This preview shows page 1. Sign up to view the full content.
Unformatted text preview: as detemmined on the
basis of meeting measures of supply efficiency, such as
percent of demands met. This tended to lead to
suboptimal resource requirements from the system's
point of view. More modem models of logistics
supportability base re- NASA Systems Engineering Handbook
Systems Analysis and Modeling Issues
Measures of Availability
Availability can be calculated as the ratio of operating time to total time, where the denominator, total time, can be
divided into operating time ("uptime") and "downtime." System availability depends on any factor that contributes to
downtime. Underpinning system availability, then, are the reliability and maintainability attributes of the system design,
but other logistics support factors can also play significant roles. If these attributes and support factors, and the operating environment of the system are unchanging, then several measures of steady-state availability can be readily calculated. (When steady-state conditions do not apply, availability can be calculated, but is made considerably more
complex by the dynamic nature of the underlying conditions.) The system engineer should be familiar with the
equations below describing three concepts of steady-state availability for systems that can be repaired.
• Inherent = MTTF / (MTTF + MTTR)
Achieved = MTTMA / (MTTMA + MMT)
Operational = MTTMA / (MTTMA + MMT + MLDT)
= MTTMA / (MTTMA + MDT)
MTTF = Mean time to failure
MTTR = Mean time to repair (corrective)
MTTMA = Mean time to a maintenance action (corrective and preventive)
MMT = Mean (active) maintenance time (corrective and preventative)
MLDT = Mean logistics delay time (includes downtime due to administrative delays, and waiting for spares,
maintenance personnel, or supplies)
MDT = Mean downtime (includes downtime due to (active) maintenance and logistics delays) Availability measures can be also calculated at a point in time, or as an average over a period of time. A further,
but manageable, complication in calculating availability takes into account degraded modes of operation for redundant
systems. For systems that cannot be repaired, availability and reliability are equal. (See sidebar on page 92.) source requirements on the system availability effects.
(See sidebar on logistics supportability models.)
Some availability models can be used to
determine a logistics resource requirement by
computing the quantity of that resource...
View Full Document