Derived Safety Requirements

Section 06: System Safety & Functional Safety

Definition

Requirements that are generated through the safety assessment process (PSSA, SSA) rather than being directly traceable to a higher-level requirement or regulation. Derived safety requirements emerge from the architecture and implementation decisions made to achieve safety objectives. Examples include requirements for failure monitoring (to detect latent failures), requirements for dissimilarity between redundant channels, independence requirements for power supplies to redundant systems, exposure time limits for maintenance intervals, and requirements for crew annunciation of degraded states.

Where This Shows Up

Derived safety requirements are a critical output of the PSSA. When a fault tree shows that meeting a safety objective depends on a specific architectural feature (e.g., independence between channels), that dependency is captured as a derived safety requirement. These requirements must be fed back into the system, hardware, and software requirements to ensure they are implemented and verified. Failure to capture and implement derived safety requirements is a common certification finding.

Primary Sources

SAE ARP4754B — Development of Civil Aircraft and Systems

Defines derived safety requirements and the process for their identification, allocation, and verification.

SAE ARP4761A — Safety Assessment Guidelines

Describes how derived safety requirements emerge from the safety assessment process.

Related Terms

Preliminary System Safety Assessment (PSSA)System Safety Assessment (SSA)Safety Objectives and Quantitative Probability TargetsSAE ARP4754B — Guidelines for Development of Civil Aircraft and Systems (ARP4754B)

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