A colloquial way to refer to ginger ale.
Referred to as such due to the fact that it's a readily available drink option to have while riding a plane, whereas other soft drinks are offered more often under other circumstances, such as at restaurants and supermarkets.
When
one wants a soft drink, the first drinks that come to
mind are, more often than not, everything else but ginger ale. However, since it's offered at plane rides, ginger ale
sometimes associated with them, hence "plane juice".