Southwest Airlines is the world’s largest operator of Boeing 737 aircraft. According to Flightradar24, it is the world’s fourth-largest airline and has a fleet of 810 planes. Because the airline operates a point-to-point model and many shorter flights, many of its aircraft can fly several legs per day, sometimes up to seven. According to a story from the Daily Mail, one of Southwest’s planes recently flew as much as 4,000 miles in 19 hours.