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The Maintainability is the characteristic represents the degree of effectiveness and efficiency with which a product or system can be modified to improve it, correct it or adapt it to changes in environment, and in requirements. This characteristic is composed of the following sub-characteristics:¹⁾

• Modularity - Degree to which a system or computer program is composed of discrete components such that a change to one component has minimal impact on other components.
• Reusability - Degree to which an asset can be used in more than one system, or in building other assets.
• Analysability - Degree of effectiveness and efficiency with which it is possible to assess the impact on a product or system of an intended change to one or more of its parts, or to diagnose a product for deficiencies or causes of failures, or to identify parts to be modified.
• Modifiability - Degree to which a product or system can be effectively and efficiently modified without introducing defects or degrading existing product quality.
• Testability - Degree of effectiveness and efficiency with which test criteria can be established for a system, product or component and tests can be performed to determine whether those criteria have been met.

Maintainability is a characteristic of a system to resume full operation after a failure in a component of the system. Components are mission critical for the system and can include equipment, machine, power, air conditioning, software, etc. Maintainability is expressed as a probability of recovery based on a specified time frame, usually this is done in terms of Five Nines (i.e., 99%, 99.9%, 99.99%, and 99.999%). For example, a 99.999% maintainability would be for 5 minutes, 15 seconds or less of downtime in a year. The downtime must include all the steps required to recover with full operational capabilities, so the time must include removal, diagnostics, assembly of resources required to perform the maintenance (i.e, parts, bays, tools, personnel, etc.) and the re-installation of the component.^{2) 3)}

There are two main aspects of Maintainability that need to be addressed for all systems or components:

• Servicability - the ease of conducting scheduled inspections and servicing
• Repairability - the ease of restoring service after a failure

Maintainability is a projection of the downtime of a system, obviously, as a projection it is not a guarantee that a system will only be down for a certain amount of time. Maintainability therefore must rely on models to calculate the failures for the components which are based on actual failure rates for components in the past or on test results of the components.

There are a wide range of models that estimate and predict reliability (Meeker and Escobar 1998). Simple models, such as exponential distribution, can be useful for “back of the envelope” calculations.

System models are used to:

(1) combine probabilities or their surrogates, failure rates and restoration times, at the component level to find a system level probability or

(2) to evaluate a system for maintainability, single points of failure, and failure propagation. The three most common are reliability block diagrams, fault trees, and failure modes and effects analyses.

There are more sophisticated probability models used for life data analysis. These are best characterized by their failure rate behavior, which is defined as the probability that a unit fails in the next small interval of time, given it has lived until the beginning of the interval, and divided by the length of the interval. See: Upkeep's discussion of models in Maintainability Definition & Calculation ⁴⁾

All these models are abstractions of reality, and so at best approximations to reality. To the extent they provide useful insights, they are still very valuable. The more complicated the model, the more data necessary to estimate it precisely. The greater the extrapolation required for a prediction, the greater the imprecision. However, obtaining all the data required as input to the models is difficult, time consuming and the data that is available is not very accurate.

Measuring the Mean Time To Repair (MTTR), is also used as part of Availability ( see 4.3.2.2 Availability ).

The MTTR, identifies the average time to restore a system or component after experiencing a failure or breakdown to the expected (i.e., specified) operating conditions. The formula for MTTR is:

The lower MTTR value correspond to a higher level of maintainability and consequently, maintainable systems take less time to repair.

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