Southwest Airlines is famous for only operating Boeing 737 aircraft. According to Planespotters.net, Southwest has 780 Boeing 737s in service and these are on average 11.6 years old. Southwest is one of the four largest US-based airlines and is considered a hybrid carrier mixing full-service and low-cost models. It only operates short-haul domestic and international leisure flights to Mexico and the Caribbean (it doesn’t fly to Canada as its system isn’t set up to handle Canadian dollars).