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 failure of an engine. 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.
The second has one engine mounted on each side of the rear fuselage, close to its empennage, used by many business jets.
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
The first twinjet to fly was the German fighter prototype Heinkel He 280, flying in April 1941 with a pair of nacelled Heinkel HeS 8 axial-flow turbojets.
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 and fill the high-capacity aircraft, and lost passengers to airlines operating more frequent narrow-body flights. However, thanks to 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 Boeing 737 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 (out of 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.
Efficiency
Twin-jets tend to be more fuel-efficient than trijet (three engine) and quad-jet (four engine) aircraft. As fuel efficiency in airliners is a high priority, many airlines have been increasingly retiring trijet and quad-jet designs in favor of twinjets in the twenty-first century. The trijet designs were phased out first, in particular due to the more complicated design and maintenance issues of the middle engine mounted on the stabilizer. Early twinjets were not permitted by ETOPS restrictions to fly long-haul trans-oceanic routes, as it was thought that they were unsafe in the event of failure of one engine, so quad-jets were used. Quad-jets also had higher carrying capacity than comparable earlier twinjets. However, later twinjets such as the Boeing 777, Boeing 787 and Airbus A350 have matched or surpassed older quad-jet designs such as the Boeing 747 and Airbus A340 in these aspects, and twinjets have been more successful in terms of sales than quad-jets.