I was flying to LFMN in the 736 NG at FL400 with VNAV and LNAV active. All was fine. I then dialed in FL380 to the ALT window to try and cruise at a lower ALT for some of the flight and clicked on ALT INTV to descend to FL380. It descended me to FL380 but now I am in ECON PATH DESC mode and locked out of the FMC's CRZ window! I cannot find a way to adjust things in the FMC to reflect a new cruise ALT of 38,000'. I've climbed back to FL400, but I am still in ECON PATH DES and still locked out of CRZ mode and at a DESC SPD of M.771.This is confusing. I thought I could climb and descend using this procedure while in VNAV and that the adjustments would appear in the FMC automatically. What have I done wrong?!Thanks.Jonathan

Sorry, forgot to state that after departing CRZ FL400 for FL380, once I reached FL380, I was unable to click on VNAV to re-activate it. I still cannot reactivate VNAV even though I climbed back up to FL400 (using LVLCH).Jonathan

Jonathan,You need to do some research on VNAV operations on this forum. Please be sure to read the manuals provided by PMDG as well. You can climb and descend during the CRUISE phase. This is a technique known as CRUISE CLIMB or CRUISE DESCENT. It should be mentioned in the manual.The logics of the FMS in the NG divides the flight into 5(?) subsequent phases of the flight. TAKE-OFF, CLIMB, CRUISE, DESCENT, LANDING. THe reason for you beiung locked out of the CRZ page is that the flow of these flight phases in unidirectional. You have to TAKE-OFF before you CLIMB, you will CLIMB before you reach CRUIS/ZE and so on and so forth.Hope it helps,

Mats:The FMC user's manual talks a little about this in FMC Cruise Operations but it does not seem to provide details of what to do once the "descent has commenced." I did everything in the correct sequence from takeoff. All I wanted to do was change my CRZ FL from 400 to 380. Somehow, in the process, I got thrown off VNAV and am now locked out of it. I am trying to get back into it but the VNAV button is now inactive.I understand the flow and the process but VNAV is eternally confounding (!)Thanks for posting anyway. I'll surf around searching for "VNAV."Jonathan

Jonathan,Try to change your CRZ ALT to FL380 on your CRZ page and reset the MCP to 38000. That should be all it takes if you are in VNAV.Hope it helps, :-beerchug

I then dialed in FL380 to the ALT window to try and cruise at a lower ALT for some of the flight and clicked on ALT INTV to descend to FL380 As one who tested this area to death I can tell you that one cannot LOWER HIS CRIUSE ATITUDE WITH ALT INTV. WHat happens is exactly what happened to you, early descent, just like the real deal. Now, to lower one's crz altitude if beyond 50 NM of TOD simply have the crz page open then re-set the MCP altitude window (this will place it into the CDU's strach-pad) line select into crz alt and EXECU, the aircraft will perform a CRZ DESC and remain in VNAV...Best Wishes,[h4]Randy J. Smith[/h4]http://www.rawbw.com/~bdoolin/shinault/southparkcartmad.gif[h3]I WANT MY 747![/h3]Caution! Not a real pilot, but do play one on TV ;-)

Randy:Great, thanks for this tip. I shall try it.Thanks to Mats once again too for your tip.Jonathan