AC 25.1309-1
AC 25.1309–1 is an FAA Advisory Circular (AC) (Subject: System Design and Analysis) that identifies acceptable means for showing compliance with the airworthiness requirements of § 25.1309 of the Federal Aviation Regulations, which requires that civil aviation equipment, systems, and installations "perform their intended function under foreseeable operating conditions." The present Revision B was released in August 2024. 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.