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)TABLE 7-1. TYPICAL FAULT INSERTION TESTRESULTS VERSUS FIELD RESULTS (Ref. 9)ronmental testing may be particularly useful for discoveringfalse alarm indications such as induced intermittentand transients.2. Increase the number of repair parts and the timebudgeted in the laboratory to investigate diagnosticanomalies.3. Expand the set of faults inserted.4. Increase the allowable cost of demonstrations toinclude repair costs. This action will permit the insertionof a better cross section of faults.5. Develop a library of computer simulation modelsto test BIT (hardware and software).6. Adopt comparability analysis as a useful tool foridentifying a realistic set of faults for insertion.7. Develop improved demonstration techniques topredict diagnostic performance in the field.Field maturation is essential to achieve inherent diagnosticpotential. When a system is first fielded, it is commonto find that not all the hardware and software provisionsof the diagnostics have been fully implemented. Inaddition, the operational use patterns and the environmentproduce new failure modes and diagnostic indications.Unfortunately, these new indications which theBIT equipment may not deal with properly- are resolvedby the judgment of operators and maintainers (who maynot have been trained to deal with them) with the aid oftechnical data that may not have been developed toaddress them. Thus a structured diagnostic maturationeffort must be resorted to for the purpose of bringing thediagnostic capability to its full potential. The key featuresof successful programs should be used in structuringfuture diagnostic maturation efforts for complex equipment.Unless this is done, diagnostics, and BITE in particular.could be the weak link in the support chain (Ref.l0).7-3.47-3.4.1DIAGNOSTICS FOR MECHANICALSYSTEMSCondition Monitoring of MechanicalSystemsDiagnostics, or “condition monitoring of mechanicalsystems”, combines the measurement of performance andthe detection of damage with the possibilities of prognosticsto eliminate unnecessary and premature removal ofsystem components. A proven technology base exists anda variety of successful demonstrations have been conductedthat indicate reliability and readiness improvementscan be obtained by application of this technologyto current or future systems. Propulsion, transmission.and structural components are significant contributors toperformance and readiness of weapon systems and platforms,and they provide the focal points for the study ofcondition monitoring of mechanical systems (Ref. 11).Unlike some of the other technologies, the issues in thecondition monitoring of mechanical systems center aroundthe development of the following specific conditionmonitoringtechnologies:7-6
Downloaded from http://www.everyspec.com on 2011-10-29T14:56:01.1. Techniques for the electrostatic monitoring ofengines2. Ultrasonic wear particle sensors3. Advanced engine diagnostic technology4. Integrated transmission monitoring5. Structural testing6. Computer program techniques for likelinessanalysis (see par. 7-6.1).These technologies need refinement, demonstration, andintegration with automatic data processing equipment.Preliminary demonstrations have shown that each of thelisted technologies has the potential to correct some of thedefects in existing condition-monitoring technology. Forexample, the late consideration of condition monitoringin mechanical systems results in additional costs, weightpenalties, and unnecessary false alarms. If properlyincorporated into the initial design, these devices canmature with the system design, will minimize the weightpenalty, and can reduce costs of the initial testing of thesubsystem prior to production and fielding (Ref. 11).Condition-monitoring techniques are being successfullyapplied to the T700 BLACK HAWK engine in thefactory and in the field. Of particular value has been theengine history recorder for comparing the relative severityof field testing of engine operation with specificationendurance test cycles. The engine chip detector hasproven to be an effective means of detecting incipientoil-wetted part failures. Borescope inspection has alsoproven to be useful and easy to do in the factory and onthe wing. Ground use of the diagnostic connector fortroubleshooting has been effective even though the currentlyavailable test box is only a nonpowered resistancechecker. Condition monitoring coupled with line-replaceableunit installation and rigging with no requiredadjustments contribute to the overall mission readiness ofthe T700 engine. As a result of the demonstrated reliabilityof the engine. only a 10-h inspection check, which isaccomplished in 3 rein, and a periodic inspection performedat 500 flight-hour intervals, which can be performedon-wing in 1 h, are required (Ref. 12).7-3.4.2 Nondestructive EvaluationNondestructive evaluation methods have been valuabletools for maintenance for decades. During the past severalyears, however, work with these methods has shownoutstanding promise for reducing maintenance costs,maintenance time, and manpower requirements and foreliminating operational hazards. A body of knowledgenow exists of the benefits obtained from nondestructivetesting technologies that could be successfully applied totanks and rotary-wing aircraft. Examples of these technologiesare (Ref. 11)1. Automated ultrasonic inspection techniques forcomposites2. Automated nondestructive testing of software3. X-ray diffraction techniques for measuring residualstresses on torsion bars and track pins4. Advanced techniques for weld monitoring.DOD-HDBK-791(AM)7-3.5 DIAGNOSTIC TECHNIQUES7-3.5.1 ImportanceIncorrect diagnoses of failures can result in the unnecessaryremoval of serviceable parts and repetitive maintenanceactions, both of which reduce the efficiency ofmaintenance actions by increasing costs and decreasingsystem availability. For example (Ref. 13), repetitivemaintenance actions on helicopters were found to occurat a rate of 0.32 per flight hour and “diagnosticsincludingtest equipment, troubleshooting, and standardmaintenance practices–-were identified as causing over50% of all repetitive maintenance actions”. If automaticmonitoring and/or alarms had been provided instead ofrelying on operator sensing, it is estimated that equipmentnonavailability due to maintenance could have beenreduced by as much as 25% per action. Obviously, thelessons learned were introduced into the design of theT700 BLACK HAWK engine (see par. 7-3.4.1). Althoughthese data are relative to helicopter maintenance, it isreasonable to assume similar data exist for other materielcategories and thus illustrate the effectiveness of improveddiagnostic techniques.The time required to locate a fault is a function oftechniques such as integrated performance monitoringand maintenance tests. On-line monitoring of performanceis designed into most Army equipment—forexample, an automotive vehicle in which the temperatureof the engine coolant, oil pressure, fuel level, and batteryor alternator output are monitored constantly. Theautomotive vehicle also serves as an example for theconduct of familiar maintenance tests such as checkingfuel, oil, coolant, battery, and other fluid levels beforedriving the vehicle. These simple performance indicationsand maintenance procedures usually localize a fault toone or several components. Diagnostic techniques takeadvantage of the gross localization aids to isolate thecomponent that requires repair or replacement.Trying to locate a fault without proper diagnostic techniquesusually requires more time than any other task inthe maintenance cycle. This is because trial-and-errortroubleshooting often results in an erroneous outcome;therefore, the process must be prolonged or delayed andrepeated. A simple illustration of this fact is the costassociated with the repair of a TV set—unless the picturetube is replaced, 90% of the charge is for labor to determinethe faulty component. Accurate automated maintenanceprovisions can significantly reduce the frequency oferrors through their consistent logic and their avoidanceof human volition and training problems. Artificial intelligenceand expert systems (see par. 7-3.5.3) can contributeto automated diagnostic processes.7-3.5.2 Types of Diagnostic TechniquesAll diagnostic techniques can essentially be dividedinto three broad categories, i.e., those employing1. Logic flow2. Automatic test equipment7-7
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