B787-8 analysis

Lissys, the software firm providing the PIANO software, did an analysis for early-built B787's and compared the fuel burn for a 5000nm mission carrying 22 metric tonnes of payload with the B767-300ERW (where W is for winglets). The analysis showed that the B787 with it's "near EIS" performance would burn only marginal less fuel than the B767 (50.1t vs. 50.7t, a difference of 1.18%).
But as the author says, the B787 cruises at Ma0.85, where the normal cruise speed of the B767 is Ma0.8. If one lets the B767 fly at Ma0.85, that would result in a fuelburn of 62.7t, so the difference at same speed is 20%. Of course this is not the orĂ­ginal claim from Boeing (well, at least I hope so...).

But let us look further into the model of the B787 and why the fuelburn is so high relative to what one should expect from that brand-new aircraft with state-of-the-art engines.
Lissys also provides a baseline model of the B787 from 2008, where the original performance Boeing designed the aicraft for is implemented.
We can now take this baseline model and do a "Point Performance" (click this option in the "Output" box)comparison: say, at 35.000ft, Ma0.85 and a weight of 180 metric tonnes.
Here is an excerpt of the output for the baseline model:
 Overall Lift / Drag Ratio = 20.25
 Total Lift Force     1765206. newtons
 Total Drag Force       87169. newtons (43584.newtons per engine)
 Engine / Airframe Performance:
 Total Fuel Flow at Thrust=Drag:       4791. kg/hr
 Specific Fuel Consumption (SFC)       0.5390 kg/hr/kgf
 Specific Air Range (SAR)              .10226 nm/kg
 Emissions Index, NOx                  11.79 g/kg.

And here is the same output for the "near EIS" model:
 Overall Lift / Drag Ratio = 19.39
 Total Lift Force     1765206. newtons
 Total Drag Force       91049. newtons (45525.newtons per engine)
 Engine / Airframe Performance:
 Total Fuel Flow at Thrust=Drag:       5116. kg/hr
 Specific Fuel Consumption (SFC)       0.5511 kg/hr/kgf
 Specific Air Range (SAR)              .09576 nm/kg
 Emissions Index, NOx                  12.03 g/kg.

What do we see?
  1. Lift/Drag Ratio decreased from 20.25 to 19.39. This is about 4.24%. In other words, the aircraft needs 4.24% more thrust to hold that speed (Ma0.85) with that weight (180 t) at that height (35kft).
  2. Specific Fuel Consumption is 0.5511kg/hr/kgf for the near EIS model, where it was 0.539kg/hr/kgf for the 4.24% lower thrust. This is an increase of 2.2%
  3. Accordingly, fuel burn per hour is almost 6.8% higher for the near EIS variant.
From all what we know about the Trent1000 and the GEnx engines, the 2.2% miss in SFC is probably a good guess for the early build B787's.
What I do not understand is the 4.24% decrease in Lift/Drag ratio. That would mean that the wings of the B787 have an aerodynamic problem. If I remember correctly, Boeing onces stated that the wing of the B747-8 (which was designed in the same timeframe) is aerodynamically better than expected, so I would guess that the B787 wing is at least as good as it was planned to be.
So I have my doubts about that part of the analysis.

On the other hand:
If we compare the baseline B787 model and the B767-300ERW, the difference in fuel burn for the 5000nm mission is about 9.5% - significant, of course, but far from being 20% as Boeing always claimed.
Even if we add 4% fuel burn for the "non-wingletted" B767-300ER, we are not near the 20%.

So there is the old wisdom, that you need the right aircraft for the right mission...


  1. It is hard to get the same number of seats into both aircraft. Boeing's claim is probably "per seat", not entire block fuel (which would be a pretty pointless claim).
    ANA has 216 seats in its B767-300. The B787-8 of ANA has 264 seats.
    B767@216: 50.6t/216PAX = .234 t/PAX
    B787@264: 50.1t/264PAX = .190 t/PAX

    Now, the payload of the B787 would be higher, 26.5t to be specific. But the fuel burn scales weak with weight at this range, 4t more payload increase the fuel burn by ~100-110kg/hour, yields roughly 51-52t fuel on the B787.

  2. And if you assume 52t of block fuel for the 264PAX B787, the block fuel per pax is .197t/PAX.
    Or ~15-16% lower than the B767-300ERW.

    Add the additional SFC and take away the winglets of the B767-300ERW, the 20% should nearly materialize.

    The lower lift over drag might also be due to insufficient lift generated. I see a lift of 176t. The best lift over drag is attained at a single lift coefficient. Maybe Lissy people shifted the cruise CL slightly up or down to match the aircraft true performance.

  3. where did you get the data for the specific fuel consumption (SFC)?