The Federal Aviation Administration, Office of Environment and Energy (FAA-AEE) has developed the Aviation Environmental Design Tool (AEDT) version 2a software system with the support of the following development team: FAA, National Aeronautics and Space Administration (NASA), U.S. DOT Volpe National Transportation Systems Center (Volpe Center), ATAC Corporation, Metron Aviation, Wyle Laboratories, CSSI, Inc., Foliage, MIT, and Georgia Tech. AEDT 2a is designed to dynamically model aircraft performance in space and time to compute aircraft noise, emissions, and fuel burn. AEDT 2a software runs on PCs using a minimum hardware configuration of a Microsoft Windows XP or 7 operating systems, Dual-core w/ 1.6 GHz FSB and 512KB L2 Cache processor, 4 GB RAM, and 500 GB hard disk storage. In the U.S., the model is used to evaluate aircraft noise, emissions, and fuel burn of proposed air traffic airspace actions under the current version of FAA Order 1050.1E. This document is the User Guide to setup and run analyses with AEDT 2a.