One thing you almost never see when an airplane lands is a blowout. Think about that, again and again, the tires hit the tarmac at 170 miles per hour and bear the weight of a modest office building, and they nail it, every time.
Aircraft tires are amazing, when you think about it, the typical airliner tire can handle a 38-ton load, it can meet the ground 500 times before needing a re-tread and can be refreshed seven times in its life.
A Boeing 777 uses 14 tires, an Airbus A380 carries 22, and the enormous Antonov An-225 demands 32. The key to their remarkable durability is maximising the air pressure, the high-flying rubber is typically inflated to 200 psi, roughly six times what you put in an automobile tire, and the tires on an F-16 fighter are pumped to 320 psi. It’s really pressurised air that’s so strong.
The tires themselves aren’t terribly large, a Boeing 737 rides on 27×7.75 R15 rubber. In english, that means it is 27 inches in diameter, 7.75 inches wide, and wrapped around a 15-inch wheel. The sidewalls aren’t terribly thick, and the strength of the tire lies in the cords embedded below the tread. They’re typically nylon, and more recently a variety known as aramid. Each layer of the casing contributes to its load bearing and air pressure resisting capabilities. Of course, tires can fail, especially when under-inflated or overloaded. Treads can come off and casings can blow out.
In the first moments after a plane touches down, the tires are skidding, not rolling. The airplane essentially drags them down the runway until their rotational velocity matches the velocity of the plane. That’s why they smoke upon landing, and why Michelin uses grooves instead of the block patterns seen on your car’s rubber, blocks would simply break off. (Most tire wear comes from this moment of contact, where the rubber meets the runway.) The stoutest tires are rated for speeds of up to 288 mph.
Airliner wheels are subjected to the daily punishment of multiple take-offs and landings. Tires are exposed to temperatures below -40°C during cruise. At touchdown, rubber temperatures can momentarily exceed 200°C.
Wheels must handle the most extreme torture in aviation, a maximum weight, high-speed rejected take-off, a fully loaded aircraft accelerates to take-off speed, then stops on the remaining runway. Tires withstand extreme heat and stress until the aircraft is safely stopped.
Aircraft tires are too rigid to be forced onto a rim like automotive tires. Aircraft wheel hubs come in two parts. The inboard and outboard hubs are bolted together with the tire in the centre, then pressurised with nitrogen.
Nitrogen Instead of Air
A gas station air pump is fine for filling car tires, but large airliner tires must be filled with an inert, dry gas. Nitrogen is inexpensive and perfect for the job.
Nitrogen-filled tires reduce the chance of fire or explosion. Tire rubber is flammable and wheel brakes reach very high temperatures. A large tire with 200 psi of atmospheric air would provide a lot of oxidising power to feed a fire. Nitrogen does not support combustion, greatly reducing the risk of a tire fire or explosion.
Other Benefits of Nitrogen
Dry nitrogen contains no water vapour. The lack of moisture reduces tire pressure variations at temperature extremes (water density varies significantly at different temperatures). With the effects of moisture eliminated, change in tire pressure due to temperature is linear and predictable
Oxygen and moisture in atmospheric air cause corrosion to aluminium and steel wheels. Dry nitrogen eliminates this problem.
Air and moisture cause oxidation of a tire’s inner liner. Nitrogen won’t degrade the rubber.
Due to their larger effective molecular size, nitrogen molecules permeate through tire rubber at a slightly slower rate than oxygen molecules. Using nitrogen may marginally contribute to reductions in tire inflation loss by permeation.
Large airliners are heavy right? A Boeing 767 has a max takeoff weight of over 400,000 pounds. A fully loaded 747-8 weighs nearly a million pounds. All that weight rides on a handful of tires.
Automobile tires are pressurised to around 30-40 psi. If large aircraft tires were filled with 35 psi, they would be flat under the weight. Large aircraft tire pressures are ridiculously high. A Boeing 767-300 main wheel is inflated to 205 psi. The high pressure supports the tire’s maximum rated load of 51,000 lbs.
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