MAINTAINABILITY DESIGN TECHNIQUES METRIC - AcqNotes.com

Downloaded from http://www.everyspec.com on 2011-10-29T14:56:01.DOD-HDBK-791(AM)and durability (RAM-D) of the equipment and the operationalenvironment. The calendar time, i.e., when themaintenance action was performed, makes that actionscheduled or unscheduled.The unscheduled interruption of a planned operation isalways undesirable and usually costly; in the extreme caseit could be catastrophic. Although unreliability is usuallythe primary case of failure and thus governs the frequencywith which maintenance actions are necessary, the ease ofmaintenance and the skill of maintenance personnel governthe duration of the action. The easier it is to maintainan item of equipment, the fewer will be the demands onboth the skill and number of personnel and, in general.the greater the reduction of equipment downtime. Accordingly,since the time required for maintenance actions is afunction of the maintainability characteristics of theequipment, effectiveness of built-in testing and physicaldesign features that affect the speed and ease with whichmaintenance can be performed should be addressed.Design features are discussed in par. 2-5.In addition to physical design features. personnel andhuman factor considerations are of prime importance.These considerations include the experience of the technician,training required. skill level, supervision required,supervision available, techniques used, physical coordinationand strength and number of technicians, andteamwork requirements. Personnel and human factorsare emphasized because the Army---as well as the otherServices—is imposing a strength cap on the number ofmilitary personnel and restricting the availability of fundsfor development and training, and procurement of trainingaids. Additionally, the scenario under which the newArmy counters threats requires the deployment of lightinfantry. Thus the impact on engineering design must bethat of ease of maintenance, adequate man machineinterface, minimal maintenance, and maximum survivability.In no single area of weapon engineering are thepotential rewards as great as those which could beachieved by simplifying the human functions needed tomaintain the weapon system.This brief introduction highlights the distinctionbetween maintainability and maintenance. In summary.maintainability is a design characteristic that makes possiblethe accomplishment of operational objectives withminimal expenditure of support effort and resources andis a prime responsibility of the maintainability engineerworking in cooperation with the designer: maintenance isthe actions necessary for retaining materiel in or restoringit to a serviceable condition.1-3 MEASURES OF MAINTAINABILlTY1-3.1 GENERALIn par. l-1 it was pointed out that the maintainabilitycharacteristic had to be expressed quantitatively to bemeaningful. This characteristic is expressed as the proba-bility that an item will be retained in, or restored to, aspecified condition within a given time period if prescribedprocedures and resources are followed. There areseveral measurable parameters that can be used to quantifythe maintainability characteristic, ease of maintenance.Ease of maintenance characterizes the maintainabilitydesigned into an equipment and can be measuredby the elapsed time in which the maintenance can beperformed. Thus the maintenance time required to correctequipment performance deviations, such as failure ordegradation, is is a good measure of how well the equipmenthas been designed for maintainability.When maintenance time as a design parameter is measured,active time only should be considered. The emphasisis on the word "active” since there are administrativeand logistic delays—e.g., absence of proper instructionsand waiting for a repair part that bear no relationship toequipment design.Active-type maintenance time for corrective maintenanceactions usually consists of three sequential steps,i.e.,1. Time to locate the parts requiring repair2. Time to perform the repair3. Time to verify that the repair has been performedsuccessful).For preventive-type maintenance, the first step if eliminatedbecause the equipment maintenance area ispredetermined.Attributes of the equipment that cause variations inrepair time result from the physical characteristics of thefailed parts, their location and mounting arrangements inthe equipment, and thus their accessibility and replaceability.Variations in the diagnostic time result from troubleshootingprocedures, location of test points and kindof test equipment, and the sequence in which troubleshootingis performed. Also technicians’ skills, theirdegree of familiarity with the equipment, and the environmentin which maintenance is being performed affectboth diagnostic time and repair time. Even identicalmaintenance actions, caused by identical failures in identicalequipment and performed by the same repairman orrepair crew, will have varying maintenance timed. A largenumber of measurements probably would yield a continuousdistribution of maintenance time for this maintenanceaction even if it were performed by the same technicianor the same repair crew. The distribution of maintenancetimes for the same maintentince action with adifferent repair crew would also be different. Fig. l-lillustrates this phenomenon for two different crews.Crew 1 performs the maintenance action with a meantime m 1; Crew 2 takes a consideraibly longer mean timem 2. Also the variance of maintenance time with whichCrew 2 works is substantially greater than that of Crew 1.Assume that both crews performed the identical maintenanceoperation under identical environmental condi-1-2