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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 world's navigable airspace is divided into three-dimensional segments, each of which is assigned to a specific class. Most nations adhere to the classification specified by the International Civil Aviation Organization (ICAO) and described below, though they might use only some of the classes defined below, and significantly alter the exact rules and requirements.
The exact requirements vary by type of airspace, whether it is day or night (for countries that permit night VFR), and from country to country. Typical visibility requirements vary from one statute mile to five statute miles (many countries define these in metric units as 1,500 m to 8 km).
Use of instrument flight rules is also required when flying in "Class A" airspace regardless of weather conditions. Class A airspace extends from 18,000 feet above mean sea level to flight level 600 (60,000 feet pressure altitude) above the contiguous 48 United States and overlying the waters within 12 miles thereof. [8]
Separation can also apply to terrain, obstacles, and controlled airspace, wherein an aircraft must stay at a minimum distance from a block of airspace; as an example, all aircraft must be approved by the controller who "owns" the airspace before the aircraft is approved to enter that sector. Separation at cruising altitude (aircraft passing below).
Class B airspace areas are designed to improve aviation safety by reducing the risk of midair collisions in the airspace surrounding airports with high-density air traffic operations. [1] Aircraft operating in these airspace areas are subject to certain operating rules and equipment requirements. [1] Class B airspace protects the approach and ...
If the airspace is not Class A, B, C, or D, and is controlled airspace, then it is Class E airspace. Class E airspace extends upward from either the surface or a designated altitude to the overlying or adjacent controlled airspace. When designated as a surface area, the airspace is configured to contain all instrument procedures.
Under visual meteorological conditions, the minimum visual range, distance from clouds, or cloud clearance requirements to be maintained above ground vary by jurisdiction, and may also vary according to the airspace in which the aircraft is operating. [4] In some countries, VFR flight is permitted at night, and is known as night VFR.