The United States Federal Aviation Administration (FAA) has a system for categorizing public-use airports (along with heliports and other aviation bases) that is primarily based on the level of commercial passenger traffic through each facility. It is used to determine whether an airport is eligible for funding through the federal government's Airport Improvement Program (AIP). Fewer than 20% of airports in the U.S. qualify for the program, though most that don't qualify are private-use-only airports.