getting to grips with COLD WEATHER OPERATIONS 寒冷天气运行(18)

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The following graph gives an idea of the variety of shapes that could be encountered in a variety of clouds, which would offer icing conditions at all temperatures, from 0 °C down
30
to -40 °C.

Local effect of speed

This figure shows: 1- the envelope of conditions, which are prone to icing, as a function of outside air temperature and aircraft speed. Only negative temperatures should be considered, but kinetic heating (KH, or ram effect) moves the maximum temperature for icing towards more negative values when speed is high. Limit is the red curve, based on the law of temperature recovery for altitudes around FL 100. However, conditions around the red curve may be those where the most difficult ice accretion cases can be met. Yellow sector is icing free. Blue sector is where icing may be encountered. Blue and yellow sectors are limited around -40°C, because practically no supercooled water might be present at colder temperatures.
2- the graph tends also to show how icing conditions may affect the ice accretion shape. Variability of shapes is great around the red curve, as shown by an example of speed effect around - 10°C. In such a regime, shapes do vary very rapidly. In general, some runback ice accretion may be met just above the red curve at intermediate speeds. At lower temperatures, which means farther under the red curve, accretions are much whiter and tend to be of a pointed shape.
Figure A2

Observation of ice accreting in flight strongly suggests that the ice which effectively
accretes is the differential result of incoming supercooled water (plus possible ice crystals) and outgoing amount of water due to a mix of erosion, evaporation and sublimation. The combined effect of those three is never negligible and, sometimes, one of them is so dominant in the overall icing process that no significant ice is accreting.
In that context, it is very difficult to describe, classify and predict ice shapes. Therefore, measures taken for aircraft protection against icing need to be based on some sort of definition of a .worst case. or an .envelope case. scenario. Such cases will be used in ice protection systems design and in certification.
Consequences of the above, include the following rules of thumb:
●  
Icing conditions are far more frequent than effective ice accretion for a given aircraft. It is not because icing has been reported ahead of you that your aircraft will also ice.

●  
Increase of speed decreases the amount of ice accreted.

●  
If rapid ice accretion is met, a moderate change of altitude is normally enough to decrease or stop the ice to accrete. The ATC controllers must immediately accept such a pilot request.

A1.4 - Other types of contamination
On ground, aircraft parked outside collect all types of precipitation which do not flow off: frost, condensation, freezing drizzle, freezing rain, slush, sleet, wet and dry snow. It would be useless to enumerate differences between those different cases. Very often, wings are covered with a mixture of different things. That, in itself, has two causes:
　
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