What are Expenses and Costs Facing Today's Air Industry?

Please provide some examples and a minimum of three sources that can be utilized in my research paper.

Labor is the main cost of the airline industry. Fuel comes in second, with commissions to travel agents coming in third. Most of the operating costs are fixed. https://people.hofstra.edu/geotrans/eng/ch3en/conc3en/airlinecosts.html http://www.investopedia.com/features/industryhandbook/airline.asp http://www.investopedia.com/features/industryhandbook/airline.asp