Availability with respect to Reliability
Significance of Availability
The Ability Of An Item To
Perform A Required
Function Under Stated
Operating & Environmental
Conditions For A Given
Period Of Time.
A Percentage Measure Of The
Degree To Which Machinery
And Equipment Is In An
Operable And Committable
State At The Point In Time
When It Is Needed.
Instantaneous (or Point) Availability
Average Up-Time Availability
Steady State Availability.
Instantaneous (or point)
availability is the probability
that a system (or component)
will be operational (up and
running) at any random time.
Inherent availability is the steady state availability when considering
only the corrective downtime of the system.
It is defined as the expected level of availability for the performance
of corrective maintenance only.
Inherent availability is determined purely by the design of the
It assumes that spare parts and manpower are 100 percent available
with no delays.
It excludes logistics time, waiting or administrative downtime, and
preventive maintenance downtime.
It includes corrective maintenance downtime. Inherent availability is
generally derived from analysis of an engineering design.
The probability that an item will operate satisfactorily at a given point in time when
used under stated conditions in an ideal support environment (i.e., that personnel,
tools, spares, etc. are instantaneously available).
It excludes logistics time and waiting or administrative downtime.
It includes active preventive and corrective maintenance downtime. Achieved
availability is defined as the achieved level of availability for the performance of
corrective and preventive maintenance.
Achieved availability is determined by the hard design of the equipment and the
Also assumes that spare parts and manpower are 100 percent available with no
Achieved availability is very similar to inherent availability with the exception that
preventive maintenance (PM) downtimes are also included.
Availability = A = U/ U+D ; Where U = Up time ,
during which the machine is in working order; D =
Down time , During which the machine is faulty and
A = M/M+R = 1/ λ / (1/ λ +1/µ ) = µ/ µ+ λ ;
Where λ = failure rate = 1/M or M = 1/λ and R =
mean repair time . Repair rate = µ = 1/R or R = 1/ µ
Unavailability = B = D/ U+D
B = R/M+R = λ/ λ +µ.
A+B = (U/U+D + D/ U+D) = 1
TIME AVAILABILITY is the percentage of
operating time that an equipment is
EQUIPMENT AVAILABILITY is the percentage
of equipments which will be available for use
after t hours of operation
MISSION AVAILABILITY is the percentage of
time t which will have any failure which
cannot be restored within a specified
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