A twinjet or twin-engine jet is a jet aircraft powered by two engines. A twinjet is able to fly well enough to land with a single working engine, making it safer than a single-engine aircraft in the event of an engine failure. Fuel efficiency of a twinjet is better than that of aircraft with more engines. These considerations have led to the widespread use of aircraft of all types with twin engines, including airliners, fixed-wing military aircraft, and others.
Aircraft configurations There are three common configurations of twinjet aircraft. The first, common on large aircraft such as airliners, has a podded engine usually mounted beneath, or occasionally above or within, each wing. Most notable examples of such a configuration are the Boeing 737 and Airbus A320. The second has one engine mounted on each side of the rear fuselage, close to its empennage, used by many business jets, although some airliners like the Fokker 70, Douglas DC-9 and COMAC ARJ21 utilise such a design as well. In the third configuration both engines are within the fuselage, side-by-side, used by most fighters since the 1960s. Later fighters using this configuration include the Su-27 'Flanker', the F-15 Eagle, and the F-22 Raptor.
History
As jet aircraft emerged in the 1940s, many early airframes would be twinjets, as contemporary engines were not generally powerful and reliable enough to power airframes via a single engine. However, the first jet aircraft to fly, Germany’s Heinkel He 178, would be single engine-powered; its relatively small form, low weight, and its status as a prototype vice an operational aircraft meant that considerations for speed, reliability, and safety were secondary. The first twinjet to fly was the German fighter-prototype, the Heinkel He 280, flying on 30 March 1941 with a pair of nacelle-mounted Heinkel HeS 8 turbojets. The first operational jet fighter aircraft, the German Messerschmitt Me 262, mounted two Junkers Jumo 004 engines. On 15 May 1941 the first British jet, the Gloster E.28/39 made its first flight and was powered by a single engine. However, the first British jet to enter production was a twinjet, the Gloster Meteor. The United States of America’s first jet was a twinjet; the Bell P-59 Airacomet would first fly on 1 October 1942. However, the first American production jet, the Lockheed P-80 Shooting Star, was a single-jet aircraft. The twinjet configuration was used for short-range narrow-bodied aircraft such as the McDonnell Douglas DC-9 and Boeing 737. The Airbus A300 was initially not successful when first produced as a short-range widebody, as airlines operating the A300 on short-haul routes had to reduce frequencies to try to fill the high-capacity aircraft, and lost passengers to airlines operating more frequent narrow-body flights. However, after the introduction of ETOPS rules that allowed twin-engine jets to fly long-distance routes that were previously off-limits to them, Airbus was able to further develop the A300 as a medium- to long-range airliner to increased sales; Boeing launched its widebody twinjet, the Boeing 767, in response. In the 1980s the Boeing 727 was discontinued, as its central engine bay would require a prohibitively expensive redesign to accommodate quieter high-bypass turbofans, and it was soon supplanted by twinjets for the narrow-body market; Airbus with the A320, and Boeing with the 757 and updated "classic" variants of the 737. During that decade only McDonnell Douglas continued development of the trijet design with an update to the DC-10, the MD-11, which initially had a range advantage over its closest medium wide-body competitors which were twinjets, the in-production Boeing 767 and Airbus A300/A310. In contrast to McDonnell Douglas sticking with their existing trijet configuration, Airbus (which never produced a trijet aircraft) and Boeing worked on new widebody twinjet designs that would become the Airbus A330 and Boeing 777, respectively. The MD-11's long range advantage was brief as it was soon nullified by the Airbus A330-300 and the extended-range Boeing 767-300ER and Boeing 777-200ER.
The Airbus A320 twinjet stands out as the most produced jet airliner. The Boeing 777X is the world's largest twinjet, and the 777-200LR variant has the world's second longest aircraft range (behind Airbus A350-900 ULR). Other Boeing twinjets include the 767, 757 (With the latter having stopped production, but still in commercial service) and 787. Competitor Airbus produces the A320 family, the A330, and the A350. Some modern commercial airplanes still use four engines (quad-jets) like the Airbus A380 and Boeing 747-8, which are classified as very large aircraft (over 400 seats in mixed-class configurations). Four engines are still used on the largest cargo aircraft capable of transporting outsize cargo, including strategic airlifters.
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