<P> SFO is the largest airport in Northern California and the second - busiest in California, after Los Angeles International Airport . In 2016, it was the seventh - busiest in the United States and the twenty - third - busiest airport in the world by passenger count . It is the fifth - largest hub for United Airlines and functions as United Airlines' primary transpacific gateway . It also serves as Virgin America's principal base of operations but then will become as Alaska Airlines on April 25, 2018 . It is the sole maintenance hub of United Airlines, and houses the Louis A. Turpen Aviation Museum and Library . </P> <P> SFO is owned and policed by the City and County of San Francisco, but is in San Mateo County . Between 1999 and 2004 the San Francisco Airport Commission operated city - owned SFO Enterprises, Inc., to oversee its business purchases and operations of ventures . </P> <P> The City and County of San Francisco first leased 150 acres (61 ha) at the present airport site on March 15, 1927 for what was then to be a temporary and experimental airport project . San Francisco held a dedication ceremony at the airfield, officially named for Mills Field Municipal Airport of San Francisco, on May 7, 1927 on the 150 - acre cow pasture . The land was leased from the Mills Estate in an agreement made with Ogden L. Mills who oversaw the large tracts of property originally acquired by his grandfather, the banker Darius O. Mills . San Francisco purchased the property and the surrounding area expanding the site to 1,112 acres (450 ha) beginning in August 1930 . The airport's name was officially changed to San Francisco International Airport in 1931, upon the purchase of the land, and "International" was added at the end of World War II as overseas service rapidly expanded . </P> <P> The earliest scheduled carriers at the airport included Western Air Express, Maddox Air Lines, and Century Pacific Lines . United Airlines was formed in 1934 and quickly became the key carrier at the airport, with Douglas DC - 3 service to Los Angeles and New York beginning in January 1937 . A new passenger terminal opened in 1937, constructed with Public Works Administration funding . The March 1939 Official Aviation Guide shows 18 airline departures on weekdays--seventeen United flights and one TWA flight . The aerial view c. 1940 looks west along the runway that is now 28R; the seaplane harbor at right is still recognizable north of the airport . Earlier aerial looking NW 1943 vertical aerial (enlargeable) The August 1952 chart shows runway 1L 7,000 feet long, 1R 7,750 feet, 28L 6,500 feet, and 28R 8,870 feet . </P>

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