MIL-STD-1798C
5.2.12.2 Other design concepts.
For those safety-critical components (and mission-, and durability-critical components determined to be feasible economically to prevent in-service failure) where damage tolerance design concepts do not apply or are not practical, other design approaches are available and shall be utilized. Use guidance provided in AWB-013 and NAVAIRINST 5000.21B to assess the component's rate of failure, the consequence of failure, and determine the hazard risk index (HRI) value. Risks that cannot be acceptably mitigated may be reviewed and accepted by the appropriate risk acceptance authority within the program risk management process. The following other design concepts are available to prevent in-service critical failures:
a. Redundant (dual/multiple) components or duplication of function to provide operational capability (without degradation) upon failure of a single component or part. Failure of a single component or part must be detectable (i.e., system is both fail operational and fail evident. Redundancy also may apply to a component or function that has redundant features, each able to sustain the critical function in event of failure of redundant part. Individual component reliability shall result in an overall rate of loss of aircraft or inadvertant store release in the improbable range per AWB-013 and
NAVAIRINST 5000.21B. When redundancy is used to reduce the risk for loss of a flight- critical function, the individual components providing the redundancy shall be classified as "mission-critical." Emergency systems are those systems that provide a back-up to continue a safety-critical function or provide a means to mitigate failure of a safety- critical system. Emergency systems shall be designated as "safety-critical" and have functional failure frequency not higher than "Remote" per AWB-013 and
NAVAIRINST 5000.21B. Redundant capability is acceptable for use (to maintain safety- critical function) in all cases, including those where damage tolerance is practical.
b. The safe-life design methodology may be used on a limited basis. When used, safe-life methodology shall establish replacement times for specifically-approved components. Damage tolerance evaluations shall be required for all safe-life designed components. These evaluations shall define critical areas, fracture characteristics, stress spectra, maximum probable initial material and/or manufacturing defect sizes, and options for either eliminating defective components or otherwise mitigating threats to structural failure of the component. Such options may include design features, manufacturing processes, or inspections. The damage tolerance evaluation shall establish individual component tracking requirements so that the safe-life component replacement times and any scheduled safety inspections can be adjusted based on actual usage. Use of a
"safe-life" approach for a MECSIP components shall be identified in the MECSIP Master Plan. A Hazard Analysis shall be performed for each safety-critical component designed per Safe Live design criteria (at the planned maximum life) and the resultant HRI accepted at the appropriate level per AWB-013 and NAVAIRINST 5000.21B.
c. Inherently improbable occurrence of failure per AWB-013 and NAVAIRINST 5000.21B.
This concept is most applicable to generic classes of components which are very similar across a wide range of aircraft (including installation loading and environment).
Examples of components for which "Improbable Occurrence" fits best are: fuel lines, fuel line couplings, and wire harnesses. In cases where "improbable occurrence" is used,
the program should still look at potential for induced damage to the safety-critical parts from damage mechanisms such as chaffing. This look can be accomplished through actions such as recurring inspections or other appropriate means. Use of an "inherently
improbable" approach for a MECSIP component shall be identified in the MECSIP Master Plan.
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