What is a Trijet?

Alan Rankin

A trijet is a jet-powered aircraft with three engines, usually a jetliner used for transporting cargo or passengers. For safety reasons, most large aircraft are required to have at least two engines. During the 1970s and 1980s, the three-engine trijet design was considered safest for extended flights between distant commercial airports. In the 21st century, technological advances have led to a decline in trijet designs.

The Boeing 727 was a commercial trijet that was developed in the 1960s.
The Boeing 727 was a commercial trijet that was developed in the 1960s.

Early aircraft engines were unreliable, leading manufacturers to install at least two engines on any given aircraft, even if one engine would be sufficient to power the vehicle. The reasoning was that if an engine failed in mid-flight, the remaining engine would still be able to provide a safe landing. For similar reasons, in 1953 the U.S. Federal Aviation Administration (FAA) required twin-engine aircraft to be within 60 minutes’ travel time of an airport or landing field at any time. In 1964, they revised this regulation, exempting aircraft with three or more engines.

As with most trijets, the third engine on a DC-10 is located in the vertical stabilizer on the aircraft's tail.
As with most trijets, the third engine on a DC-10 is located in the vertical stabilizer on the aircraft's tail.

As a result, commercial airlines began moving away from double-engined “twinjets.” By stocking their fleets with trijets, they could provide long-distance service over bodies of water or uninhabited areas, such as arctic regions. By the late 1970s, the trijet outnumbered the twinjet in U.S.-based airline fleets by a ratio of 2:1. In the late 20th century, safer technology and an increasing number of airports allowed for the easing of such restrictions, and the twinjet became popular once more.

The trijet is somewhat problematic, compared to aircraft with two or four engines, because of the placement of the third engine. The engine must be placed in the center of the aircraft, as an off-center location would disrupt the airplane's delicate balance. In many trijets, the third engine is placed high up on the tail, but this placement makes it difficult for maintenance personnel to service the engine. A trijet must often be taken to a hangar where aircraft technicians can use special equipment to reach the third engine; such techniques are not usually required for wing-mounted engines.

Other trijets have an “S-duct” design for the tail-mounted third engine. In this design, the front of the third engine is above the tail, but the engine body takes an S-shape, so its end comes out below the tail at the back of the aircraft. This allowed easier access to the engine, but also made for a complicated design that was more expensive to build and maintain. Many S-duct engine parts could not be used interchangeably with those of the wing-mounted engines.

Twin-engine airplanes were originally developed for safety reasons.
Twin-engine airplanes were originally developed for safety reasons.

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