De-Icing / Anti-Icing and Visibility in Snow vs. Snowfall Intensity
De-Icing / Anti-Icing
Introduction
Low temperatures and freezing precipitation may present specific problems to flight crews.
To deal with these critical weather conditions, special cold weather procedures have been established, for both flight and ground crews. They supplement the normal procedures and shall be observed when applicable. The intention of this section is to define general and specific information. Flight crew should consult the relevant sections of the FCOM for specific details.
Definitions
Active Frost: Condition when frost is forming. Active frost occurs when aeroplane surface temperature is at or below 0ºC and at or below the dew point.
Anti-Icing: a Precautionary procedure which provides protection against the formation of frost or ice and accumulation of snow or slush on treated surfaces of the aeroplane for a limited period of time (Hold Over Time).
Anti-Icing Fluid:
Check: An examination of an item against a relevant standard by a trained and qualified person.
Clear Ice: Coating of ice, generally clear and smooth, but with some air pockets. It is formed on exposed objects at temperatures below, or slightly above, freezing temperature, with the freezing of super-cooled drizzle, droplets or raindrops.
Cold-Soak Effect: Even in ambient temperature between -2°C and at least +15°C, ice or frost can form in the presence of visible moisture or high humidity if the aircraft structure remains at 0ºC or below. Anytime precipitation falls on a cold-soaked aircraft, while on ground, clear icing may occur. The wings of the aeroplane are said to be cold-soaked when they contain very cold fuel as a result of having just landed after a flight at high altitude or from having been re-fuelled with very cold fuel.
The following factors contribute to cold-soaking:
Contamination Check: A check of aeroplane surfaces for contamination to establish the need for de-icing.
Damp Runway: A runway is considered damp when the surface is not dry, but when the moisture on it does not give it a shiny appearance.
De-Icing: Procedure by which frost, ice, slush or snow is removed from an aeroplane in order to provide clean surfaces.
De-Icing/Anti-Icing: Combination of the de-icing and anti-icing procedures. It may be performed in one or two steps.
De-Icing Fluid:
Dew Point: Temperature at which water vapour starts to condense.
Freezing Drizzle: Fairly uniform precipitation composed exclusively of fine drops (diameter less than 0.5 mm) very close together which freezes upon impact with the ground or other exposed objects. Metar code: FZDZ.
Freezing Fog: A suspension of numerous minute water droplets which freezes upon impact with the ground or other exposed objects; generally reducing the horizontal visibility at the earth's surface to less than 1 km. Metar code: FZFG.
Freezing Rain: Precipitation of liquid water particles which freezes upon impact with the ground or other exposed objects, either in the form of drops of more than 0.5 mm (0.02 inch) diameter or smaller drops which, in contrast to drizzle, are widely separated. Metar code: FZRA.
Friction Coefficient: Relationship between the friction force acting on the wheel and the normal force on the wheel. The normal force depends on the weight of the aircraft and the lift of the wings.
Frost/Hoar Frost: Ice crystals that form from ice saturated air at temperatures below 0°C by direct sublimation on the ground or other exposed objects.
Hail: Precipitation of small balls or pieces of ice with a diameter ranging from 5 to >50 mm, falling either separately or agglomerated. Metar code: GR.
Hold Over Time (HOT): Estimated time for which an anti-icing fluid will prevent the formation of frost or ice and the accumulation of snow on the protected surfaces of an aeroplane, under specified weather conditions.
Ice Pellets: Precipitation of transparent (grains of ice), or translucent (small hail) pellets of ice, which are spherical or irregular, and which have a diameter of 5 mm or less. The pellets of ice usually bounce when hitting hard ground. Metar code: PE.
Icing Conditions: May be expected when the OAT (on the ground and for takeoff) or when TAT (in flight) is at or below 10ºC, and there is visible moisture in the air (such as clouds, fog with low visibility of one mile or less, rain, snow, sleet, ice crystals) or standing water, slush, ice or snow is present on the taxiways or runways.
Icy Runway: A runway is considered icy when its friction coefficient is 0.05 or below.
Light Freezing Rain: Precipitation of liquid water particles which freezes upon impact with the ground or other exposed objects, either in the form of drops of more than 0.5 mm or smaller drops which, in contrast to drizzle, are widely separated. The measured intensity of liquid water particles is up to 2.5 mm/hour or 25 grammes/dm2/hour with a maximum of 0.25 mm in 6 minutes.
Lowest Operational Use Temperature (LOUT): The lowest operational use temperature (LOUT) is the higher (warmer) of:
Precipitation: Liquid or frozen water that falls from clouds as rain, drizzle, snow, hail, or sleet.
Continuous Precipitation: Intensity changes gradually, if at all.
Intermittent Precipitation: Intensity changes gradually, if at all, but precipitation stops and starts at least once within the hour preceding the observation.
Precipitation Intensity: Indication of the amount of precipitation falling at the time of observation.
It is expressed as light, moderate or heavy. Each intensity is defined with respect to the type of precipitation occurring, based either on a rate of fall for rain and ice pellets or visibility for snow and drizzle. The rate of fall criteria is based on time and does not accurately describe the intensity at the time of observation.
Radiation Cooling: A process by which temperature decreases, due to an excess of emitted radiation over absorbed radiation. On a typical calm clear night aeroplane surfaces emit long-wave radiation, however, there is no solar radiation (shortwave) coming in at night and this long-wave emission will represent a constant net energy loss. Under these conditions, the aeroplane surface temperatures may be up to 4°C or more below that of the surrounding air.
Rain or High Humidity on Cold Soaked Wing: Water, visible moisture or humidity forming ice or frost on the wing surface, when the temperature of the aeroplane wing surface is at or below 0°C.
Rain: Precipitation of liquid water particles either in the form of drops of more than 0.5 mm (0.02 inch) diameter or of smaller widely scattered drops. Metar code: RA.
Rain and Snow: Precipitation in the form of a mixture of rain and snow.
Rime: A rough white covering of ice deposited from fog at the temperature below freezing). As the fog usually consists of super-cooled water drops, which only solidify on contact with a solid object, rime may form only on the windward side or edges and not on the surfaces. It can generally be removed by brushing, but when surfaces, as well as edges, are covered, it will be necessary to use an approved de-icing fluid.
Saturation: Maximum amount of water vapour allowable in the air. It is about 0.5 g/m3 at - 30ºC and 5 g/m3 at 0ºC for moderate altitudes.
Shear force: Force applied laterally on an anti-icing fluid. When applied to a Type II or IV fluids, the shear force will reduce the viscosity of the fluid; when the shear force is no longer applied, the anti-icing fluid should recover its viscosity. For instance, shear forces are applied whenever the fluid is pumped, forced through an orifice or when subjected to airflow. If excessive shear force is applied, the thickener system could be permanently degraded and the anti-icing fluid viscosity may not recover and may be at an unacceptable level.
Sleet: Precipitation in the form of a mixture of rain and snow. For operation in light sleet treat it as light freezing rain.
Slush: Snow or ice that has been reduced to a soft watery mixture.
Snow: Precipitation of ice crystals, most of which are branched, star-shaped or mixed with unbranched crystals. At temperatures higher than -5°C, the crystals are generally agglomerated into snowflakes. Metar code: SN.
Dry Snow: Snow which can be blown if loose or, if compacted by hand, will fall apart upon release. Specific gravity: up to but not including 0.35. Dry snow is normally experienced when the temperature is below freezing and can be brushed off easily from the aircraft.
Wet Snow: Snow which, if compacted by hand, will stick together and tend to or form a snowball. Specific gravity: 0.35 up to but not including 0.5. Wet snow is normally experienced when a temperature is above freezing and is more difficult to remove from the aircraft structure than dry snow being sufficiently wet to adhere.
Compacted Snow: Snow which has been compressed into a solid mass that resists further compression and will hold together or break up into chunks if picked up. Specific gravity: 0.5 and over.
Snow Grains: Precipitation of very small white and opaque particles of ice that are fairly flat or elongated with a diameter of less than 1 mm. When snow grains hit hard ground, they do not bounce or shatter. For hold over time purposes treat snow grains as snow. Metar code: SG.
Snow Pellets: Precipitation of white opaque particles of ice. The particles are round or sometimes conical; their diameter range from about 2-5 mm. Snow pellets are brittle, easily crushed; they do bounce and may break on the hard ground. Metar code: GS.
Supercooled Water Droplets: Condition where water remains liquid at negative Celsius temperature. Supercooled drops and droplets are unstable and freeze upon impact.
Visible Moisture: Fog, rain, snow, sleet, high humidity (condensation on surfaces), ice crystals or when taxiways and/or runways are contaminated by water, slush or snow.
Visual Meteorological Conditions: Meteorological conditions expressed in terms of visibility, distance from cloud, and ceiling, equal to or better than specified minima.
Visibility in Snow vs. Snowfall Intensity
Source: Transport Canada - Relationship between Visibility and Snowfall Intensity, and Theoretical Considerations in the Estimation of Snowfall Rate using Visibility.
Using the table as a guideline may help the flight crew in a determination of a snowfall intensity. This determination may be of assistance when using holdover tables.
Introduction
Low temperatures and freezing precipitation may present specific problems to flight crews.
To deal with these critical weather conditions, special cold weather procedures have been established, for both flight and ground crews. They supplement the normal procedures and shall be observed when applicable. The intention of this section is to define general and specific information. Flight crew should consult the relevant sections of the FCOM for specific details.
Definitions
Active Frost: Condition when frost is forming. Active frost occurs when aeroplane surface temperature is at or below 0ºC and at or below the dew point.
Anti-Icing: a Precautionary procedure which provides protection against the formation of frost or ice and accumulation of snow or slush on treated surfaces of the aeroplane for a limited period of time (Hold Over Time).
Anti-Icing Fluid:
- Type I fluid;
- Mixture of water and Type I fluid;
- Type II fluid, Type III fluid, or Type IV fluid;
- Mixture of water and Type II fluid, Type III fluid, or Type IV fluid.
Check: An examination of an item against a relevant standard by a trained and qualified person.
Clear Ice: Coating of ice, generally clear and smooth, but with some air pockets. It is formed on exposed objects at temperatures below, or slightly above, freezing temperature, with the freezing of super-cooled drizzle, droplets or raindrops.
Cold-Soak Effect: Even in ambient temperature between -2°C and at least +15°C, ice or frost can form in the presence of visible moisture or high humidity if the aircraft structure remains at 0ºC or below. Anytime precipitation falls on a cold-soaked aircraft, while on ground, clear icing may occur. The wings of the aeroplane are said to be cold-soaked when they contain very cold fuel as a result of having just landed after a flight at high altitude or from having been re-fuelled with very cold fuel.
The following factors contribute to cold-soaking:
- Temperature and quantity of fuel in fuel cells.
- Type and location of fuel cells.
- The length of time at high altitude flights.
- The temperature of re-fuelled fuel and time since refuelling.
Contamination Check: A check of aeroplane surfaces for contamination to establish the need for de-icing.
Damp Runway: A runway is considered damp when the surface is not dry, but when the moisture on it does not give it a shiny appearance.
De-Icing: Procedure by which frost, ice, slush or snow is removed from an aeroplane in order to provide clean surfaces.
De-Icing/Anti-Icing: Combination of the de-icing and anti-icing procedures. It may be performed in one or two steps.
De-Icing Fluid:
- Heated water;
- Type I fluid;
- Mixture of water and Type I fluid;
- Type II, Type III, or Type IV fluid;
- A mixture of water and Type II, Type III, or Type IV fluid.
Dew Point: Temperature at which water vapour starts to condense.
Freezing Drizzle: Fairly uniform precipitation composed exclusively of fine drops (diameter less than 0.5 mm) very close together which freezes upon impact with the ground or other exposed objects. Metar code: FZDZ.
Freezing Fog: A suspension of numerous minute water droplets which freezes upon impact with the ground or other exposed objects; generally reducing the horizontal visibility at the earth's surface to less than 1 km. Metar code: FZFG.
Freezing Rain: Precipitation of liquid water particles which freezes upon impact with the ground or other exposed objects, either in the form of drops of more than 0.5 mm (0.02 inch) diameter or smaller drops which, in contrast to drizzle, are widely separated. Metar code: FZRA.
Friction Coefficient: Relationship between the friction force acting on the wheel and the normal force on the wheel. The normal force depends on the weight of the aircraft and the lift of the wings.
Frost/Hoar Frost: Ice crystals that form from ice saturated air at temperatures below 0°C by direct sublimation on the ground or other exposed objects.
Hail: Precipitation of small balls or pieces of ice with a diameter ranging from 5 to >50 mm, falling either separately or agglomerated. Metar code: GR.
Hold Over Time (HOT): Estimated time for which an anti-icing fluid will prevent the formation of frost or ice and the accumulation of snow on the protected surfaces of an aeroplane, under specified weather conditions.
Ice Pellets: Precipitation of transparent (grains of ice), or translucent (small hail) pellets of ice, which are spherical or irregular, and which have a diameter of 5 mm or less. The pellets of ice usually bounce when hitting hard ground. Metar code: PE.
Icing Conditions: May be expected when the OAT (on the ground and for takeoff) or when TAT (in flight) is at or below 10ºC, and there is visible moisture in the air (such as clouds, fog with low visibility of one mile or less, rain, snow, sleet, ice crystals) or standing water, slush, ice or snow is present on the taxiways or runways.
Icy Runway: A runway is considered icy when its friction coefficient is 0.05 or below.
Light Freezing Rain: Precipitation of liquid water particles which freezes upon impact with the ground or other exposed objects, either in the form of drops of more than 0.5 mm or smaller drops which, in contrast to drizzle, are widely separated. The measured intensity of liquid water particles is up to 2.5 mm/hour or 25 grammes/dm2/hour with a maximum of 0.25 mm in 6 minutes.
Lowest Operational Use Temperature (LOUT): The lowest operational use temperature (LOUT) is the higher (warmer) of:
- The lowest temperature at which the fluid meets the aerodynamic acceptance test (according to AS5900) for a given type (high speed or low speed) of aeroplane, or
- The freezing point of the fluid plus the freezing point buffer of 10°C for Type I fluid and 7°C for Type II, III or IV fluids.
Precipitation: Liquid or frozen water that falls from clouds as rain, drizzle, snow, hail, or sleet.
Continuous Precipitation: Intensity changes gradually, if at all.
Intermittent Precipitation: Intensity changes gradually, if at all, but precipitation stops and starts at least once within the hour preceding the observation.
Precipitation Intensity: Indication of the amount of precipitation falling at the time of observation.
It is expressed as light, moderate or heavy. Each intensity is defined with respect to the type of precipitation occurring, based either on a rate of fall for rain and ice pellets or visibility for snow and drizzle. The rate of fall criteria is based on time and does not accurately describe the intensity at the time of observation.
Radiation Cooling: A process by which temperature decreases, due to an excess of emitted radiation over absorbed radiation. On a typical calm clear night aeroplane surfaces emit long-wave radiation, however, there is no solar radiation (shortwave) coming in at night and this long-wave emission will represent a constant net energy loss. Under these conditions, the aeroplane surface temperatures may be up to 4°C or more below that of the surrounding air.
Rain or High Humidity on Cold Soaked Wing: Water, visible moisture or humidity forming ice or frost on the wing surface, when the temperature of the aeroplane wing surface is at or below 0°C.
Rain: Precipitation of liquid water particles either in the form of drops of more than 0.5 mm (0.02 inch) diameter or of smaller widely scattered drops. Metar code: RA.
Rain and Snow: Precipitation in the form of a mixture of rain and snow.
Rime: A rough white covering of ice deposited from fog at the temperature below freezing). As the fog usually consists of super-cooled water drops, which only solidify on contact with a solid object, rime may form only on the windward side or edges and not on the surfaces. It can generally be removed by brushing, but when surfaces, as well as edges, are covered, it will be necessary to use an approved de-icing fluid.
Saturation: Maximum amount of water vapour allowable in the air. It is about 0.5 g/m3 at - 30ºC and 5 g/m3 at 0ºC for moderate altitudes.
Shear force: Force applied laterally on an anti-icing fluid. When applied to a Type II or IV fluids, the shear force will reduce the viscosity of the fluid; when the shear force is no longer applied, the anti-icing fluid should recover its viscosity. For instance, shear forces are applied whenever the fluid is pumped, forced through an orifice or when subjected to airflow. If excessive shear force is applied, the thickener system could be permanently degraded and the anti-icing fluid viscosity may not recover and may be at an unacceptable level.
Sleet: Precipitation in the form of a mixture of rain and snow. For operation in light sleet treat it as light freezing rain.
Slush: Snow or ice that has been reduced to a soft watery mixture.
Snow: Precipitation of ice crystals, most of which are branched, star-shaped or mixed with unbranched crystals. At temperatures higher than -5°C, the crystals are generally agglomerated into snowflakes. Metar code: SN.
Dry Snow: Snow which can be blown if loose or, if compacted by hand, will fall apart upon release. Specific gravity: up to but not including 0.35. Dry snow is normally experienced when the temperature is below freezing and can be brushed off easily from the aircraft.
Wet Snow: Snow which, if compacted by hand, will stick together and tend to or form a snowball. Specific gravity: 0.35 up to but not including 0.5. Wet snow is normally experienced when a temperature is above freezing and is more difficult to remove from the aircraft structure than dry snow being sufficiently wet to adhere.
Compacted Snow: Snow which has been compressed into a solid mass that resists further compression and will hold together or break up into chunks if picked up. Specific gravity: 0.5 and over.
Snow Grains: Precipitation of very small white and opaque particles of ice that are fairly flat or elongated with a diameter of less than 1 mm. When snow grains hit hard ground, they do not bounce or shatter. For hold over time purposes treat snow grains as snow. Metar code: SG.
Snow Pellets: Precipitation of white opaque particles of ice. The particles are round or sometimes conical; their diameter range from about 2-5 mm. Snow pellets are brittle, easily crushed; they do bounce and may break on the hard ground. Metar code: GS.
Supercooled Water Droplets: Condition where water remains liquid at negative Celsius temperature. Supercooled drops and droplets are unstable and freeze upon impact.
Visible Moisture: Fog, rain, snow, sleet, high humidity (condensation on surfaces), ice crystals or when taxiways and/or runways are contaminated by water, slush or snow.
Visual Meteorological Conditions: Meteorological conditions expressed in terms of visibility, distance from cloud, and ceiling, equal to or better than specified minima.
Visibility in Snow vs. Snowfall Intensity
Source: Transport Canada - Relationship between Visibility and Snowfall Intensity, and Theoretical Considerations in the Estimation of Snowfall Rate using Visibility.
Using the table as a guideline may help the flight crew in a determination of a snowfall intensity. This determination may be of assistance when using holdover tables.
De-Icing / Anti-Icing and Visibility in Snow vs. Snowfall Intensity
Reviewed by Aviation Lesson
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Reviewed by Aviation Lesson
on
3:07 PM
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