Clear icing: An brief explanation of how it forms, why water does not always freeze at 0 degrees Celsius, and the adverse effect on clear ice accumulation on the aerodynamic capabilities of an aircraft.

     I had a simple question. How does supercooled droplets of water exist? How does water not freeze at 0 C? I did my research and this is what I found. “We also determined the number of water molecules N* in the critical ice nuclei (including their sheath of intermediate-ice) through the mean first passage time (MFPT) method.” (Moore, 2011) Not surprisingly, this is one of the easier sentences to understand.

    Pure water does not freeze at 0C. It needs a nucleus to form a crystal around. According to Smithsonian magazine (which helpfully summarized the Moore Nature entry) water will not spontaneously form a nucleus until -40 degrees. (Zielinski, 2011). When this supercooled water contacts the aircraft it instantly freezes into clear ice, having a non-water material to catalyze the crystallization process. This icing is most common at temperatures between 0 and 15 degrees Celsius. (“Pilot’s,” 2016) Supercooled droplets of very pure water are found in the low part of turbulent cloud systems. (Airboyd, 2010)

    When clear ice forms on the various forward surfaces it will cause a variety of detrimental effects. Blockage of intakes and carburetors will restrict airflow, and ice on pitot tubes will cause improper reading of airspeed. But most critically the aerodynamics of the vehicle itself will be negatively changed. It is not the weight of the ice, but the shape which is the concern. Clear ice builds up around the aerodynamic surfaces, thickening and deforming the shaped of the airfoil. This significantly reduces the lift capability of both propellers and the leading edges of wings.  (Airboyd, 2010)

     The two primary ways to counteract any icing on the leading edge of an airfoil are using a deicing boot or electricity. A deicing boot is inflated by drawing bleed air off of the engine, that is air that is is taken from the compression cycle of the turbine engine. Alternatively a deicing boot can be inflated using an engine driven pneumatic pump. Propeller blades are deiced with electrical heaters which run along the leading edge of the blade. (Pilot's, 2016)

I think that is pretty a pretty cool fact.

Airboyd. (2010, January 22). Ice Formation On Aircraft (1960). Retrieved December 11, 2020, from https://www.youtube.com/watch?v=9I40DQcK_6U

Moore, E. B., & Molinero, V. (2011). Structural transformation in supercooled water controls the crystallization rate of ice. Nature, 479(7374), 506-8. http://ezproxy.libproxy.db.erau.edu/login?url=https://www-proquest-com.ezproxy.libproxy.db.erau.edu/scholarly-journals/structural-transformation-supercooled-water/docview/910128750/se-2?accountid=27203

Pilot's handbook of aeronautical knowledge: FAA-H-8083-25B. (2016). Aviation Supplies & Academics. Retrieved 2016, from https://www.faa.gov/regulations_policies/handbooks_manuals/aviation/phak/media/pilot_handbook.pdf

Zielinski, S. (2011, November 30). At What Temperature Does Water Freeze? Retrieved December 11, 2020, from https://www.smithsonianmag.com/science-nature/at-what-temperature-does-water-freeze-1120813/