alliances have been most successful at purchasing invisible supplies and services, such as fuel. Airlines usually prefer to purchase items visible to their passengers to differentiate themselves from local competitors. If an airline's main domestic competitor flies Boeing airliners, then the airline may prefer to use Airbus aircraft regardless of what the rest of the alliance chooses. Largest airlines The world's largest airlines can be defined in several ways. , American Airlines Group was the largest by fleet size, passengers carried and revenue passenger mile. Delta Air Lines was the largest by revenue, assets value and market capitalization. Lufthansa Group was the largest by number of employees, FedEx Express by freight tonne-kilometres, Turkish Airlines by number of countries served and UPS Airlines by number of destinations served (though United Airlines was the largest passenger airline by number of destinations served). State support Historically, air travel has survived largely through state support, whether in the form of equity or subsidies. The airline industry as a whole has made a cumulative loss during its 100-year history. One argument is that positive externalities, such as higher growth due to global mobility, outweigh the microeconomic losses and justify continuing government intervention. A historically high level of government intervention in the airline industry can be seen as part of a wider political consensus on strategic forms of transport, such as highways and railways, both of which receive public funding in most parts of the world. Although many countries continue to operate state-owned or parastatal airlines, many large airlines today are privately owned and are therefore governed by microeconomic principles to maximize shareholder profit. In December 1991, the collapse of Pan Am, an airline often credited for shaping the international airline industry, highlighted the financial complexities faced by major airline companies. Following the 1978 deregulation, U.S. carriers did not manage to make an aggregate profit for 12 years in 31, including four years where combined losses amounted to $10 billion, but rebounded with eight consecutive years of profits since 2010, including its four with over $10 billion profits. They drop loss-making routes, avoid fare wars and market share battles, limit capacity growth, add hub feed with regional jets to increase their profitability. They change schedules to create more connections, buy used aircraft, reduce international frequencies and leverage partnerships to optimize capacities and benefit from overseas connectivity. Environment Aircraft engines emit noise pollution, gases and particulate emissions,