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 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, have some advantages of the trijet design, such as the wings located further aft and a more rearward center of gravity.)
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 (
), whereas the DC-10's tail is on top of the engine.
eliminated the S-duct design, instead positioning the engine straight through the tail.
One issue with trijets is positioning the central engine. On most trijets this is placed at the tail along the centerline, 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,
Lockheed Tristar and, more recently, the
Dassault_Falcon_7X. An S-duct has the advantage of reduced drag. Additionally, since the engine outlet is located lower down, the aircraft will be easier to handle in the event of an engine failure. However, S-ducts are complicated and costly designs. 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, this has inferior aerodynamic properties when compared to the S-duct designs. Since the engine outlet is located much higher up than the wing-mounted engines, an engine failure will produce a greater moment about the aircraft's lateral axis, making it more difficult to control.
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
MD-11 mount an engine underneath each wing.
Trijets are more efficient and cheaper than quadjets (four-engine aircraft, whose engines are usually under the wing), as the engines are the most expensive part of the plane and having more engines consumes more fuel, particularly if quadjets and trijets had engines of similar power, making the trijet configuration more suited to a shorter-range medium payload airliner compared to quadjets which are long-range and larger-sized. However, the difficulty and complexity of mounting the center engine through the tail in a trijet design will somewhat negate this advantage. The main disadvantage with trijets is fuel efficiency, as a trijet design will almost always consume more fuel than a comparable twinjets design, resulting in higher operating costs. However this was worth the trade off 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, as the additional power from the third engine gave trijets advantages in longer range and/or heavier payload over a comparable twinjet design. Since the 1990s, with further advancements in high-bypass turbofan technology, large twinjets have been equipped with purpose-designed engines like the
General Electric GE90 and those two engines would match or exceed the output of trijets and even quadjets.