Preliminary System Safety Assessment
PSSASection 06: System Safety & Functional Safety
Definition
A systematic evaluation of a proposed system architecture to determine how failures within the architecture could lead to the failure conditions identified in the FHA, and whether the proposed architecture can meet the safety objectives. The PSSA examines the system design at an early stage using qualitative and preliminary quantitative methods, such as preliminary fault trees, dependency diagrams, and Markov models. The PSSA establishes safety requirements for the system elements — including hardware, software, and human factors — that must be met to achieve the system-level safety objectives. These derived safety requirements are then allocated to lower-level items.
Where This Shows Up
The PSSA bridges the gap between the FHA (which identifies what must be safe) and the SSA (which demonstrates that the final design is safe). The PSSA is iterative: as the design matures, the PSSA is refined to reflect design decisions. Key outputs include derived safety requirements (e.g., requirements for independence, redundancy, monitoring, and dissimilarity) and preliminary quantitative assessments showing whether the proposed architecture is expected to meet probability targets.
Primary Sources
Artifacts Produced
Document containing the preliminary safety analysis of the proposed system architecture, including preliminary fault trees, safety requirement allocations, and identification of common cause concerns.
Related Terms
Need help navigating certification?
Understanding the terminology is the first step. If you need expert guidance on DO-178C, DO-254, ARP4754B, or any aspect of FAA, EASA, or TCCA certification, our team is here to help.