In reliability engineering, the term availability has the following meanings: The degree to which a system, subsystem or equipment is in a specified operable and committable state at the start of a mission, when the mission is called for at an unknown, i.e. a random, time. The probability that an item will operate satisfactorily at a given point in time when used under stated conditions in an ideal support environment. Normally high availability systems might be specified as 99.98%, 99.999% or 99.9996%. Representation The simplest representation of availability (A) is a ratio of the expected value of the uptime of a system to the aggregate of the expected values of up and down time (that results in the "total amount of time" C of the observation window) Another equation for availability (A) is a ratio of the Mean Time To Failure (MTTF) and Mean Time To Repair (MTTR), or If we define the status function as therefore, the availability A(t) at time t > 0 is represented by Average availability must be defined on an interval of the real line. If we consider an arbitrary constant , then average availability is represented as Limiting (or steady-state) availability is represented by Limiting average availability is also defined on an interval as, Availability is the probability that an item will be in an operable and committable state at the start of a mission when the mission is called for at a random time, and is generally defined as uptime divided by total time (uptime plus downtime). Methods and techniques to model availability Reliability Block Diagrams or Fault Tree Analysis are developed to calculate availability of a system or a functional failure condition within a system including many factors like: Reliability models Maintainability models Maintenance concepts Redundancy Common cause failure Diagnostics Level of repair Repair status Dormant failures Test coverage Active operational times / missions / sub system states Logistical aspects like; spare part (stocking) levels at different depots, transport times, repair times at different repair lines, manpower availability and more. Uncertainty in parameters Furthermore, these methods are capable to identify the most critical items and failure modes or events that impact availability. Definitions within systems engineering Availability, inherent (Ai) The probability that an item will operate satisfactorily at a given point in time when used under stated conditions in an ideal support environment. It excludes logistics time, waiting or administrative downtime, and