High Quality Content by WIKIPEDIA articles! Frontier Airlines, Inc., is an American airline headquartered in Indianapolis, Indiana, United States. The carrier, which is a subsidiary and operating brand of Republic Airways Holdings, operates flights to 83 destinations throughout the United States, Mexico, and Costa Rica and maintains hubs at Denver International Airport, Milwaukee`s General Mitchell International Airport, and Kansas City International Airport. It provides regional service to the surrounding Rocky Mountain States through a code-share agreement with Great Lakes Airlines. Frontier is not a member of an air carrier alliance.