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In which of the following situations is an aircraft most susceptible to icing?
  • A
    Level flight below a rain-producing cloud when OAT is below zero degrees C.
  • B
    Flying in dense cirrus clouds.
  • C
    Level flight in snowfall below a nimbostratus layer.
  • D
    Flying in heavy drizzle.

Refer to figure.

The correct option describes the ideal conditions for Freezing Rain.

Freezing Rain occurs when precipitation, in the form of rain, passes from a warm air mass into a relative cold air mass of an air temperature less than 0°C. The rain maintaining its liquid state in sub-zero temperatures renders it super-cooled. These supercooled water droplets freeze when they come into contact with the ground or other exposed surface, if the surface temperature is below 0°C.

Now, freezing Rain can cause significant accumulations of clear ice. Clear ice is the most hazardous form of airframe icing. The raindrops spread and freeze on contact with a cold airframe. When the supercooled drop hits the airframe, it freezes relatively slowly due to the latent heat released in the freezing process. This allows part of the drop to spread backwards before it too freezes.

  • Clear ice is the most serious of the various forms of ice because it has the fastest rate of accumulation and is more difficult to remove than rime ice.
  • Clear ice forms a sheet of solid, clear, glazed ice with very little air trapped in it which can dramatically alter the aerodynamic properties of an aerofoil. There may also be a significant increased weight penalty associated with clear ice.

"Level flight in snowfall below a nimbostratus layer." INCORRECT. Snow is solid precipitation in the form of ice crystals. Remember that for airframe icing to form water must be present in liquid state.
"Flying in dense cirrus clouds." INCORRECT. You are least likely to encounter icing in high clouds, because these clouds consist of ice crystals and this already-frozen moisture will not adhere to your airframe.

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