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Winter Operation Definitions

Wikis > Winter Operation Definitions
Winter Operation Definitions

Active Frost
Active frost is a condition when frost is forming. Active frost occurs when aeroplane surface temperature is:

    • at or below 0°C (32°F); and
    • at or below dew point

Anti-Icing
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 (holdover time).

Anti-Icing Fluid
a) mixture of water and Type I fluid;
b) Premix Type I fluid;
c) Type II fluid, Type III fluid, or Type IV fluid;
d) mixture of water and Type I fluid, Type III fluid, or Type IV fluid.

    Note: Fluids mentioned in a) or b) must be heated to ensure a temperature of 60°C (140°F) minimum at the nozzle.

Check
An examination of an item against a relevant standard by a trained and qualified person.

Cold-Soak Effect
The wings of an 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. Whenever precipitation falls on a cold-soaked aeroplane when on the ground, clear icing may occur. Even in ambient temperatures between -2°C and +15°C (28°F and 59°F), ice or frost can form in the presence of visible moisture or high humidity if the aeroplane structure remains at 0°C (32°F) or below. Clear ice is very difficult to be detected visually and may break loose during or after take-off. The following factors contribute to cold- soaking: temperature and quantity of fuel in fuel cells, type and location of fuel cells, length of time at high altitude flights, temperature of re-fuelled fuel and time since re-fuelling.

Contamination
Contamination is understood as all forms of frozen or semi-frozen moisture such as frost, snow, ice or slush.

Contamination Check
Check of aeroplane surfaces for contamination to establish the need for de-icing.

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 procedures “de-icing” and “anti-icing”. It may be performed in one or tow steps.

De-Icing Fluid
a) heated water;
b) mixture of water and Type I fluid;
c) Premix Type I fluid;
d) Type II, Type III, or Type IV fluid;
e) Mixture of water and Type II, Type III, or Type IV fluid.

    Note: De-icing fluid is normally applied heated in order to ensure maximum efficiency.

Freezing Drizzle
Fairly uniform precipitation composed exclusively of fine drops (diameter less than 0.5mm (0.02 IN)) very close together which freezes upon impact with the ground or other exposed objects.

Freezing Fog
A suspension of numerous very small water droplets which freezes upon impact with ground or other exposed objects, generally reducing the horizontal visibility at the earth’s surface to less that 1km (5/8 mile).

Frost/Hoar Frost
Ice crystals that form ice saturated air at temperatures below 0°C (32°F) 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 >50mm (0.2 to >2.0 IN) falling either separately or agglomerated.

Holdover Time
From the commencement of the final step of the de-icing/anti-icing procedure, the 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 weather conditions as specified in the holdover time table.

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 5mm (0.2 IN) or less. The pellets of ice usually bounce when hitting hard ground.

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 0.5mm (0.02 IN) or smaller drops which, in contrast to drizzle, are widely separated. Measured intensity of liquid water particles is up to 2.5mm/hour (0.10 IN/hour) or 25 grams/dm2/hour with a maximum of 0.25mm (0.01 IN) in 6 MIN.

Lowest Operational Use Temperature (LOUT)
The Lowest Operational Use Temperature (LOUT) is the higher (warmer) of

    a) 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
    b) the freezing point of the fluid plus the freezing point buffer of 10°C (18°F) for Type I fluid and 7°C (13°F) for Type II, III, or IV fluids.

For applicable values refer to the fluid manufacturer’s documentation.

Moderate and Heavy 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 0.5mm (0.02 IN) or smaller drops which, in contrast to drizzle, are widely separated. Measured intensity of liquid water particles is more than 2.5mm/hour (0.10 IN/hour) or 25 grams/dm2/hour.

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 (32°F).

Rain and Snow
Precipitation in the form of a mixture of rain and snow.

Rime Ice
Small frozen water droplets, spherical opaque/milky granular appearance looking similar to frost in a freezer. Typically rime ice has low adhesion to the surface and its surrounding rime ice particles.

Snow
Precipitation of ice crystals, most of which are branched, star-shaped or mixed with unbranched crystals. At temperatures higher than -5°C (23°F), the crystals are generally agglomerated into snowflakes.

Snow Grains
Precipitation of very small white and opaque particles of ice that are fairly flat or elongated with a diameter of less than 1mm (0.04 IN). When snow grains hit hard ground, they do not bounce or shatter.

    Note: For holdover time purposes treat snow grains as snow.

Snow Pellets
Precipitation of white, opaque particles of ice. The particles are round or sometimes conical; their diameter ranges from about 2-5mm (0.08 – 0.2 IN). Snow pellets are brittle, easily crushed; they do bounce and may break on hard ground.

    Note: For holdover time purposes treat snow pellets as snow.

Slush
Snow or ice that has been reduced to a soft watery mixture.

Source: ICAO

See also:
Precipitation, Icing
Anti-Icing Codes
Winter Operation Definitions

 
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