Requirements
VFR require a pilot to be able to see outside the cockpit, to control the aircraft's attitude, navigate, and avoid obstacles and other aircraft. Governing agencies establish specific requirements for VFR flight, including minimum visibility, and distance from clouds, to ensure that aircraft operating under VFR are visible from enough distance to ensure safety.
Under Visual meteorological conditions the minimum visual range, distance from cloud, and heights to be maintained above ground vary by jurisdiction, and may also vary according to the airspace in which the aircraft is operating.
The VFR pilot is required to "see and avoid" obstacles and other aircraft. Pilots flying under VFR assume responsibility for their separation from all other aircraft and are generally not assigned routes or altitudes by air traffic control (ATC). Depending on the category of airspace in which the flight is being conducted, VFR aircraft may be required to have a transponder to help Air Traffic Control identify the aircraft on radar in order that ATC can provide separation to IFR aircraft.
In the United States, a pilot operating VFR outside of class B airspace can request "VFR flight following" from ATC. This service is provided by ATC if workload permits it, but is an advisory service only. The responsibility for maintaining separation with other aircraft and proper navigation still remains with the pilot. In the United Kingdom, a pilot can request for "Deconfliction Service", which is similar to flight following.
Meteorological conditions that meet the minimum requirements for VFR flight are termed visual meteorological conditions (VMC). If they are not met, the conditions are considered instrument meteorological conditions (IMC), and a flight may only operate under IFR. IFR operations have specific training requirements and certification required of the pilot, and increased equipment requirements for the aircraft. Additionally, an IFR flight plan must usually be filed in advance. For efficiency of operations, some ATC operations will routinely provide "pop-up" IFR clearances for aircraft operating VFR, but that are arriving at an airport that does not meet VMC requirements. For example, in the United States, California's Oakland (KOAK), Monterey (KMRY) and Santa Ana (KSNA) airports routinely grant temporary IFR clearance when a low coastal overcast forces instrument approaches, while the rest of the state is still under visual flight rules.
In most if not all countries of the world, VFR pilots also have an option for requesting Special VFR when meteorological conditions at an airport are below normal VMC minima, but above Special VFR requirements. Special VFR is only intended to enable takeoffs and landings from airports that are near to VMC conditions, and may in some States only be performed during daytime hours if a pilot does not possess an instrument rating.
VFR flight is not allowed in airspace known as class A, regardless of the meteorological conditions except after failure of two way radio communications. In the United States, class A airspace begins at 18,000 feet msl, and extends to an altitude of 60,000 feet msl.
Read more about this topic: Visual Flight Rules