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3.0 MAINTAINABILITY AND AVAILABILITY

LECTURE NOTE / INSTRUCTION NOTE

AZIANASHIMA HEEDAYU BINTI ZAINAL

3.1 DEFINITION OF MAINTAINABILITY

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The probability that a failed unit will be repaired within a given

amount of time. The term is also used to denote the discipline of

studying and improving the maintainability of products, primarily

by reducing the amount of time required to diagnose and repair

failures.

Where P(T<t) is the probability of completing the repairs in time

< T , and f(x) is the repair time probability density function.

Availability is the probability that an item will be able to

function (i.e. not failed or undergoing repair) when called

upon to do so. This measure takes into account an item’s

reliability (how quickly it fails) and its maintainability (how

quickly it can be repaired).Steady state availability: )(A

3.2 DEFINITION OF AVAILABILITY

Introduction to Maintainability and Availability

Many systems are repairable; when the system fails — whether it is

an automobile, a dishwasher, production equipment, etc. — it is

repaired. Maintainability is a measure of the difficulty to repair the

system.

MEASUREMENT OF MAINTAINABILITY AND AVAILABILITY

More specifically, maintainability is:

The measure of the ability of a system to be retained in, or

restored to, a specified condition when maintenance is

performed by personnel having specified skill levels, using

prescribed procedures and resources, at each prescribed level of

maintenance and repair.

• Elemental Activities are simple maintenance actions of

short duration and relatively small variance that do not

vary appreciably from one system to another. An example

of an elemental activity is the opening and shutting of a

door.

SIX COMPONENTS OF MAINTAINABILITY

STRUCTURE

• Malfunction Active Repair Time consists of:

a. Preparation time

b. Malfunction verification time

c. Fault location time

d. Part procurement time

e. Repair time

f. Final malfunction test time

Items a–f above are composed of elemental activities.

•Malfunction Repair Time consists of:

a. Malfunction active repair time

b. Malfunction administrative time

• System Repair Time is the product of malfunction repair time

and the number of malfunctions.

• System Downtime includes:

a. System logistic time

b. System repair time

c. System final test time

• Total System Downtime is a combination of the distributions of

a. Initial delay

b. System downtime

Availability is a measure of the readiness of a system. More

specifically, availability is:

• A measure of the degree to which a system is in an operable and

committable state at the start of a mission when the mission is

called for at a random time.

There are three categories of availability.

1. Inherent Availability is the ideal state for analyzing availability.

It is a function only of the mean time to fail, MTBF, and the mean

time to repair, MTTR; preventive maintenance is not considered.

Inherent availability is defined as:

n

i i

n

i iiMTTR1

1

i

i

where n is the number of sub-systems

- is the failure rate of the I-th subsystem, and

- is the mean time to repair the I-the unit

• Achieved Availability includes preventive maintenance as well as

corrective maintenance. It is a function of the mean time between

maintenance actions, MTMA, and the mean maintenance time,

MMT. Achieved availability is defined as:

• Operational Availability includes preventive maintenance,

corrective maintenance, and delay time before maintenance begins,

such as waiting for parts or personnel. It is a function of the mean

time between maintenance actions and the mean down time, MDT,

and is defined as:

END OF TOPIC