Instrument meteorological conditions

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A pilot's view of the runway just before landing in thick fog at night CAT IIIA Landing.jpg
A pilot's view of the runway just before landing in thick fog at night

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.

Contents

Distinction from Visual Meteorological Conditions

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. [1] 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. [2]

Summary of United States VMC minima (14 CFR 91.155), providing requirements for minimum visibility and separation from clouds for different airspace classes. In this example, for airspace classes C/D/E up to an altitude of 10,000 ft (3,000 m) AMSL, whether day or night, VMC minima require 3 miles (5 km) of visibility (denoted "3 SM") and cloud clearance of 1,000 ft (300 m) above, 500 ft (150 m) below, and 2,000 ft (610 m) horizontally ("152"). US airspace visibility 2016.jpg
Summary of United States VMC minima ( 14 CFR 91.155 ), providing requirements for minimum visibility and separation from clouds for different airspace classes. In this example, for airspace classes C/D/E up to an altitude of 10,000 ft (3,000 m) AMSL, whether day or night, VMC minima require 3 miles (5 km) of visibility (denoted "3 SM") and cloud clearance of 1,000 ft (300 m) above, 500 ft (150 m) below, and 2,000 ft (610 m) horizontally ("152").

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.

Visibility and separation from clouds

With good visibility, pilots can determine the aircraft attitude by utilising visual cues from outside the aircraft, most significantly the horizon. Without such external visual cues, pilots may be subject to sensory illusions and must use an alternative reference for the attitude, which is usually provided by gyroscopically-driven instruments such as the attitude indicator ("artificial horizon"). The availability of a good horizon cue is controlled by meteorological visibility, hence minimum visibility limits feature in the VMC minima.

Because the basic traffic avoidance principle of flying under visual flight rules (VFR) is to "see and avoid", it follows that distance from clouds is an important factor in the VMC minima: as aircraft flying in clouds cannot be seen, a buffer zone from clouds established by the minimum separation requirements provides for time to react to an unseen/unknown aircraft exiting the clouds, especially when air traffic control may not be enforcing aircraft separation (as in airspace classes C-G).

Use of flight instruments under VMC

IMC should not be confused 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; for example, in the United States, flight under VFR in class A airspaces is prohibited except in emergencies. Indeed by far the majority of commercial flights are operated solely under IFR.

It is possible to be flying under VFR in conditions that are legally considered VMC, but still be forced to rely on flight instruments for attitude control because there is no distinct external horizon; for example, at night over water, which may create a so-called black hole effect if the sky and ground are equally dark, or when lights on the water cannot be distinguished from stars in the sky.

Inadvertent entry into Instrument Meteorological Conditions

If weather deteriorates during flight or the aircraft flies into clouds, a flight that started out under VFR may turn into a flight under IMC. This is known as inadvertent entry into instrument meteorological conditions (IIMC), or more briefly VFR into IMC. IIMC is a potentially dangerous situation that has resulted in many accidents, [3] as pilots may succumb to spatial disorientation, leading to loss of control or controlled flight into terrain. [4] Statistics from the Federal Aviation Administration indicate that spatial disorientation is a factor in approximately 15% of general aviation accidents; of those, approximately 90% are fatal. [5] Other statistics indicate that 4% of general aviation accidents were attributable to weather; of those weather-related accidents, 50% resulted from VFR into IMC, and 72% of the VFR into IMC accidents were fatal. [6]

In the 180—Degree Turn Experiment conducted in 1954 by the University of Illinois, twenty student pilots flew from VFR into simulated IMC; after entry, all of them eventually reached a dangerous flight condition or attitude [lower-alpha 1] over a period ranging from 20 to 480 seconds. [7] : 16  The average time to reach a dangerous condition was 178 seconds, echoed in the title of the "178 Seconds to Live" article distributed by the Federal Aviation Administration in 1993; [8] however, the original 1954 study was noted for simulating an aircraft the subjects had little to no experience with, and only providing a partial instrument panel. [4] In addition, the "178 seconds" average time was extracted from the preliminary evaluation; after training for a standardized procedure to exit IMC, each student pilot was tested three times, and 59 of the 60 resulting simulated flights successfully resulted in a controlled descent out of the cloud deck without reaching a dangerous condition. [9]

See also

Notes

  1. ↑ The authors defined the dangerous flight condition or attitude as one of the following four situations: [7] : 8 
    1. A stall, either normal or accelerated
    2. A bank exceeding 45°
    3. An excessive speed (more than normal fast cruise)
    4. Obvious or prolonged loss of altitude or directional orientation

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In aviation, instrument flight rules (IFR) is one of two sets of regulations governing all aspects of civil aviation aircraft operations; the other is visual flight rules (VFR).

In aviation, visual flight rules (VFR) are a set of regulations under which a pilot operates 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 minima, i.e., in visual meteorological conditions (VMC), as specified in the rules of the relevant aviation authority. The pilot must be able to operate the aircraft with visual reference to the ground, and by visually avoiding obstructions and other aircraft.

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<span class="mw-page-title-main">Visual meteorological conditions</span> Flight category allowing pilots to fly using vision as well as instruments

In aviation, visual meteorological conditions (VMC) is an aviation flight category 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 and are defined by: visibility, cloud ceilings, and cloud clearances.

Instrument rating refers to the qualifications that a pilot must have in order to fly under instrument flight rules (IFR). It requires specific training and instruction beyond what is required for a private pilot certificate or commercial pilot certificate, including rules and procedures specific to instrument flying, additional instruction in meteorology, and more intensive training in flight solely by reference to instruments.

In general aviation, scud running is a practice in which pilots lower their altitude to avoid clouds or instrument meteorological conditions (IMC). The goal of scud running is to stay clear of weather to continue flying with visual, rather than instrument, references. This practice is widely accepted to be dangerous, and has led to death in many cases from pilots flying into terrain or obstacles, such as masts and towers, normally referred to as controlled flight into terrain, or CFIT; however, even instrument-rated pilots sometimes elect to take the risk to avoid icing or embedded thunderstorms in cloud, or in situations where the minimum instrument altitudes are too high for their aircraft.

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 (ICAO) and described below, though they might use only some of the classes defined below, and significantly alter the exact rules and requirements. Similarly, individual nations may also designate special use airspace (SUA) with further rules for reasons of national security or safety.

An instrument rating is an authorization required for a pilot to fly under instrument flight rules (IFR). In the United States, the rating is issued by the Federal Aviation Administration.

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In United States aviation VFR-on-top is defined as air traffic control authorization for an aircraft operating under instrument flight rules (IFR) to do so in visual meteorological conditions (VMC) at any appropriate visual flight rules (VFR) altitude. A pilot receiving this authorization must comply with the VFR visibility, distance from cloud criteria, and the minimum IFR altitudes. The use of this term does not relieve controllers of their responsibility to separate aircraft in Class B and Class C airspace or TRSAs.

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1991 Vallejo helicopter crash

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<span class="mw-page-title-main">1963 Camden PA-24 crash</span> Aviation crash in 1963

On March 5, 1963, American country music performers Patsy Cline, Cowboy Copas, and Hawkshaw Hawkins were killed in an airplane crash near Camden, Tennessee, United States, along with the pilot Randy Hughes. The accident occurred as the three artists were returning home to Nashville, Tennessee, after performing in Kansas City, Kansas.

Continued VFR into IMC is when an aircraft operating under visual flight rules intentionally or unintentionally enters into instrument meteorological conditions. Flying an aircraft without visual reference to the ground can lead to a phenomenon known as spatial disorientation, which can cause the pilot to misperceive the angle, altitude, and speed they are traveling. This is considered a very serious safety hazard in general aviation. According to AOPA’s Nall Report, approximately 4% of general aviation accidents are weather related, yet these accidents account for more than 25% of all fatalities.

References

  1. ↑ "Pilot/Controller Glossary". Archived from the original on 2009-09-27. Retrieved 2009-09-21.
  2. ↑ "Spatial Disorientation Accidents: VFR in VMC". AOPA . Retrieved 15 July 2023.
  3. ↑ "Training Fact Sheet – Inadvertent Entry Into Instrument Meteorological Conditions (IIMC)" (PDF). Inadvertent entry into IMC is a situation where deteriorating weather prevents you from flying under visual meteorological conditions when you were planning to fly under VFR.{{cite web}}: CS1 maint: url-status (link)
  4. 1 2 Rowland, David (February 5, 2017). "Surviving VFR into IMC". AV Web. Retrieved 26 February 2021.
  5. ↑ LeCompte, Tom (September 2008). "The Disorient Express". Air & Space. Retrieved 26 February 2021.
  6. ↑ Ison, David (February 6, 2016). "Understanding VFR Into IMC Accidents". Plane & Pilot. Retrieved 26 February 2021.
  7. 1 2 Aulls Bryan, Leslie; Stonecipher, Jesse W.; Aron, Karl (1954). 180-degree turn experiment. University of Illinois. ASIN   B0007EXGMI. LCCN   a54009717. OCLC   4736008. OL   207786M.
  8. ↑ "178 Seconds to Live" (PDF). Federal Aviation Administration. Retrieved 26 February 2021.
  9. ↑ Dubois, William E. (February 4, 2016). "The lost lessons of '178 seconds to live'". P&E: Proficiency [blog]. Aircraft Owners & Pilots Association. Retrieved 26 February 2021.
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