This is an expression that is used by the travel industry in the U.S to refer to short commuter flights between small cities or towns. The aircraft used on such routes are usually very small. Very often the flights are rough, forcing the passengers to hold on to their seats as tightly as possible. In the process, the knuckles turn white! Hence the term ‘white knuckler’.
*The short flight turned out to be a white knuckler.
The term is now being used to refer to any scary or tense situation.
*It was white knuckle time for all those sitting in the waiting room of the hospital.