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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 in the 1980 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. 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. Other variations of three-engine designs are trimotors, which are aircraft with three piston engines. ==History== 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.〔(Table 1-13: Active U.S. Air Carrier and General Aviation Fleet by Type of Aircraft ) Bureau of Transportation Statistics〕 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.〔http://www.flightglobal.com/articles/2005/12/20/203709/clipped-wings.html〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「trijet」の詳細全文を読む スポンサード リンク
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