page_title: Maintenance philosophy
section_title: Periodic
hierarchical_section_title: Maintenance philosophy / Definition / Failure Detection / Periodic
caption_attribution_description: English: Periodic maintenance to restore operation.
context_page_description: Maintenance Philosophy is the mix of strategies that ensure an item works as expected when needed.
context_section_description: 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.
caption: Probability of operational failure accumulates as time passes until diagnostic or preventative maintenance actions eliminate any actual failures.