Lets presume that the the weight, weather, fuel etc data that I intoduce into the FMS - be it a LevelD, an iFly, a PMDG - is correct, so the FMS does its business and gives a set of data like Opt Flight level, climb and cruise target speeds, etc.So how come so often my aircraft seems incapable of reaching the Crz level I set, let alone the Opt Alt, teven after burning off some fuel, and even dropping FPM to 400 or so the aircraft loses speed - waaay lower than the target climb speed;AT this moment the Opt Alt is FL380 and I am stuggling to reach FL310 and ir would take weeks to get higher.Are the figures that the FMS produces nothing more than recomendations - "This would be good" - but not necessary achievable? If so why does the FMS set parameters that it knows that curruent circumstances - weight, weather etc - will not allow?Or am I not understanding anything of whats going on?

Are you sure your following the units posted from the loader sheet that comes with the mentioned planes? Are you sure the acutal weight loaded and what your punching into the FMC match?I have no issues getting to my altitude in the LVD or PMDG birds.With either plane as you burn off fuel you should be offered a step climb as the fuel weight starts to burn off allowing the plane to climb higher.Another thing that will alter your speed is what you set for your Cost Index. I also let the plane fly in VNAV and set my constraints on the MCP so I do not bust an assigned altitude.

northridge, what is your is TAT and SAT? you may be feeding the FMC correct info but your weather program may be outputting wrong high alt temps.Regards

The temperatures - AHA!Thanks.