Originally Posted by
tuna hp
Airlines are still buying plenty of 4 engine planes. How do the A330 and A340 compare? Again, as I said, I can't see how 3 engines would make sense with the way that larger airliners have evolved to be built. It would have to be something smaller that couldn't fit the engines on the wings.
The A330 is a twin.
ETOPS (two engine over-water) flights are limited to something like three hours to a divert field (used to be two hours, but I think that changed for some operations).
Pacific long-haul operations require more than two engines for regulatory reasons, so the larger airplanes which are designed for that have four.
I think there are currently technical limitations on designing an engine large enough to only need two for a 747 or A380. Also the fan diameter, and hence landing gear length, would be very large which would create issues for ground handling (nobody could reach the fuselage).
Originally Posted by
tuna hp
Correct me if I'm wrong, but I believe that the reason larger aircraft mid-mounted wings is for ground clearance for the engines. Airplanes that have fuselage engines can have a 1 piece wing with the fuselage resting on top. Its supposed to be more structurally efficient.
Like I said before, I think the weight savings would be minimal. Carrying the spars all the way through might be a little more efficient, but engineers can do wonders with complex structures and arched structural objects (like a barrel-shaped wing-box) are inherently strong.