Instrument meteorological conditions (IMC), sometimes referred to as
Blind flying, is an aviation term that describes weather conditions that normally require pilots to fly primarily by reference to instruments, and therefore under
Instrument Flight RulesInstrument flight rules are regulations and procedures for flying aircraft by referring only to the aircraft instrument panel for navigation. Even if nothing can be seen outside the cockpit windows, an IFR-rated pilot can fly while looking only at the instrument panel...
(IFR), rather than by outside visual references under
Visual Flight RulesVisual flight rules are a set of regulations which allow a pilot to operate an aircraft in weather conditions generally clear enough to allow the pilot to see where the aircraft is going. Specifically, the weather must be better than Basic VFR Weather Minimums, as specified in the rules of the...
(VFR). Typically, this means flying in
cloudA cloud is a visible mass of droplets or frozen crystals suspended in the atmosphere above the surface of the Earth or another planetary body. A cloud is also a visible mass attracted by gravity, such as masses of material in space called interstellar clouds and nebulae...
, bad
weatherWeather is a set of all the phenomena occurring in a given atmosphere at a given time. Weather phenomena lie in the troposphere. Weather refers, generally, to day-to-day temperature and precipitation activity, whereas climate is the term for the average atmospheric conditions over longer periods...
or at night. Pilots normally train to fly in these conditions with the aid of products like Blockalls, which simulate zero visibility.
The weather conditions required for flight under VFR are known as
Visual Meteorological ConditionsIn aviation, visual meteorological conditions are those in which visual flight rules 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...
(VMC).
Instrument meteorological conditions (IMC), sometimes referred to as
Blind flying, is an aviation term that describes weather conditions that normally require pilots to fly primarily by reference to instruments, and therefore under
Instrument Flight RulesInstrument flight rules are regulations and procedures for flying aircraft by referring only to the aircraft instrument panel for navigation. Even if nothing can be seen outside the cockpit windows, an IFR-rated pilot can fly while looking only at the instrument panel...
(IFR), rather than by outside visual references under
Visual Flight RulesVisual flight rules are a set of regulations which allow a pilot to operate an aircraft in weather conditions generally clear enough to allow the pilot to see where the aircraft is going. Specifically, the weather must be better than Basic VFR Weather Minimums, as specified in the rules of the...
(VFR). Typically, this means flying in
cloudA cloud is a visible mass of droplets or frozen crystals suspended in the atmosphere above the surface of the Earth or another planetary body. A cloud is also a visible mass attracted by gravity, such as masses of material in space called interstellar clouds and nebulae...
, bad
weatherWeather is a set of all the phenomena occurring in a given atmosphere at a given time. Weather phenomena lie in the troposphere. Weather refers, generally, to day-to-day temperature and precipitation activity, whereas climate is the term for the average atmospheric conditions over longer periods...
or at night. Pilots normally train to fly in these conditions with the aid of products like Blockalls, which simulate zero visibility.
The weather conditions required for flight under VFR are known as
Visual Meteorological ConditionsIn aviation, visual meteorological conditions are those in which visual flight rules 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...
(VMC). IMC and VMC are obviously mutually exclusive. In fact Instrument meteorological conditions are defined as less than the minima specified for visual meteorological conditions. The boundary criteria between VMC and IMC are known as the VMC minima.
With good visibility, pilots can determine the attitude of the aircraft by utilising visual cues from outside the aircraft, most significantly the
horizonThe horizon is the apparent line that separates earth from sky.It is the line that divides all visible directions into two categories: those that intersect the Earth's surface, and those that do not...
. Without such external visual cues, pilots must use an internal cue of attitude, which is provided by gyroscopically-driven instruments such as the Attitude Indicator (or "Artificial Horizon"). The availability of a good horizon cue is controlled by meteorological visibility, hence minimum visibility limits feature in the VMC minima. Visibility is also important in the avoidance of terrain.
Since the basic traffic avoidance principle of flying under
Visual Flight RulesVisual flight rules are a set of regulations which allow a pilot to operate an aircraft in weather conditions generally clear enough to allow the pilot to see where the aircraft is going. Specifically, the weather must be better than Basic VFR Weather Minimums, as specified in the rules of the...
(VFR) is "see and avoid", it also follows that distance from cloud is an important factor in the VMC minima: as aircraft in cloud cannot be seen, a buffer zone from cloud is required.
ICAO recommends the VMC minima internationally; they are defined in national regulations, which rarely significantly vary from ICAO. the main variation is in the units of measurement as different states often use different units of measurement in aviation. The criteria tend to be stricter in controlled airspace, where there is a lot of 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
Class A and B airspaceThe world’s navigable airspace is divided into three-dimensional segments, each of which is assigned to a specific class. Most nations adhere to the classification specified by the International Civil Aviation Organization and described below...
where all aircraft are provided with standard 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 distance from cloud minima.
It is important not to confuse IMC with
Instrument flight rulesInstrument flight rules are regulations and procedures for flying aircraft by referring only to the aircraft instrument panel for navigation. Even if nothing can be seen outside the cockpit windows, an IFR-rated pilot can fly while looking only at the instrument panel...
(IFR) -- "IMC" describes the actual weather conditions, while "IFR" describes the rules under which the aircraft is flying. Aircraft can (and often do) fly IFR in clear weather, for operational reasons, or when flying in airspace where flight under VFR is not permitted; indeed by far the majority of commercial flights are operated solely under IFR.
It is possible to be flying in VFR / VMC conditions and have to rely on flight instruments for attitude control. Two examples would be on a dark night over water, or a clear night with lights on the water and stars in the sky looking the same, ie. no distinct horizon to fly by.