Rime, Clear and Mixed Icing
One of the major weather hazards to aviation is icing. Icing is the formation of ice on parts of a vehicle. Pilots and controllers need to be aware of the icing process, under what conditions ice will form on an aircraft, the different forms it takes on an aircraft and its effects on the aircraft’s flight characteristics. Icing occurs when an aircraft flies through visible water and the temperature at the point where the moisture strikes the aircraft is 32° F (0° C) or colder. Even though the air temperature around the airplane may be a few degrees warmer than freezing, aerodynamic cooling can occur (due to the rapid movement of the airplane through the air creating a wind chill effect) and lower the temperature of the airplane’s surface thus inducing icing.
Supercooled water increases the rate of icing. As a supercooled water droplet hits the airplane’s surface a part of it freezes instantaneously. The manner in which the remaining portion of the water droplet freezes determines whether the ice formation is clear ice, rime ice or mixed ice.
Clear ice – After the initial impact of supercooled droplets from large raindrops strike the surface, the remaining liquefied portion flows out over the surface and freezes gradually. This freezes as a smooth sheet of sold ice. It is hard and heavy and is difficult to remove.
Rime ice – formed from small supercooled droplets when the remaining liquefied portion after initial impact freezes rapidly before the drop has time to spread over the surface. This traps air between the droplets, and gives the ice a white appearance. It is lighter in weight than clear ice. Its formation is irregular and its surface is rough. It is brittle and more easily removed than clear ice.
Mixed ice – formed when supercooled water droplets are of various sizes or are intermingled with snow or ice particles. After initial impact, the remaining portion freezes rapidly and forms a mushroom shape on the leading edges of a wing. Ice particles are embedded in clear ice and form a hard and rough-edged mass.
Icing is considered a cumulative hazard as it takes time for the ice to build up on the aircraft and increasingly changes the aircraft’s flight characteristics.
The type of ice an aircraft may encounter is predominately a function of temperature and the size of the water droplets in the atmosphere. Clear ice is generally found in temperatures of 0 to -10 deg C and in an atmosphere where the droplets are large.
Rime ice is generally found in temperatures between -10 and -40 deg C in atmospheres where water droplets are small or the moisture is in snow form.
Between these ranges of temperatures and ranges of moisture particle sizes there is a transition zone where the combination of clear ice and rime ice can coexist forming mixed ice.
Ice which accretes on the external parts of an aircraft is most often the result of the impact of supercooled water droplets of various sizes on that aircraft. This may happen within cloud or when flying through precipitation. The reason why water droplets do not all freeze as soon as the ambient temperature falls below 0°C is the release of latent heat as water changes state to ice. So much latent heat is released that the change of state is slowed down so that it takes place progressively as temperature continues to fall. This continues until, by about -20°C, most of the by now supercooled water has turned to ice. Ice grains which have already fully formed and are dry when they impact an aircraft do not adhere but simply bounce off. So the relative severity of ice accretion can be expected to progressively decrease as ambient temperature reduces below 0°C so that little if any remains below -20°C.
– For every one degree of supercooling one eightieth of a super cooled water drop will change to ice on impact. The rest running back over the airframe as water and then freezing.
– Highest proportion of dangerous clear ice forms in cloud at temperatures just below 0 deg C.
– At lower temperatures there will be a higher proportion of rime ice.
– By about -25 deg C only smallest water drops will remain unfrozen as super cooled water drop.
– From -25 deg C down, all icing will be rime and light rime.
– No icing above 0 deg C or below – 45 deg C.
– The zero degree isotherm in the “Polar Regions” is at ground level.
– The zero degree isotherm in “Temperate Regions” is at mean sea level in winters and at about 10,000 feet in summers.
– The zero degree isotherm in “Tropics” is at about 14,000 feet in winters and 16,000 feet in summers.
– Inside CU and CB the freezing level can be substantially above or below the mean level in the clear air due to updrafts and downdrafts.
– Worst airframe icing in a Tropical CB at temperatures just below 0 deg C.
Exam Question Tips:
At what degree of icing can ICAO “No change of course and altitude necessary” recommendation be followed? — “Light”
At what degree of icing should ICAOs change of course and/or altitude “desirable recommendation” be followed? — “Moderate”
At what degree of icing should ICAOs change of course and/or altitude “immediately instruction” be followed? — “Severe”
Clear ice forms on an aircraft by the freezing of Large (not small) super cooled water droplets.
Rime ice forms through the freezing onto aircraft surfaces of small (not large) super cooled water drops.
The type of icing most likely to occur when an aircraft flies into super cooled rain in an area with a temperature below 0 deg C is “Clear Ice”.
Intensity and type of airframe icing most likely to occur when flying at FL 170 in AS with the outside air temperature at -20 deg C is “Light Rime”.
Severe airframe icing is unlikely under conditions of flying for some time in dense layered cloud when the outside air temperature is -25 deg C.
Ice most likely to form on the aircraft’s surface is in the temperature band of 0 to -10 deg C.
In mature CB’s the probability of severe icing, according to meteorological rules, is greatest in the temperature range of 0 to -23 deg C
A thick wing has a bigger stagnation point, so it sends out bigger pressure waves ahead of the wing. This causes the airflow to divide well ahead of the wing so a lot of the moisture will miss the wing. A thin wing does not give as much warning to the air ahead of the wing, so a lot of the supercooled drops will collide with the wing, resulting in a heavier ice accretion than a thick wing. (http://www.atpforum.eu/showthread.php?t=2188)
A vertical temperature profile indicates the possibility of severe icing when the temperature profile intersects the 0 deg C isotherm twice. The freezing level, or 0°C (zero-degree) isotherm, represents the altitude in which the temperature is at 0°C (the freezing point of water) in a free atmosphere.
The icing in cloud which forms over hills is likely to be more severe than in the same type of cloud over level terrain because enforced ascent of air releases more water, which is retained in the cloud by the increased upward components.
You are flying with an OAT of -12 deg C and a TAS of 250 kt at FL 150 through 8 octas NS. In clouds pushed up against the mountains, moderate to severe mixed icing is most probable.