Instrument meteorological conditions
Encyclopedia
Instrument meteorological conditions (IMC) is an aviation flight category that describes weather conditions that require pilots to fly primarily by reference to instruments, and therefore under Instrument Flight Rules
(IFR), rather than by outside visual references under Visual Flight Rules
(VFR). Typically, this means flying in cloud
or bad weather
. Pilots sometimes train to fly in these conditions with the aid of products like Foggles, specialized glasses that restrict outside vision, forcing the student to rely on instrument indications only.
The weather conditions required for flight under VFR are known as Visual Meteorological Conditions
(VMC). IMC and VMC are 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, around which are known as "marginal VMC".
With good visibility, pilots can determine the attitude of the aircraft by utilising visual cues from outside the aircraft, most significantly the horizon
. 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 Rules
(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 airspace
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 IFR (Instrument flight rules
) - 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 VFR in conditions that are legally considered VMC and have to rely on flight instruments for attitude control. Two examples would be on a dark night over water (black hole effect), or a clear night with lights on the water and stars in the sky looking the same, i.e. no distinct horizon to fly by.
Instrument flight rules
Instrument flight rules are one of two sets of regulations governing all aspects of civil aviation aircraft operations; the other are visual flight rules ....
(IFR), rather than by outside visual references under Visual Flight Rules
Visual flight rules
Visual 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 cloud
Cloud
A cloud is a visible mass of liquid droplets or frozen crystals made of water and/or various chemicals suspended in the atmosphere above the surface of a planetary body. They are also known as aerosols. Clouds in Earth's atmosphere are studied in the cloud physics branch of meteorology...
or bad weather
Weather
Weather is the state of the atmosphere, to the degree that it is hot or cold, wet or dry, calm or stormy, clear or cloudy. Most weather phenomena occur in the troposphere, just below the stratosphere. Weather refers, generally, to day-to-day temperature and precipitation activity, whereas climate...
. Pilots sometimes train to fly in these conditions with the aid of products like Foggles, specialized glasses that restrict outside vision, forcing the student to rely on instrument indications only.
The weather conditions required for flight under VFR are known as Visual Meteorological Conditions
Visual meteorological conditions
In aviation, visual meteorological conditions is an aviation flight category 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...
(VMC). IMC and VMC are 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, around which are known as "marginal VMC".
With good visibility, pilots can determine the attitude of the aircraft by utilising visual cues from outside the aircraft, most significantly the horizon
Horizon
The horizon is the apparent line that separates earth from sky, the line that divides all visible directions into two categories: those that intersect the Earth's surface, and those that do not. At many locations, the true horizon is obscured by trees, buildings, mountains, etc., and the resulting...
. 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 Rules
Visual flight rules
Visual 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 airspace
Airspace class
The 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 IFR (Instrument flight rules
Instrument flight rules
Instrument flight rules are one of two sets of regulations governing all aspects of civil aviation aircraft operations; the other are visual flight rules ....
) - 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 VFR in conditions that are legally considered VMC and have to rely on flight instruments for attitude control. Two examples would be on a dark night over water (black hole effect), or a clear night with lights on the water and stars in the sky looking the same, i.e. no distinct horizon to fly by.
External Links
- "Up Through The Soup", February 1931, Popular Mechanics Early system for night-bad weather flying by mail pilots