Why did big airplanes go from four engines to two engines?
In: Engineering
First, ETOPS. It’s been explained elsewhere. But as engines have gotten stupidly powerful at a given rate of fuel consumption, and engines are ridiculously expensive, less is more baby!
Also, PART 25. Let’s just look at US regs for now. 14 CFR part 25 is the standard for certification of a transport category airplane, which your airliners are. It says things like, there must be a separate fuel cell for each installed engine, each engine should be able to get fuel from any cell if its own runs dry, and any one cell should be able to run multiple engines at once. So imagine a fuel system to meet those criteria for a 2-engine airplane. Now for a 4-engine airplane. See how much more complex the latter is, how many more valves and pumps and lines are required? Twin-engine planes are simpler, and therefore cheaper, to build.
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