MAINTAINABILITY DESIGN TECHNIQUES METRIC - AcqNotes.com

Downloaded from http://www.everyspec.com on 2011-10-29T14:56:01.DOD-HDBK-791(AM)CHAPTER 4ACCESSIBILITYAccessibility, as a significant attribute of maintainability, is discussed in detail. Considerations affecting theaccessibility for three types of maintenance—visual inspection, tools and test equipment, and physical repairand replacement-are presented. Access parameters of size, location, and type are addressed. Specificguidance to access components of major classes of materiel is given. A checklist for maintainability engineers isprovided.4-1 INTRODUCTIONAccessibility is defined as a design feature that affectsthe ease of admission to an area for the performance ofvisual and manipulative maintenance. Thus accessibilityrelates to the hardware configuration, i.e., packaging. Ifan item can be reached quickly requiring the use of a fewstandard tools and only a few simple steps the item isaccessible. However, if an item can be reached quickly butrequires many tools, special tools, many or difficult operationsto reach it, the item is really nit accessible. Also themere fact that a technician can “get at something” doesnot mean that he can maintain it. If an awkward bodyposition must be assumed to reach the item. requiring therepairman to be a contortionist, the item is not consideredaccessible and maintainable because the repairman maynot be able to exert the forces required to make the repair.Furthermore, the disassembly or removal of parts thatinterfere with easy access to a component requiring maintenanceis highly undesirable especially if the maintenanceaction is required at the unit level adequate spaceusually is not available for laying out parts as they areremoved. This increases the possibility that the parts willbecome lost, damaged, or contaminated with dirt andmud, and that further malfunctions will be introducedinto the system.Inaccessibility also embodies psychological effects.Controls: checkpoints; inspection windows; and lubrication,pneumatic, and hydraulic replenishment points aredesigned into the equipment to keep it operating at peakperformance. If it is difficult to access these features, theoperator or repairman will postpone or neglect theseoperations in favor of more convenient tasks.The maintainability engineer, in his desire to maximizeaccessibility, should be aware of at least two limitingconditions, i.e.,1. Level of Accessibility. Accessibility should beavailable only to the throwaway and discard level. If theitem is to be discarded at the unit level, the task is simple.However. it the replaced item is to be returned for repairat the intermediate level, accessibility poses additionalconsiderations.2. Safety. Accessibility must be consistent with theassociated system safety plan required by MIL-STD-882(Ref. 1). The purpose of the safety plan is to minimize oreliminate hazards to which the operator or maintenancepersonnel will be exposed. The safety plan also addresseshazards to the system which may be introduced by maintenanceactions. Maintainability design must interfacewith ground operations, weapon loading. on-loading ofmunitions, refueling, and mission function. For reasonsof safety, maintenance personnel from a psychologicalpoint of view—would rather avoid working in areasexposed to live ordnance and easily ignited gaseousvapors. Also requirements for exposing the technician topotential hazards—moving parts. hot components, buildupof electric charge—should be avoided or minimized.Improved safety associated with maintenance operationswill reduce maintenance time and, consequently, increaseavailability.The variability in the physical size of personnel isdirectly related to accessibility. What is within easy reachof a 6.5-ft individual may be out of reach for a shorterperson. Conversely, the larger individual may experiencedifficulty placing his larger hands around a component,particularly if he is wearing arctic mittens. Whether oflarge or small stature, maintenance personnel must haveaccess to parts and room enough to operate on the parts-–test, adjust, or replace. Accordingly, the qualitativeaspects of human physiology and psychology as theyaffect access times will be addressed. The material in thischapter is closely related to Chapter 9. “Human Factors”,which presents quantitative aspects of anthropometricmeasures, visibility measures, and frequency of measurementerrors. A mutual relationship exists between themaintainability engineer and the human factors engineer:1. The maintainability engineer is responsible forquantifying inherent downtime and inherent availability.2. The human factors engineer provides the quantitativeinformation the maintainability engineer needs toinsure that personnel who fall within the 5th and 95thpercentiles can function within the access design.The final design is a result of cost-effective trade-offs4-1