This document provides processes used to develop civil aircraft and systems. It has been revised to incorporate known errata and specific industry comments necessary to maintain the recommendations consistent with industry state-of-the-art. The guidelines herein are industry best practices for the development of aircraft and of systems. Modern aircraft typically comprise a large integrated environment consisting of multiple systems with significant dependencies and interactions. Frequently portions of these systems are developed by separate individuals, groups, or organizations. These systems require design discipline and systematic development to ensure that safety and operational requirements can be fully realized and substantiated. Adherence to these guidelines is recommended for development of all aircraft systems, especially those that may contribute to failure conditions with the potential to affect safety. Work carried out jointly with SAE S-18