I supervise dissertation students albeit not in Geography. I am assuming that you are working at undergraduate level and so at first glance I think that your subject is too wide; you are going to have to complete a huge research project in order to compare and contrast between low-cost carriers and the mainstream airlines. If you want to look at budget airlines then consider a narrower topic but one that you can do some in-depth analysis with. How about considering the effect that low-cost airlines have had on social mobility in Europe, or the contribution of low-cost airlines to the rise of the 'tiger' economies in the Far East?