Air service in the United States is highly concentrated, with 88 percent of all passenger boardings at the 62 large- or medium-hub airports. Small airports refer to small-hub, nonhub, and commercial-service nonprimary airports, each with less than 0.25 percent of all annual passenger boardings, or less than 1.8 million total boardings in 2012. Many small communities across the United States have access to the more than 450 small airports with scheduled passenger service, provided mostly by regional airlines that are under contract with mainline network airlines, like Delta Air Lines or United Airlines. The airport categories in figure 1 also determine the allocation of Airport Improvement Program (AIP) grants for airport capital improvements. FAA awarded nearly $3 billion in grants to all airports in fiscal year 2013, for safety, capacity, and environmental capital improvements. The grants offset the fees that airports charge users, so they are critical for small airports hoping to retain or attract airport users. For example, any airport with at least 10,000 passengers is assured at least $1 million in annual grant funding.