let me make a guess... after landing, the pilot stops near a refuel area, so the fuel will be 100%, so it will be greater than the initial value... and the Flight Logger gives a negative value as a result. Or, the pilot 'refuels' during flight, and the same problem occurs. Maybe Armando can give a solution to it (e.g. the logger will stop recording the fuel amount if it becomes greater than the previous value).