The purpose of this AIR is to provide aircraft ECS engineers and their managers with information about a generalized computer program method for calculating steady state thermodynamic performance of an aircraft environmental control system. Many current computer programs are company proprietary or tend to be specifically oriented to a particular ECS configuration or system. This AIR describes a computer program general enough to allow analysis of essentially all types of environmental control systems. It is available to companies in the environmental control system industry. The general nature of the computer solution is useful for sizing activities and trade study during a preliminary design phase, as well as for detailed system performance analysis. The intent of this AIR is improved communication between aircraft user, aircraft manufacturer, and ECS equipment suppliers by use of an industry-wide computer program. This AIR describes the Advanced Environmental Control System Computer Program, and suggests further improvements to expand its capabilities.