SAE ARP4754B / ED-79B - Guidelines for Development of Civil Aircraft and Systems 5.2.3.2.2: 5.2.3.2.2 FDAL and IDAL assignment process

Assignment is top-down: from the FHA severity, the PASA or PSSA assigns the top-level FDAL, then sub-function FDALs after decomposition, then IDALs for the items the sub-functions are allocated to; reused, previously certificated functions and items have their FDAL and IDAL shown to satisfy the general principles. If the architecture contains development errors through two or more independent members, levels may reflect that containment, with members found within the FFS by the safety assessment and their independence assessed in common mode analysis; independence among FFS members is substantiated at the FDAL of the top-level failure condition, and members may receive lower FDALs only if functional independence holds. Interactions of systems forming an aircraft function are assessed at the aircraft function FDAL. The process is repeated for every top-level failure condition of each function and the most stringent result assigned; the applicant chooses the option within the general principles. IDAL assignment always follows FDAL assignment, consistently with it, and becomes the input to DO-178C or DO-254; IDAL options may be used only where the FFS has item development independence, and the final combination always satisfies the general principles and the cases in ARP4761A/ED-135.