Why do commercial airliners use nitrogen in their tires?

On a commercial aircraft tire filled to around 200 psi, if air was used the O2 partial pressure would be 42 psi.  Given the heat generated from braking during landings, the use of nitrogen on aircraft tires has more to do with eliminating fire/explosion hazards than it does with preventing tire aging; with the large internal volume of commercial aircraft tires and the large amount of O2 that would be present if filled with air, any fire that gets started inside the tire is likely to evolve a lot of heat and pressure before being snuffed out.  Nitrogen truly is a critical safety item in this setting, as was tragically illustrated by the crash of Mexicana Airlines flight 940 back in 1986, when a tire mistakenly filled with air caught fire and exploded shortly after takoff.

A similar situation exists on the space shuttle, albeit even more extreme.   The shuttle’s tires are filled with nitrogen to 315 psi, and get replaced after every single landing; clearly NASA is not interested in nitrogen for the purpose of extending tire life.  (if you want to see what crazy things can happen in an oxygen-rich environment, read up on Apollo 1.  Also, for more information on the matter from a purported aircraft mechanic, click here and check out comment #2.)