Four engines are best for very long ranges
The reason why a four-engined aircraft is better at long ranges is essentialy twofold. First, all airliners are designed to continue to fly safely after suffering an engine failure during take-off. This means that each engine of a twin must have enough power available to allow the aircraft to continue its take-off. Carrying such an excess of power (in the form of heavier engine with more drag) as a marginal effect over moderate ranges, but becomes an unacceptable penalty over longer ranges. By contrast, a four engines aircraft loses only a quarter of its power when it suffers an engine failure, so the additional power required from each of the remaining three is not as great as from the remaining one on a twin.
Secondly, the structure of the wing of a four engine can be lighter than that of a comparable twin, since the location of the four engines better compensates the normal upward bending of the wing-root during flight. Conversely, with a common wing the MTOW of the four engined version can be 20 per cent higher that of the twin, as is the case with A340 and A330.
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