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 this kind of an airplane. One has an engine mounted beneath each wing, and another has one engine mounted on each side of the rear fuselage, close to its empennage. In the third configuration, both jet engines are within the fuselage, side-by-side. The third configuration is notable for being used on most fighters since 1960s, and still continuing, for example in the Su-27 'Flanker', the F-15 Eagle, and the F-22 Raptor.
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. Power is not an issue. One of the engines is more than powerful enough to keep the aircraft aloft. Mostly, it is about 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.
Twin jets tend to be more fuel-efficient than aircraft with three or more engines. Fuel efficiency in airliners is a high priority, and a high percentage of airliners use two engines. The Boeing 737 twinjet stands out as the most produced jet airliner. The Boeing 777 is the world's largest twinjet, and a variant is the world's longest range airliner. Other Boeing twin jets include the Boeing 767, 757 (discontinued from production but still in service) and 787. Competitor Airbus produces the Airbus A320 and A330, and the upcoming A350.
One of the reasons for the twin-only jets is cost of the engines(engine is the key price of the plane final cost), and required service, paperwork, certificates - if only one engine has problems it must be repaired or in worst cases bought new. 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.
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 and transpacific 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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