rgr - i'll try itOK now i get it... that kind of step save me 0.2% only... maybe the reason not to step att all :) thanks

I think there is somewhere a bug or so. Like Sagi said it tells in CRZ page UNABLE and 0,0% savings when enter a higher altitude on the step climb entry, especially that the OPT crz level is higher than the altitude entered in step climb.If I enter the higher CRZ altitude @CRZ ALT it thinks it is ok and tells me for example 2.28 % savings. It looks like that it doesn't care if I enter a possible step climb altitude. edit: I always tried being in the air

I get "UNABLE" much more often then not, thats on long flights with good % savings and a TOD hours away. I have been just doing the step climb anyway without issues.

I understand. But where is the problem then? Is it a matter of wrong calculation on the FMC side?

I think it is even more subtle then that. Say you are at FL340, optimum is FL370 and you enter FL360 as your desired target. The FMC says x% savings but that saving is what you would get if you were already flying at that alt. If sometime between where you are now and 200 miles short of T/D, the fuel used to climb to your target, and the fuel saved flying at your target is less then the fuel you will use flying at your current level, then it will tell you to initiate that climb and when to do it.

I know what you mean. But i don't think this is the point on this issue. Today for example, I was around 1000nm away from my T/D.There should be enough savings to afford the climb.

I know what you mean. But i don't think this is the point on this issue. Today for example, I was around 1000nm away from my T/D.There should be enough savings to afford the climb.

Not knowing your actual figures, I can only make some guesses. But if you were quite heavy, then the cost of climbing to a higher altitude now, or later in the cruise, might still exceed any savings from flying at a higher altitude.

Tried this today while cruising at FL360, 975 nm from TOD with FL378 as OPT on the CRZ page. Entering a step climb to 380 gave me "UNABLE", however entering 380 at left LSK1 predicted a saving of 4.5%. After climbing at cruising at FL380, predicted fuel at my destination is 200Kg more than it was at FL360, so there does appear to be an issue with the step climb calculation.