i have climbed to my first clearance altitude in VNAV (VNAV light disengages). when i input the cruise altitude into FMC together with a "cross at" altitude for the next waypoint why can i not re engage VNAV ? the aircraft does climb but in speed mode at 250 kts and resists any command to go to VNAV. can somebody please explain. thanksneil c

>i have climbed to my first clearance altitude in VNAV (VNAV>light disengages). when i input the cruise altitude into FMC>together with a "cross at" altitude for the next waypoint why>can i not re engage VNAV ? the aircraft does climb but in>speed mode at 250 kts and resists any command to go to VNAV. >can somebody please explain. thanks>neil c Hello Neil, Sorry but you have left out the most important part of your inquiry - namely what aircraft you are using VNAV with. Typically VNAV with enter what is called VNAV ALT which is a temporary level off mode but still retains VNAV (Sort of a limbo mode =)) when one holds at an altitude other than the FMC crz altitude. Of course I cannot say this applies across the boardfor all aircraft but you will see this on Boeing 744 and 737s although not all of them depending upon their FMC software.................Randy J. Smith................CAUTION! My views represent no one but my own. While I do help companies test products - this in no way means I represent them in ANY fashion.[h4]Evolution is a process that results in heritable changes in a population spread over many generations[/h4]

Hi. By way of further info im flying a 737-800 and am cleared to climb to my cruise altitude of FL270 and cross a particular fix at 15000ft. I enter my cruise alt in the MCP and engage VNAV. No problem. When I then enter the alt constraint in the FMC Legs page VNAV disengages and the MCP changes to IAS and SPD. The aircraft continues to climb to FL270 albeit at a constant 250kts. The FMC then displays the message "DISABLE NEXT ALT". All very frustrating. Can you help me understand ? regards neil c