The C~ icago-re? ion airspace system analysis conducted as part of the~ 1te sele~ tion phase of the Illinois-Indiana. Regional Airport study 1s descnbed. The analysis provided data on performance measures 1: 1sed to evaluate the five proposed alternative sites, from the airspace feasibility standpoint. A comparative analysis approach considering alternatives and scenarios was conducted using data generated from SIMMOD simulation. The region, nature of study, structure and characteristics of the region's airspace, elements of the analysis methodology, input data require-me~ ts an~ collection, simulation procedure, and interpretation of simulation output data are described and discussed. The performance measures used as criteria to assess the feasibility of the alternative airport sites from regional airspace system are discussed. Results of the analysis are presented, evaluation of performance measures on the airport and system levels are discussed a~ d the operation~ l characteristics of the regional airspace syste~ with each of the five alternative sites included are described.