The trijet, a three-engine aircraft, was once popular for long-haul flights due to safety regulations and unreliable early aircraft engines. However, technological advances and increasing numbers of airports have led to a decline in trijet designs. The placement of the third engine also presents maintenance challenges.
A trijet is a jet-powered aircraft with three engines, usually an aircraft used to carry 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 safer for extended flights between distant commercial airports. In the 21st century, technological advances have led to a decline in trijet designs.
Early aircraft engines were unreliable, leading manufacturers to fit at least two engines to any given aircraft, even if one engine was sufficient to power the vehicle. The reasoning was that if one engine failed mid-flight, the remaining engine could still provide a safe landing. For similar reasons, in 1953 the US Federal Aviation Administration (FAA) required twin-engine aircraft to be within a 60-minute drive of an airport or landing field at any time. In 1964, they revised this regulation, exempting aircraft with three or more engines.
As a result, commercial airlines began to move away from twin-engine “twins.” By stocking their fleets with trijets, they could provide long-haul service over bodies of water or uninhabited areas, such as arctic regions. By the late 1970s, the trijet outnumbered the twinjet in US-based airline fleets by a ratio of 2:1. By the late 20th century, safer technology and increasing numbers of airports allowed 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, due to the placement of the third engine. The engine should be placed in the center of the aircraft, as an off-center location would disrupt the aircraft’s delicate balance. On many trijets, the third engine is placed at the top of the tail, but this location makes it difficult for maintenance personnel to service the engine. A trijet often must be taken to a hangar where aircraft technicians can use special equipment to reach the third engine; Such techniques are not usually necessary 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 body of the engine takes an S-shape so that its end exits below the tail at the rear of the aircraft. This allowed easier access to the engine, but also created a complicated design that was more expensive to build and maintain. Many parts of the S-duct engine could not be used interchangeably with those of the wing-mounted engines.
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