Tyre inflation.
Reference CAP747 Generic requirement no 16. ( Previously issued as AWN no 70).
".... all braked wheels of retractable landing gear units on aeroplanes defined in para 1 will be required to have tyres inflated with Nitrogen or other suitable inert gas, and maintained such as to limit the Oxygen content of the compressed gases to not greater that 5% by Volume."
key points seem to be braked wheels and retractracable gears,
regards,
om15.