silent failure modes that require periodic preventative maintenance actions. Periodic Operational failure will eventually occur when an item is used in its normal mode of operation if there is no intervention. The procedures associated with periodic maintenance are generally called a Periodic Maintenance System (PMS). There is risk that the system will not work as expected, and this risk grows as time passes due to increasing possibility of silent faults that cause operational failure. Periodic maintenance actions control risk of operational failure. This relies on invasive procedures that renders a system inoperable for a brief period while users run manual diagnostic or preventative procedures. The following are a few examples. Calibration Built In Test (BIT) External Diagnostics (instrumentation) System Operational Test (SOT) The item is down and is unavailable for normal operation during the time while a periodic maintenance procedure is being performed. Failure is statistical. There is a random chance that the system or item will not function when required. Reliability declines as the time passes, and probability of failure increases until action is taken. The item will eventually fail if there is no intervention. Periodic maintenance increasingly reduces operational failure risks as procedure are used more often. Average reliability improves as the time between maintenance actions is reduced. As an example, an item with no CBM features will work as expected about 90% of the time if periodic maintenance is performed about 5 times more frequently than the MTBF. Fault Isolation Fault Isolation is the strategy used to identify the root cause for a failure. There are two methods. Automatic Fault Isolation Manual Fault Isolation Automatic Fault Isolation Automatic Fault Isolation identifies the root cause for failure with no manual intervention. This is generally used to control redundant items when it is necessary to automatically bypass failures. Manual Fault Isolation Manual Fault Isolation is when maintenance personnel must identify root cause for a failure. This usually requires the following. Manual diagnostic tests Test equipment Spare parts Documentation Training Device instrumentation used with CBM is generally used to reduce the time and effort required to isolate root cause. Corrective Action Corrective Action is the activity that restores performance for the item or system after a failure. There are two kinds of corrective action. Automatic Manual Automatic Corrective Action Automatic correction is possible for redundant systems when fault-detection, fault-isolation, and fault-bypass are all automatic. Automatic corrective action is also called