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An aircraft flying from Oslo Gardermoen Airport (ENGM) in Norway, to Stockholm Arlanda Airport (ESSA) in Sweden, encounters a squall line. It will not be able to avoid flying through the Cumulonimbus/Towering Cumulus clouds en-route. Which levels would give the most severe accretion of clear ice?
  • A
    The levels where the temperature is below -25°C.
  • B
    The levels where the temperature is between -10°C and -25°C.
  • C
    The levels where the temperature is between 0°C and -10°C.
  • D
    The levels where the temperature is the lowest.

Refer to figure.
Clear Ice/Glaze Ice
If a large, supercooled water droplet strikes an aircraft, it will start to freeze and this will release latent heat. This will delay the freezing process whilst part of the supercooled water droplet flows back over the impact surface forming clear ice. The amount of a supercooled water droplet that freezes on impact is approximately 1/80th of the droplet for each degree below freezing.

Clear ice is a transparent form of ice formed by large, supercooled water droplets, and it can be very dangerous. There can be a lot of flowback and the ice appears transparent because there is no air trapped under the flowback icing.

The ice will easily modify and destroy the shape of the aerofoil and its weight can cause problems with control, especially if the build-up is uneven. It is illustrated in the figure above.

Propeller icing can cause severe vibrations and because the ice adheres strongly, when it breaks off, the pieces can be large and cause skin damage.

Clear ice forms in Ns, Cu and mostly Cb clouds at temperatures from 0°C to -20 °C. The worst icing occurs just below 0°C, as this is when the water droplet will stay warmest and run backwards over the wing for the longest.

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