Southwest was able to sustain a four-aircraft schedule only three aircraft by utilizing the 'ten-minute turn" Southwest turned its first annual profit in 1973 Southwest's expansion strategy flourished after 1973. By 1977 Southwest operated six airplanes and had transported five million customers." New flight destinations were added such as Rio Grande Valley, Austin, Corpus Christi, El Paso, Lubbock, and Midland/odessa.Between 1979 and 1980, Southwest established its first interstate service to New Orleans and Albuquerque, followed by Oklahoma City and Tulsa. Southwest expanded west in 1982, when it added service to Phoenix, Las Vegas, and San Diego. In late 1984 the first 737-300 was added to its fleet and was used to service flights to Chicago Midway and St. Louis.