Visual flight rules (VFR) are a set of aviation regulations under which a pilot may operate an aircraft in weather conditions sufficient to allow the pilot, by visual reference to the environment outside the cockpit, to control the aircraft's attitude, navigate, and maintain safe separation from obstacles such as terrain, buildings, and other aircraft. A VFR flight is a "flight conducted in accordance with the visual flight rules".
The essential collision safety principle guiding the VFR pilot is "see and avoid." 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. Near busier airports, and while operating within certain types of airspace classifications, VFR aircraft in Class B & Class C airspace are required to have a transponder. Governing agencies establish specific requirements for VFR flight, consisting of minimum visibility, distance from clouds, and altitude to ensure that aircraft operating under VFR can be seen from a far enough distance to ensure safety.
From a regulatory perspective, airspace is categorized as controlled and uncontrolled. In controlled airspace known as class B, air traffic control (ATC) will separate VFR aircraft from all other aircraft. In most other types of controlled airspace, ATC is only required to maintain separation to aircraft operating under instrument flight rules (IFR), but workload permitting will assist all aircraft. In the United States, a pilot operating VFR outside of class B airspace can request "VFR traffic following" from air traffic control (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.
