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Downloaded from http://www.everyspec.com on 2011-10-29T14:56:01.DOD-HDBK-791(AM)repairability as required by the maintainability concept, ments are not sacrificed for expediency or budgetarymanpower, and skill restraints?considerations.5. Are design trade-offs considered with maintain- The means for implementing these design features areability requirements? Exercise care so that the require- discussed in the chapters that follow.REFERENCES1. MIL-STD-721 C, Definitions of Effectiveness Terms 6. T. F. Pleska, F. L. Jew, and J. E. Angus, MaintainforReliability, Maintainability, Human Factors, and ability Prediction and Analysis Report, Hughes Air-Safety, 12 June 1981.craft Company Report, RADC-TR-78-169, Rome2. MIL-HDBK-472, Maintainability Prediction, 12 Air Development Center, Air Force Systems Com-January 1984. mand, Griffiss Air Force Base, NY, July 1978.3. AMCP 706-133, Engineering Design Handbook, 7. DOD Directive No. 5000.1, Major System Acquisi-Maintainability Engineering Theory and Practice, tions, 29 March 1981.January 1976.8. AR 702-3, Army Materiel Systems Reliability, A vail-4. DOD Directive No. 5000.40, Reliability and Main- ability, and Maintainability (RAM), 1 May 1982.trainability, 24 February 1982.9. MIL- STD-470A, Maintainability Program Require-5. MlL-STD-471 A, Maintainability Demonstration, 8 ments (For Systems and Equipments), 3 JanuaryDecember 1978. 1983.BIBLIOGRAPHYMIL-STD-2165, Testability Program for Electronic Sys- MIL-STD-2077, Test Program Sets, General Requiretemsand Equipments, 26 January 1985. ments for, 9 March 1978.1-8
Downloaded from http://www.everyspec.com on 2011-10-29T14:56:01.DOD-HDBK-791(AM)CHAPTER 2SIMPLIFICATIONSimplification is defined and the importance of it as a factor in maintainability is emphasized. Fivetechniques for achieving simplification are discussed:(1) Coordination of Equipment and Job Design, (2) PartReduction, (3) Function Consolidation, (4) Access Improvement, and(5) Maintenance Procedure Streamlining.Part reduction and function consolidation are described as functions of system-level trade-offs and valueengineering. Features to enhance an approved support plan are presented. Examples of complex modulesredesigned to achieve simplicity are included, and a simplification design checklist is provided.2-1 INTRODUCTIONGeneral Maxwell D. Taylor, while chairman of theJoint Chiefs of Staff, commented on the modern Army asfollows: “our Army must be able to disperse and hide,and converge and fight. It must be able to shoot, move,and communicate. If we are to attain this concept ofmobility, we must reduce our requirements for logisticalsupport.”. Despite the fact that this statement was made20 yr ago, it is still relevant today the “shoot and scoot”tactic still applies.For many years roughly 11 cents of every dollar in thedefense budget was expended for maintenance; the supportcosts of some systems exceeded their acquisitioncosts many fold, and the number of maintenance techniciansrequired for support exceeded the number of operatingpersonnel. The continued development and deploymentof highly complex systems such as guided missiles,communication networks, computers, and reconnaissancedevices—require a mass of on-site support equipmentwhich in turn demands massive logistic support.The common denominator of all support costs appearsto be complexity --not solely system complexity but alsocomplexity of operation and maintenance. If this is true,then design for both simplicity of operation and maintenanceis the area that offers the most promise for reductionof support costs. A certain amount of equipmentcomplexity is necessary, and the designer should try toachieve ease of maintainability in spite of complexity. Themajor support costs could be greatly reduced if thedesigner working in conjunction with the maintainabilityengineer were constantly aware of the limitations of operatorand maintenance personnel and of the adverse environmentin which such personnel must perform - personalfatigue, blackout conditions, cold, rain, mud, dust, etc. Amaintainability feature that reduces the number of maintenancepersonnel, however, does not tell the completestory. This feature may also result in the elimination of anitem of test equipment which in turn eliminates therequirement for manuals to operate and repair the testitem and for backup facilities to repair the item—i.e., asnowball effect. This same scenario could be applied tothe elimination of a part.Simplification is easy to mandate but is probably themost difficult maintainability characteristic to achieve.The rewards are great because the results are a significantdriving factor in the reduction of life cycle costs. Simplicityis worth the effort invested to achieve it and should bethe constant goal of every design engineer.2-2 DESIGN TECHNIQUESThere is a general tendency on the part of designers ofequipment to produce an overly complex product. Inmany cases the equipment uses too many parts, has tooclose operating tolerances, is too expensive to build, andis difficult and expensive to maintain. The resolution ofthese factors, to develop a simple design, is the result ofcompromises and trade-offs among the user, designer,and maintainability engineer but never at the expense ofsystem availability or effectiveness. For example, if for agiven system the desired degree of availability cannoteconomically be achieved by the incorporation of reliabilityin its design, then it can be achieved only by increasedemphasis on maintainability characteristics that willreduce downtime. Maintainability, however, should notbe used as a crutch for reliability. Trade-offs in maintainabilityshould encompass reliability, support, cost, andstate-of-the-art design for testability using built-in testequipment and automatic test equipment.Design techniques for achieving simplification include1. Coordination of equipment and job design2. Reduction in number of parts. 3.Value engineering4. Consolidation of functions5. Improved access to parts6. Streamlined maintenance procedures7. Software maintenance.In applying these techniques, the necessity for trade-offsand compromises previously discussed must be considered.Equally important is the impact of these techniqueson the logistical support plan. The resultant equipmentdesign should represent the simplest configuration possibleconsistent with functional requirements and expectedservice and performance conditions.Each of these techniques is discussed in the paragraphsthat follow.2-1
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