Airspace class (United States)

The United States airspace system's classification scheme is to provide maximum pilot flexibility with 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.

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 airspace extends from 18,000 feet (5,500 m) mean sea level MSL to FL600 (approximately 60,000 feet (18,000 m) MSL) throughout the continental 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" Hg 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.

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