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It is now possible to add Fuel data in litres or US gallons in the Journey Log panel.
In a section Settings > Fleet, edit the aircraft and set up a default fuel unit as l or USGAL
Once the default fuel unit is set up, you can add fuel in litres or US gallons in the Journey Log panel (as on screenshot below) - the default unit set up applies to: Block fuel, Used fuel, Remaining fuel and Uplift.
The same defined fuel unit shows in the panel (suitable for adding JL data for multiple sector trip)