Trijet

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.

Generally, 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.

Other variations of three-engined designs are trimotors, which are aircraft with three piston engines.

History

The Hawker Siddeley Trident featured an S-duct.

The McDonnell Douglas DC-10 featured no S-duct, instead positioning an engine straight through the tail.

Two McDonnell Douglas MD-11 of Lufthansa Cargo and World Airways.

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 aeroplanes 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 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.

MDC was planning a new series of DC-10 family trijets called the MD-XX (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. The trijet design is currently only built on a very limited production run of Tupolev Tu-154M narrowbody airliners, and the Dassault Falcon 7X and Dassault Falcon 900 business jets, all three of which feature S-ducts.

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.

Notable examples

• Boeing 727
• Dassault Falcon 50
• Dassault Falcon 900
• Dassault Falcon 7X
• Hawker Siddeley Trident
• Lockheed L-1011 Tristar
• 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

• Trimotor
• Twinjet
• S-duct
• Wide-body aircraft

References

• Modern Commercial Aircraft Willian Green, Gordon Swanborough and John Mowinski, 1987

External links

• Stanford University Aircraft Aerodynamics and Design Group Engine Placement Accessed 2007-03-13
• Undeveloped MD-11/MD-12 models page