Why doesn’t a twinjet double decker airplane exist?

Question

I read an article about the Airbus A380 (Let’s call it an A380 from now on) and why the production of A380s ended. The article cited 2 reasons for end of production of A380s: Point-to-point transit is more common in aviation nowadays, which didn’t make sense for me because, in reality, most airlines (With the exception of some budget airlines) use hub-and-spoke transit instead; And the fact that the A380 is a quadjet, which makes because twinjets are cheaper for airlines and ETOPS exist. With both the A380 and the Boeing 747 out of production, twinjets (The Airbus A350 and the Boeing 777X in particular) have taken over and they sadly, however, have only one deck, and that explains the title question. Sorry for the post being long

In: Other

Answer 1

There were some talks in redesigning the quadjets into twinjets years ago but there were several issues:

The load distribution is highly affected on the original wings so they’d have to be scrapped and made from scratch to properly support only one engine per wing.

The wing box that connects the wings to the rest of the aircraft frame would also be affected since the loads it was designed for are not the same and could lead to catastrophic failure.

The fuselage would probably need some modifications at best, or be scrapped at worst.

There’s also the issue with engine size. For it to have more power and remain at least as efficient to carry such loads it would have to have a much larger diameter. To prevent the engine from hitting the ground and ingesting damaging debris it would need to be mounted a little higher and possibly forward (see 737 max) and this would offset the balance enough to create a different feel in control of the aircraft at best, and impossible to maintain stable flight or perform emergency or as extreme maneuvers as before at worst.

At that point so much of the original aircraft has been modified or swapped completely and none of the negative side effects fully fixed that you’re left with a rather expensive and inefficient aircraft to operate. Building a new aircraft from scratch entirely with the specs you look for would be cheaper and safer and a lack of safety can be just as expensive as developing an aircraft.