Orangedog433, thats not true. The mass in the TMD file is for a full tank (look at TMD files) and only the required percentage is used depending on what the tank level is, just like a real aircraft or helicopter.
As I said above fuel systems are experimental and prone to leaks. Fuel usage can be modelled and using fuel does change the weight and balance. A full fuel system doesn't just have a tank, it has switches, fuel lines, fuel pressure, connections, return lines, fuel pumps or fadec systems, governors (if needed), fuel usage rates and instrumentation to show the fuel tank quantities etc. Like all the systems in FS2 or FS4, its a complex business if you want to be accurate. We also need a working electrical system, an engine or two and the list goes on.
If the TMD file is set correctly running out of fuel will also cause an engine failure. Also, not setting the desired switches can cause fuel to not be correctly pumped around the system, once again effecting CofG and engine failures.
FS2 and FS4 do allow for very accurate systems implementation, not just fuel but electrics, hydraulics, radios, flight parameters etc etc. A full study level aircraft can be created by any user wanting to get his hands dirty, sure it's not easy nor is it quick but it can be done.
Jetpack is a very busy guy as he looks after all the aircraft TMD files for the systems we all take for granted, implementing new systems, programming all the MFD screens and quite often answering questions from developers and users alike.