and maintain air services: labor, fuel, airplanes, engines, spares and parts, IT services and networks, airport equipment, airport handling services, booking commissions, advertising, catering, training, aviation insurance and other costs. Thus all but a small percentage of the income from ticket sales is paid out to a wide variety of external providers or internal cost centers. Moreover, the industry is structured so that airlines often act as tax collectors. Airline fuel is untaxed because of a series of treaties existing between countries. Ticket prices include a number of fees, taxes and surcharges beyond the control of airlines. Airlines are also responsible for enforcing government regulations. If airlines carry passengers without proper documentation on an international flight, they are responsible for returning them back to the original country. Analysis of the 1992–1996 period shows that every player in the air transport chain is far more profitable than the airlines, who collect and pass through fees and revenues to them from ticket sales. While airlines as a whole earned 6% return on capital employed (2–3.5% less than the cost of capital), airports earned 10%, catering companies 10–13%, handling companies 11–14%, aircraft lessors 15%, aircraft manufacturers 16%, and global distribution companies more than 30%. There has been continuing cost competition from low cost airlines. Many companies emulate Southwest Airlines in various respects. The lines between full-service and low-cost airlines have become blurred – e.g., with most "full service" airlines introducing baggage check fees despite Southwest not doing so. Many airlines in the U.S. and elsewhere have experienced business difficulty. U.S. airlines that have declared Chapter 11 bankruptcy since 1990 have included American Airlines, Continental Airlines (twice), Delta Air Lines, Northwest Airlines, Pan Am, United Airlines and US Airways (twice). Where an airline has established an engineering base at an airport, then there may be considerable economic advantages in using that same airport as a preferred focus (or "hub") for its scheduled flights. Fuel hedging is a contractual tool used by transportation companies like airlines to reduce their exposure to volatile and potentially rising fuel costs. Several low-cost carriers such as Southwest Airlines adopt this practice. Southwest is credited with maintaining strong business profits between 1999 and the early 2000s due to its fuel hedging policy. Many other airlines are replicating Southwest's hedging policy to control their fuel costs. Operating costs for US major airlines are primarily aircraft operating expense including jet fuel, aircraft