Before we begin, here is a brief review of airspace.
Class A airspace altitudes are from 18,000 feet up to and including 60,000 feet. VFR flight, including VFR-ON-TOP, is not permitted in Class A airspace.
A clearance is always required to enter Class B airspace. In addition to a two way radio and mode C transponder, the aircraft must also be equipped with a VOR for flight under IFR in a Class B. A private pilot certificate or greater is required to enter a Class B. Certain Class B airspace areas permit student pilots, however, if several regulatory requirements are met. A pilot can operate in or around a Class B airspace area without a transponder, so long as it is approved by ATC. A request to deviate from transponder requirements must be made at least 1 hour prior to the proposed operation. Large turbine powered airplanes must remain above the floor of a Class B at all times.
To enter a Class C airspace, two way radio communications must be established with air traffic control. Communications must be maintained while inside the airspace. A mode C transponder is required when inside or above a Class C airspace.
Class D airspace typically extends to 4 nm (5 sm) and up to 2,500 feet above the elevation of the primary airport, but can be tailored, as necessary. Two way radio communications must be established and maintained with ATC, and ATC must authorize all operations inside the airspace. When a part time tower closes, its Class D airspace reverts to Class E, Class G, or a combination of both.
Class E is controlled airspace. Air traffic control exercises control of IFR aircraft in Class E airspace. Class E airspace exists over the entire country from 14,500 to 17,999 feet. It extends down to 1,200 feet AGL over large areas of the United States. To protect IFR arrivals and departures, Class E is often extended downward to protect specific portions of instrument approach or departure routes. For these transition areas, Class E is often extended downward to 1,200 AGL, 700 feet AGL, or all the way to the surface.
Class G is uncontrolled airspace. ATC has no authority or responsibility for operations in uncontrolled airspace.
ATC routes IFR traffic around prohibited and restricted airspace, which contains unusual, often invisible, hazards to flight.
ATC will also route IFR traffic around active military operations areas. An MOA is established to separate certain military activities from IFR traffic. Additionally, the presence of an MOA makes VFR pilots aware of the location of these military activities.
An alert area is an area that contains a high volume of pilot training or some unusual activity.
A warning area contains the same hazardous activities as a restricted area. However, the government cannot designate restricted areas in international airspace. As a result, these areas are simply designated as warning areas.
A terminal radar service area, or TRSA, is not a classification of airspace or special use airspace. It is simply an area for which ATC is present and provides separation services between all IFR and participatin VFR aircraft. Participation for VFR pilots is not mandatory, but encouraged.
A local advisory area is not an airspace classification or special use airspace. It refers to the area for which the a flight service station provides airport advisory services. A flight service station will provide this airport advisory service for the airport at which the FSS is located, when the control tower for that airport is closed.
Cloud Clearance and Visibility Requirements (FAR 91.155)
VFR weather minimums do not apply for Class A airspace, since VFR flight is not permitted there. For Class B airspace, VFR minimums are 1 sm and clear of clouds.
In Class C and D airspace, weather minimums are 3 sm, 500 feet below, 1000 feet above, and 2000 feet horizontally from clouds.
For Class E airpace below 10,000 feet, you need 3 sm and must stay 500 feet below, 1000 feet above, or 2000 feet horizontally from clouds. At or above 10,000 feet, 5 sm and 1000 below, 1000 feet above, or 1 sm horizontally from clouds is required.
Weather minimums for Class G airspace at night are the same as those for Class E. During the day, the visibility below 10,000 feet is reduced from 3 sm to 1 sm. Also, during the day, the VFR weather minimums for Class G are reduced to 1 sm and clear of clouds when operating at or below 1,200 feet AGL, regardless of your MSL altitude.