Airspace Classes and VFR
Authorities use the ICAO definitions to derive additional rules for VFR cloud clearance, visibility, and equipment requirements.
For example, consider Class E airspace. It is possible that an aircraft operating under VFR is not be communication with ATC, so it is imperative that its pilot be able to see and avoid other aircraft (and vice versa). That includes IFR flights emerging from a cloud, so the VFR flight must keep a designated distance from the edges of clouds above, below, and laterally, and must maintain at least a designated visibility, to give the two aircraft time to observe and avoid each other. The low-level speed limit of 250 knots does not apply above 10,000 feet (3,000 m), so the visibility requirements are higher.
On the other hand, in Class B and Class C airspaces, separation is provided by ATC to all aircraft. Now the VFR pilot only needs to see where it is going, so visibility requirements are reduced and there is no designated minimum distance from clouds.
Similar considerations determine whether a VFR aircraft must use a two-way radio and/or a transponder.
Read more about this topic: Airspace Class
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