A trijet is a jet aircraft powered by three jet engines. 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. Trijets are more efficient than quadjets, but not as efficient as twinjets, which replaced trijets as larger and more reliable turbofan engines became available.
Types still in production Chengdu J-36 Venom Dassault Falcon 7X Dassault Falcon 8X Dassault Falcon 900
Design
One consideration with trijets is positioning the central engine. This is usually accomplished by placing the engine along the centerline but still poses 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, Boeing 727, Tupolev Tu-154, Lockheed L-1011 TriStar, and, more recently, the Dassault Falcon 7X. The S-duct has low drag, and since the third engine is mounted closer to the centerline, the aircraft will normally be easy to handle in the event of an engine failure. However, S-duct designs are more complex and costlier, particularly for an airliner. Furthermore, the central engine bay would require structural changes in the event of a major re-engining (remodeling of the engine). For example, the 727's central bay was only wide enough to fit a low-bypass turbofan and not the newer high-bypass turbofans which were quieter and more powerful. Boeing decided that a redesign was too expensive and ended its production instead of pursuing further development. The Lockheed Tristar's tail section was too short to fit an existing two-spool engine as it was designed only to accommodate the new three-spool Rolls-Royce RB211 engine, and delays in the RB211's development, in turn, pushed back the TriStar's entry into service which affected sales.
The McDonnell Douglas DC-10 and related MD-11 use an alternative "straight-through" central engine layout, which allows for easier installation, modification, and access. It also has the additional benefit of being much easier to re-engine. However, this sacrifices aerodynamics compared to the S-duct. Also, as the engine is located much higher up than the wing-mounted engines, engine failure will produce a greater pitching moment, making it more difficult to control.
The placement of the remaining two engines varies. Most smaller aircraft, such as the Hawker Siddeley Trident and the Boeing 727, as well as the intermediate-sized Tupolev Tu-154, have two side-mount engine pylons in a T-tail configuration. The larger widebody Lockheed TriStar and DC-10/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. Additionally, in the late 1990s, Boeing, which had taken over McDonnell Douglas, considered removing the tail engine from the MD-11 to make it a twinjet but instead cancelled MD-11 production altogether.
Advantages and drawbacks Trijets are more efficient and cheaper than four-engine aircraft, 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. For widebody aircraft, this advantage makes the trijet configuration more suited to a mid-size airliner compared to the quadjet layout for jumbo jets (i.e., the DC-10 versus the quadjet Boeing 747). However, the difficulty and complexity of mounting the third engine through the tail will somewhat negate the cost/efficiency advantage. Nonetheless, this was worth the trade-off in the 1960s to the 1990s when widebody trijets and twinjets shared engines of similar output, such as when the DC-10, MD-11, Boeing's 767, and Airbus's A300, A310, and A330 were all powered by the General Electric CF6, and the additional power from the third engine gave the DC-10/MD-11 advantages in longer range and/or heavier payload over the A300/A310/A330 twinjet. Since the 1990s, with further advancements in high-bypass turbofan technology, large twinjets have been equipped with purpose-designed engines such as the Boeing 777's General Electric GE90, allowing twinjets to perform the same tasks as most trijets and even many quadjets but more efficiently. Due to their added thrust, trijets will have slightly improved takeoff performance compared to twinjets if an engine fails. Because takeoff performance for aircraft is usually calculated to include an extra margin to account for a possible engine failure, trijets are better able to take off from hot and high airports or those where terrain clearance near the runway is an issue.
Unlike twinjets, trijets are not required to land immediately at the nearest suitable airport if one engine fails. (This advantage is also shared with quadjets.) This is advantageous if the aircraft is not near one of the operator's maintenance bases, as the pilots may then continue the flight and land at an airport where it is more suitable to perform repairs. Additionally, for trijets on the ground with one engine inoperative, approval can be granted to perform two-engine ferry flights. Prior to the introduction of ETOPS, only trijets and quadjets were able to perform long international flights over areas without any diversion airports. However, this advantage has largely disappeared recently as ETOPS-certified twin-engined aircraft are able to do so as well. Another major advantage of the trijet design is that the wings can be located further aft on the fuselage, compared to twinjets and quadjets 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. However, a disadvantage is that the rear-mounted engines shift the aircraft's center of gravity rearwards. This forces the wings to be aft-mounted and reduces the moment arm between the horizontal stabilizer and the center of gravity. Consequently, abnormally large stabilizers are required to maintain longitudinal stability, seen on the McDonnell Douglas DC-10 and Lockheed L-1011, resulting in reduced fuel efficiency. The successor of the DC-10, the MD-11, attempted to mitigate this by reducing the size of the horizontal stabilizer, causing the aircraft to be slightly less stable and more complex to handle during takeoff and landing.
History
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