Availability refers to the ability of an item to perform its required function under stated operating conditions over a period of time. It is measured as a percentage and indicates the degree to which machinery and equipment is operable and usable when needed. There are several types of availability including inherent availability, achieved availability, and operational availability. Inherent availability considers only corrective downtime and assumes unlimited spare parts and support, while achieved availability includes preventive maintenance downtimes as well. Operational availability reflects actual field conditions and accounts for logistics, wait times, and other delays. Availability is calculated based on up time versus total time, and is impacted by failure and repair rates.

1. Availability with respect to Reliability
Significance of Availability

2. The Ability Of An Item To
Perform A Required
Function Under Stated
Operating & Environmental
Conditions For A Given
Period Of Time.

3. 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.

5. Instantaneous (or Point) Availability
Average Up-Time Availability
Steady State Availability.
Inherent Availability
Achieved Availability
Operational Availability
AVAILABILITY CLASSIFICATIONS

6. Instantaneous (or point)
availability is the probability
that a system (or component)
will be operational (up and
running) at any random time.

9. 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
equipment.
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.

10. 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
facility.
Also assumes that spare parts and manpower are 100 percent available with no
delays.
Achieved availability is very similar to inherent availability with the exception that
preventive maintenance (PM) downtimes are also included.

12.  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
being repaired.
 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

13.  TIME AVAILABILITY is the percentage of
operating time that an equipment is
operational
 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
maximum down-time