Southwest Airlines Co. is an American low-cost airline with its largest focus city at Las Vegas' McCarran International Airport. Southwest is the largest airline in the United States by number of passengers carried domestically per year (as of December 31, 2007). Southwest, the 6th largest U.S. airline by revenue, maintains the second-largest passenger fleet of aircraft among all of the world's commercial airlines. As of July 12, 2008, Southwest operates approximately 3,500 flights daily. Southwest is headquartered at 2702 Love Field Drive in the Love Field area of Dallas, Texas, adjacent to Love Field airport. Southwest Airlines has carried more customers than any other U.S. airline since August 2006 for combined domestic and international passengers according to the U.S. Department of Transportations Bureau of Transportation Statistics. Southwest Airlines is one of the worlds most profitable airlines, posting a profit for the 36th consecutive year in January 2009. Southwest has built a successful business on an unusual model: flying multiple short, quick trips into the secondary (more efficient and less costly) airports of major cities, using primarily only one aircraft type, the Boeing 737.