A trijet is an aircraft powered by three jet engines. Early twin-jet designs were limited by the FAA's " 60-minute rule", whereby the flight path of twin-engined jetliners was restricted to within 60 minutes' flying time from a suitable airport, in case of engine failure. In 1964 this rule was lifted for trijet designs, as they had a greater safety margin. This led to a flurry of trijet designs, which by 1980 had become the most popular airliner configuration.
Today, both narrow-body and wide-body trijet production has ceased in the West, being replaced by twinjets which are quieter and more efficient, and with ETOPS restrictions eased, became more suitable for long-haul overwater operation. With modern engines having extremely low failure rates and increased power output, more than two engines are no longer necessary except for "very large aircraft," such as the Airbus A380 or Boeing 747.
A real disadvantage with trijets is positioning the central engine. On most trijets they are placed at the tail along the middle, producing some technical difficulties. A "straight" layout such as the DC-10 and MD-11 leaves the engine high above the ground, making access difficult. Another option is an S-shaped duct like that found on the Boeing 727, Tupolev 154 and Lockheed Tristar, which is a complicated and costly design. One major advantage of the trijet design is that the wings can be located further aft on the fuselage, allowing main cabin exit and entry doors to be more centrally located for quicker embarkation and disembarkation, ensuring faster turnaround times.
During the late 1970s and early 1980s, three was the most common number of engines on US jet airlines, making up a majority of all such aeroplanes in 1980. From 1985 to 2003 the number of such planes in service has sunk from 1488 to 602. The number of twin-jets has more than quadrupled in the same period.
MDC was planning a new series of DC-10 family trijets called the MD-XX (Lengthened versions of the MD-11). The MD-XX Long Range would have been capable of travelling distances up to 8,320 nautical miles and a wing span of 213 feet. The project was cancelled in 1996, one year before McDonnell Douglas merged with Boeing.