Five-9's (99.999%) means less than 5 minutes when the system is not operating correctly over the span of one year.
As an example, availability of 99.9% means nothing after the only known source stops manufacturing a critical replacement part.
For observation, these reflect the different areas of the organization, such as maintenance personnel and documentation for MTTR, and manufacturers and shippers for MLDT.
Such systems will continue without noticeable interruption when these conditions are satisfied unless there are secondary failures.
This is called active redundancy, which requires no maintenance to prevent mission failure.
Systems with spare parts that are energized but that lack automatic fault bypass are not actually redundant because human action is required to restore operation after every failure.
PMS works is periodic maintenance, like when you perform diagnostic tests on your car every 90 days (or 3,000 miles).
A failure may occur any time during the 90 days, such as a broken light, but you will not become aware until you perform diagnostic test.
Mean Time To Discover is statistical when PMS is the dominant maintenance philosophy.
The following table shows the anticipated down-time for different availabilities for a mission time of one year.
Mission failure is the result of trying to use a system in its normal mode when it is not working.
This article incorporates public domain material from Federal Standard 1037C.