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.)
and DC-10 share a similar configuration, however the biggest difference is the mounting of the tail engine. The Tristar's tail-mounted engine, as with the 727, is located in the rear fuselage and fed through an
does not use the S-duct design, instead its tail engine is mounted above the fuselage in a "straight" layout.
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 most common configuration is having the central engine located in the rear fuselage and 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. The S-duct design 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 and Lockheed TriStar. The Boeing 727's 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 new noise regulations of the 1980s, but such a redesign was prohibitively expensive and so Boeing ended production of the 727 instead of pursuing further development. The Lockheed Tristar's S-duct cross section was too small to fit an existing double spool engine as it was designed only to accommodate the new triple spool
RB211 engine, and delays in RB211 development in turn pushed back the TriStar's entry into service which affected sales.
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. Also, as 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 engine pylons in a
T-tail configuration. The larger widebody
Lockheed Tristar and
MD-11 mount an engine underneath each wing. Preliminary studies were done on the TriStar to reuse the fuselage and wing for a twinjet design though these never materialized due to Lockheed's lack of funds, while in the late 1990s Boeing which had taken over McDonnell Douglas had considered removing the tail engine from the MD-11 to make it a twinjet but ending up instead cancelled MD-11 production altogether.
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 share engines of similar power, making the trijet configuration more suited to a mid-range medium size airliner compared to long-range large quadjets. 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 twinjet design, resulting in higher operating costs. However this was worth the trade off during the 1970-1990s when trijets and twinjets shared engines of similar output, such as when the DC-10/MD-11 trijets and Airbus A300/A330 twinjets were all powered by the
General Electric CF6, where the additional power from the third engine gave the DC-10/MD-11 advantages in longer range and/or heavier payload over the A300/A330 twinjet. 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.