No aircraft would ever fly then. You'd be surprised how much equipment on every single flight is inoperative or scheduled for deferred maintenance. There's an

No aircraft would ever fly then. You'd be surprised how much equipment on every single flight is inoperative or scheduled for deferred maintenance. There's an engineered threshold at which an aircraft is deemed not airworthy. On airliners, this is managed through a Minimum Equipment List (MEL), which is a formal checklist that tells the crew which equipment is allowed to be broken (and for how long). This threshold is less than 100% by design, because little things break all the time. It can take days/weeks in the shop to fix issues, so you need to batch up all the maintenance. (Can you imagine if you had to take every aircraft offline for several days after every flight?) And that's okay, because there's still a large safety margin built in as long as the MEL is followed.