If you add the following line to the fuel section of the aircraft.cfg file you can use the fuel dump feature of FSX.

fuel_dump_rate = 0.0167

Pressing Ctrl-Shift-D will dump it, but don't forget to stop dumping before the tanks get too low! At the rate above a full tank will go in a minute (not real figure). Handy for when you are carrying a bit too much weight for a carrier landing.