because of diagnostic testing that requires down time. For example, an automobile that requires 1 day of maintenance every 90 days has a Mean Active Maintenance Down Time of just over 1%. This is separate from the type of down time associated with repair activities. Nines Availability expectations are described in terms of nines'''. The following table shows the anticipated down-time for different availabilities for a mission time of one year. This is the typical time-span used with commercial systems. Supportability Systems that require maintenance are said to be supportable if they satisfy the following criteria. Adequate labor for PMS diagnostics or fault indicators for failure modes that lack PMS Access to manufacturing sources for replacement parts Sufficient resources to sustain access to replacement parts and maintenance labor (money, lodging, ...) Training and maintenance documentation suitable to restore operation Transportation services to deliver replacement parts Systems that lack any of these requirements are said to be unsupportable. Mission Failure Mission failure is the result of trying to use a system in its normal mode when it is not working. Apart from human error, mission failure results from the following causes. Lack of instrumentation for the fault in the form of an indicator suitable for CBM Failure after PMS diagnostic testing has been completed but before operation is attempted Lack of diagnostic PMS testing for any failure mode that lacks CBM See also High availability List of system quality attributes Spurious trip level References External links Reliability and Availability Basics System Reliability and Availability Systems engineering