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)diagnostic approach and then implementing that appreach.The analysis includes BIT analysis, nodal analysisfor partitioning, test point design, fault simulation.and diagnostic preparation for all levels of maintenance.To be effective, testability analysis must be used todefine diagnostic requirements before the detailed designof the major equipment is begun. During the detaileddesign, test point analysis and nodal analysis for partitioningshould be major inputs to the layout or packagingdesign that contributes to modularization. Analysis ofprojected BIT performance and fault simulation studiesshould be used to evaluate the process to determinewhether the BIT objective is being met and as inputs to aBIT maturation program. Design improvement datashould also be extracted from test and operational data.For example, system components coming off the productionline should be tested with equipment designed foreventual use by repair personnel. An analysis of the testresults will reveal whether the test equipment can confidentlyand satisfactorily perform its diagnostic purpose.Chapter 13, “Test and Evaluation (T&E)”, AMC-TRADOC Materiel Acquisition Handbook (Ref. 6), setsforth procedures used to plan, evaluate, and report on thetest and evaluation of materiel systems and or items. thetypes of test and evaluation performed. and the responsibilitiesof the US Army Materiel Command (AMC), USArmy Training and Doctrine Command (TRADOC),operational tester, and other organizations in the test andevaluation process. The purposes and time frames as theyrelate to the various test programs from operational testingto production testing are defined.Guidelines for conducting the testability analysis are1. Question the process by which the developerdefined his testability approach—how, why, and logicemployed.2. Insure testability design is concurrent with majorequipment design.3. Insure test point selection and design, and testabilitypartitioning play a major role in system layout andpackaging.4. Insure that a failure modes and effects analysiswas available and used as part of the testability analysis.5. Insure that BIT analysis and fault simulation wereused to evaluate the coverage and effectiveness of the BITdesign if BIT is used. BIT development, integration, andcheckout must be concurrent with system performancedevelopment.6. Insure that the testability design approach evolvesas information is obtained from analysis and test experience.Compare the results with the requirements.7-2.3 INTERFACE RELATIONSHIPSEquipment availability. measured as a probability, hastwo elements, i.e.,1. The probability that an item is operable because ithas not failed, i.e., is reliable2. The probability that, if the item has failed or isdown for maintenance, it can be restored to serviceablecondition within the time permitted by mission constraints.Any failed condition that is present and undetectable isnot corrected and is, therefore, a part of the unreliabilityof a mission. This highlights the importance of testabilitybecause the incidence of undetectable failures can bereduced by designing equipment for a high degree ofreliable testability. This attribute of the system is referredto as test effectiveness, or BIT efficiency, i.e., the percentof all faults or faults that the BIT system detects. one ofthe more complex tasks in the acquisition of modernweapon systems is the specifying of performance measuresor figures of merit for BIT. It is becoming commonpractice that contracts for electronic subsystems andcomponents specify the false alarm rate and the percent offailures that can be detected and isolated. Ref. 7 discussesthe analysis performed on a complex digital data systemwherein the BIT specifications required that 98% of allpossible failures be detectable and that 90% of all failuresbe isolatable to one plug-in assembly. Par. 7.2-4 discussesand illustrates by example characteristics external to BIT.It is obvious that testability interfaces with reliability,modularization, end-item configuration, space allocation,BIT, automatic test equipment (ATE), and logistics.Trade-off studies involving these factors will result in themost cost-effective method for achieving availabilitygoals consistent with the mission of the system. Diagnostictechniques are important inputs to this analysis (seepar. 7-2.2), and in some cases diagnostic techniquesevolve from the analysis.In the trade-off between reliability and testability,availability may be enhanced by designing a piece part toa higher level of reliability or by providing standbyredundancy. Testability, with its BIT, introduces anothcrcomponent into the system that may fail, i.e.. the BITequipment may not reveal an undetected fault or maysignal a false alarm. However, added redundancy mayraise the reliability of a component to a level where testingis considered unnecessary. Consider the following redundantsystemwhere the reliability R cof a component is 0.8. The equationfor calculating the improved reliability of this parallelredundant system, where k = 2, isSince the reliability of the redundant system has beenincreased from 80% to 96%, which enhances overall systemavailability, testing of the circuit may be consideredunnecessary and the BITE eliminated. Unfortunately,redundancy almost always increases the maintenance7-2
Downloaded from http://www.everyspec.com on 2011-10-29T14:56:01.DOD-HDBK-791(AM)workload, complicates fault detection, and increases cost.Depending upon the size of the added component, theincreased weight and volume also may be a factor. Offsettingthese negative aspects are the cost of providing testequipment, the time to conduct the test, and the timerequired to repair a mission-critical end-item.7-2.4 CHARACTERISTICS EXTERNAL TOBIT (Ref. 8)There are two important considerations external toBITE that must be addressed in any discussion of BITEand diagnostics, namely,1. Reliable performance of the weapon system determines,to a large extent, the criticality of BIT performance.Therefore, if the basic system is very reliable,more than expected, a shortfall in the BIT performancemay have very limited impact on the operational utility ofthe system.2. All system faults that are correctable by maintenanceaction must eventually be detected and isolated.The Failure Modes, Effects, and Criticality Analysis(FMECA) is an effective tool for evaluating BIT effectiveness.The FMECA can be used in defining test andcheckout procedures to insure that all essential parameters,functions, and modes are verified. Therefore, thetechniques, tools, manuals, test equipment, and personnelrequired to isolate non-BIT detectable faults can be amajor maintenance consideration.Example 7-1, which follows, illustrates the impact ofBITE on the overall maintenance planning effort. It alsoillustrates the effect of external factors on BIT equipmentperformance.Example 7-1:Assume the radar of an attack aircraft is composed offive line-replaceable units (LRUS) with the followingBITE and system characteristics:System:Five LRUSMean Time to Repair (MTTR) B— with BITE:2-h—includes failures that have been bothdetected and isolatedMean Time to Repair (MTTR) NB—non-BITE:5-h—includes failures that may not have beenisolated but may have been detectedMean Flying Hours Between Failures MFHBF50 flying hoursTime Period of Interest TPI:2500 flying hoursBIT Specifications:Percent detect ion R delect= 90%Percent isolation R isol= 90% (to LRU level)False alarm rate R FA= 5% (of all BITE indications).For this example, operating time is assumed to be flighttime.Before beginning the analysis, note that a relativelyhigh BIT system capability has been specified. A casualexamination would likely conclude that, with this extensiveBIT coverage, there is minimal maintenance actionrequired.The notationfollows:AFIC =(F B) detect=(F B) LRU=F FA=(F B) total=F T=R EA=MFHBF =(MTTR) B=(MTTR)NB=R detect=R isol=(T FA) total=(T LRU)total=(T NB)total =(T NB+FA) total=T total=TPI =used in the conduct of the analysisautomatic fault isolation capability, %number of failures of the total numberof failures F Tthat BITE will detect astrue, failuresnumber of BITE detected failures thatcan be isolated to LRU level, failuresnumber of false alarms expected duringTPI, dimensionlesstotal BITE indications of failure, i.e.,true failure plus false alarms, failurestotal failures expected during TPI,failuresfalse alarm rate indicated by BITE, %mean flying hours between failures, flyinghoursmean time to repair BITE detected andisolated failures hmean time to repair non-BITE detectedand isolated failures, hBITE detection rate, %BITE isolation rate to LRU level, %total maintenance time to resolve falsealarms, htotal corrective maintenance time torepair BITE detected and isolated failures,htotal corrective maintenance time torepair non-BITE detected and isolatedfailures, htotal non-BITE corrective maintenancetime to repair non-BITE detected andisolated failures plus false alarm maintenancetime, htotal corrective maintenance time duringTPI, i.e., sum of BITE and non-BITE corrective maintenance time, htime period of interest, 2500 flying hours.The analysis follows:1. Total number of failures F totalto be experienced,on the average, arewhereTPI = time period of interest, flying hoursMFHBF = mean flight hours between failures,flying hours.7-3
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