A trijet is a jet aircraft powered by three jet engines. Early 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. This led to a flurry of trijet designs, which in their heyday of the 1980s were the most popular airliner configuration. However, subsequent improvements in engine reliability and a corresponding relaxation in airline safety rules made the trijet obsolete for normal passenger services, and no manufacturer now produces three-engine airliners.
The first three-jet designs to fly were the Hawker Siddeley Trident (1962) and the Boeing 727 (1963). Both were compromises to meet airline requirements. In the case of the Trident to meet BEA's changing needs, for the 727 to be acceptable for three different airlines. Although collaboration between the manufacturers was considered, it did not come about.
During the late 1970s and early 1980s, three was the most common number of engines on US jet airlines, making up a majority of all such aircraft in 1980. From 1985 to 2003 the number of such planes in service had sunk from 1488 to 602. The number of twin-jets had more than quadrupled in the same period.
One issue with trijets is positioning the central engine. On most trijets they are placed at the tail along the middle, producing some technical difficulties. The central engine is most commonly supplied with air by an S-shaped duct – this is used on the Hawker Siddeley Trident, Boeing 727, Tupolev Tu-154, and Lockheed Tristar and is a complicated and costly design. The DC-10 and MD-11 use an alternative "straight" layout, which allows for easier engine installation, modification, and access, however, has inferior aerodynamic properties when compared to the S-duct designs. One major advantage of the trijet design is that the wings can be located further aft on the fuselage, 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 also 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. Placement of the remaining two engines varies. Most smaller aircraft, like the Hawker Siddeley Trident, the Boeing 727 and the Tupolev Tu-154 have all three engines mounted at the tail, whereas the larger Lockheed Tristar and DC-10/MD-11 have one tail-mounted engine and an engine mounted underneath each wing.
The main disadvantage with trijets is fuel efficiency, as a trijet design will almost always consume more fuel than a comparable twin engine design. This results in higher operating costs and reduced range. Although trijets are more efficient than four-engine aircraft, the difficulty and complexity of mounting the center engine through the tail will somewhat negate this advantage.
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 two-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, 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.
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 wing span of 213 feet. The project was cancelled in 1996, one year before McDonnell Douglas was taken over by Boeing.
Today, 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, the most widely used trijets are the DC-10 and the MD-11, mostly operated by UPS Airlines and FedEx Express as cargo planes.
Future of trijets
Airbus filed a patent in 2009 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 SBJ
- Sukhoi-Gulfstream S-21
- 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.
- Table 1-13: Active U.S. Air Carrier and General Aviation Fleet by Type of Aircraft Bureau of Transportation Statistics
- http://www.flightglobal.com/articles/2008/04/18/223089/airbus-files-patent-for-new-trijet-design.html Airbus files patent for new trijet design, FlightGlobal.com, Retrieved 2008-12-11.
- Modern Commercial Aircraft Willian Green, Gordon Swanborough and John Mowinski, 1987
- Stanford University Aircraft Aerodynamics and Design Group Engine Placement Accessed 2007-03-13
- Undeveloped MD-11/MD-12 models page
- Patent for a triple engine fighter
- Patent for a triple engine fighter
- NR-349 interceptor proposal