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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).
Title 14 CFR – Aeronautics and Space is one of the fifty titles that make up the United States Code of Federal Regulations (CFR). Title 14 is the principal set of rules and regulations (sometimes called administrative law) issued by the Department of Transportation and Federal Aviation Administration, federal agencies of the United States which oversee Aeronautics and Space.
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.
It usually happens when the aircraft is inside controlled airspace, and the local weather is less than the minimums required for flight under visual flight rules (VFR) within the airspace in question. [5] Note that an aircraft might be able to fly under SVFR even in Class A airspace, where instrument flight rules (IFR) flight is the norm. [6]