On misinterpretation of the asymptotic property of system time-between-failures

If a system, not necessarily one in series, is composed of components having various time-to-failure distributions and components are replaced good-as-new as they fail, then the system time-between-failure distribution tends toward the exponential. Many practicing reliability engineers, incorrectly invoking this property, model their systems with an exponential time-to-failure. We show, under two conditions, using a hypothetical fleet of vehicles, the severity of this error. Modeling time-to-failure as exponential results in gross over-sparing and high unavailability costs as well.