If the 737 loads up with a full fuel load how can the plane be refuelled,do,s it need to be empty in the first place,
Well, of course you have. If you start the flight already full, what would be the point of asking for refueling in the first place ? When the truck arrives, it will see there's nothing to do, and will go back.
- If you are landing after a flight, you probably will need refueling
- If you are starting a new flight, maybe it would be better if you saved your startup situation with very little fuel.
Note that, in the time passing between the moment you call the GSX refueling and the truck arrives, you can do any changes to the fuel quantity, and GSX will ignore them (so, this might be a good time to empty the plane, if you don't start empty by default), but from the moment it tells you "Please use the refueling system of your aircraft", THAT counts to decide if there's fuel to load.
This means, for example, if you started with 15 tons, called the GSX refueling, and while it arrives, you set the fuel at 5t, then, *after* it tells you to use the refueling system of your aircraft, you set it again at 12t, the truck will load 7t.
If the plane have a progressive refueling simulation, like the PMDG 777 or the Aerosoft Airbus, the refueling speed will be controlled by the plane (GSX will just wait there and display the counter accordingly).
If the plane doesn't have a progressive refueling simulation, like the 737 ( I think ) or the Aerosoft Airbus when using the "Load Instant" option, GSX will display the fuel counter at a very fast speed, just to confirm you that something happened, but at a faster speed, since the plane did an "Instant" load.
Basically, the only time when GSX controls the refueling and the refueling speed, is with planes that don't have a custom fuel system.