01. Icing Severity Guidelines
Ice accretion on aircraft has been a hazard ever since prolonged flight at freezing temperatures became possible. When sufficient ice forms on an aircraft, either on the airframe or in the engine system, the aircraft may be affected to such an extent that continued safe flight is impossible unless action is taken to remove the accumulation of ice or to fly in atmospheric conditions where ice accretion will cease.
Various methods are available to prevent ice formation or to remove deposits of ice after formation. The DC-10 anti-icing systems for instance, protect the windshields, engine inlet lips, wing slats, pitot and TAT probes, various drain masts and the upper VHF antenna.
The rate at which ice accumulates on an aircraft, and the form and texture which the deposit assumes, vary with meteorological condition. In extreme cases it may be dense, solid clear ice (glaze ice), or it may be an opaque porous or spongy deposit of low density (rime ice). In all cases, to a greater or lesser degree, the diminished aerodynamic properties of various parts of the airframe may be affected with a tendency toward reduced lift, increased drag and a higher stalling speed, or decreased engine performance.
With the advent of larger and faster aircraft, the problems of ice accretion are reduced; but some problems persist. These problems may be aggravated as a result of the more rigid control of aircraft necessary with the increased traffic, particularly in holding patterns, where a pilot may not be free to deviate from the prescribed flight plan in order to leave the icing conditions.
The basic parameters which dictate the rate at which ice forms on an aircraft surface are as follows:
- liquid water content (LWC) of the cloud through which the aircraft is flying;
- water droplet size (diameter);
- surface geometry;
- surface collection efficiency;
- aircraft speed.
The collection efficiency of a body is a function of the flow field around the surface. Small bodies exhibit a high collection efficiency. The flow field associated with larger bodies produces a smaller collection efficiency.
The duration of the exposure of an airplane to icing conditions is dependent on the aircraft speed and the effects of the above-mentioned parameters. However, icing tunnel tests show that the accumulation of ice on a surface are not always in a linear relationship with time, especially for the accretion of glaze ice, because ice growth changes the airfoil characteristics and causes a continual change in collection characteristics with respect to the initial rate.
TAT (on some aircraft types: RAT) is one basic criterion for judging whether icing is likely in flight. Implicit reliance, however, cannot be placed on the TAT reading at temperatures near 0°C, because the TAT probe (by design) has a higher temperature recovery than ice-collecting surfaces on the airframe, that is, there may be local areas on the airframe that are experiencing lower total temperatures than those measured by the TAT probe. This is one reason why operating manuals dictate that the ice protection system be turned on already at temperatures well above zero (mostly between -5°C and 10°C) in the presence of visible moisture.
If the liquid or frozen water particles are large and numerous enough, radar could detect them and provide another indication of impending icing. With experience, one is able to recognize the kind of cloud associated with the various types of echo which appear on the radar screen. A clear-cut echo of high intensity frequently indicates a cloud of the convective type. A line of such echoes may indicate a line of cumulus or cumulonimbus clouds which may be embedded in other clouds which do not produce an echo. (In cumulus, cumulonimbus and nimbostratus clouds, the chance of icing increases and the severity may be heavy. In stratocumulus, altocumulus, and altocumulus associated with altostratus, the probability of icing is about 50 percent and the severity will exceed a moderate rate only occasionally.) Warm-front clouds usually give rise to radar echoes which are neither so well defined nor of such high intensity. It must be emphasized that the absence of a radar echo does not imply freedom from icing. Although the concentration and size of liquid drops may be insufficient to produce an echo, clouds may be present in which icing may occur if the temperature is below 0°C. On the other hand, radar echoes from snow and large ice crystals may be received when the liquid water content is small.
Sometimes, under favourable conditions, a pilot may obtain some indication as to whether a particular cloud at very low temperatures consists of ice or water because of associated optical phenomena. A halo – i.e. a circle, or part of a circle – around the sun or moon and usually of white light (but sometimes coloured) is an indication of ice-crystal cloud. On the other hand a corona (a coloured ring, usually much smaller than the halo around the sun or moon) indicates a cloud of water droplets.
A pilot may also recognize a water cloud from above by the presence of a coloured ring similar to a corona surrounding the shadow of his aircraft on the cloud.
Although the visibility in a cloud may frequently be a guide to the amount of water per unit volume, it does not necessarily indicate the severity of icing to be expected if the temperature is appropriate.
On those aircraft where external surfaces are not easily seen from the cockpit, the corners of the windshield will frequently provide evidence of icing conditions.
If the deposit is opaque (appears light in texture) and grows out into the airstream, the conditions are probably not severe. If glaze ice, however, is observed to be forming, the rate of growth may be rapid, in which case immediate action is necessary.
When the aircraft subsequently enters air where the TAT is above 0°C, the ice will be shed. If clear air is entered with temperatures still below 0°C, the ice will sublimate (i.e. change from solid to vapour state) but it will take a long time to do so. Increasing the airspeed, however, will increase the TAT which may speed this sublimation process.
Guidelines with respect to icing conditions:
Trace
Ice becomes perceptible. Rate of accumulation is slightly greater than the rate of sublimation. It is not hazardous even though de-icing/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). Occasional use of de-icing/anti-icing equipment removes/presents accumulation. It does not present a problem if the de-icing/anti-icing equipment is used.
Moderate
The rate of accumulation is such that even short encounters become potentially hazardous and use of de-icing/anti-icing equipment or flight diversion is necessary.
Severe
The rate of accumulation is such that de-icing/anti-icing equipment fails to reduce or control the hazard. Immediate flight diversion is necessary.
02. Icing in High-Level Cloud Layers
Cirrus clouds do not generally cause icing because the cloud particles are ice crystals. Altostratus clouds also frequently consist of ice crystals, but the possible presence of super-cooled water must not be ignored.
Altocumulus clouds will usually have water drops predominating down to about -30°C. In both altostratus and altocumulus clouds, the water content is likely to be low, and the rate of ice accretion also low. Prolonged flight in such clouds may cause appreciable accretion.