Fail-Safe Design
Section 06: System Safety & Functional Safety
Definition
A design philosophy in which the occurrence of any single failure, or likely combination of failures, results in a safe condition or allows continued safe flight and landing. In a fail-safe design, failures are accommodated through a combination of redundancy, designed failure paths, detectability, and crew procedures. The fail-safe concept was the original safety philosophy for transport aircraft structure (fail-safe structure permits damage or partial failure without catastrophic structural failure) and has been extended to systems design. Under 14 CFR/CS 25.1309, the fail-safe design concept requires that no single failure results in a catastrophic failure condition.
Where This Shows Up
Fail-safe is the foundational safety philosophy for transport category aircraft. It does not mean failures cannot occur; rather, it means the design accommodates failures safely. For structures, this includes damage-tolerant design with multiple load paths. For systems, it includes redundancy with failure detection and the ability to continue safe flight and landing. Fail-safe differs from fail-operational and fail-passive in the specific behavior after failure.
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Primary Sources
Requires that the airplane and its systems be designed to a fail-safe design concept.
Elaborates on the fail-safe design concept and its application to systems.
Related Terms
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