Hey, I was just flying the 737-700 from Stansted to Nice. After setting up the fmc, it gave me a recomended cruise altitude of FL310 which is what i selected as the flight level. Just before reaching the top of climb i reacieved a message in the fmc saying "VNAV disconnect" and "unable to cruise alt" the vnav then disconected and wouldnt allow me to turn it back on.Any ideas why this happened?Thanks in advace.

Check the CRUISE page, it provides optimum and maximum flight levels that will clue you into the reasoning behind it's "unable" message. The recommended altitude that it comes up with on the INIT PERF page usually works (but not always for some reason). I've seen it do what you describe on rare occasions (most my flights are short hops).

Hello, You must take into consideration the fact that the FMC is providing you with opt/max altitudes following performance based upon your current A/C weight, it is not giving you a "crz alt" per se. If I had to guess I would venture to assume that the aircraft could not make it to the TOC (Your set crz alt)without already needing to DESCEND in VNAV to remain on the VNAV path. The 2 points (T/C-E/D) are overlapping so lower your crz altitude and it should work.Best,Randy J. Smith<<>>

Thanks for the help, I tried another flight and it worked fine so i think that it was probably an error on my part.thanks again,Dave