Although air is 78% nitrogen, most aircraft tires are filled with pure nitrogen. There are many tire and automotive shops with nitrogen generators to fill tires. The advantage of using nitrogen is that the tank is dry. Often a compressed air tank will have water in it that comes from atmospheric water vapor condensing in the tank after leaving the air compressor. Nitrogen maintains a more stable pressure when heated and cooled as a result of being dry and doesn't permeate the tire as easily due to being a slightly larger molecule (155 pm) than O2 (152 pm).