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A twinjet or twin jet is a jet aircraft powered by two engines. Such configuration of an aircraft is the most popular today for commercial airliners, for fighters, and many other kinds, because while offering safety from a single engine failure, it is also acceptably fuel-efficient.
As of today, there are three most common configurations of twinjet aircraft. The first has a podded engine usually mounted beneath, or occasionally above or within, each wing, usually for standard narrow-body and wide-body airliners.
In the third configuration, both jet engines are within the fuselage, side-by-side, which is notable for being used on most fighters since the 1960s, and still continuing, for example in the Su-27 'Flanker', the F-15 Eagle, and the F-22 Raptor.
The twinjet configuration was originally suitable for the short-range narrow-body such as the 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 were forced to reduce frequencies to try and fill the aircraft and so they lost passengers to airlines operating more frequent narrow body flights. However, thanks to the introduction of ETOPs 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/long range airliner to increased sales, and 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 upcoming Airbus 330. 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's four-engine derivative, the Airbus A340, and the extended-range Boeing 767-300ER and Boeing 777-200ER.
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 are increasingly retiring trijet and quad-jet designs in favor of twinjets since the 2000s. 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. The quad-jet design was favored over early twinjets for long-haul trans-oceanic routes due to its immunity from ETOPS restrictions, and these also tended to have higher passenger/cargo capacities as well, however modern twinjets such as the Boeing 777 have matched or surpassed older quad-jet designs such as the Boeing 747 in these aspects.
One of the reasons for the twin-only jets is cost of the engines itself which make up a significant proportion of the plane's final cost. Each engine also require service, paperwork, and certificates, so fewer engines means less to repair or replace. This is also the reason why private aviation generally consists of aircraft with only 1-2 engines. The stability and fewer errors of modern engines leads naturally to as few engines as possible.
The Boeing 737 twinjet stands out as the most produced jet airliner. The Boeing 777 is the world's largest twinjet, and the 777-200LR variant is the world's longest range airliner. Other Boeing twinjets include the Boeing 767, 757 (discontinued from production but still in service) and 787. Competitor Airbus produces the Airbus A320 and 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. There are plans to revive production of 6 engine giants like the Antonov An-225 Mriya.
When flying far from diversionary airports (so called ETOPS/LROPS flights), the aircraft must be able to reach an alternate on the remaining engine within a specified time in case of one engine failure. When aircraft are certified according to ETOPS standards, thrust is not an issue, as one of the engines is usually more than powerful enough to keep the aircraft aloft. Mostly, ETOPS certification involves maintenance and design requirements ensuring that a failure of one engine cannot make the other one fail also. The engines and related systems need to be independent and (in essence) independently maintained. ETOPS/LROPS is often incorrectly thought to apply only to long overwater flights, but it applies to any flight more than a specified distances from an available diversion airport. Overwater flights near diversion airports need not be ETOPS/LROPS-compliant.
In the event of an engine failure, the remaining engine must provide enough thrust to keep the airplane in flight, even if the failure occurs during take-off at a point where it is too late to reject the take-off. In other words, a fully laden twinjet must be able to climb on one engine.
Due to the lack of engine redundancy, in the event of volcanic ash ingestion as happened with the air travel disruption after the 2010 Eyjafjallajökull eruption, airline operators of twinjets must be equally as cautious and safety-conscious as operators of aircraft with three or more engines in any areas affected by aerial ash fallout. Thus far in the course of modern aviation history, two four-engine passenger aircraft, a British Airways Flight 9 in 1982 and a KLM Flight 867 in 1989, suffered engine failures due to volcanic dust ingestion.
Introduction to transoceanic flights
Since the 1990s, airlines have increasingly turned from four-engine or three-engine airliners to twin-engine airliners to operate Transatlantic flight and Transpacific flight routes. On a nonstop flight from America to Asia the long-range aircraft usually follows the great circle route. Hence, in case of an engine failure in a twinjet (like Boeing 777), it is never too far from an emergency landing field in western Canada, Alaska, or eastern Russia. The Boeing 777 has also been approved by the Federal Aviation Administration for flights between North America and Hawaii, which is the world's longest regular airline route with no emergency landing fields along the way.
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