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Concept
Instrument meteorological conditions
Summary
In aviation, instrument meteorological conditions (IMC) are weather conditions that require pilots to fly primarily by reference to flight instruments, and therefore under instrument flight rules (IFR), as opposed to flying by outside visual references under visual flight rules (VFR). Typically, this means flying in cloud or poor weather, where little or nothing can be seen or recognised when looking out of the window. Simulated IMC can be achieved for training purposes by wearing view-limiting devices, which restrict outside vision and force the trainee to rely on instrument indications only.
The weather conditions required for flight under VFR are known as visual meteorological conditions (VMC). The boundary criteria between VMC and IMC are known as VMC minima. IMC and VMC are mutually exclusive. In fact, instrument meteorological conditions are defined as less than the minima specified for visual meteorological conditions. Conditions that are above VMC minima but relatively close to one or more of them are sometimes referred to as marginal VMC, and flight in such conditions is referred to as marginal VFR.
ICAO recommends the VMC minima internationally; they are defined and enforced by national regulations, which rarely significantly vary from ICAO. The typical variation is in the units of measurement as different regulatory authorities use different units of measurement in aviation.
The VMC minima tend to be stricter in controlled airspace, where there is increased air traffic, therefore greater visibility and cloud clearance is desirable. The degree of separation provided by air traffic control is also a factor. For example, in strictly-controlled class A and B airspace, where all aircraft are provided with positive separation, the VMC minima feature visibility limits only, whereas in classes C–G airspace, where some or all aircraft are not separated from each other by air traffic control, the VMC minima also feature cloud separation criteria.
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