards.
The main disadvantage with trijets when compared to twinjets is fuel efficiency, as a trijet design will almost always consume more fuel than a comparable twinjet design, resulting in higher operating costs. However this was worth the trade off during the 1970-1990s when trijets and twinjets shared engines of similar output, such as when the DC-10/MD-11 trijets and Airbus A300/A330 twinjets were all powered by the General Electric CF6, where the additional power from the third engine gave the DC-10/MD-11 advantages in longer range and/or heavier payload over the A300/A330 twinjet. Since the 1990s, with further advancements in high-bypass turbofan technology, large twinjets have been equipped with purpose-designed engines like the Boeing 777's General Electric GE90 and those two engines would match or exceed the output of trijets and even quadjets.
The first trijet design to fly was the Tupolev Tu-73 bomber prototype, first flown in 1947. 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.
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. 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 airlines.
However, improvements in engine reliability and a subsequent relaxation in airline safety rules made the trijet obsolete for normal passenger services, and no manufacturer now produces three-engine airliners. From 1985 to 2003 the number of such planes in service had sunk from 1488 to 602. The number of twinjets had more than quadrupled in the same period. Both Lockheed and McDonnell Douglas were financially weakened competing in the trijet widebody market, which led to Lockheed ending production of the L-1011 TriStar in 1984 after producing only half the units needed to break even, while DC-10 sales also slowed. In the 1980s the Boeing 727 was discontinued, as its S-duct central engine bay would require a prohibitively expensive redesign to accommodate quieter high-bypass turbofans, and it was soon supplanted by twinjets for the narrow-body market; Airbus with the A320, and Boeing with the 757 and updated "classic" variants of the 737.
During the 1980s, McDonnell Douglas was the only aircraft 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 series of DC-10 family trijets called the MD-XX, which were lengthened versions of the MD-11. The MD-XX Long Range would have been capable of travelling distances up to 8,320 nautical miles and had a wingspan of 213 feet. The project was cancelled 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 Boeing's own offerings. A study to remove the MD-11's tail-mounted engine (which would have made it a twinjet) never came to fruition, furthermore the MD-11 had nothing in common in terms of design or type rating with other Boeing jet airliners. 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 Airbus A330 and Boeing 777, respectively. The MD-11's long range advantage was brief as it was soon nullified by the Airbus A330's four-engine derivative, the Airbus A340, and the extended-range Boeing 767-300ER and Boeing 777-200ER.
Modern engines have extremely low failure rates and increased power output. This makes twinjets more suitable for long-haul overwater operation, 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. As such, having more than two engines is no longer considered necessary, except for very large aircraft such as the Boeing 747 and Airbus A380 (over 400 seats in a mixed-class configuration), 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 is over Antarctica.
Today[when?], both narrow-body and wide-body trijet production has ceased for almost all commercial aircraft, being replaced by twinjets. As of 2016, the Dassault Falcon 7X, Dassault Falcon 8X, and Dassault Falcon 900 business jets, all of which feature S-ducts, are the only trijets in production. Some old trijets, such as the Boeing 727, Tupolev Tu-154, McDonnell Douglas DC-10, and MD-11, have found second careers as cargo aircraft, as well as limited charter, governmental, and military service. Currently[when?], the most widely used trijets are the DC-10 and the MD-11, mostly operated by UPS Airlines and FedEx Express as cargo planes.
For private and corporate operators, where fuel efficiency is often less important than for airlines, trijets are sometimes more desirable than twinjets due to their immunity from ETOPS and ability to take off from shorter runways. As a result, a sizable number of trijets (including retired airline 727s and newly-built Dassault Falcons) are in use by private operators and corporate flight departments.
Future of trijets
Airbus filed a patent in 2008 for a new, twin-tail trijet design, but it is unknown if this will ever be developed or produced. The proposed Boeing X-48 Blended wing body design also has three engines. Some proposed supersonic aircraft designs feature a trijet configuration, such as Lockheed's N+2 design study and Aerion's AS2 supersonic business jet. The latter is currently taking orders and a wooden mockup has been constructed.
- Boeing 727
- Boeing X-48
- Dassault Falcon 50
- Dassault Falcon 900
- Dassault Falcon 7X
- Dassault Falcon 8X
- Hawker Siddeley Trident
- Lockheed L-1011 TriStar
- Martin XB-51
- McDonnell Douglas DC-10
- McDonnell Douglas MD-11
- Tupolev Tu-154
- Yakovlev Yak-40
- Yakovlev Yak-42
Proposed or suspended trijet developments
- Boeing 747-300 Trijet - downsized 747 to compete with the DC-10 and L-1011, cancelled
- Blended Wing Body Trijet - proposed design based on the Boeing X-48
- McDonnell Douglas MD-XX - stretched derivative of the DC-10, project shelved
- North American NR-349 - proposed interceptor derivative of the A-5 Vigilante, cancelled
- Airbus twin-tail trijet, - status unknown
- Dassault Supersonic Business Jet - suspended
- Aerion AS2
- Sukhoi-Gulfstream S-21
- Boom Technology's - Boom Supersonic Transport
- Boeing 777 - Originally envisioned as a trijet in the 1970s to compete with the DC-10 and the L-1011. Later became a twin engine design.
- "Commercial Jets". Modern Marvels. Season A149. January 16, 2001. approx. 15 minutes in.
- Table 1-13: Active U.S. Air Carrier and General Aviation Fleet by Type of Aircraft Bureau of Transportation Statistics
- Airbus files patent for new trijet design, FlightGlobal.com, Retrieved 2008-12-11.
- Modern Commercial Aircraft Willian Green, Gordon Swanborough and John Mowinski, 1987