A-D,
I handed my 747 books back a few years ago. (albeit of the -400 variety) so I cannot be too sure of the figures, but I struggle to believe that M0.86 at 290 is efficient. The only definite example I have is an experience I had coming back from Florida. Getting our clearance we were held down at FL290 because of traffic crossing to Portugal. We had M0.86 selected ready for our expected higher level, and our arrival fuel which had been calculated as approx 10 t (based on FL370), dropped to -1.5 T when we changed the cruise level in the FMC.
If the FMC in this case is like the -400 one then the CI is being ignored anyway as it is in SEL SPD. Being close to the optimum cruise level does not mean you are flying the aircraft in the most efficient manner it just means that you are flying at the optimum level for the speed selected. Anyway as said before there must a reason to fly that fast low down, probably a delayed flight or even maybe ATC speed contraint.
Apologies for the thread creep.
