MAINTAINABILITY DESIGN TECHNIQUES METRIC - AcqNotes.com

Downloaded from http://www.everyspec.com on 2011-10-29T14:56:01.DOD-HDBK-791(AM)istic. The object of RCM is to preserve the inherent designlevels of reliability and accomplish it at minimum cost.The RCM concept uses decision logic to evaluate andconstruct maintenance tasks which are based on theequipment functions and failure modes. Evaluation ofequipment designs in accordance with RCM techniqueswill also determine when it is cost-effective to employpreventive maintenance and when it is not cost-effective.It is essential that the RCM analysis be integrated with themaintainability concepts for testability and diagnostictechniques.8-2.3 RELIABILITY CENTEREDMAINTENANCE LOGICRCM logic is intended for application when the failuremode, effect, and criticality of component failure hasbeen identified. Fig. 8-2 (Ref. 4) illustrates the RCManalysis process—a process that is applied to each repairableitem in the equipment or system. The logic isdesigned to accomplish the following (Ref. 4):1. By using data from the system safety and reliabilityprograms, identify components in the system orequipment that are critical in terms of mission or operatingsafety.2. Provide a logical analysis process to determinethe feasibility and desirability of scheduled maintenancetask alternatives.3. Highlight maintenance problem areas for designreview consideration.4. Provide the supporting justification for scheduledmaintenance task requirements.Par. 4-9, Ref. 4, presents a detailed discussion on theapplication of the logic steps identified in Fig. 8-2.The functional failure identified by the application ofRCM logic can be assessed for consequence of failure andis processed according to its severity category—i.e., catastrophic,critical, marginal, or minor. The logic process isbased on the following (Ref. 4):1. Minor or Marginal Severity Categories. Scheduledmaintenance tasks should be performed only whenperformance of the scheduled task will reduce the lifecycle cost of the equipment or system.2. Critical and Catastrophic Severity Categories.Scheduled maintenance tasks should be performed whensuch tasks will prevent a decrease in reliability or deteriorationof safety to unacceptable levels or when the taskwill reduce the life cycle cost of ownership of the equipmentor system.Reliability (R) and Maintainability (M) Characteristicsform, Fig. 8-3, is used to record the disposition. Instructionsfor locating the data on the form are contained inpar. 4-8 (Ref. 4).8-3 PREVENTIVE MAINTENANCEEVALUATION PARAMETERSAND TRADE-OFFSThe relative values of preventive maintenance actionsmay be determined as a function of several performanceand cost parameters. The value should be quantifiedwhenever possible to remove the decision from subjectivejudgment. However, performance and cost must never betraded off with safety.Preventive maintenance actions incur cost in terms ofmaterials, man-hours, and equipment downtime; hencethe detailed analysis discussed in par. 8-2. The cost ofman-hours and repair parts and materials can be determinedrather easily. The cost of equipment downtime,however, is a function of the operational value of theparticular equipment in the situation that exists when it isdown. Accordingly, maintenance is scheduled into theoperations when there is the minimum risk that theequipment will be required for an operational assignment.The two chief parameters—availability and cost ofownership—relative to an optimum preventive maintenanceprogram are discussed in the paragraphs thatfollow.8-3.1 AVAILABILITYAchieved availability is the probability that a system orequipment, when used under stated conditions in an idealsupport environment—i.e., available tools, parts, manpower,manuals, etc.—shall operate satisfactorily at agiven time. Note that supply downtime and waiting oradministrative downtime are excluded. Two equationsfor achieved availability are given8-2.4 DISPOSITION OF RELIABILITYCENTERED MAINTENANCEANALYSISEach failure analyzed by the application of the RCMlogic leads the analysts to make a decision on the dispositionof each failure mode. The Data Record B: Item8-3