AC 25.1309-1
AC 25.1309–1 is an FAA Advisory Circular (AC) (Subject: System Design and Analysis) that describes acceptable means for showing compliance with the airworthiness requirements of § 25.1309 of the Federal Aviation Regulations. The present unreleased but working draft of AC 25.1309–1 is the Aviation Rulemaking Advisory Committee recommended revision B-Arsenal Draft (2002); the present released version is A (1988). The FAA and EASA have accepted proposals by type certificate applicants to use the Arsenal Draft on recent development programs.
AC 25.1309–1 establishes the principle that the more severe the hazard resulting from a system or equipment failure, the less likely that failure must be. Failures that are catastrophic must be extremely improbable.
The airworthiness requirements for transport category airplanes are contained in Title 14, Code of Federal Regulations (14 CFR) part 25 (commonly referred to as part 25 of the Federal Aviation Regulations (FAR)). Manufacturers of transport category airplanes must show that each airplane they produce of a given type design complies with the relevant standards of part 25.
AC 25.1309–1 describes acceptable means for showing compliance with those airworthiness requirements. It recognizes Aerospace Recommended Practices ARP4754 and ARP4761 (or their successors) as such means:
AC 25.1309–1 provides background for important concepts and issues within airplane system design and analysis.
The circular provides a rationale for the upper limit for the Average Probability per Flight Hour for Catastrophic Failure Conditions of 1 x 10−9 or "Extremely Improbable". Failure Conditions having less severe effects could be relatively more likely to occur; that is, an inverse relationship between severity and likelihood.
This AC presents the FAA Fail-Safe Design Concept, which applies basic objectives pertaining to failures:
The AC lists design principles or techniques used to ensure a safe design. Usually, a combination of at least two safe design techniques are needed to provide a fail-safe design; i.e. to ensure that Major Failure Conditions are Remote, Hazardous Failure Conditions are Extremely Remote, and Catastrophic Failure Conditions are Extremely Improbable.
With emergence of highly integrated systems that perform complex and interrelated functions, particularly through the use of electronic technology and software-based techniques [e.g., Integrated Modular Avionics (IMA) ], concerns arose that traditionally quantitative functional-level design and analysis techniques previously applied to simpler systems were no longer adequate. As such the AC includes expanded, methodical approaches, both qualitative and quantitative, that consider the integration of the "whole airplane and its systems".
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