Icing and Performance

Icing is one of the most dangerous environmental conditions an aircraft can face. Icing is most commonly formed in supercooled clouds and can form at temperatures at or slightly above the freezing point. Icing forms as a result of supercooled droplets make contact with a control surface. The control surface itself may be colder than 0 degrees Celsius, and as a result, when the supercooled droplet makes contact, ice forms (FAA, Advisory Circular AC 91-74B.pdf).  

 

Icing is dangerous for all aircraft due to its ability to directly affect the aerodynamic performance of a lifting surface. When an aircraft starts accumulating ice, the chord line of the wing increases, weight increases, drag increases while the laminar flow is disrupted, and the aircraft's critical angle of attack decreases. This means that the pilot will have to increase power and pitch to produce the same amount of lift to keep the aircraft flying at the same altitude and maintain directional control (FAA, Advisory Circular AC 91-74B.pdf). 

 

There are numerous ways to prevent ice build-up, however, depending on the aircraft, some of the best methods are not available to smaller aircraft. Since ice most commonly forms on the leading edges of airfoils, a pneumatic boot can be used to clear the leading edge mechanically. Another method is using ethanol to clear icing on your propellers. Since ethanol has a much lower freezing point than water, you can chemically wash ice off of surfaces. These methods usually are not available to general aviation aircraft due to their complexity. A Cessna 172 for example, only has Pitot heat, a windshield defroster, and carb heat, which by itself is not enough for safe flight into the icing (4: Aerodynamic Factors 2012). 

 

I have personally had a close call experience with icing when flying cross country. I was in hard IFR, moderate precipitation, and a low outside air temperature, and had to request a lower altitude to stay below the icing level since air temperature decreases with an increase in altitude and vice versa. My partner and I were able to monitor the outside air temperature in flight and decided to descend once the O.A.T. read 2 degrees Celsius. In hindsight, we took the proper precautions by letting ATC know that we didn't have any de-icing systems on our plane and would be requesting a lower altitude as soon as the temperature starts becoming an issue. But the next day was clear skies and VFR, so we could have always flown the next day and avoided all of it.  

 

References

4: Aerodynamic Factors. (2012). In Instrument flying handbook: FAA-H-8083-15B (pp. 13-17). Washington, D.C.: Federal Aviation Administration.

FAA. (2015, October 8). Advisory Circular AC 91-74B.pdf [Advisory Circular].