The United States airspace system's classification scheme is intended to maximize pilot flexibility within acceptable levels of risk appropriate to the type of operation and traffic density within that class of airspace – in particular to provide separation and active control in areas of dense or high-speed flight operations.
The Albert Roper (1919-10-13 The Paris Convention) implementation of International Civil Aviation Organization (ICAO) airspace classes defines classes A through G (with the exception of class F which is not used in the United States). The other U.S. implementations are described below. The United States also defines categories of airspace that may overlap with classes of airspace. Classes of airspace are mutually exclusive. Thus, airspace can be "class E" and "restricted" at the same time, but it cannot be both "class E" and "class B" at the same location and at the same time.
Note: All airspace classes except class G require air traffic control (ATC) clearance for instrument flight rules (IFR) operations.
Airspace classes
In the U.S., airspace is categorized as regulatory and non-regulatory. Within these categories exist: controlled (classes A, B, C, D, and E) and uncontrolled (class G) airspace, based on which air traffic control service is provided to IFR flights and some VFR flights. Class F is not used in the U.S. Besides controlled and uncontrolled airspace, other types of airspace include "special use" and "other airspace".
Class A
Class A airspace extends from mean sea level MSL to FL600 (approximately MSL) throughout the contiguous United States and Alaska. Unlike the altitude measurements used in other airspace classes, the FLnnn flight levels used in class A airspace are pressure altitudes referenced to a standardized altimeter setting of 29.92" InHg and thus the true altitudes depend on local atmospheric pressure variations.
Unless otherwise authorized by ATC, all flight operations in class A airspace must be under ATC control, and must be operating IFR, under a clearance received prior to entry. An example of an exception to IFR-only flight in this airspace is the use of wave windows. These specific airspace blocks may be opened by ATC to allow sailplanes to fly in the lee waves of mountains.
Since class A airspace is normally restricted to instrument flight only, there are no minimum visibility requirements.
Class A airspace was formerly known as Positive Control Airspace (PCA).
Class B
Class B airspace is defined around key airport traffic areas, usually airspace surrounding the busiest airports in the US according to the number of IFR operations and passengers served. The exact shape of the airspace varies from one class B area to another, but in most cases it has the shape of an inverted wedding cake, with a series of circular "shelves" of airspace of several thousand feet in thickness centered on a specific airport. Each shelf is larger than the one beneath it. Class B airspace normally begins at the surface in the immediate area of the airport, and successive shelves of greater and greater radius begin at higher and higher altitudes at greater distances from the airport. Many class B airspaces diverge from this model to accommodate traffic patterns or local topological or other features. The upper limit of class B airspace is normally MSL.
All aircraft entering class B airspace must obtain ATC clearance prior to entry and must be prepared for denial of clearance. Aircraft must be equipped with a two-way radio for communications with ATC, an operating Mode C transponder and automatic altitude reporting equipment. Furthermore, aircraft overflying the upper limit of any class B airspace must have an operating Mode C transponder. Visual flight rules (VFR) flights may proceed under their own navigation after obtaining clearance but must obey any explicit instructions given by ATC. Some class B airspaces include special transition routes for VFR flight that require communication with ATC but may not require an explicit clearance. Other class B airspaces include VFR corridors through which VFR flights may pass without clearance (and without technically entering the class B airspace).
VFR flights operating in class B airspace must have three miles (5 km) of visibility and must remain clear of clouds (no minimum distance).