In aviation, visual meteorological conditions (or VMC) are those in which visual flight rules (VFR) flight is permitted—that is, conditions in which pilots have sufficient visibility to fly the aircraft maintaining visual separation from terrain and other aircraft. They are the opposite of Instrument Meteorological Conditions (IMC). The boundary criteria between IMC and VMC are known as the VMC minima. As part of private pilot training, pilots are required to demonstrate some instrument flying skills. This is usually done in the aircraft with simulated IMC conditions using products like blockalls.

Visual meteorological conditions are usually defined by certain visibility minimums, cloud ceilings (for takeoffs and landings), and cloud clearances.

The exact requirements vary by type of airspace, whether it is day or night (for countries that permit night VFR), and from country to country. Typical visibility requirements vary from one statute mile to five statute miles (many countries define these in metric units as 1,500m to 8km). Typical cloud clearance requirements vary from merely remaining clear of clouds to remaining at least one mile away (1,500m in some countries) from clouds horizontally and one thousand feet away from clouds vertically. For instance, in Australia, VMC minima outside controlled airspace are clear of cloud with 5,000m visibility below 3,000ft AMSL or 1,000ft AGL (whichever is higher), and 1,000ft vertical/1,500m horizontal separation from cloud above these altitudes or in controlled airspace. Above 10,000ft, 8,000m visibility is required to maintain VMC. 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 3,000m.

Generally, VMC requires greater visibility and cloud clearance in controlled airspace than in uncontrolled airspace. In uncontrolled airspace there is less risk of a VFR aircraft colliding with an IFR aircraft emerging from a cloud, so aircraft are permitted to fly closer to clouds. An exception to this rule is class B airspace class, in which ATC separates VFR traffic from other VFR traffic and from IFR traffic, which is why in Class B Airspace lower cloud clearance is required.

UK Weather Minima for VFR Flight within Controlled Airspace (Classes C to E)

• At and above FL 100

8 km flight visibility, 1500m horizontally from cloud, 1000ft vertically from cloud

• Below FL 100

5 km flight visibility, 1500m horizontally from cloud, 1000ft vertically from cloud

• At or below 3000ft

Same as (b) above

For fixed wing aircraft operating at 140kt or less 5 km flight visibility Clear of cloud and in sight of surface

For helicopter Clear of cloud and in sight of surface

UK Weather minima for VFR flight outside Controlled Airspace (Classes F and G Airspace)

• At and above FL 100

8 km flight visibility, 1500m horizontally from cloud, 1000ft vertically from cloud

• Below FL 100

5 km flight visibility, 1500m horizontally from cloud, 1000ft vertically from cloud

• At or below 3000ft

As in (b) above or: for any aircraft: 5 km flight visibility, Clear of cloud and with the surface in sight

or

for an aircraft, other than a helicopter, operating at 140kt or less: 1500 m flight visibility, Clear of cloud and with the surface in sight.

for helicopters: 1500m horizontally from cloud Clear of cloud, with the surface in sight and in a flight visibility of at least 1500m.

[edit] See also

• List of meteorology topics

[edit] External links

• U.S. Federal Aviation Regulations defining visual meteorological conditions.

• Guide to Visual Flight Rules in the UK