Preliminary System Safety Assessment

• The PSSA is an iterative analysis that should begin in the conceptual phases of a project, and is initially based on the failure conditions (FC) listed in the system functional hazard assessment (FHA) as well as the basic events shown in the aircraft (or super system) level FTAs.
• Common Cause Analysis (CCA) should be performed during the PSSA to validate.
• The PSSA begins with the allocation of aircraft level functional requirements to the system level.

PSSAs are most often used to:

1. generate a complete list of system level FCs (incl derived safety requirements)
2. generate a list of independence principles to guide the design
3. evaluate design alternatives / approaches from a safety perspective identify architectural mitigation strategies, BIT detection requirements, etc.
4. allocate failure budgets to the subsystems, units & ckts comprising the system establish the safety critical subsystem and item level failure effects provide safety inputs for the SSA and System, H/W & S/W requirements docs, etc
5. show that the system will meet the qualitative & quantitative system safety rqmts.