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The following is an excerpt from The Reliability Engineering Handbook by Bryan Dodson and Dennis Nolan, © Quality Publishing. It may be ordered from the Quality Publishing Order Form..

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. 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.

Military Handbook 472 (MIL-HDBK-472) defines six components of maintainability, which are discussed below.

1. 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.

2. Malfunction Active Repair Time consists of:

   1. Preparation time

   2. Malfunction verification time

   3. Fault location time

   4. Part procurement time

   5. Repair time

   6. Final malfunction test time

   Items a–f above are composed of elemental activities.

3. Malfunction Repair Time consists of:

   1. Malfunction active repair time

   2. Malfunction administrative time

4. System Repair Time is the product of malfunction repair time and the number of malfunctions.

5. System Downtime includes:

   1. System logistic time

   2. System repair time

   3. System final test time

6. Total System Downtime is a combination of the distributions of

   1. Initial delay

   2. System downtime

MIL-HDBK-472 provides a procedure for predicting maintainability based on the structure described above. The philosophy of the procedure is based on the principles of synthesis and transferability. The synthesis principle involves a buildup of downtimes, step-by-step, progressing from the distribution of downtimes of elemental activities through various stages culminating finally with the distribution of system downtime. The transferability principle embodies the concept that data applicable to one type of system can be applied to similar systems, under like conditions of use and environment, to predict system maintainability.

Other useful maintainability references are Military Standard 470, which describes a maintainability program for systems and equipment, and Military Standard 471, which provides procedures for maintainability verification, demonstration, and evaluation.

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 comitable 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

   

2. 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

   

3. 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

   

   It is important to note that the type of availability being described is often not distinguished. Many authors simply refer to "availability," MTTR may be the equivalent of MMT or MDT, and MTBF may be the equivalent MTMA.