GLOSSARY OF TERMS


Active frost. A condition when frost is forming. Active frost occurs when the surface temperature is at or below 0oC and at or below the dew point.


Anti-icing. Anti-icing is a precautionary procedure by which clean aircraft surfaces are protected against the formation of ice and frost and the accumulation of snow and slush for a limited period of time.


Cold-soak effect. The wings of aeroplanes 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 refueled 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, length of time at high altitude, temperature of refueled fuel and time since refueling.


Clear ice. 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, ice or frost can form in the presence of visible moisture or high humidity if the aeroplane structure remains at 0°C or below. Clear ice is very difficult to detect visually and may break loose during or after take-off.


Critical surfaces. Wings, control surfaces, propellers, horizontal stabilizers, vertical stabilizers or any other stabilizing surface on an aircraft. These surfaces should be completely free of ice, snow, slush or frost before take-off. The critical surfaces should be determined by the aircraft manufacturer.


De-icing. The process which removes ice, snow, slush or frost from aircraft surfaces. De-icing/anti-icing. A procedure combining both the de-icing process and the anti-icing process and which can be performed in one or two steps:


One-step de-icing/anti-icing. This procedure is carried out with heated anti-icing fluid. The fluid is used to de-ice the aircraft and remains on the surfaces to provide anti-icing capability.


Two-step de-icing/anti-icing. This procedure contains two distinct steps. The first step, de-icing, is followed by the second step, anti-icing, as a separate fluid application. After de-icing, a separate overspray of anti-icing fluid is applied to protect the aircraft’s critical surfaces, thus providing maximum anti-icing protection.


Drizzle. Fairly uniform precipitation composed exclusively of fine drops (diameter less than 0.5 mm (0.02 in)) very close together. Drizzle appears to float while following air currents although, unlike fog droplets, drizzle falls to the ground.


Fog and ground fog. A visible aggregate of minute water particles (droplets) in the air reducing the horizontal visibility at the Earth’s surface to less than 1 kilometer.


Freezing fog. A fog formed of supercooled water droplets which freeze upon contact with exposed objects and form a coating of rime/clear ice.


Freezing rain and freezing drizzle. Rain or drizzle in the form of supercooled water drops which freeze upon impact with any surface.


Surface Contamination 22


Frost. A deposit of small, white ice crystals formed on the ground or other surfaces. Frost is formed by sublimation, i.e. when water vapour is deposited upon a surface whose temperature is at or below freezing.


High humidity. An atmospheric condition where the relative humidity is close to saturation.


Hoar frost. A greyish-white crystalline deposit of frozen water vapour formed on surfaces in clear, still weather. Holdover time. Holdover time (HOT) is the estimated time the anti-icing fluid will prevent the formation of ice and frost and the accumulation of snow on the protected (treated) surfaces of an aeroplane.


Precipitation intensity. Intensity of precipitation is an indication of the amount of precipitation collected per unit time interval. It is expressed as light, moderate or heavy. Intensity is defined with respect to the type of precipitation occurring, based either on rate of fall for rain and ice pellets or visibility for snow and drizzle. The rate of fall criterion is based on time and does not accurately describe the intensity at a particular time of observation.


Rain. Precipitation of liquid water particles, either in the form of drops of more than 0.5 mm in diameter or smaller drops which, in contrast to drizzle, are widely separated.


Rime. A deposit of ice, produced by freezing of supercooled fog or cloud droplets on objects at temperatures below or slightly above freezing. It is composed of grains separated by air, sometimes adorned with crystalline branches.


Shear force. Shear force is a force applied laterally on an anti-icing fluid. When applied to a Type II, III or IV fluid, 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 fluid viscosity may fall outside the range set by the manufacturer and tested for certification. Fluid degraded in this manner should not be used for operational purposes.


Slush. Water-saturated snow which with a heel-and-toe slap-down motion against the ground will be displaced with a splatter. Snow. Precipitation of ice crystals, mostly branched in the form of six-pointed stars. The crystals are isolated or agglomerated


to form snowflakes. Dry snow. Snow from which a snowball cannot readily be made and which has a temperature less than 0°C. Wet snow. Snow which contains a great deal of liquid water.


Visible moisture. Fog, rain, snow, sleet, high humidity (condensation on surfaces) and ice crystals can all produce visible moisture on aircraft, taxiways and runways exposed to and contaminated by these conditions.


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