St. Louis, MO STL to Atlanta, GA ATL flight with frontier airlines

United Airlines, established in 1926, is a major U.S. airline headquartered in Chicago, Illinois. Operating an extensive domestic and international network, United serves more destinations than any other airline, connecting travelers across six continents. The airline's eight hubs include Chicago–O'Hare, Denver, Houston–Intercontinental, Los Angeles, Newark, San Francisco, Washington–Dulles, and Guam. As a founding member of the Star Alliance, United offers a comprehensive range of services, including regional flights through United Express, catering to both leisure and business travelers. The airline is committed to sustainability, investing in sustainable aviation fuel and innovative technologies to reduce its environmental impact.

American Airlines is one of the largest airlines in the world, connecting travelers to hundreds of destinations across the United States, Latin America, Europe, Asia, and beyond. Operating from major hubs such as Dallas/Fort Worth, Miami, and Charlotte, American offers a wide range of travel options. Passengers can choose from Economy with affordable fares, Premium Economy with extra space, or Business and First Class featuring flat-bed seats, premium dining, and lounge access. With a modern fleet and membership in the oneworld alliance, American Airlines combines global reach with comfort and reliability.

Spirit Airlines is an American ultra-low-cost carrier, headquartered in Dania Beach, Florida, operating scheduled flights throughout the United States, the Caribbean, and Latin America. Known for its distinctive bright yellow planes, Spirit's business model traditionally focuses on offering low base fares by unbundling services, allowing passengers to pay only for the amenities they choose, such as baggage and seat selection. As of August 2024, Spirit is evolving its offerings, introducing new fare tiers like "Go Big," "Go Comfy," "Go Savvy," and "Go," which include various bundled perks such as wider seats, extra legroom, priority boarding, and in-flight refreshments. Spirit was the seventh largest passenger carrier in North America as of 2023.