DALLAS - LaGuardia Airport is the smallest of the three major airports in the New York area, with just two main runways. Planes often sit in long lines on the tarmac waiting their turn to take off.

So why would Southwest Airlines Co., a carrier that boasts about its on-time prowess, want to go there? In many ways, because it must.

Southwest prospered by offering low fares to leisure travelers whose only other affordable option was a car trip. It flew primarily to America's secondary airports, where costs are low and productivity is high because incoming planes can land, drop off passengers, take on the next group, and get back in the air quickly.

On Sunday, Southwest starts service at LaGuardia, one of the nation's most congested airports. This should bring cheaper ticket prices to New York area vacationers flying to Chicago, Baltimore, and beyond.

But the move also is part of a risky transition that Southwest knows it has to make to win the loyalty of business travelers, who increasingly will dictate its future prospects for success.

Southwest started flying in 1971 with three planes. It did not offer the amenities found on other airlines, but it outlived early rivals such as Braniff by sticking to a core philosophy: Give people low fares and great service.

The Dallas carrier still sees itself as an underdog today, even as it serves 65 cities and carries more than 100 million U.S. passengers per year, more than any other airline.

There are still no first-class cabins and no assigned seats on Southwest, giving it the air of a carrier for penny-pinching vacationers.

"We're very dependent on business travelers, so we're not a leisure airline like some of our smaller competitors are," chief executive officer Gary C. Kelly said in an interview. He said company surveys showed that in normal economic times, at least 40 percent of his customers were traveling on business.

Airlines covet business travelers because they make repeat trips and often pay higher fares for booking at the last minute.

Southwest needs that revenue now. The airline has been profitable for 36 straight years, but it has been in the red since last fall. Traffic is down, and costs are rising.

So while it is cutting flights across its system, Southwest is entering New York and three other big cities, including Boston's Logan Airport.

Robert Crandall, who competed against Southwest when he ran American Airlines in the 1980s and '90s, said Southwest had stuck to a well-defined business model of low fares and low costs at secondary airports.

"Going into LaGuardia is a change to that model," Crandall said, "but they've decided they don't have any choice - they need the [passenger] volume to grow."