The true gain of increased use of CFRP or other lighter materials vs Al has not been easily visible for the 787 or 350 generation of frames. I had expected a decrease in OEW to MTOW ratio but this has not appeared, the 788 expected final ratio of 50,2% is clearly worse then the 15 years older 77W at 47,7%.

Astuteman brought me on the track that the reduced fuel consumption makes the OEW/MTOW ratio stay the same (or even slightly worse ) but the whole aircraft is "lighter".

This is of course correct, the ratio that gets better is payload/range vs MTOW and fuel burned. Thereby we have a very simple first measure of efficiency for a frame:

1. SIMILAR RANGE

Within the same range domain one can simply divide the payload with MTOW, this gives the first order efficiency factor at that range. As OEMs typically state the MTOW, the range at full typical cabin with no cargo (=fullPnoC) and the 210lb per pax+bag is set we can calculate this factor easily and correctly:

For the typical 8000nm class:

___________fullPnoC %_________Range nm @fullPnoC

788..........................9,4................~8000

789........................10,6................~8000

358..........................9,9................8490

359........................11,2................8100

351........................11,2................7990

77W........................9,9................7930

For the typical 6500nm class:

___________fullPnoC %_________Range nm @fullPnoC

781........................12,8................~6500

333........................11,8................5850

763E......................11,1................5975

The above says nothing about the earning vs. cost side which of course is the real measures (look at LAXDESIs analysis for that), but as a first check it is useful and we can use the first typical data the framers state .

2. COMPENSATED FOR RANGE DIFFERENCES

If one want one can start to compensate for the range differences, in that case one must use or assume an OEW. fullPnoC fuel used per nm is given as 90% (10% reserves) of MTOW-(OEW+fullPnoC) divided by range. If we don't have OEW we use 50% of MTOW as discussed in other threads. This gives:

___________fuel kg/nm_________Range comp vs 8k nm_____Comp fullCnoP %

788.......................10,4................~0.....................................9,4

789.......................11,3................~0.....................................10,6

358.......................11,4................-5,4t..................................12,0

359.......................11,6................-1,2t..................................11,6

351.......................13,0................~0.....................................11,1

77W......................16,9................1,2t...................................11.0

and for the midrange bunch:

___________fuel kg/nm_________Range comp vs 6k nm_____Comp fullCnoP %

781.......................12,9................-6,5t.................................15,4

333.......................13,1................2t.....................................11,0

763E.....................11,5................0......................................11,0

CAVEATS:

- Clearly one can see the deficiency of this simple ratio check, e.g. the 763E comes out equal to 333 but the ratio does not look at any costs or revenue i.e. those LD3 positions at anything other then fullPnoC range is not considered.

- The fuel average over distance is OK to do as the climb and descent fuel with 60% higher consumption is only for 4% of the distance, a wash.

- "fullPnoC ratio" is perhaps a bit cryptic, if we find it usefull one can also call it "Payload ratio".

Thoughts? Faults?

[Edited 2011-05-22 01:07:21]