I cannot understand why I always get this message on the FMC when I ask for a higher altitude. The OPT/MAX figures are well above what I am asking for, so why do I get the UNABLE message? I must be doing something wrong. Chris Hall

Yea, this is an on-going bug. The calc works fine before you reach the cruise alt, but as soon as you do, it goes into UNABLE.

Why are you trying to step to FL370 when FL410 is already the OPT and MAX? Isn't it just telling you there is no step?

Also, I believe as we don't know how long his route is, it could simply be that he's already too close to his current T/D, which would make another climb impossible, hence UNABLE.

Agree with badderjet, your distance to SC is further than TD. John,Romania