SFO is the largest airport in the San Francisco Bay Area and the second busiest in California, after LAX. In 2017, it was the seventh-busiest airport in the United States and the 24th-busiest in the world by passenger count. It is the fifth-largest hub for United Airlines, and it is operated in Terminal 3 and the International Terminal. SFO functions as United's primary transpacific gateway. It also serves as a hub for Alaska Airlines, which operates in Terminal 2. It is a major maintenance hub for United Airlines and houses the Louis A. Turpen Aviation Museum and Library.