MAINTAINABILITY DESIGN TECHNIQUES METRIC - AcqNotes.com

Downloaded from http://www.everyspec.com on 2011-10-29T14:56:01.From Example 7-1 it is apparent that even with a highAFIC of 81%, the non-BITE-oriented corrective maintenancerepresents 60/140 = 43%—i.e., (T BN) total/ T total—ofthe total anticipated corrective maintenance hours. Theexample did not consider the impact of any scheduledmaintenance since it is not associated with BITE. AlsoExample 7-1 has been greatly simplified in that it ignoredBITE-addressable errors such as cable connectors. In thisplanning type of example, it was assumed that the BITAFIC will be 81%. If, in fact, the AFIC is 81%, then80/140 = 57%—i.e., (TLRU)total/Ttotal —of the maintenanceeffort will be oriented toward BITE detected and isolatedfailures. However, if the true AFIC is determined to belower than 81%, it may be necessary to reevaluate theoverall effectiveness of the entire maintenance and logisticprograms as well as total mission effectiveness.7-3 DIAGNOSTICS7-3.1 GENERAL (Ref. 7)Diagnostics refers to the functions performed and thetechniques used in determining and isolating the cause ofmalfunctions in an operating system or in determining itsoperational status. The primary objective of the maintainabilityengineer in the field of diagnostics is an overallreduction of system downtime by providing a strategy forthe rapid location of faults.Observations from various case studies of materiel nowin the inventory reveal that diagnostic system developmentis an immature discipline when compared to reliabilityor maintainability. One of the chief reasons is thatthere are no accepted definitions of requirements that aredirectly understandable and that can be related to fieldperformance. Diagnostic tests also are less mature—although fault insertion tests can be diagnosed in thelaboratory, they are poor predictors of field performance.A comparison of results from laboratory fault insertiontests and field operational tests for the F-16, APG-66 FireControl Radar, is shown in Table 7-1 (Ref. 9). Table 7-1indicates that a successful demonstration in a laboratorysetting is no guarantee of success in the field. Demonstrationby fault insertions is necessary, but not sufficient, tovalidate a diagnostic design.DOD-HDBK-791(AM)Lack of knowledge in the diagnostic area presents asignificant challenge to the developer to improve thediagnostics of current weapon systems and acquisitionmethods for improved diagnostics in future weapon systems.Where a mature diagnostic capability has beenachieved in systems that required sophisticated techniques,it is obvious that success was no accident; successevolved as the direct result of a carefully defined processconsisting of the following elements:1. Planning2. Management strategy3. Motivation4. Technical activity and innovation5. Adequate funding that spanned system acquisitionfrom the definition of requirements through deployment.As indicated in par. 7-1, the user’s requirements shouldaddress diagnostic capability in the larger context of theoperational mission and environment as well as the supportconstraints of manpower, skill-level maintenanceconcept, deployment, and logistic burden. The requirements,constraints, environment, and economics shouldthen drive the architecture of the system with diagnosticsbeing one of the fundamental characteristics.7-3.2 DESIGN NEEDS (Ref. 9)In the area of design of diagnostic systems, case studieshave identified the following design needs:1. Strategies to minimize “cannot duplicate”, “benchcheckdserviceable", "retest OK”, and false alarm conditions2. Techniques to maximize vertical testability. i.e.,from system, to subsystem, to subassembly, to part level3. Flexible diagnostic systems that will permitchanges to be incorporated in diagnostic algorithms, displays,and tolerances with minimal hardware impact4. Fault-free software development techniques5. Techniques to enable more concurrent hardwareand software development, and earlier integration of thetwo6. Computer-aided engineering techniques forenhancing design for testability. Some techniques such asLOGMOD and STAMP may already be available tomeet this need.7. Experienced engineers who understand how toachieve good diagnostic techniques8. Tools for predicting, measuring, and managingdiagnostic designs9. Better design practices such as the control of timingmargins in high-speed circuits and systems.7-3.3 DEVELOPMENT ANDDEMONSTRATION TESTING (Ref. 9)Case studies have shown that improvements in developmentand demonstration testing will aid in diagnosticdevelopment. The following guidelines have been suggestedby the experts:1. Use reliability and other test events as opportunitiesto discover problems with BITE performance. Envi-7-5