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ALC-33: Inflight Icing
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Structural Icing Type

Outside Air Temp. Range Icing Type
2 ° C to -10° C Clear
-10° C to -15° C Mixed
-15° C to -20 ° C Rime

There are three types of structural icing:  clear, rime, and mixed. In most cases, the type of structural ice is most dependent on the air temperature.  However, the likelihood of clear ice increases with droplet size.

Clear ice typically forms when temperatures are around 2 ° C. to -10° C. and with the presence of large water droplets freezing drizzle, or freezing rain.  Clear ice is the most dangerous type of structural ice not only because it is hard to see, but also because it can change the shape of the airfoil.  In addition, clear ice often forms well beyond the ice-protected areas of the aircraft.
clear.jpg clearicetest.jpg

The more hazardous clear ice shapes, as in the two examples above, tend to form at temperatures closer to 0°C.  In warmer temperatures, supercooled water droplets first impact a surface and then flow aft before freezing. This process often forms "horns" or other shapes that can substantially disrupt airflow over the wing.

mixed.jpgMixed ice, a combination of clear ice and rime ice that has the worst characteristics of both, can form rapidly when ice particles become embedded in clear ice and build a very rough accumulation. Mixed ice is most likely to form at temperatures between -10° C to -15° C. 

Rime ice forms when small droplets freeze immediately rime.jpgon contact with the aircraft surface.  It typically occurs with temperatures between -15° C. and -20 ° C.  Rime ice has a milky, opaque appearance resulting from air trapped when it strikes the leading edge of an airfoil and freezes.   It is less dense, and usually easier to remove than clear ice. Rime ice tends to form wedge-shaped accretions that do not disturb airflow as much as clear ice.

Did you know? Any type of structural icing can block the pitot tube and static ports and cause the breakage of antennas on the aircraft. This can cause a pilot to lose or receive erroneous indications from various instruments such as the airspeed indicator and altimeter and can cause loss of communications and radio navigation capabilities.

Application & Risk Management
  • To evaluate the risk of a structural icing encounter, remember that temperature, moisture content, and droplet size determine the shape and structure of the ice.
  • The ice shape and structure in turn determine how it affects the aerodynamic and handling qualities of the aircraft.