| Within the United States, flight can be undertaken under one of two different sets of regulations (similar regulations apply in all ICAO countries). These are the Visual Flight Rules and the Instrument Flight Rules. Visual Meteorological Conditions, aka VMC, must prevail for operation under the VFR rules.
VMC means that the visibility (distance one can see) and cloud separation meets certain minimum requirements that allow a pilot to visually see and avoid other aircraft. If conditions are below these minima, Instrument Meteorological Conditions prevail and only operation under IFR is authorized (with consequent increased regulation). Under certain circumstances Special VFR operation may also be authorized.
These requirements are set forth in 14CFR91.155, and are:
In Class B airspace: visibility at least 3 statute miles, and clear of clouds.
In Class C airspace, Class D airspace, and Class E airspace less than 10,000 feet MSL: visibility at least 3 statute miles, and at least 500 feet below, 1000 feet above, and 2000 feet horizontal separation from clouds.
In Class E airspace, at or above 10,000 feet MSL: visibility at least 5 statute miles, 1000 feet above, 1000 feet below and 1 statute mile horizontal separation from clouds.
In Class G airspace, the rules are more complicated:
- 1200 feet or less AGL, day: visibility at least 1 statute mile, and clear of clouds.
- 1200 feet or less AGL, night: visibility at least 3 statute miles, and at least 500 feet below, 1000 feet above, and 2000 feet horizontal separation from clouds.
- More than 1200 feet AGL and less than 10,000 feet MSL, day: visibility at least 1 statute mile, and at least 500 feet below, 1000 feet above, and 2000 feet horizontal separation from clouds.
- More than 1200 feet AGL and less than 10,000 feet MSL, night: visibility at least 3 statute miles, and at least 500 feet below, 1000 feet above, and 2000 feet horizontal separation from clouds.
- More than 1200 feet AGL and above 10,000 feet MSL: visibility at least 5 statute miles, 1000 feet above, 1000 feet below and 1 statute mile horizontal separation from clouds.
Because VFR operations are never permitted in Class A airspace (except under very obscure NORDO situations), there are no VFR minima for Class A airspace.
For airspace definitions see Airspace classifications.
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