Classification of Availability
There are three forms availability based on the classification of down time viz. Working time, stand by time and lost time in routing preventive maintenance.
- Intrinsic Availability (Ai)
This is the probability that system is operating satisfactorily at any point of time, when under stated conditions, where the time considered is operating time and active repair time.
Design Accuracy
It is the probability that the system will successfully accomplish its mission given the system is operating within design specifications.
Ai = Operating time/(Operate time + Actual repair time)
= MTBF / (MTBF + MTTR)
- Achieved Availability (Aa)
It is inclusive version of the inherent availability and includes factors influencing the availability of a system which are directly related with the designed maintainability and reliability characteristics of the system. This includes corrective and preventive maintenance and the mean maintenance time. Administrative or supply delay times are not included as down time for the calculation of availability.
Aa = Mean time between maintenance/ Total elapsed time exclusive of administrative and logistic delays
- Operational Availability (Ao)
It differs from the achieved availability in the sense that in case of operational availability as the total operational time, the total down time including administrative delays is also taken into account.