just want to know why when in both lnav and vnav mode the plane does not start descending according to the info on the legs page of a route?? do I have to manually set an altitude on the AFDS??

U have to select a lower altitude on the MCP?

so u are saying yes i do have to set a lower altitude in the MCP altitude window for it to start descending??

Yes- while at cruise the MCP ALT is the altitude at which descent below is prohibited (in V-NAV). Bruce.

Hey gooper,Yes, you have to set the altitude lower in the window for the aircraft to start descending. In fact, you should have gotten a message on the FMC to remind you to do so a few miles before you reached your T/D (Top of Descent) point.The autopilot sees the altitude set in the window as a constraint that it will not go past, whether you are climbing or descending.So even if you are in VNAV, and the FMC has programmed your next waypoint for 10,000, and you have 20,000 set in the window and are descending thru 25,000...you will level off at 20,000. It will then switch out of VNAV into altitude HOLD and stay that way until you change the altitude in the MCP window. You then have to re-engage VNAV for it to continue descending using the FMC profile.It works the same way when climbing. You should also hear a warning horn 1,000 feet before you reach the altitude set in the window. This is another cue to reset the altitude in the window if you want to fly to a different altitude. Regards,Steve Dra

>>It works the same way when climbing. You should also hear a warning horn 1,000 feet before you reach the altitude set in the window. This is another cue to reset the altitude in the window if you want to fly to a different altitude.;)Denis