While Southwest did not say what cities it will serve from Milwaukee, its five largest airports include Las Vegas, Chicago Midway, Phoenix, Baltimore and Houston.

Meanwhile, Midwest is shrinking in the face of the troubled economy and competition from AirTran. When AirTran first bid for the carrier in December 2006, Midwest carried about 50% of the airport traffic, including about 12% on its regional subsidiary, according to airport statistics. In March, Midwest carried about a third of the traffic, split nearly evenly between the mainline carrier and two independent regional affiliates.

"For years, Southwest wouldn't compete with Midwest," Healy says. "Southwest was successful because they managed expectations (for amenities) to the lowest level, while Midwest had a quality product and a reputation for the best in-flight service. They were at the two extremes of the spectrum. But that Midwest is largely gone."

Now, General Mitchell Field is positioned to emerge as a low-fare airport for the northern Chicago area. "The airport folks have been trying to build the Northern Illinois market, and this will bring even greater attention, and what's good for Milwaukee is good for AirTran," Healy said.

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