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Flight under SVFR is only allowed in controlled airspace, and always requires clearance from air traffic control (ATC). [4] 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]
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. [3] 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.
Air traffic control may also issue a "special VFR" clearance to VFR aircraft, to allow departure from a control zone in less than VMC – this reduces the visibility minimum to 1,600 m. Generally, VMC requires greater visibility and cloud clearance in controlled airspace than in uncontrolled airspace.
VFR visibility and cloud clearance requirements are the same as for class C and D airspaces when below 10,000 feet (3,000 m) MSL. Above 10,000 ft MSL, the visibility requirement is extended to 5 miles (8 km) and the cloud clearance requirement is extended to 1,000 feet (300 m) below clouds, 1,000 feet (300 m) above, and 1 mile (1.6 km) laterally.
VFR and IFR aircraft now require taxi clearance in the "manoeuvring area" of the aerodrome, but can still taxi within set apron areas without a clearance. IFR aircraft now receive slot times and the visibility requirements of Special VFR are reduced from 3000m visibility to 1600m.
In some countries pilots are required to get an air traffic control (ATC) clearance, or in absence of a controller, advise the nearest flight service station or center. Pilots are required to adhere to VFR minima when climbing and descending over the clouds. Flight in cloud is not permitted. [1]
A clearance always specifies a clearance limit, which is the farthest the aircraft can fly without a new clearance. In addition, a clearance typically provides a heading or route to follow, altitude, and communication parameters, such as frequencies and transponder codes. In uncontrolled airspace, ATC clearances are unavailable.
VFR over-the-top is different from VFR-on-top, in that VFR-on-top is an IFR clearance that allows the pilot to fly VFR altitudes, while VFR over-the-top is strictly a VFR operation in which the pilot maintains VFR cloud clearance requirements while operating on top of an undercast layer.