Operational availability

Operational availability is used to evaluate the following performance characteristic.

For a system that is expected to be available constantly, the below operational availability figures translate to the system being unavailable for approximately the following lengths of time (when all outages during a year are added together):

${\displaystyle A_{o}=0.9\approx 877\ hours\ down\ time\ per\ year}$

${\displaystyle A_{o}=0.99\approx 87\ hours\ down\ time\ per\ year}$

${\displaystyle A_{o}=0.999\approx 8\ hours\ down\ time\ per\ year}$

${\displaystyle A_{o}=0.9999\approx 52\ minutes\ down\ time\ per\ year}$

${\displaystyle A_{o}=0.99999\approx 5\ minutes\ down\ time\ per\ year}$

The following data is collected for maintenance actions while in operation to prioritize corrective funding.

• Diagnostic down time is required to identify the amount of time spent performing maintenance when fault reporting does not support condition-based maintenance.
• Criticality identifies level of risk associated with loss of mission, injury or loss of life, and capital equipment.
• Fault isolation down time is required to identify the amount of time spent locating a failure.
• Logistics delay down time is required to identify the amount of time required to obtain replacement parts or software.
• Corrective maintenance down time is required to identify the amount of time required to install and reconfigure replacement parts and software.

This data is applied to the reliability block diagram to evaluate individual availability reduction contributions using the following formulas.

${\displaystyle A_{o}^{D}=\left({\frac {Total\ Time}{Total\ Time+Diagnostic\ Down\ Time}}\right)}$

${\displaystyle A_{o}^{FI}=\left({\frac {Total\ Time}{Total\ Time+Fault\ Isolation\ Down\ Time}}\right)}$

${\displaystyle A_{o}^{L}=\left({\frac {Total\ Time}{Total\ Time+Logistics\ Down\ Time}}\right)}$

${\displaystyle A_{o}^{C}=\left({\frac {Total\ Time}{Total\ Time+Corrective\ Down\ Time}}\right)}$

Redundant items do not contribute to availability reduction unless all of the redundant components fail simultaneously.

Operational availability is the overall availability considering each of these contributions.

${\displaystyle A_{o}=A_{o}^{D}\times A_{o}^{FI}\times A_{o}^{L}\times A_{o}^{C}}$

• Active redundancy
• Availability
• Downtime
• Reliability block diagram