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".