Ideal maintenance simply means that the maintenance activities do prevent all failures. Consequently, corrective maintenance (CM) is not required and only preventive maintenance (PM) suffices. Non-ideal CBM means that even though the operating condition of the component is monitored, some failures still occur unexpectedly and therefore corrective maintenance is also required (Zio 2009). In practice, failures may occur due to the incorrect diagnoses or prognoses of failures due to human, software, or hardware errors in the CBM application. Non-ideal ABR means that unexpected failures may occur within the scheduled replacement interval, which lead to corrective maintenance. A further comparison of ideal and non-ideal maintenance policies is summarised in Table 1.