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)Aid) (Ref. 15). Contracts were awarded by the Army in1985 for the development of AI technology to assist in thetroubleshooting and maintenance of helicopters andrelated aviation systems. Specifically, AI is being harnessedto diagnostic test equipment for the AH-64APACHE attack helicopter. Software developed usingAI will be applied to a device referred to as an “intelligentfault locator”—a van-mounted computer that can servicehelicopters on a flight line or in the field. The systemenables crew chiefs to enter a fault description into acomputer, which will then take the crew chief through aseries of checks that will identify the problem.7-4 FUNCTIONAL TESTS7-4.1 TYPESFunctional testing performed with diagnostic equipmentis for the purpose of1. Fault Localization, Isolation, and Prediction.The identification of replaceable units or componentsthat have caused a known fault and, where practicable, anindication of an impending fault2. Verification Testing. Testing performed to insurereadiness before the start of the mission and aftermaintenance3. Testing for Hidden Faults. Testing of diagnosticmonitors to insure readiness.Each of these functions is discussed further in the paragraphsthat follow.7-4.1.1 Fault Localization, Isolation, andPredictionThe primary test functions include fault detection, faultlocalization or isolation, and fault prediction. The desiredfunction must be defined and its resulting complexityconsidered in relation to the equipment being tested. Themaintainability plan should establish whether the testfunction is to be fault detection alone; fault detection andisolation; or fault detection, then isolation; and, whenpracticable, prediction. Neither the inability of the automatictest feature to predict impending failure shouldprevent it from being useful in isolating the fault once amalfunction has occurred nor should failure of the automatictest feature to isolate a fault prevent it from beinguseful for detecting the fault and signifying a “down-forrepair”condition. In general, fault isolation should beconsistent with the maintainability plan and should notgo below the cost-effective level of detection.7-4.1.2 Verification TestingTesting frequently is used to verify the performance orcondition of the system when no fault is present or afterfault correction. For example, most missiles are checkedfor proper operation prior to firing; normally the sametest equipment is used to verify that the missile is operatingproperly following the replacement of an unserviceablecomponent. The test designer must decide whetherstimuli—the inputs or signals from the test set to the itembeing tested to allow simulation of its operation undermission conditions—are required to interrogate the system.The stimuli must be realistic and controlled—e.g., ifcurrent is introduced into a system to check circuit continuity,care must be exercised to insure that the magnitudeof the current will not inadvertently activate sensitivecomponents. Accordingly, stimuli application must becarefully considered relative to the functions exercised inthe item under test. The selection of the test set stimuli, ifrequired, must be made at the same time the transducersare selected.7-4.1.3 Testing for Hidden FaultsHidden function faults are failures within the testequipment or condition monitors, which would preventthe detection of a system component or failure for operationreadiness assessment or during a mission. A premissiontest of the monitor must be designed into the testequipment if detection of the system or component failureis critical.7-4.2 PERCENTAGE OF FAILURESDETECTABLEDiagnostic test features must themselves be the subjectto determine that a specified percentage of component orsystem failures can be detected by the techniques and testequipment employed. The basis for this verification is thedocumentation developed during the equipment testabilityanalysis (see par. 7-2.2). The test equipment designerinitially verifies analytically that sufficient failure modeswill be detected by the test features to meet the percentageof troubleshooting requirements. The actual testing tosubstantiate this analysis usually is done on a randomsampling basis for complex equipment. i.e., a certainfraction of the failure modes are identified randomly.These modes are then simulated in the prime equipmentto determine whether the test features detect and isolatethe source of failure. The results of the test sampling arethen used to accept or reject the hypothesis that the testfeatures meet the specified requirements. Caution must beexercised, however, because the laboratory results maynot be consistent with those experienced in the field (seeTable 7-1).7-5 DIAGNOSTIC EQUIPMENT DESIGNCONSIDERATIONSDesign considerations for diagnostic equipment are afunction of many parameters:1. Type of equipment to be used and factors leadingto this decision2. Diagnostic software considerations3. Test point identification4. Test sequence considerations5. Selection of transducer or sensor6. Stimuli selection7. Output format8. Diagram use.Each of the parameters together with their applicationis discussed in the pars, 7-5.3 through 7-5.10.7-10
Downloaded from http://www.everyspec.com on 2011-10-29T14:56:01.7-5.1 CLASSIFICATIONIn this discussion it is assumed that the maintainabilityplan has dictated that the test procedures and equipmentmust be automated, i.e., manual troubleshooting employingoff-the-shelf devices will not suffice. Thus diagnosticequipment can be classified as generalized or specialized.The decision depends on whether the test equipment willserve several subsystems of the same systems or othersimilar systems or whether it will be unique to the subsystemor system under design. If either a generalized orspecialized test system will satisfy the time restraints ofthe maintainability plan, then the decision should be measuredin relationship to1. Newly furnished capabilities or techniques2. Efficiency of operation3. Effect on operating system(s) serviced, i.e., canthe on-line system be conducted, or must one system waitwhile another is being tested?4. Effect on skill level, number, and type of techniciansrequired5. Total cost implications.DOD-HDBK-791(AM)7-5.2 SNEAK CIRCUITS (Ref. 16)To cope with the challenge of maintaining complexweapon systems, the maintainability engineer is forced torely on the increasing use of BITE and ATE. Their use—since the equipment itself embodies electrical hardware orsoftware systems—may introduce sneak circuits into theprime system or the sneak circuit may be indigenous tothe test equipment. A sneak circuit is an unexpected pathor logic flow within a system which, under certain conditions,can initiate an undesired function or inhibit adesired function. The path may consist of hardware,software, operator actions, or combinations of these factors.Sneak circuits are not the result of hardware failurebut are latent conditions inadvertently designed into thesystem or coded into the software program, which cancause it to malfunction under certain conditions. Sneakcircuit analysis is the analytical technique used to identifysneak circuits in systems.The causes of sneak circuits are system complexity,system changes, and user operations. Hardware complexityresults in numerous interfaces between the prime systemand the BITE or ATE, which may obscure theintended function or produce unintended functions. Theeffects of even a minor wiring or software change to aspecific component may result in undesired system operations.Also a system that is relatively sneak free can bemade to circumvent desired functions or generate undesiredfunctions as a consequence of improper test operationsor procedures, e.g., a test performed out of sequence.The identification of a sneak circuit, however, does notalways indicate an undesirable condition; in fact, somehave been used to accomplish tasks when other circuitryhas failed (Ref. 17). The implications of a sneak circuit,therefore, must be explored and its impact on the circuitfunction determined before any corrective action is taken.Categories of sneak circuits are defined as1. Sneak Paths. Unexpected paths along which current,energy, or logical sequence flow in an unintendeddirection or to an unintended destination2. Sneak Timing. A situation in which events occurin an unexpected or conflicting sequence3. Sneak Indication. An ambiguous or false displayof system operating condition that may result in an operator’staking an undesired action4. Sneak Label. incorrectly labeled system function—e.g., system inputs, controls, or displays that may causean operator or technician to apply an incorrect stimulus.Sneak circuit analysis requires a lot of computer timeand may be expensive. Therefore, the analysis should beconsidered for components—hardware and software—that are critical to mission success and safety. However, iftest equipment is yielding spurious information, i.e., anexcessive number of “cannot duplicates”, “retest OKs”,bench-checked serviceable, or false alarms—sneak circuitanalysis may reveal the source or cause of theanomalies.7-5.3 BUILT-IN TEST EQUIPMENT VERSUSAUTOMATIC TEST EQUIPMENTBITE is built-in, integral with the subsystem or systemto be tested. ATE is not integral with the subsystem orsystem; ATE must be moved to the test item or the itemmust be moved to the ATE. Some of the factors that mustbe considered in choosing between BITE and ATE are1. Technicians’ Skill Level. In general, a technicianwill require a higher skill level to use ATE because detectionis not usually of the simple “go” or “no go” type. Thetechnician will have to hook up the ATE, apply selectivestimuli, and perhaps interpret readouts.2. Physical Factors. To minimize weight and sizerequirements, it may be necessary to employ ATE; this isparticularly important in airborne systems. However, restrictedaccess to the item to be tested would favor BITequipment.3. Maintainability and Reliability. These attributesof the system must be considered in the decision process.BITE could impose another element into an alreadyoverly complex system and thus detract from the overallreliability of the system. Since it is present in the system,BITE may increase the maintenance load on the system—downtime for BITE maintenance reduces the availabilityof the system. On the other hand, since specific BITE willbe less complex than multipurpose ATE, BITE shouldrequire less maintenance and be more reliable.4. Logistics. The numerous pieces of BITE—with itsunique purpose and function—will add to the number ofrepair parts required together with an increased need foroperational and maintenance manuals. A centralizedATE that consolidates many test functions reduces thelogistic burden.5. Frequency of Application. If a test must be conductedfrequently to determine the operational readinessor status of a system—particularly if the test must beconducted on-line—BITE would be the likely candidate.The immediate nonavailability, movement, and the requiredhookup could render ATE unmanageable.7-11
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