Depends on the Aircraft and make, some tyres have wear limits to the cord layers which is when you change them, they look like scrap but are perfectly safe............ remember the pilot and crew have the same tbuilt in urge to live as well so just query it politly, they will reassure you if you are worried and will take an extra look.............. If the woman on the Aloha 737 had chirped up at her concern then perhaps the odd person would still be with us today
