> A big one is having four jet engines rather than two. If you had two equivalent aircraft but one a twin jet the other a quad, the twin would have higher efficiency.
One can make a good case for turbine engines having better efficiency the larger they are (less losses due to boundary layer friction etc.), so yes, if you have two otherwise equivalent aircraft I'd assume fewer bigger engines would be better. But here we don't have equivalent aircraft, as the A380 is bigger than, well, pretty much any other passenger airliner out there.
The A380 engines produce around 350 kN, which is slightly bigger than, say, the B787 engines. Admittedly the A350 engines are a bit bigger than that, but not by a huge amount. But the data seems to suggest both the 787 and 350 are much more fuel efficient than the A380?
So to make an equivalent comparison, one would take an engine, make a twin engine plane and a quad engine plane with that exact same engine. Naively one could expect the quad version to take twice the number of passengers or cargo, and have roughly the same fuel economy/seat. If not, why not? Which scaling laws are non-linear here?
> The engines on the A380 aren't actually that modern with the two options being 15-20 year old designs. One of the reasons it was discontinued is that no engine manufacturer was willing to make new engines for it, so they weren't able to produce a modern fuel efficient variant.
> And it also makes limited use of composites in its design, so it's heavier than it would be if designed today.
This explanation I can certainly buy. Also that the biggest turbofans are nowadays so large that a twin engine plane is already so big there's a very small market for quads.