hisometimes in the FMC i read messages that i don't understandi searched in the manual, but nothingthese messages are:-UNABLE NEXT ALT, when i engage VNAV after initialising the FMC with a SID;-MAX ALT FLXXX, it happens sometimes (XXX is a variable number)and another issue:i usually calculate the fuel with the tables and the tutorial provided in the fuel planning page of the manuali calculated the fuel based on a ZFW and a trip lenght,and i calculated a reserve of 45000 lbs of fuelbut when i setup the FMC, i go to the progress page and i see that the planned fuel at destination is 0.0 lbs, so in the reserve section of the PERF INIT page in the FMC i have to insert 0.0 to make that "FUEL INSUFFICIENT" message go awayhow can you explain these things??Giulio Dondi, from italy

-UNABLE NEXT ALT, when i engage VNAV after initialising the FMC with a SID;

This message means simply that your plane is not able to reach the next alt constraint in your sid, due to heavy weight or really strange altitudes

-MAX ALT FLXXX, it happens sometimes (XXX is a variable number)

The plane is unable to reach your desired cruise altitude - this may also be a problem with the weight of the plane

i usually calculate the fuel with the tables and the tutorial provided in the fuel planning page of the manuali calculated the fuel based on a ZFW and a trip lenght,and i calculated a reserve of 45000 lbs of fuelbut when i setup the FMC, i go to the progress page and i see that the planned fuel at destination is 0.0 lbs, so in the reserve section of the PERF INIT page in the FMC i have to insert 0.0 to make that "FUEL INSUFFICIENT" message go awayhow can you explain these things??

I don't know how exact your calculation really is, but you have definitely calculated too less fuel - check your legs page if there is a wrong waypoint with the same name as your planned waypoint, but eventually a few thousand miles away ... there are a lot of wayopints which have the same name ... so your fuel can be too less when the plane tries to reach the wrong waypoint.

You can get the 'MAX_ALTITUDE xxx' as well if the sim loses focus - like by opening the map view, etc or swapping windows - just hit vnav twice.Maybe that is your trip fuel - you have also to add alternate fuel, landing fuel, contingency fuel and taxi fuel as well plus any extra discresionary? fuel.A pretty goo fuel planner at http://fuel-planner-for-the-pmdg-747-400x....e.informer.com/John Ellison

UNABLE NEXT ALT on a SID can usually be ignored... it will go away in most cases once you're on you way to getting above the constraint.INSUFFICIENT FUEL: If you are confident in your fuel on board for the trip, the warning could be due to either not having winds programmed into the FMS or an error in the flight path (use PLAN view on the ND to verify your route). Sometimes, decreasing your reserve value in the FMS will also fix this.

well, for further information, i can give you my fuel calculations for that flightall weights are in poundsPax weight was 60840, Cargo was 87500 and ZFW was 542428 (value given by the PMDG load manager) route lenght was 5472 miles, calculated with super Flight planner for FSX (actually, when i programmed the FMC it gave me two waypoints out of the course, but in the planner they weren't) anyway, i calculated 250000 punds of fuel, i added 20000 pounds for winds and that gave me 270000 poundsthen the manual says to calculate MLF, ALT fuel and contingency fuelMLF=24000 poundsALT=15000 poundsContingency=13500 poundsthe PLW was at this point 594928 poundsthen i adjusted the flight plan fuel with the values provided in the manual (i calculate the medium value, which is 740 lbs/hour)the flight plan fuel was then 279000 poundsthe PTOW was 873928 (just below MTOW) and thew total fuel to put in the plane was 331500 (originally it was more than that but i had to adjust to fit in the max fuel provided in tha load manager)the initial altitude was FL300 and the FMC reserve was 45.5i think i did the maths correctly, if not, tell me where i did wrongGiulio Dondi from Italy

I'm not going to check your calculations, but I can advise that you can safely reduce the number you put into the FMS and this might eliminate the warning message.I always use NAM instead of trip length.. see AOM 2-12.