After the terrorist attacks on the United States on September 11, 2001, federal agencies took immediate steps to secure U.S. airspace. FAA grounded all air traffic and DOD ordered Air Force fighter jets to fly patrols over selected U.S. cites to deter and respond to any additional attacks. In the months after the attacks, the President developed certain national strategies and directives, and Congress established TSA and gave it the responsibility to provide security for all modes of transportation. Congress also later passed and the President signed legislation to protect the homeland against air, land, and maritime threats, including creating the DHS to coordinate and lead the national homeland security effort. After the attacks, interagency coordination increased as FAA, NORAD, TSA, their parent cabinet departments, and other agencies with homeland air defense or security roles and missions worked together to meet the overall goal of protecting U.S. airspace. NORAD and the FAA have historically been the main contributors to protecting U.S. airspace. FAA’s primary mission is to safely manage the flow of air traffic in the United States, but it contributes to air security through its control of U.S. airspace. About 17,000 FAA controllers monitor and manage airspace, support the coordination of security operations, and provide information to military and law enforcement agencies when needed. Within NORAD, Continental North American Aerospace Defense Command Region personnel monitor radar data on aircraft entering and operating within continental U.S. airspace. NORAD also conducts air patrols in U.S. airspace. According to FAA, the agency divides airspace into four categories: controlled, uncontrolled, special use, and other. Controlled airspace may include special flight restrictions and will have specific defined dimensions, including altitude ranges, or vertical boundaries, and surface area, or horizontal boundaries. Any aircraft operating within controlled airspace must comply with rules governing that airspace or be subject to enforcement action. Controlled airspace is further divided into classes ranging from A through E. Each class of airspace has its own level of Air Traffic Control services and operational requirements that pilots must follow in order to enter and operate in it. For example, to operate in class A airspace, pilots must have air traffic controller clearance to enter and must have communication equipment on board to permit communication with air traffic controllers. In lesser-restricted airspace, pilots can navigate by landmarks. Controlled airspace can be further classified with special flight restrictions. In uncontrolled airspace, class G, air traffic controllers have no authority or responsibility to control air traffic. FAA also reserves airspace for special purposes, called Special Use Airspace, which is normally established to protect important infrastructure, including military installations. An Air Defense Identification Zone is restricted airspace in which the ready notification, location, and control of aircraft are required for national security reasons. FAA’s other airspace category includes national security areas, military training routes, and temporary flight restriction areas. A temporary flight restriction typically restricts flights over specified areas for a specified period of time. These zones can be established over critical infrastructure, military operations areas, National Security Special Events, and United States Secret Service protectees (e.g. such as the President, whose airspace is protected as he moves throughout the United States). FAA notifies pilots of temporary flight restrictions through its Notices to Airmen program. Pilots are required to check for notices before beginning their flights to avoid any temporary flight restriction zones during their flights. If pilots violate such a zone, FAA can take actions against them ranging from suspending the pilot’s certificate to fly in response to a one- time, first-time violation to revocation of the certificate when the violation is deliberate or otherwise shows a disregard for the regulations. Temporary flight restrictions are one component of a tiered security aviation system. The system includes ground procedures, such as TSA passenger screening procedures, and in-flight security procedures, including reinforced cockpit doors and Federal Air Marshals on selected domestic and international flights. Temporary flight restrictions are considered passive air space control measures intended to keep the flying public out of the protected airspace so that agencies can more readily identify and respond to pilots exhibiting hostile intent. A temporary flight restriction alone will not prevent pilots from entering the protected airspace. FAA monitors national airspace traffic to ensure safety and has established triggers to help identify suspicious aircraft and pilots. According to FAA procedures, FAA controllers are to advise the pilots to change their course or altitude if they are on a course toward prohibited or restricted airspace without authorization, or if they are circling or loitering over a sensitive area. Sensitive areas include airspace over dams, nuclear and electrical power plants, chemical storage sites, the location of the President, or military facilities. Various forms of suspicious pilot and aircraft activity are being monitored. If a violation is imminent or underway, responding agencies have only limited time in which to decide what actions to take. Nonetheless, the agencies need sufficient time to try to determine the pilot’s intent and, if necessary, to order, scramble, and launch DOD or DHS aircraft to intercept the violator. The response to a violation is managed using a process of recognition, assessment and warning, interdiction, recovery, and follow-up; which agency takes these actions depends on the specific nature of the violation. FAA can report a violation of restricted airspace based on radar tracking. If the offending aircraft deviates from its planned flight path but is not heading directly toward the protected asset, FAA may monitor the aircraft and try to contact the pilot but not interdict the aircraft. Conversely, if NORAD or FAA perceives the aircraft to be a threat based on its speed, direction, or other information, NORAD can alert its aircraft and attempt to intercept the violator. If successfully diverted away from the protected asset or restricted airspace, Secret Service, FAA, TSA, or local law enforcement officers may meet the aircraft and interview the pilot upon landing, to identify any hostile intent. On the other hand, if the offending pilot does not divert and proceeds to operate in a manner perceived as threatening, the NORAD pilot can be ordered by the appropriate authorities to engage the violating aircraft. Figure 1 shows an aircraft deviating from its planned flight path and shows more highly threatening and, conversely, less threatening violations of restricted airspace. Our review of an FAA database found about 3,400 reported violations of restricted airspace from September 12, 2001, to December 31, 2004, most of which were committed by general aviation pilots. According to FAA, violations occur because (1) pilots may divert from their planned flight path to avoid bad weather, or may make navigational errors and consequently enter restricted airspace; (2) FAA may establish airspace restrictions with little warning, and pilots already in the air may be unaware of the new restrictions; or (3) pilots may not check for notices of new restrictions as required by FAA and may consequently enter restricted airspace without authorization. In addition, terrorists might deliberately enter restricted airspace to observe the government’s response or to carry out an attack. FAA investigates pilot deviations into restricted airspace to determine the reasons for an incident and to determine whether the pilot’s certificate should be temporarily suspended or permanently revoked. As the scope of restricted airspace increases, the number of violations generally also increases. In addition, a greater concentration of air traffic, such as in the eastern United States, would affect the number of violations. FAA has worked with the aviation community to inform them of the additional restricted areas. Figure 2 shows the percentage of violations of restricted airspace by area of the United States. General aviation aircraft pilots accounted for about 88 percent of all violations of restricted U.S. airspace between September 12, 2001, and December 31, 2004. Figure 3 shows the percentage of incursions by type of aircraft. According to FAA data, pilot error is the biggest contributor to restricted airspace violations. Pilots may not check for FAA Notices to Airmen that indicate the location of restricted airspace, or FAA may establish such airspace with little warning, and pilots may consequently enter the airspace. Airspace restrictions can move, such as when the President travels. Notices on the location of newly restricted airspace may be issued quickly, and pilots may already be in their aircraft or in the air when the restriction is announced and implemented. Moreover, pilots may fly around bad weather or may experience equipment problems and consequently enter restricted airspace to maintain safe operations. To reduce violations, FAA has conducted safety seminars, provided a toll- free number for pilots to call and check for restricted airspace, identified the location of restricted airspace on its Web site, and encouraged pilots to check for and be attentive to notices on restricted airspace. When a pilot enters restricted airspace without authorization, FAA investigates and decides what actions to take against the pilot. After the September 2001 attacks, FAA strengthened the actions that it could take. For example, FAA no longer issues warning notices or letters of correction to pilots. Instead, FAA will now suspend a pilot’s certificate for 30 to 90 days for a single, inadvertent, first-time violation of a temporary flight restriction area that was established with a notice. The temporary suspension’s length depends on the degree of danger to other aircraft and persons or property on the ground, the pilot’s level of experience, prior violations record, and certain other factors. If a pilot deliberately enters restricted airspace without authorization, FAA will revoke the pilot’s certificate.