Why did big airplanes go from four engines to two engines?
In: Engineering
Cost, efficiency, improved technology and some rule changes (in the US).
It costs less to maintain 2 engines rather than 4 other things being equal. Also fewer but larger engines are more fuel efficient. Technology has also improved such that engines now rarely fail.
In the past, there was a rule (in the US) that flights that had to go a certain distance from an airfield or over water had to have more than 2 engines (in case of failure of an engine, the plane must be able to remain flying). Once the rule changed, airlines and plane manufacturers made the switch to 2 engine planes. (nowadays a 2 engine plane must still be able to fly if one engine fails)
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