Trijet

Boeing 727-200 Advanced of Champion Airlines at Los Angeles

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.

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.

Design

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 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. (The McDonnell Douglas DC-9 twinjet and its derivatives, whose engines are mounted on pylons near the rear empennage, enjoys some advantages of the trijet design such as the wings located further after and a more rearward center of gravity.)

The Hawker Siddeley Trident featured an S-duct.
The Lockheed Tristar looks almost identical to the DC-10, with difference in how the tail-mounted engine looks. The Tristar's tail-mounted engine, as with the 727, is within the tail ( S-duct), whereas the DC-10's tail is on top of the engine.
The McDonnell Douglas DC-10 did away with the S-duct design, instead positioning the engine straight through the tail.

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, which has reduced drag and improved stability but this is a complicated and costly design. Furthermore, the central engine bay would require structural changes in order accommodate future larger-diameter engines; in the case of the Boeing 727 the 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 noise regulations, but such a redesign was prohibitively expensive and so Boeing ended production of the 727 instead of pursuing further development. The DC-10 and related 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.

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 pylons near the tail, whereas the larger Lockheed Tristar and DC-10/ MD-11 mount an engine 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, resulting in higher operating costs. During the 1970s when trijets and twinjets shared engines of similar output, such as when the DC-10 trijet and Airbus A300 twinjet were both equipped with the General Electric CF6, the additional power from the third engine gave trijets advantages in longer range and/or heavier payload. 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 quad-jets. Although trijets are more efficient than quad-jets (four-engine aircraft, whose engines are usually under the wing), the difficulty and complexity of mounting the center engine through the tail will somewhat negate this advantage.