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A trijet is an aircraft powered by three jet engines. Early twin-jet designs were limited by the FAA's 60-minute rule, whereby the flight path of twin-engined 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 by 1980 had become the most popular airliner configuration.
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 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 has sunk from 1488 to 602. The number of twin-jets has more than quadrupled in the same period.
A real disadvantage 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 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 is more efficient, but leaves the engine high above the ground, making access difficult. 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 embarkation and disembarkation, ensuring faster turnaround times.
With ETOPS restrictions eased, twinjets became more suitable for long-haul overwater operation. With modern engines having extremely low failure rates and increased power output, more than two engines are no longer necessary except for large aircraft such as the An 225, Airbus A340-600, Airbus A380 or Boeing 747.
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 a wing span of 213 feet. The project was cancelled in 1996, one year before McDonnell Douglas was taken over by Boeing.
Current status 
Today, both narrow-body and wide-body trijet production has ceased for almost all commercial aircraft, being replaced by twinjets. As of 2013, the Dassault Falcon 7X and Dassault Falcon 900 business jets, both of which feature S-ducts, are the only trijets in production. Some trijets, such as the Boeing 727-200, Tupolev Tu-154M, McDonnell Douglas DC-10, and McDonnell Douglas MD-11 have found second careers as cargo aircraft, as well as limited passenger, governmental, and military service. Currently, the most widely-used trijets are MD-11 freighters.
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. Boeing also said that they would make a Boeing 747 trijet. But again it is unknown if this will ever be developed or produced.
Notable examples 
- Boeing 727
- Boeing X-48
- Dassault Falcon 50
- Dassault Falcon 900
- Dassault Falcon 7X
- 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: never produced.
- Blended Wing Body Trijet
- McDonnell Douglas MD-XX: never produced.
- Airbus twin-tail trijet, status unknown.
- Dassault Supersonic Business Jet (suspended)
See also 
- 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