UPDATE: Thank you for the comment! Of course I picked a wrong MTOW for the 777-300ER. I corrected it and the values coming out of it. The basic conclusion keeps the same, especially as the announced SFC reduction of the GE9X will be 10% rather than 7%.
Of course, even with the small increase in inner diameter of the B777-X a ten-abreast seating will be less comfortable than a 9 abreast-seating in the A350. And more capacity is always higher risk (for revenue vs. costs), but for all current B747-400 routes the B777-9 would be the ideal replacement. The A350-1000 will fly with lower costs per flight, so if you can fill the A350-1000 it will be a profit maker as well.
The proposed B777-X models, the larger B777-9 holding about 405 passengers and the B777-8 with about 350 seats, should slash fuel burn per seat by 15% (B777-9 vs. B777-300ER) and about 10% (B777-8 vs. B777-200LR).
Is that possible? Let's have a look by taking a very simple approach and concentrate on the comparison
between the B777-9 and the B777-300ER:
First of all, the B777-9 will hold about 11% more passengers. This leads to a total fuel burn reduction of 5.7%, as 85%*(403/365)= 94.3%
Fuel burn in one given point can be defined as:
Fuel Burn = SFC * M *(1(L/D)), where
SFC is the specific fuel burn (installed) at that point (defined by thrust, altitude and temperature)
M is the Mass (Mass Force to be correct) of the aircraft
L/D is the ratio between lift force of the wing (or the whole aircraft to be precise) and the drag of the aircraft
If we label the values for the B777-300ER aircraft with ER and the values for the B777-9 with X, this leads to
SFCX= 94.3%*SFCER * (MER/MX) * ((L/D)X)/(L/D)ER)
What does that mean?
First of all, this equation is valid for the same flight points in a given flight mission. Take a 4000nm mission of both aircraft and compare the values after half of the distance is flown. The aircraft are in cruise condition then and let's say there are at the same height.
Now let's have a look at the weight ratio: the lower the weight of the B777-9 will be, the higher is the ratio of the weights. MTOW for the B777-300ER is 351.5t and Flightglobal reported a MTOW target of the B777-9 of 344t, giving a ration of 1.02. But this ratio is only true for the beginning of the respective MTOW missions; for a shorter 4000nm mission the ratio of the weights will be lower at the start of the mission. And as the B777-9 should (by definition) burn approx. 5.7% less fuel, the weight ratio gets smaller the longer the flight lasts. The ration should nevertheless always be larger than 1.
Next is lift-to-drag ratio: the B777-9 will get a wing that is approximately 10% larger and will have a significantly larger wingspan than the wing of the B777-300ER. This will increase the lift-to-drag ratio, leading also to a ratio of larger than 1.
Conclusion: the 15% reduction in fuel burn per seat does not mean any magic SFC number for the engine, as in the worst case, when both the weight ratio and the L/D ratio should be "just" 1, the engine SFC has to be just 94.3% of the GE90-115B or about 5.7% better. GE already announced that their GE9X engine will be about 7% better than the GE90-115B, so the 15% fuel burn reduction target looks conservative as of now.
What does it mean for the A350-1000? This aircraft is heavily challenged, as it's capacity is significantly smaller and thus fuel burn per seat cannot meet the performance of the B777-9, even if the SFC of the Trent XWB would be the same as that of the GE9X (or another engine that might find it's way onto the B777-9 wing). The Trent XWB might have the best SFC when it enters service, but the GE9X and respective engines will have a technology advantage of at least two years. Weight per passenger of the A350-1000 is also a disadvantage (308t./350pax=0.88t./pax vs. 344t./405pax=0.85t./pax).
Is there any growth potential left in the A350-1000? As it looks now, a final stretch (-1100) would be the only solution for Airbus to make the largest A350 variant look attractive again...