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)diagnosis of equipment malfunctionsb. Physical Requirements. Normal color vision,night vision, near vision, auditory acuity. hand-eye coordination,manual dexterity, and clarity of speechc. Occupational Qualifications. Knowledge of thefunctioning, assembly, testing, and maintenance of a varietyof mechanical, electronic, electrical, hydraulic, andpneumatic components of missile weapon systems. Thesesystems include missile guidance, acquisition radar, targettrack radar, missile track radar, digital and analog computers,and fire distribution systems.9-5.3 HUMAN ERROR9-5.3.1 GeneralHuman error can be defined as any personnel actionthat is inconsistent with behavioral patterns considered tobe normal or any action that differs from prescribedprocedures. Human error includes (Ref. 18)1. Failing to perform a task (omission)2. Incorrectly performing a task3. Performing a task not required4. Performing a task out of sequence5. Failing to perform a task within the allocated time6. Responding inadequately to a contingency.Human errors always have been made and will continueto be made, as stated by the following variations ofMurphy’s Law (“Anything that can go wrong will gowrong.”) (Ref. 18):1. “Any task that can be done incorrectly, no matterhow remote the possibility is, will someday be doneincorrectly. ”2. “No matter how difficult it is to damage equipment,a way will be found to do so.”3. “At some time, instructions will be ignored whenthe most complicated task is being performed. ”The need to avoid human error increases with the size,complexity, and yield of the weapon system. The greaterthe size and complexity, the greater the number of maintenancetasks, and the more chances there are for humanerror. The larger the weapon system yield, the greater theaccident potential in the event of a human error.Complex equipment does not of necessity requiregreater skill to operate nor is it more difficult to service;complex equipment can be designed for simplicity ofoperation and maintenance. However, the more complexthese actions are, the more vulnerable they are to humanerror—particularly when the user is under tension oremotional stress. This can be a critical problem in combator emergency situations. An Army study (Ref. 19) subjectedrecruits to simulated emergencies, such as theincreasing proximity of falling mortar projectiles to theircommand posts, in a manner that they believed the situationsto be real. As many as one third of the new recruitsfled in panic; they did not perform the assigned task thatwould have ended the mortar attack. These studies indicatethe devastating effects that very high stress levels canhave on the performance of even thoroughly trained,reliable personnel.9-5.3.2 CausesRegardless of thorough training and high skill levels, atechnician will make mistakes, and errors frequentlycause equipment malfunction with varying consequences.For example, a driver fails to fill the radiator of a truck,the engine overheats, and the truck stops on the roadinconvenient but not serious. A technician fails to put acotter pin in a castellate nut in the flight control linkage ofan aircraft, control of the plane is lost in flight, the planecrashes, and all aboard are killed very serious.Maintenance requirements are so demanding that theyoften leave no room for human error, yet mistakes will bemade. For example, a report by one of the military servicesrevealed that in a 15-month period errors made inaircraft maintenance contributed to 475 accidents andincidents in flight and ground operations. Ninety-six aircraftwere seriously damaged or destroyed, and 14 liveswere lost (Ref. 14). A study of these accidents revealedthat many of the failures that caused the accidentsoccurred shortly after periodic inspections. The reportconcluded that these human failures were caused by1. Inadequate basic training in the relevant maintenancepractices, policies, and procedures2. Lack of training in maintenance of the types andmodules of equipment being maintained3. Inadequate or improper supervision4. Inadequate inspection.9-5.3.3 Contributing FactorsKnowledge about human error can reduce the probabilityof damaged equipment or personnel injury byimposing human factors constraints on the equipmentdesign. The characteristics that follow contribute tohuman errors and diminish the safety of person machinerelationships (Ref. 7):1. Population Stereotypes. A population stereotypeis “the way most people in the population expect somethingto be”. People expect, for example, that when acontrol is turned clockwise (except for flow controlvalves), the controlled function should increase. and viceversa.2. Performance Requirements in Excess of HumanCapablility for the Full Range of Maintainers. Equipmentdesign that exceeds the physical and psychological limitsof human capability creates a high likelihood of accidents.For example, a design may require pitch or visualdiscrimination beyond the capability of human senses.3. Designs That Promote Fatigue. Any design thatmakes personnel work harder than normally expected islikely to promote fatigue and increase error. For example,inadequate lighting produces eyestrain and fatigue. andexcessive noise in the work environment increases the rateof fatigue.4. Inadequate Facilities or Information. When personnelmust perform tasks in inadequate facilities orwithout proper information, errors are likely to occur.For example, if the tolerances for instrument readings arenot provided, personnel tend to assume tolerances theybelieve to be reasonable. Operator action based on9-14
Downloaded from http://www.everyspec.com on 2011-10-29T14:56:01.DOD-HDBK-791(AM)instrument indication may be too late or too inaccurate tobe of value.5. Unnecessarily Difficult or Unpleasant Tasks.When design results in tasks that are unpleasant or complex,personnel may not devote the proper amount oftime and attention to attaining satisfactory performance.For example, if two adjustments of equipment interrelateso that precise setting and resetting are required to attainthe proper value, personnel making the adjustments aremore likely to stop short of the proper value than if theadjustments were independent. Also tasks that may getthe technician excessively dirty or wet, such as crawlingunder a vehicle in a muddy field, will frequently be“overlooked”.6. Necessarily Dangerous Tasks. Motivationalcharacteristics, rather than performance capabilities, mayintercede when personnel perform dangerous tasks. Forexample, personnel exposed to high voltages during preventivemaintenance operations are less likely to performtasks thoroughly, and as frequently as they would if thedanger did not exist.7. Unpleasant Environments. Proper environmentalconditions must be maintained as much as possible sothat the capability of the body’s regulatory mechanisms tosustain a constant internal environment is not strained.Optimum environmental conditions make minimal demandson the body’s self-regulatory mechanisms. (Thisrelates to performance of unpleasant tasks discussed inItem 5.)To minimize the possibility of human error in accomplishingany procedure involving a nuclear device, theArmy has developed the “two-person concept”. Two ormore persons, each capable of undertaking the prescribedtasks and of detecting incorrect or unauthorized proceduresare involved. One person performs the task whilethe other checks to make sure the task has been performedcorrectly.9-5.3.4 QuantificationThere is a need to understand and predict the contributionof human error to reliability and maintainabilityparameters such as mean time between failure and meantime to repair. Both of these are characteristics of thehardware, but both are also influenced by human performance.In fact, there are some analyses that indicate thatthe majority of system failures are attributable to humansand not to hardware.Much work has been done in human performance reliability(HPR). Yet a basic problem remains, i.e., lack of agood data base of human error and performance (Ref.20). The models available are fairly sophisticated, butconsidering the poor quality of the data input to themodels, the output should be used with caution. There areseveral HPR indices that differ both in scope and in typeof model used. Two types––the technique for humanerror rate (THERP) and the Siegel-Wolf model—will bediscussed.The analytical or simulation THERP can be used topredict the total system or subsystem failure rate resultingfrom human errors (Ref. 21). The THERP methodologybegins with a task analysis that breaks the system into aseries of personnel-equipment functional (PEF) units.The system being analyzed is then described by a functionalflow diagram. Prediction data are assigned to eachPEF. A computer program calculates the reliability oftask accomplishment and performance completion timeand takes into account dependent and redundant relationships.The Siegel-Wolf digital simulation model is orientedtoward the effects of time stress on the successful completionof the task. The model outputs are (Ref. 21)1. Average time expended2. Average peak stress3. Average final stress4. Probability of task success5. Average waiting time6. Sum of subtasks ignored7. Sum of subtasks failed.A good start on a data bank of human error rates hasbeen made by the American Institute of Research (AIR)(Ref. 22). The AIR estimates of error rates are for average,trained military personnel with average motivationwho are operating under normal conditions. However,very little work has been done to quantify the degradationof human performance under operational stress.A second source of data results from an analysis of themaintenance data in the Army maintenance managementsystem (TAMMS). Maintenance actions reported throughTAM MS are analyzed and supplemented with independentjudgments and arrive at quantitative values forhuman error data. Table 9-8 presents the results of a studythat used this method, Human error rate estimates for alarge system were derived from existing data of poorquality by modifying the data with the independentjudgments of human reliability analysts. These judgmentswere made after reviewing information on personnel skilllevels; previous jobs held by these personnel; procedures;and design of the control, displays, and other equipmentread or manipulated by the personnel.To date, the primary source of HPR information issubjective data based on expert opinion or objective datasupplemented as necessary with subjective judgments.Techniques for developing expert estimates include theDelphi technique (Ref. 21).9-6 PHYSICAL FACTORSPhysical factors are relevant in designing for maintainability;it is important that maintenance personnel bephysically capable of performing required tasks. If theycannot, maintenance efficiency suffers.Maintenance proficiency is directly affected by a widevariety of natural and induced environmental elementsthat degrade performance by interfering with a sensoryprocess or by creating physiological or psychologicalstress. Stress is a function of many factors—e.g., fatigue,lack of training, worry, fear—that is exacerbated by theirritants of noise, vibration, and inclement temperature.The stress level rises to a point at which the person’sability to perform in a satisfactory manner declines suddenlyand markedly. Logically, the more severe the stress,9-15
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