In aviation, visual flight rules (VFR) is a set of regulations under which a pilot operates an aircraft in weather conditions generally clear enough to allow the pilot to see where the aircraft is going. Specifically, the weather must be better than basic VFR weather minima, i.e., in visual meteorological conditions (VMC), as specified in the rules of the relevant aviation authority. The pilot must be able to operate the aircraft with visual reference to the ground, and by visually avoiding obstructions and other aircraft. If the weather is less than VMC, pilots are required to use instrument flight rules, and operation of the aircraft will be primarily through referencing the instruments rather than visual reference. In a control zone, a VFR flight may obtain a clearance from air traffic control to operate as Special VFR.
Requirements
VFR requires a pilot to be able to see outside the cockpit to control the aircraft's altitude, 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 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. In some countries, VFR flight is permitted at night, and is known as night VFR. This is generally permitted only under more restrictive conditions, such as maintaining minimum safe altitudes, and may require additional training as a pilot at night may not be able to see and avoid obstacles. 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 from IFR aircraft. 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. For pilots without an instrument rating who cannot legally fly by instrument flight rules, the restrictions of VMC minima can be troublesome in locations where weather conditions can change suddenly and unexpectedly or when weather events are highly localized. For instance, a small cloud forming over the airport at less than 1000 feet technically requires the airport to allow only IFR flights using instrument approaches/departures. A VFR flight intending to land there would normally be denied clearance, and would either have to divert to another field with VMC, or declare an emergency and override the denial of clearance, which can prompt an inquiry and possibly result in adverse consequences for the pilot. To avoid these scenarios, VFR flights intending to land at or take off from an airport experiencing localized conditions marginally below VMC minima may request Special VFR clearance from the tower. SVFR flight is only allowed while within the portion of an airport's controlled airspace (class B, C, or D) that extends to the surface, and it must be explicitly requested by the pilot and granted by the tower (some airports, primarily large Class B facilities, do not allow SVFR operation in their airspace at all). Visibility on the ground must still be greater than one statute mile, but most other VMC minima such as ceiling are waived. The pilot is required to maintain VFR separation distances from other aircraft and, by requesting SVFR, asserts that they can do so despite the marginal conditions. For departing flights, the pilot must be flying in VMC by the time they leave the surface footprint of the controlled airspace (otherwise they would be flying VFR in IMC which is illegal). VFR flight is not allowed in airspace known as class A, regardless of the meteorological conditions except after failure of two way radio communications or during declared emergencies such as VFR traffic attempting to avoid severe weather formations. In the United States, class A airspace is measured using flight levels, and begins at FL180 up to FL600, which is about 18,000 to 60,000 feet as measured using an altimeter at standard pressure (29.92 inHg, 1013 mbar). An exception to IFR-only flying in this airspace is the occasional allowance of sailplanes within designated wave windows which may be opened by air traffic control when high altitude flights are to be conducted into mountain lee waves.
Traffic advisories In the United States, Canada, and Australia, a pilot operating under VFR outside Class B, C, D airspace can request "flight following" from ATC, to receive continuous verbal updates on air traffic. This service is provided by ATC if workload permits, but it is an advisory service only. The responsibility for maintaining separation with other aircraft and proper navigation still remains with the pilot in command (PIC). In the United Kingdom, this is known as a "Traffic Service". In other countries, it is known as "Flight Information Service".
Pilot certifications In the United States and Canada, any certified pilot who meets specific recency of experience criteria may operate an airworthy aircraft under VFR.
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