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.

Inherent Availability

Achieved Availability

Operational Availability

AVAILABILITY CLASSIFICATIONS

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 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.

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.

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

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