After each successful flight, X-Plane stores a whole lot of information from the flight of each aircraft you have flown with, which is used as starting data for the next flight. Its worth having a look into this file and see exactly what is saved there. The entries are sometimes somewhat cryptic, but it’s easy to figure them out. You can find this file in the …X-Plane 12\Output\preferences folder.
These entries can happily be changed (It’s just a text file) if you are unhappy with the numbers, or even deleted of they are screwed up. X-Plane will replace anything deleted with it’s default value.
For example, the fuel left over from the previous flight (shown in red), or the payloads (shown in Blue) of a particular aircraft (in this case the Cessna 172) is stored in these lines:
P _weight_and_balance _AircraftLaminarResearchCessna172SPCessna172SPG1000acf {“weight”:{“deicefluid_in_liters”:0.0,”deice_holdover_time_in_minutes”:0.0,”fueltank_weight_in_kilograms”:[39.641891479492188,39.641849517822266,0.0,0.0,0.0,0.0,0.0,0.0,0.0],”oxygen_pressure_in_millibars”:0.0,”payload_weight_in_kilograms”:[41.788509368896484,41.788509368896484,34.190597534179688,34.190597534179688,22.793731689453125,9.4973878860473633]}}
You can either change these values (don’t worry about the numbers after the decimal point),
or simply delete the whole entry and when you begin a flight with that particular aircraft X-Plane will load the aircraft with it’s default fuel (usually half-full tanks) and default payload.