Achieving Reliable, Maintainable & Available Systems
Note: The following guidance is based on experience in weapon system acquisition. It is generally applicable to automated information systems as well. Additional guidance, pertaining to automated information systems will be developed for the next Deskbook release.
1.) Reliable, maintainable and available systems are achieved through a disciplined systems engineering approach employing the best design, manufacturing and support practices. In order to achieve the user reliability, maintainability and availability requirements, emphasis should be on:
(a) Understanding the user’s system readiness and mission performance requirements, physical environments (during use, maintenance, storage, transportation, etc.) the resources (people, dollars, etc.) available to support the mission, the risks associated with these requirements, and translating them into system requirements that can be implemented in design and verified;
(b) Managing the contributions to system reliability, maintainability and availability that are made by hardware, software, and human elements of the system;
(c) Preventing design deficiencies (including single point failures), precluding the selection of unsuitable parts and materials, and minimizing the effects of variability in the manufacturing and support processes; and
(d) Developing robust systems, insensitive to the environments experienced throughout the system’s life cycle and capable of being repaired under adverse or challenging conditions.
2.) Reliability, maintainability and availability design analyses should be part of an iterative process of continually assessing and improving the design. A design reference mission profile should be developed that includes functional and environmental profiles that:
(a) Define the boundaries of the performance envelope,
(b) Provide the timelines (environmental conditions and applied or induced stresses over time) typical of operations within the envelope, and
(c) Identify all constraints (including conditions of storage, maintenance, transportation, and operational use), where appropriate.
3.) Reliability, maintainability and availability objectives should be translated into quantifiable and verifiable contractual terms and allocated through the system design hierarchy.
(a) Contractual requirements should be traceable to operational requirements and capable of verification.
(b) Estimated or measured reliability, maintainability and availability characteristics should be used to evaluate the design.
(c) Achievement of contractual requirements should be verified through a combination of engineering analysis and test results. Determination of contractual compliance based on engineering analysis without supporting test data can lead to erroneous conclusions.
4.) Single point failures should be avoided.
(a) If a mission or safety critical single point failure cannot be eliminated through design, the design should be made robust (insensitive to the cause of failure) or redundant.
(b) Fault tree analysis and failure modes, effects, and criticality analysis (FMECA) are tools that should be used to help identify where degradation or failure could compromise the mission or the safety of the operator or maintainer.
Thermal, shock, vibration (including resonant frequency), corrosion, durability, and other analyses or tests have proven beneficial design aids for electronic and mechanical equipment. These analyses and tests should be performed as an integral part of design evolution and validation and not as “after-the-fact” inspections.
Dormant reliability analyses should be done and an aging and surveillance program established for pyrotechnics, explosives, rocket motors, and other items that have shelf-life (dormant reliability) requirements or are susceptible to long term storage degradation.
Systems requiring fault detection and isolation capability should complete a FMECA. The results from the analyses and any lessons learned should be used to develop specific reliability, maintainability and availability design criteria. Prevention and elimination of unverified indications of failure (false alarms, “could not duplicates,” etc.) should be an integral part of the system design process.
The design should be based on established parts selection practices and guidelines. Past component history, physical and environmental stresses, and component criticality should be considered in the part selection process.
Design criteria should specify that maintenance tasks will be performed with a minimum number of common and peculiar tools.
The system should be designed such that it maintains minimum acceptable performance despite variations due to the manufacturing process, life-cycle environment, and component degradation or drift.
Government or contractor furnished or off-the-shelf items should be operationally suitable for their intended use and capable of meeting their allocated requirements.
The reliability, maintainability and availability effort should be closely coordinated with other systems engineering efforts, especially acquisition logistics, safety, quality, producibility, test, and manufacturing.
Battle damage repair techniques should be identified and, if any are required, be developed concurrently with the weapon system design. They should be demonstrated before entering the production, deployment, and operational support phase.
The following sections are intended to provide a representative listing of reliability, maintainability and availability focus areas by acquisition phase.
Phase 0, Concept Exploration
(a) Efforts should focus on developing measurable values for baseline parameters for each system reliability, maintainability and availability objective that applies to each alternative system concept.
(b) Engineering analyses performed to ensure a reliable, maintainable and available system should use operational and support experience with similar systems to help identify and avoid existing shortfall.
(c) A system life profile should be defined to include mission profiles.
(d) Tentative operational objectives should be responsive to documented needs of the mission area but also be realistically achievable in comparison to baseline values.
Phase I, Program Definition and Risk Reduction
(a) Contractor furnished items should be designed to prevent operational reliability, maintainability and availability deficiencies typical of similar field items or the items being replaced.
(b) Government furnished and off-the-shelf commercial items should have met, or should be required to meet, their allocated reliability, maintainability and availability goals for the new system under environmental stresses defined for the new system.
(c) Operating and support concepts should be tailored to prevent, to the extent possible, operational reliability, maintainability and availability deficiencies.
(d) Thresholds and objectives for reliability, maintainability and availability, at the system and critical subsystem levels, should be established at Milestone II. They should be translated into specific values in contracts for both contractor and Government furnished equipment.
Phase II, Engineering and Manufacturing Development
(a) Reliability, maintainability and availability maturation should be assessed and enforced to ensure reliability, maintainability and availability objectives are met well before the production, deployment, and operational support phase.
(b) Design corrections should have been verified under natural and induced environmental conditions no less severe than design requirements.
(i) Improvements in reliability, maintainability and availability resulting from proposed design corrections should not be considered is estimating reliability, maintainability and availability levels unless their effectiveness has been verified or specified provisions have been made to verify their effectiveness.
(ii) The recurrence of failures due to weak parts and workmanship defects should be precluded by specific quality control provisions in the production contracts.
Phase III, Production Fielding/Deployment, and Operational Support
(a) The acquiring agency should continue to correct operational reliability, maintainability and availability deficiencies due to materiel design and quality, to ensure that reliability, maintainability and availability objectives reaffirmed at the production decision are achieved in service.
(b) Responsibility for the correction of operational reliability, maintainability and availability deficiencies caused by operating or support concepts should be clearly defined.