[scrumper]

Is this because of something analogous to an expected value calculation? Like the increased probability of failure of a single engine when carrying 4, combined with the lower excess power per engine, means that the safety margin of a 4 engine plane is worse? Or is it something to do with the doubling of the number of critical parts that could fail? I'm struggling to come up with a workable argument why 2 is safer, it seems counterintuitive. I don't have enough of the puzzle.

EDIT: Never mind, I see you wrote more down below about this. It's increased risk of a catastrophic engine failure bringing down the entire plane, not an engine simply dying and forcing a landing. Four engines = twice as many chances for that to happen.

[WalterBright]

There's an enormous amount of energy in a jet engine with that spinning turbine. Then there's all the heat, fuel, and fire in it. When that energy lets go, you don't want to be anywhere near it.

It's amazing that jet engines work at all! Yet the safety record of the engines is incredibly good.

[dotancohen]

But twins typically have even larger turbines, no?

[WalterBright]

High bypass fans are newer engine designs, nothing to do with being used in twins.

Though in general, a new engine design necessitates a new wing design, which means a new airplane.