Even though Southwest ( LUV) has expanded the range of airports it is willing to serve, some prominent airports still don't make the cut.

Long known for avoiding both congested airports and other carriers' fortress hubs, Southwest has made a U-turn this year. It will add New York LaGuardia in June and Boston Logan in August, after starting Minneapolis service in March.

But CEO Gary Kelly said on a recent earnings conference call that four airports - Chicago O'Hare, Dallas-Fort Worth International, Miami International and Washington's Reagan National -- are not on the carrier's current list.

In the past, Southwest would not have been expected to serve any of the four. But today, with congested and hub airports on the table, their exclusion becomes noteworthy. In the case of Chicago, Dallas-Fort Worth and Miami, it reflects proximity to key Southwest outposts at Chicago Midway, Love Field in Dallas and Fort Lauderdale/Hollywood International Airport.

Regarding National, Kelly said Southwest has served Baltimore for 16 years and Washington Dulles for three. "In terms of thinking about Baltimore-Washington, we're very happy with our service level there," he said

As for LaGuardia, Logan and Minneapolis, Kelly acknowledged: "We've certainly tried to break through some paradigms.

"We can think about those opportunities in 2009 and have them make sense when they wouldn't have as late as 1999. We now carry more passengers than any other U.S. airline (and) we have a very strong, very well-known brand with customers who love us and would prefer to fly on us."

The reason why Minneapolis "is getting off to such a hot start" is that passengers from throughout Southwest's route system want to fly there, he said. "That will work the same way for LaGuardia and Boston," he said. Minneapolis is a fortress hub for Delta ( DAL) subsidiary Northwest.

Kelly acknowledged that LaGuardia congestion does not reconcile easily with Southwest's preference for high aircraft utilization, but said it's something "we are willing at this point to only expose ourselves to modestly." Out of 3,200 daily departures, only 16 will involve LaGuardia, he said.

A code share with defunct partner ATA Airlines, which served LaGuardia, convinced Southwest that its passengers want to fly there, Kelly said. Southwest service from LaGuardia to Baltimore and Chicago Midway will begin June 28. "We think it will be an enormous boost to our Chicago business," Kelly said.

Southwest's reluctance to serve National is good news for US Airways ( LCC), which carries about a third of the airport's passengers. Delta carries about 20% and American ( AMR) carries about 16%.

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