Set an assignment for the "Add Fuel" command by selecting "Menu"> "Option">"Controls", and finally "Assignments". Select "Add Fuel Quantity" and click "Change Assignment". In the pop up window, enter the desired key (for example, <). Note: You cannot use SHIFT or CTRL. Save the changes. Start the game, and load aircraft. Fill in the fuel bar to only about 11 to 15% to avoid low fuel warnings. You can click the "Add Fuel" key when you are ready to take off or after reaching your cruise altitude. Be careful not to run out of fuel or try landing with a tank 100% full. Try the following trick. Fill your tank just with the fuel you need for your trip, then once you land, press the key assigned to "Add Fuel Quantity" as many times as you needed to get the tank to 100%. Do this before you end the flight via Fs Passenger and you will be credited for the fuel you filled.