6.1 MAINTAINABILITY DEFINED

Maintainability may be defined variously as follows:

1. Maintainability is the probability of successfully performing and completing one or more, or any specified combination, of the various maintenance actions listed in Chapter 3, within a prescribed period of time t ₁.

   Once the times-to-maintain pdf is determined, based on the maintenance downtime category involved, the maintainability for the period t ₁, M( t ₁), may be calculated from

   

   where g( t) is the pdf of the times required to complete the particular maintenance action(s), for which the equipment's maintainability is desired to be quantified. This quantification of maintainability is illustrated in Fig. 6.1 based on Eq. (6.1).

   
   Figure 6.1: Quantification of maintainability given the pdf of the maintenance times.

2. Maintainability is a characteristic of design and installation which is expressed as the probability that an item can be restored to, or retained in, satisfactory operating condition within a given period of time when maintenance action is performed in accordance with prescribed procedures and resources.

3. Maintainability of a part in a system is the probability of completing system maintenance, whether by replacing or by repairing that part, in time t ₁, when that part fails or has to be replaced preventively.

4. Maintainability comprises those characteristics (both qualitative and quantitative) of materials, design and installations which make it possible to meet operational objectives with a minimum expenditure of maintenance effort (in terms of manpower, skill levels, test equipment, technical data,...