ARP4761, Guidelines for Conducting the Safety Assessment Process on Civil Aircraft, Systems, and Equipment is an Aerospace Recommended Practice from SAE International. In conjunction with ARP4754, ARP4761 is used to demonstrate compliance with 14 CFR 25.1309 in the U.S. Federal Aviation Administration (FAA) airworthiness regulations for transport category aircraft, and also harmonized international airworthiness regulations such as European Aviation Safety Agency (EASA) CS–25.1309.

This Recommended Practice defines a process for using common modeling techniques to assess the safety of a system being put together. The first 30 pages of the document covers that process. The next 140 pages give an overview of the modeling techniques and how they should be applied. The last 160 pages give an example of the process in action.

Some of the methods covered:

The general flow of the safety life cycle under ARP4761 is:

The Functional Safety process is focused on identifying functional failure conditions leading to hazards. Functional Hazard Analyses / Assessments are central to determining hazards. FHA is performed early in aircraft design, first as an Aircraft Functional Hazard Analysis (AFHA) and then as a System Functional Hazard Analysis (SFHA). Using qualitative assessment, aircraft functions and subsequently aircraft system functions are systematically analyzed for failure conditions, and each failure condition is assigned a hazard classification. Hazard classifications are closely related to Development Assurance Levels (DALs) and are aligned between ARP4761 and related aviation safety documents such as ARP4754A, 14 CFR 25.1309, and Radio Technical Commission for Aeronautics (RTCA) standards DO-254 and DO-178B.

FHA results are normally shown in spreadsheet form, with columns identifying function, failure condition, phase of flight, effect, hazard classification, DAL, means of detection, aircrew response, and related information. Each hazard is assigned a unique identifier that is tracked throughout the entire safety life cycle. One approach is to identify systems by their ATA system codes and the corresponding hazards by derivative identifiers. For example, the thrust reverser system could be identified by its ATA code 78-30. Untimely deployment of thrust reverser would be a hazard, which could be assigned an identifier based on ATA code 78-30.

FHA results are coordinated with the system design process as aircraft functions are allocated to aircraft systems. The FHA also feeds into the PSSA, which is prepared while the system architecture is developed.

The PSSA may contain qualitative FTA, which can be used to identify systems requiring redundancy so that catastrophic events do not result from a single failure (or dual failure where one is latent). A fault tree is prepared for each SFHA hazard rated hazardous or catastrophic. Fault trees may be performed for major hazards if warranted. DALs and specific safety design requirements are imposed on the subsystems. The safety design requirements are captured and traced. These may include preventive or mitigation strategies selected for particular subsystems. The PSSA and CCA generate separation requirements to identify and eliminate common mode failures. Subsystem failure rate budgets are assigned so that hazard probability limits can be met.