MIL-STD-1798C
the redundant parts to achieve an improbable level for safety-critical functional failure per AWB-013 and NAVAIRINST 5000.21B. The value shall be provided to the Reliability Analysis Team who will monitor actual reliability to verify redundancy controls rate of functional failure to an improbable level.
3. Latent (undetected) failures.
4. The resulting effects on the airplane and occupants, considering the stage of flight and operating conditions, and action required, and the capability of detecting faults.
e. Critical environmental conditions must be considered to depict compliance with failure conditions described in subparagraphs a. and b., immediately above.
5.2.12 Design criteria.
The contractor shall translate the system requirements into specific design criteria to be used for material selection, equipment sizing, design, analysis, and test. The objective is to ensure criteria which reflect the planned usage of the systems are applied to the development and verification process so that specific performance, operational, and maintenance/support requirements can be met. The task of developing design criteria begins as early as is practical
in the development cycle. The rationale for selecting design criteria must provide a justifiable basis for meeting safety, design performance and service life; while also meeting cost and
supportability requirements. Specific criteria shall be developed to support functional
performance, durability, damage tolerance, strength, vibration/dynamic response, maintenance, integrity management, and other specified requirements. Criteria shall be established to ensure
that safety-critical components can safely withstand undetected flaws, corrosion, impact
damage, and other types of damage throughout their design service life. Design criteria to prevent in-service failures of mission- and durability-critical parts should be implemented when cost effective. Figure 2 provides a summary of design approaches available to prevent in- service failures.
5.2.12.1 Damage tolerance design concepts.
Damage tolerance criteria shall be applied, where application is practical, to all safety-critical MECSIP components. Damage tolerance criteria shall also be considered for all mission- and durability-critical components. When a damage tolerance design approach is utilized, criteria shall establish a minimum critical flaw size for those locations which are difficult to inspect; such that, no inspection will be necessary in these areas for the life of the system. Subsystem damage tolerance designs shall be categorized into one of the general design concepts which follow:
a. fail-safe concepts where the required residual strength of the remaining intact component structure shall be maintained for a period of unrepaired usage through the use of multiple load paths or damage arrest features after a failure or partial failure. The period of unrepaired usage necessary to achieve fail-safety must be long enough to ensure the failure or partial failure will be detected by the inspection method selected
and repaired prior to the failure of the remaining intact structure.
b. slow damage growth concepts where flaws, defects, or other damage are not allowed to attain the size required for unstable, rapid propagation to failure. This concept must be used in single-load-path and non-fail-safe multiple load path components. No significant growth which results from manufacturing defects or from damage due to high-energy impact shall be allowed for composite components.
c. leak before burst concept where fracture mechanics analysis is used to confirm that a through crack in a fluid container will leak fluid before burst by demonstrating tolerance
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