Spirit Airlines isn’t the most beloved airline in America. Not yet anyway. However, it is one of the most popular in terms of load factor, or the percentage of people on each of the airline’s planes. Inspired by the ultra-low cost, a la carte carriers that operate in Europe, Spirit changed its business model in 2006. After a bit of turbulence, the airline has now emerged as a leader in the ultra-low cost airline space with the goal of becoming America’s go-to low-cost air....