Why do fighter jets pilots use an oxygen mask?
the pressurization system in a fighter plane might be able to furnish 10,000 ft cabin altitude at 30,000 feet, but the pilot would need supplemental oxygen when flying higher than that. In addition, if the cabin is breached above 30,000 ft the pilot would have very little time to get his or her mask on before blacking out, so the mask is worn at all times.