Ice, ice baby too cold, too cold...
— Vanilla Ice

Types of icing and their differences

  • Clear Ice− A glossy, clear, or translucent ice formed by the relatively slow freezing or large supercooled water droplets.
  • Rime Ice− Rough, milky, opaque ice formed by the instantaneous freezing of small supercooled water droplets.
  • Mixed− A mixture of clear ice and rime ice.
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Degrees of icing

Trace− Ice becomes perceptible. Deicing/anti-icing equipment is not utilized unless encountered for an extended period of time (over 1 hour).

Light− The rate of accumulation may create a problem if flight is prolonged in this environment (over 1 hour). It does not present a problem if the deicing/anti-icing equipment is used.

Moderate− The rate of accumulation is such that even short encounters become potentially hazardous and use of deicing/anti-icing equipment or flight diversion is necessary.

Severe− The rate of ice accumulation is such that ice protection systems fail to remove the accumulation of ice, or ice accumulates in locations not normally prone to icing. Immediate exit from the condition is necessary.

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Conditions prone to icing

  • Particle size & concentration
  • Shape of aircraft surfaces
  • Aircraft speed
  • Air temperature
  • Supercooled liquid water droplets or wet snowflakes
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Effect on Aircraft

  • Decrease in Lift and/or Increase in Weight can lead to aircraft losing altitude
  • Decrease in Thrust and/or Increase of Drag can result in loss of airspeed and lift
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Temperature Inversion

There are times wherein instead of consistently getting colder with altitude that an inversion later will exist which can cause temperatures to raise before getting colder again.

Supercooled Large Droplets (SLD)

What is SLD?

  • Ordinarily, temperatures decrease with altitude. However, when there is a temperature inversion, this is not the case. A layer of cold air lies under a layer of warmer air.
  • Freezing rain forms when snow falls into air that is above freezing and melts, forming liquid precipitation. These liquid water droplets continue to fall into a layer of air that is at or below freezing which can form SLD.
  • Potential for encountering SLD is most likely within the temperature inversion- which is most often from 3,000-12,000 feet.
  • Typically when a SLD strikes an airframe refreezing to ice occurs, which can quickly accumulate creating dangerous conditions.
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How can ATC help pilots in SLD

  • When advised SLD conditions exists inform aircraft on your frequency.
  • Solicit other aircraft to find areas where the outside air temperature is above freezing and free of clouds.
  • Solicit and disseminate pilot reports of icing in a timely manner.
  • Avoid issuing rapid descents for aircraft close to the ground.
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Carburetor icing

  • Float-Type Carburetors are most prone to icing.
  • Icing is a result of fuel vaporization leading to a decrease in air pressure which results in a temperature drop.
  • Carburetor icing is most likely to occur below 70F/21C with relative humidity above 80%.
  • However it is not restricted to this range and can be seen in outside air temperatures up to 100F/38C.
  • Carburetor Heat exists as a method to prevent icing.
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Carburetor icing accident (VH-TXF)

  • 14 September 2014 an experimental aircraft departed Dubbo Airport, New South Wales, Australia.
  • Witnesses recalled the aeroplane’s engine ‘splutter’, silence during the turn, and followed by a ‘rev’ followed again by silence.
  • The ATSB found that engine failure was likely due to Carburetor icing.  There were additional contributing factors.
  • Finding included encouraging pilot weather awareness and considering icing risk.
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Cold Temperature Correction
"From hot to cold, look out below."
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Cold Temperature Correction

  • When temperatures are much lower than the standard atmosphere there may be the need to adjust altitude to compensate for the effects on the altimeters.
  • When temperatures are below International Standard Atmosphere (ISA) the aircraft will be lower than anticipated; thus requiring a higher altitude for terrain and obstacle avoidance- or Cold Temperature Correction (CTC).

ADDITIONAL SOURCES OF INFORMATION:

Read more on the FAA Website

Read more on the Flight Safety Website

Read more on the Nasa Website

Read more on the SkyBrary Website

Cold Temperature Correction

ICAO Doc 4444 (PANS-ATM)

8.6.5.2 When vectoring an IFR flight and when giving an IFR flight a direct routing which takes the aircraft off an ATS route, the controller shall issue clearances such that the prescribed obstacle clearance will exist at all times until the aircraft reaches the point where the pilot will resume own navigation. When necessary, the relevant minimum vectoring altitude shall include a correction for low temperature effect.

Note 1.— When an IFR flight is being vectored, the pilot may be unable to determine the aircraft’s exact position in respect to obstacles in this area and consequently the altitude which provides the required obstacle clearance. Detailed obstacle clearance criteria are contained in PANS-OPS (Doc 8168), Volumes I and II. See also 8.6.8.2.

Note 2.— It is the responsibility of the ATS authority to provide the controller with minimum altitudes corrected for temperature effect.

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ICAO. (2020). Doc 4444, Procedures for Air Navigation Services - Air Traffic Management (PANS-ATM), 16th Ed., 9th Amendment. Montréal, Canada: International Civil Aviation Organization.

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Calculations

ICAO Doc 8168 (PANS-OPS, Volume I)

1.7.1 Temperatures lower than those of the standard atmosphere result in the actual altitude of an aircraft being lower than that indicated by the barometric altimeter. As a consequence, the minimum obstacle clearance (MOC) actually achieved could be lower than the prescribed MOC. In order to prevent this, the pilot shall correct for low temperatures. The pilot is responsible for any necessary cold temperature corrections to all published minimum altitudes/heights in both conventional and PBN procedures.

a) the altitudes/heights for the initial and intermediate segment(s);

b) the decision altitude/height (DA/H) or minimum descent altitude/height (MDA/H); and

c) subsequent missed approach altitudes/heights.

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ICAO. (2020). Doc 8168, Procedures for Air Navigation Services - Aircraft Operations (PANS-OPS), Volume I, 6th Ed., 9th Amendment. Montréal, Canada: International Civil Aviation Organization.

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4% adjustment

ICAO Doc 8168 (PANS-OPS, Volume III)

4.3.1 Requirement for temperature correction The calculated minimum safe altitudes/heights must be adjusted when the ambient temperature on the surface is much lower than that predicted by the standard atmosphere. In such conditions, an approximate correction is 4 per cent height increase for every 10°C below standard temperature as measured at the altimeter setting source. This is safe for all altimeter setting source altitudes for temperatures above –15°C.

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ICAO. (2018). Doc 8168, Procedures for Air Navigation Services - Aircraft Operations (PANS-OPS), Volume III, 1st Ed. Montréal, Canada: International Civil Aviation Organization.

Awareness note:

Please review your individual State procedures for Cold Temperature Correction; application may vary between States.

4.1.3 State’s responsibility PANS-AIM, Appendix 2 (Contents of Aeronautical Information Publication), indicates that States should publish in Section GEN 3.3.5, “The criteria used to determine minimum flight altitudes”. If nothing is published, it should be assumed that no corrections have been applied by the State.

Note.— The determination of lowest usable flight levels by air traffic control units within controlled airspace does not relieve the pilot-in-command of the responsibility for ensuring that adequate terrain clearance exists, except when an IFR flight is being vectored.

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ICAO. (2018). Doc 8168, Procedures for Air Navigation Services - Aircraft Operations (PANS-OPS), Volume III, 1st Ed. Montréal, Canada: International Civil Aviation Organization.

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Controller Actions

  • If protocols for Cold Temperature Correction (CTC) exist it is important that controllers are made aware of their existence and are trained in the responses and responsibilities associated with them.
  • When cold temperature is seasonal, annual refresher training should be provided.
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Vectoring and low temperature correction

ICAO Doc 8168 (PANS-OPS, Volume III)

4.1.4 Air traffic control (ATC) If an aircraft is cleared by ATC to an altitude which the pilot-in-command finds unacceptable due to low temperature, then the pilot-in-command should request a higher altitude. If such a request is not received, ATC will consider that the clearance has been accepted and will be complied with.

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ICAO. (2018). Doc 8168, Procedures for Air Navigation Services - Aircraft Operations (PANS-OPS), Volume III, 1st Ed. Montréal, Canada: International Civil Aviation Organization.

See also: ICAO Annex 2 and ICAO Doc 4444 (PANS-ATM), Chapter 6.

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Aircraft CTC accident

Low Temperature Correction was one of the causal factors in a 1981 Bell 212 helicopter crash.  The crash took place at high latitudes in poor weather. 

The report states, “THE PILOT DESCENDED TO 150FT AGL ON HIS ALTIMETER. THE HELICOPTER STUCK THE SEA ICE AND CRASHED. TEMP CORRECTION NOT APPLIED.”

ADDITIONAL SOURCES OF INFORMATION:

Read more on the Twin and Turbine Website

Read more on the UK CAA Website