Dr. Mary Ann O’Grady

How much do you know about aircraft icing and the conditions that cause it?  Take this quiz, and see how well you do.

Aircraft Icing Quiz Questions
  1.  _____ ice is lighter than _____ ice, has an irregular shape and a surface roughness that reduces aerodynamic efficiency.
  2. The three negative outcomes of aircraft icing on the airplane are: _____, ______, and _____ .
  3. The three types of structural icing are _____, ______, and _____ .
  4. The two ingredients for structural icing are ______ and _____ .
  5. The most important effect of ice on the wings and tail is _____ .
  6. The three types of icing intensities are _____, _____, and _____ .
  7. When this type of icing intensity occurs so that deicing or anti-icing cannot reduce or control the accumulation, the pilot’s only option is to _____ .
  8. The type of cloud that produces the most severe icing is a _____ cloud.
  9. If a pilot encounters ice in cumuliform clouds during the winter, he or she should _____, or ______ immediately
  10. The first places that a pilot should look for the formation of ice on the aircraft are _____, or ______.
Aircraft Icing Quiz Answers
  1. Answer: rime, clear
  2. Answer: decrease in thrust, reduction in lift, and an increase in drag
  3. Answer: rime, clear, mixed
  4. Answer: presence of visible moisture, temperatures at or below freezing
  5. Answer: reduction of lift
  6. Answer: trace, moderate, severe
  7. Answer: get out of the icing [conditions]
  8. Answer: cumulous
  9. Answer: divert, descent into warmer air
  10. Answer: leading edges of the airfoils, any objects that protrude into the air flow, such as antennas, OAT probe, etc.
Discussion: Aircraft Icing Conditions

The motivations underlying why rational pilots who avail themselves of all available weather information and data during their flight planning process, yet ultimately decide to deliberately fly into icing conditions are varied. But one key element may be the fact that these PICs lacked sufficient knowledge about icing conditions, and they found themselves navigating into dangerous weather conditions. Aircraft icing has long been classified as one of the greatest weather hazards to aviation. This is because icing is likely to be both cumulative and invisible which can cause the aircraft to slow down, force it downward, and/or make it go out of control. In addition, engine performance can diminish, contribute to false indications on the instruments, and result in a loss of radio communication. It can also freeze the landing gear to a point where it cannot fully extend or retract, and it can prevent the brakes from functioning properly. During the winter months, structural icing is more of a concern for pilots during a flight than induction icing, which is why it’s the focus of this quiz and subsequent discussion.

Only two ingredients are required for structural icing: visible moisture and temperatures at or below freezing. Cooling occurs when lift is produced which can reduce the aircraft surface or skin temperatures to below freezing despite the ambient air temperature being above freezing. Supercooled water is defined as water that remains in a liquid state although its temperature has dipped below freezing. When a supercooled drop of water comes into contact with a cold aircraft, a portion of that drop freezes instantly and adheres to the aircraft’s surface while the remaining portion of that drop is warmed by friction. Aerodynamic cooling can cause that drop to refreeze, however, and it is the manner in which that remaining liquid freezes that determines whether the forming ice is clear, rime, or mixed. If the supercooled large drops flow out and freeze into a smooth sheet of ice, it creates clear or translucent ice that is hard, glossy, heavy, and tenacious. As its accumulation continues, it may build up into a single or double horn-like shape on leading edges of the aircraft which increase drag and a correspondingly inverse decrease in lift.

In contrast, rime ice is created from supercooled small drops where the liquid freezes more quickly before it has had time to spread out over the aircraft’s surface which traps air between the droplets giving rime ice a rough, milky, opaque appearance. Although rime ice is lighter than clear ice, its irregular shape and surface roughness reduces aerodynamic efficiency by reducing lift, increasing drag; and it is more easily removed with aircraft deicing equipment than is clear ice which is heavier and results in a solid sheet configuration. When the supercooled water droplets vary in size or mix with snow or ice particles, a combination of clear and rime ice can form very rapidly into highly irregular shapes that build up on airfoil leading edges. Regardless of which form icing assumes, the amount of the ice accumulation is directly proportional to the amount of liquid water in the clouds with the worst case scenario being a combination of large water droplets, temperatures close to freezing, and clouds having significant water content.

The effects of icing include a reduction in lift, an increase in drag, and a decrease in thrust where the effects of these three factors become cumulative which may require a full power setting and a high angle of attack to maintain altitude. However, this attitude may result in a new problem where ice can begin forming on the underside of the wing which adds more weight and drag so the need to get out of the icing conditions then becomes the prime directive. Depending on the PIC’s experience with flying in icing conditions, any ice may be too much ice but the FAA has categorized icing into three intensities: trace, moderate, and severe. Trace ice is barely visible and is typically not a hazard unless the aircraft is exposed for one hour or more. Trace ice can usually be handled by inflight deicing/anti-icing equipment for durations of one hour or less. Moderate ice accumulates at a rate where even short encounters with it are potentially hazardous, and the use of deicing/anti-icing equipment is definitely required. Severe icing is defined as an accumulation of so much ice that deicing/anti-icing equipment cannot reduce or control its accumulation so the only option for the PIC is to get out of the icing conditions as quickly as possible.

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Reference:

Duncan, P.A. (2016). Rime and Clear and Mixed. Retrieved on March 10, 2016, from
http://avstop.com/stories/rimeandclearandmixed.htm

Additional Quiz:

Do You Know These 5 Aviation Acronyms?

Additional Resources:

Aircraft Icing Safety Advisory – AOPA

Aircraft De-Icing and Anti-Icing Equipment – AOPA

Aircraft Icing Advisory Circular – FAA

The Madness of Icing – Flying Magazine

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