aviation is accounts for approximately 5% of U.S. gross domestic product and also contributes to $1.3 trillion in annual economic activity as well as helps generate and support 10 million jobs annually. The aircraft industry contributes to the economy through many different avenues, including manufacturing and the airline industry. As of 2009, the National Air Traffic Controllers Association reported airline operations helped "generate $150.5 billion to the U.S. gross domestic product, while airport operations contributed an additional $44.6 billion." Economic Impact of Commercial Aviation on the U.S. Economy (2006-2009) Airports Operations Public airports are usually constructed and operated by local governments. The main exceptions are on military bases. Like highways and passenger rail, the federal government subsidizes air travel with $14 billion of federal funds going to airport operations in 2002. Security and regulation Air transportation security in the United States is regulated by the Transportation Security Administration (TSA), an agency of the US Department of Homeland Security. Passengers must provide a valid federal or state-issued ID in order to be allowed onto a flight. A person must also go through a pat-down procedure or a body scan before boarding a flight to ensure that they have no prohibited items. These security policies have been adopted by the US government ever since the 9/11 Terrorist Attacks in which terrorists managed to hijack several commercial airliners. Items that are prohibited on airplanes include firearms, tools, or other objects that can be used as weapons, explosive or flammable materials, and other dangerous or debilitating chemicals or substances. Airlines Passenger airlines The U.S. has no single flag carrier and passenger airlines in the United States have always been privately owned. The U.S. has three major international carriers (Delta Air Lines, American Airlines, and United Airlines) and eight additional large carriers that operate mainly domestic flights but have some international destinations (Alaska Airlines, Allegiant Air, Frontier Airlines, Hawaiian Airlines, JetBlue, Southwest Airlines, Spirit Airlines, and Sun Country Airlines). At the end of 2014, the three international carriers plus Southwest controlled more than 80% of all passenger travel in the U.S. market. There is currently no government regulation of ticket pricing, although the federal government retains jurisdiction over aircraft safety, pilot training, and accident investigations (through the Federal Aviation Administration and the National Transportation Safety Board). The Transportation Security Administration provides security at airports and the federal government provides billions of dollars annually to maintain