The main landing gear tires of the F-22 Raptor fighter are inflated to a pressure between approximately 15 and 15.8 bar. By comparison, the tires of an average passenger car usually operate at a pressure of around 2.2 bar.
Such high pressure is necessary for relatively small tires to support an aircraft weighing more than 27 tons, especially during quick contact with a concrete runway.
Atmospheric air contains approximately 21 percent oxygen, but also a certain amount of water vapor. During a high-speed landing, friction causes the tires, wheels and brakes to heat up strongly.
Why regular air is not a good choice for the F-22
The oxygen trapped inside the rubber can then react with the heated material through an oxidation process. During a large number of flights and landings, this can gradually weaken the structure of the tire and reduce its resistance. An additional problem is humidity. The F-22 regularly flies at altitudes greater than 15 kilometers, where the outside air temperature can drop below -50 degrees Celsius.
Water vapor from ordinary air in such cold conditions can condense and freeze inside the tire. The resulting ice crystals can disturb the balance of the wheel and cause changes in the internal pressure. That’s why technicians use specialized equipment that fills tires with clean and dry nitrogen. Nitrogen does not introduce moisture into the tire and reacts significantly less with the tire at high temperatures than oxygen.
This helps to keep the pressure more stable despite the large differences between the very cold conditions at altitude and the strong heating during landing. The F-22 can reach a speed of almost 300 kilometers per hour when landing. During such a landing, and especially during an emergency aborted takeoff, the temperature of the tires and brakes can rise very quickly.
If regular air were used, the presence of oxygen and moisture would increase the risk of oxidation, pressure spikes and tire damage. Pure nitrogen removes most of the oxygen and water vapor from the system, so the wheel gets greater thermal stability and a safer reserve in extreme conditions.
That’s why nitrogen is used in F-22 tires not for better grip, but for greater reliability. It reduces the impact of moisture, slows down internal oxidation and helps the tires withstand a pressure of almost 16 bar, an altitude of more than 15 kilometers and very high temperatures during landing, writes Wion.