Hugo,1. Has this problem always been present with your 74F? If you have had successful flights, have you since installed any software, navigation data, SIDS and STARS. 2. If you have not updated the FMC you may want to try it. What's the worst that could happen - it still does not work. On the other hand it just may solve your problem.3. When you input the flightplan into the FMC do you do it manually or are you using a program like FS Commander and FS Build? What ever the process, does the FMC offer any errors when imputing or importing the flightplan?JustinSorry Mats, you beet me to it, thats what happens when you answer the phone before pressing "post message".

>Should there be SPEED/ALTITUDE FMC calculation? I just want>the plane to follow the L NAV path and have the autopilot>control the speed and altitude along the route.Yes it should !!!It seems to me that you have not CONFIRMED the aircraft weights in the PERFormance page of your FMC-CDU.So, without knowledge of the weights, the FMC is not able to build a correct flight path. Do not ask me how can this affect LNAV...but maybe part of the reason for your trouble.Jose Luis

Then that's it! I indeed did not CONFIRM the a/c weighs in de PERF page.I was not used to doing this when flying the PMDG 737. It all works well now, thanks a million!!Hugo