In iOS 12, the Flight Pad app featured clearly-visible selection buttons for either imperial or metric units, and you could also choose to show fuel in mass or volume. This choice seems to have disappeared with iOS 13. In fact, the selection buttons are still there, but the unselected button is now black. We are working on an update to correct this bug.
Temporary solution: Tap into the black either to the right or to the left of the displayed unit and you will see that the other unit appears.
This bug has been corrected with the recent app update 4.9.3.
Solution: Update the Flight Pad app to 4.9.3
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