"Many different computer programs have been developed to determine performance capabilities of aircraft environmental control systems@ and to calculate size and weight tradeoffs during preliminary design. Many of these computer programs are limited in scope to a particular arrangement of components for a specific application. General techniques@ providing flexibility to handle varied types of ECS configurations and different requirements (i.e.@ during conceptual or preliminary design@ development@ testing@ production@ and operation) are designated ""company proprietary"" and are not available for industry-wide use. This document describes capabilities@ limitations@ and potentials of a particular computer program which provides a general ECS analysis capability@ and is available for use in industry. This program@ named AECS*@ was developed under the sponsorship of the U.S. Air Force Flight Dynamics Laboratory (Reference 1 and 2). The basic operating modes and organizations of the program are described. Methods of problem definition@ data inputs and outputs@ control options@ and computer system are discussed. The program's key capabilities and limitations@ and recommendations for future improvements in AECS to facilitate its use as an industry-wide acceptable method for analysis@ are also discussed. PURPOSE 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 of 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. * Initially IECS"