Southwest Airlines has reached an agreement in principle on terms that would allow a private operator to lease Midway Airport in what the city's chief financial officer described Thursday as "a very big first step" toward privatizing the Southwest Side field.

But the accord is preliminary, and Southwest vice president Bob Montgomery issued a carefully worded statement that stopped short of a final endorsement.

"With the city, Southwest welcomes the opportunity to increase our collective knowledge about airport privatization in a manner that hopefully produces a mutually beneficial outcome," Montgomery said. "We applaud the City of Chicago for carefully exploring the possibility of a potential long-term lease of Midway Airport to a private airport operator."

The Daley administration now must convince four more of the seven scheduled airlines that operate at the airport to sign on before being able to proceed with the initiative.

If the airlines agree, the city would seek bidders for a long-term lease whose terms also must be approved by the Federal Aviation Administration, the Transportation Security Administration and the City Council.

Mayor Richard Daley, who authorized research on a Midway deal, hopes that a long-term lease could bring a big payday for the city. City Hall's 99-year lease of the Chicago Skyway to a Spanish-Australian consortium in 2005 produced $1.8 billion.