Airspace Structure. The Federal Aviation Administration has established an airspace structure that provides a framework for regulating the responsibilities of different air traffic control facilities. The airspace structure is basically divided into controlled and uncontrolled airspace, according to six classes of vertical air space.
Class B airspace is controlled airspace -- from the surface to 10,000 feet MSL -- surrounding high-capacity commercial service airports (e.g., San Francisco International).
Class C airspace is controlled airspace -- from the surface to 4,000 feet MSL -- surrounding lower-activity commercial service airports (e.g., Oakland International and San Jose International) and some military airports.
Class D airspace is controlled airspace -- from the surface to 2,500 feet MSL -- surrounding airports with an air traffic control tower (e.g., Hayward, Livermore, Palo Alto, Reid-Hillview, and San Carlos airports). All aircraft operating within classes A, B, C, and D airspace must be in contact with the air traffic control tower responsible for that airspace.
Class E is any controlled airspace not classified as A, B, C, or D. The area of a class E airspace is configured to encompass all instrument approach procedures and low-altitude federal airways. (Federal airways, or Victor airways, are corridors of airspace eight miles wide that extend upward from 1,200 above ground level to 18,000 feet MSL.) The entire Metropolitan Bay Area falls within a single area of class E airspace. (Note: some class D airspaces revert to class E during periods when the air traffic control tower is closed, e.g., Hayward airport.) Only aircraft conducting instrument flights are required to be in contact with air traffic control when operating in class E airspace. While aircraft conducting visual flights in class E airspace are not required to be in radio communication with air traffic control, such flights can only be conducted if minimum visibility and cloud ceilings exist.
Class G is uncontrolled airspace -- any area not classified A, B, C, D, or E. No contact with air traffic control is required.
Air Traffic Control in the Bay Area. Aircraft arriving and departing within the Bay Area are controlled by different control facilities. The class B airspace surrounding San Francisco International is controlled by the Bay Approach Control facility. The class C airspace surrounding Oakland International is controlled by the air traffic control tower at the airport. All aircraft transiting above class B and C airspace in the Bay Area are controlled by the Oakland Air Route Traffic Control Center (ARTCC). This facility controls aircraft in a large multi-state area, providing pilots with information on altitude and aircraft separation, as well as route guidance.
FAA Air Traffic Regulations. Federal Aviation Regulations (FARs) are contained in the Code of Federal Regulations, title 14. Part 91 contains "General Operating and Flight Rules" and Part 93 contains "Special Air Traffic Rules and Airport Traffic Patterns. Minimum safe flying altitudes are defined in Part 91 section 119 (14 CFR 91.119):
Except when necessary for takeoff or landing, no person may operate an aircraft below the following altitudes:
(a) Anywhere. An altitude allowing, if a power unit fails, an emergency landing without undue hazard to persons or property on the surface.
(b) Over congested areas. Over any congested area of a city, town, or settlement, or over any open air assembly of persons, an altitude of 1,000 feet above the highest obstacle within a horizontal radius of 2,000 feet of the aircraft.
(c) Over other than congested areas. An altitude of 500 feet above the surface, except over open water or sparsely populated areas. In those cases, the aircraft may not be operated closer than 500 feet to any person, vessel, vehicle, or structure.
(d) Helicopters. Helicopters may be operated at less than the minimums prescribed in paragraph (b) or (c) of this section if the operation is conducted without hazard to persons or property on the surface. In addition, each person operating a helicopter shall comply with any routes or altitudes specifically prescribed for helicopters by the Administrator.