Low-fare giant Southwest Airlines, upset about the skyrocketing cost of flying out of Seattle-Tacoma International Airport, is considering...

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Low-fare giant Southwest Airlines, upset about the skyrocketing cost of flying out of Seattle-Tacoma International Airport, is considering moving its Seattle operations to Boeing Field.

Southwest has been quietly talking with county officials for more than a year about flying out of King County International Airport, as Boeing Field is officially known. The discussions reached the point that it was time to inform the County Council and the public of a possible agreement, County Executive Ron Sims said yesterday.

The nation’s biggest domestic carrier, Southwest has made an art of keeping its costs low by avoiding premium-priced airports and flying out of second-tier locations, such as Oakland instead of San Francisco and Chicago’s older Midway Airport instead of O’Hare.

But yesterday’s announcement is sure to cause controversy because of the potential for more noise at Boeing Field and the loss of revenue for Sea-Tac, which recently spent $4 billion to increase its capacity in part to accommodate the growth of airlines such as Southwest.

“It’s a terrible idea,” County Councilman Dwight Pelz said. “We’ve got an airport. It’s called Sea-Tac.”

Southwest, with about 40 landings a day, has the fourth-largest presence at Sea-Tac, behind Alaska Airlines, Alaska subsidiary Horizon Air, and United Airlines.

But Southwest’s ties to Seattle are much greater than its flights here. The low-cost airline is arguably Boeing’s most important customer. The airline flies 429 Boeing 737s, which are assembled in Renton and usually delivered to customers at Boeing Field. Southwest spawned the low-cost airline industry and encouraged followers to fly 737s.

“We do want to grow and expand in Seattle, and we are looking for ways to do that,” Southwest spokeswoman Marilee McInnis said.

The airline is looking at what a move would cost and what facilities would have to be built at Boeing Field, McInnis said.

Southwest and other airlines have made no secret of their unhappiness with rising fees at Sea-Tac, which is run by the Port of Seattle. To pay for the third runway, new terminal and other improvements, the airlines’ cost to use the airport has been projected to rise from about $4 a passenger in the mid-1990s to about $23 each in 2009. That, airlines say, makes Sea-Tac one of the most expensive airports in the country.

Airlines will be responsible for about half of the billion dollars it is costing to build the airport’s third runway — more than double what they originally anticipated.

But Bob Edwards, president of the Port Commission, said airlines were part of a decision 10 years ago to expand Sea-Tac rather than open another airport or send traffic to other airports.

Were airlines to pull out now, he said, costs for Sea-Tac’s remaining passengers would rise, and the overall air transportation system would be less efficient. For example, if federal security personnel had to deploy more resources to Boeing Field, that could mean more bottlenecks for Sea-Tac passengers, he said.

“I don’t see where the public wins” if Southwest moves, Edwards said.

Sims said the county was approached by Southwest as well as other airlines as early as 2003, but that Southwest was the only airline that maintained interest in flying out of Boeing Field.

As an airport operator, the county is required by federal law to accommodate airlines if it can, Sims said. But he said he’s also interested in the financial health of Boeing Field, which has 3,000 employees and is supported by fees, rent and federal funds.

Sea-Tac has been Puget Sound’s primary commercial airport since 1949. Boeing Field, used mostly for small planes and cargo carriers, hasn’t had significant commercial traffic since Horizon Air flew there in the 1970s. As a relief airport to Sea-Tac, Boeing Field is sometimes used during the winter to handle commercial flights.

It handles about 900 takeoffs and landings each day, to which Southwest would add about 80. Sims said a full environmental study would have to be done to assess the need for additional facilities and the question of additional noise.

But bigger planes aren’t necessarily noisier. The newest 737s flown by Southwest are required to meet strict federal noise standards, and in many cases may be quieter than older corporate jets flown at Boeing Field, a Boeing spokeswoman said yesterday.

Tom Boyer: 206-464-2923 or tboyer@seattletimes.com

Christina Siderius: 206-515-5066 or csiderius@seattletimes.com