Hi Rarife,
Regarding the fuel and the FMS, there are two things you have to consider:
- Weight: in normal operation the FOB is added to the ZFW, giving you the GW. If it is on ground before engine start the GW will with FOB - TAXI + ZFW. If it is in the air (or in the ground after engine start up) GW will be ZFW plus FOB. This is regardless of the FOB being fully usable or not. If the center tanks valves are stuck in the closed position, then the FOB has a half amber box drawn around it to warn the pilots to re asses the fuel available, but the GW in the FMS will still include the whole amount of FOB on board. I can't find anything in the FCOM or FCTM regarding FMS DATA INSERTION when fuel in a center tank is not usable so I think the best thing is to use the actual FOB for weight and mass calculations, leaving to the pilot the task to consider how much they actually have for the flight.
- Center of gravity: in the FMS, after the pilot receives the load sheet, he only has to input the actual ZFW, the ZFW CG, and the FOB. The FMS will determine the actual CG of the airplane by correcting the ZFW CG with the FOB index that has in his database. No intervention from the pilot is needed regarding the FOB index because the fuel index chart is establish assuming a fuel distribution in accordance with refuel distribution (I am actually paraphrasing FCOM-PER-LOADING-LOADING FUEL-LOADING WEIGHT AND BALANCE-FUEL INDEX TABLES). In other words, the refuel is done automatically by the airplane fuel system and is always done in the same fashion. so for a certain amount the index will always be the same.
In the case you are bringing here the FOB is non standard (the automated refuel system will never leave 700 or 1000 kgs of fuel in one of the center tanks), so the fuel index chart doesn't apply anymore. So, to determine the applicable fuel index in this non standard case, the pilot has to use the tables given the aforementioned chapter of the FCOM, checking the actual amount of fuel in each tank, finding the index for each of them, and adding them. This figure is what has to be used in the load and trim sheet.
So, in your case, I think the pilots should use ZFW + FOB = GW, keeping in mind that 1.000 kgs of that FOB is not usable (hence the amber box), and GW CG = ZFW CG + FOB index taken from the tables in the PER chapter of the FCOM.
I stand to be corrected if there is any other way to do it but I haven't found anything else in the FCOM or FCTM.