Sorry, I have written too briefly. I use FS2004. Flight Simulator has different types of variables used in gauges. The best known are L:Vars (public variables set in gauges) and G:Vars (those stay only inside one gauge). A: E: and P:Vars are those, you can get from FS using offsets (the list is here: http://msdn.microsoft.com/en-us/library/cc526981.aspx). In gauges, they have own names, which can be used. The last type are K:Vars, which are nothing else than controls shown in FSUIPC documentation (http://msdn.microsoft.com/en-us/library/cc526980.aspx, and in FSUIPC file: List of FS2004 controls.pdf).
One can read A, E and P variables, but changing them is possible only via K (events). It is easier in FSUIPC, because one may change offset directly. This way it isn't possible in XML gauge (as I know, but I hope to be wrong).
Now I look for control (K:Var), which will allow me to change fuel level in tanks. In list of FS2004 control I found ADD_FUEL_QUANTITY, but as written in ESP, this only adds fuel. I look for something like REMOVE_FUEL_QUANTITY.
Regards,
Slawek