The maximum length of time that a component can be left in service, before it will start to experience a rapidly increasing probability of failure. The Useful Life determines the frequency with which a Scheduled Restoration or a Scheduled Discard task should be performed. Note that for the concept of the Useful Life of a component to hold true, components must, at some consistent point in time, experience a rapidly increasing probability of failure. Research in the airline industry showed that, in this industry at least, this was only true for 11% of the components in modern aircraft