MAINTAINABILITY DESIGN TECHNIQUES METRIC - AcqNotes.com

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Downloaded from http://www.everyspec.com on 2011-10-29T14:56:01.DOD-HDBK-791(AM)1-3.2 QUANTITATIVE MAINTAINABILITYINDICESThere are many mathematical indices used to quantifymaintainability. It is important to remember, however,that these relationships merely categorize data derivedfrom planned testing. For maintainability, the test planningphase is equal in importance to the assessment phase.Testing that does not adequately demonstrate the effect ofthe physical features and personnel and support aspects(described in par. 1-2) actually provides data that effectivelyconceal the impact of these critical elements.Indices used to support maintainability analysis must1. Be composed of measurable quantities2. Provide effectiveness-oriented data3. Be readily obtainable from operational and applicabledevelopment testing.These kinds of indices provide system designers, users,and evaluators with data operational readiness, missionsuccess, maintenance manpower costs, and logistic supportcosts -that can be used to evaluate candidate systemsmore precisely.Since maintainability programs must be tailored to thespecific system, the terms used to define the various measuresshould be similarly selected: however, the termsshould be standardized for similar major systems. It isalso important that the numerical values associated withthe terms describing system maintainability)’ must beoperational values and not inherent values (Ref. 4).Because of their importance, the terms are defined:1. Operational Value. A measure of maintainabilitywhich includes the combined effects of item design. quality,installation, environment. operation, maintenance,and repair2. Inherent Value. A measure of maintainability)which includes only the effects of item design and itsapplication, and assumes an ideal operation and supportenvironment.Some common, accepted indices of maintainability are1. Mean Time to Repair2. Maximum Time to Repair1-4
Downloaded from http://www.everyspec.com on 2011-10-29T14:56:01.DOD-HDBK-791(AM)3.4.5.6.7.8.9.10.11.12.13.Mean Maintenance TimeEquipment Repair TimeGeometric Mean Time to RepairMaintenance Man-HoursRepair RatesMaintenance RatesProbability of Fault DetectionProportion of Faults IsolatableAutomatic Fault Isolation CapabilityPercent of False AlarmsPercent of False Removals.These terms are defined in the Glossary. Equations forcalculating these indices, together with examples, arefound in Refs. 3, 5, and 6.1-4 MAINTAINABILITY PROGRAM1-4.1 OBJECTIVESDoD Directive 5000.1 states, “Improved readiness andsustainability are primary objectives of the acquisitionprocess. Resources to achieve readiness will receive thesame emphasis as those required to achieve schedule orperformance objectives. As a management precept, operationalsuitability of deployed weapon systems is anobjective of equal importance with operational effectiveness.”.Operational suitability as defined by Ref. 7 is “Thedegree to which a system can be satisfactorily placed infield use, with consideration being given to...maintainability,safety, human factors, manpower supportability,logistic supportability, and training requirements.”. Today,when large masses of troops and materiel can be deployedanywhere in the world in a matter of days or even hours,system readiness demands the utmost attention.The importance of a maintainability program to thesystem acquisition process is well established by the previousdiscussion. Therefore, to accomplish its stated pur-pose, the maintainability program must contribute toachieving the following objectives (Refs. 8 and 9):1. Improved operational readiness2. Reduced maintenance manpower needs and assurancethat the fielded system can be operated and maintainedwith skills and training expected to be available tothe Army3. Reduced life cycle costs, i.e., economically operatedand maintained4. Assurance that maintainability requirements areproperly demonstrated and assessed during developmentand operational test and evaluation. Fig. 1-3 illustratesthe time phasing for maintainability verification, demonstration,and evaluation.5. Provision of data essential for management, i.e.,assurance that the results of maintainability assessmentare presented in a form suitable to support the decisionmakingprocess.It must be recognized that these objectives are interrelatedand affect both operational effectiveness and ownershipcosts. Accordingly, they must be applied skillfully toavoid being duplicative or contradictory, and at each levelat which maintenance is to be performed. These samegoals also may apply to related programs, e.g., reliability,and may require the same kind of tasks and analysis todemonstrate and verify them. To avoid duplication ofeffort, performance of such tasks or analysis will be coordinatedand whenever possible combined with similartasks called for under other program elements (Ref. 9).Coordination may require trade-offs. However, in thetrade-off process care must be exercised to insure thatmaintainability demonstration and evaluation are notsacrificed.Cost is an item of consideration at each phase in theacquisition process—concept exploration, demonstrationand validation, full-scale development, and productionand deployment. Since maintainability is dedicated toFigure 1-3. Maintainability Level of Effort1-5
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