I must be missing something ?
Comparing the 787 with the 76X cruise FF at the same GROSSWEIGHT
only tell you the AERODYNAMIC- and ENGINE EFFICIENCY difference between
the two planes. The 787 will win here but not by a huge margin.
The B787 airframe and etc composite structure is much LIGHTER than
the B76X (operational empty weight).
SO, Imagine the B764 and B787 flying the exact same trip with the exact
same payload/pax. Due to less structural weight,
the 787 will ALWAYS be lighter than the B764 for the same flight.
- It also needs to plan LESS trip FUEL than the 764, making it even lighter
- Being significantly lighter, it will cruise HIGHER than the 764 on the same trip burning even less.
- It cruises faster -> gets there sooner, which means the 764 engines
will still be burning fuel when the 787 has parked.
If we add all this up the total effiency of the acf is supperior.
It's a bit like adding winglets to a 737. The aerodynamic gain is about 2%.
However, having to plan less trip fuel and thus always being lighter & cruising higher, the total fuel
gain will be 4-5 % on the winglet acf.