hi,
for helicopter units:
<bCanMoveImpassable>1</bCanMoveImpassable>
<bCanMoveAllTerrain>1</bCanMoveAllTerrain>
this unit will move on every terrain,
this unit will move on all terrain besides mountains (i think):
<bCanMoveImpassable>0</bCanMoveImpassable>
<bCanMoveAllTerrain>1</bCanMoveAllTerrain>
every mod today uses the first setting for units such as helicopters.
see civ wiki for help on every xml tag.
so no need for flying mod anymore (saw your new mod threads - good luck).