Introduction

These terms are taken largely from ADS-4, with updates and additions. See the Resources for other, more extensive terms lists.

Selected Aircraft Icing Terms

Airfoil chord: the longest line possible between the leading edge and trailing edge of an airfoil. See figure.

Airspeed: The speed of the aircraft through the air. Sometimes measured in knots (nautical mile per hour) or KTAS. A nautical mile is about 6076 feet or 1852 m, or 1.15 statute ("standard") miles.

Angle of attack: Angle between airfoil chord and line of flight. See figure.

Anti-icing: The prevention of ice formation on airframe surfaces. The use of heat is the most common means of anti-icing.

Appendix C: Appendix C of the United States Chapter 14 Code of Federal Regulations Part 25 that defines icing conditions used for certification of large transport category aircraft.

Appendix O: Appendix O of the United States Chapter 14 Code of Federal Regulations Part 25 that defines large drop icing conditions used for certification of large transport category aircraft. It defines conditions for freezing drizzle and freezing rain.

Collection efficiency: The ratio of actual water collection rate to the water catch rate when water drop paths are straight lines. Also termed catch efficiency. See figure.

Continuous Maximum Icing: Icing category defined in Appendix C for "Stratiform Clouds", commonly used for design of wing, empennage, and windshield ice protection on transport aircraft.

Cumulus clouds: Massy, piled-up clouds with a flat base.

De-icing: The removal of ice that has formed on airframe surfaces.

Drop trajectory: The path of a water drop with respect to an airfoil. See figure.

Evaporation: The process where liquid water turns to water vapor.

Fully evaporative anti-icing: Anti-icing with sufficient power to prevention of ice formation on airframe surfaces.

Freezing Drizzle: Drizzle size water drops at temperature slightly below freezing and characterized by relatively large water drops that may freeze in contact with airframe surfaces.

Freezing fraction: That part of the water catch that freezes.

Freezing rain: Rain at temperature slightly below freezing and characterized by relatively large water drops that may freeze in contact with airframe surfaces.

Glaze ice: Clear ice with rapid growth rate and characteristic "double horn" or "mushroom" formation. (This term is also used by pilots to describe a smooth film of ice covering leading edges at temperatures just below the melting point of ice.) See figure.

Ice accretion: A growth or buildup of ice; an ice formation.

Icing wind tunnel: A wind tunnel (a tube with a fan to provide airflow) that also has a means of providing cold air and water spray to simulate supercooled cloud icing conditions.

Impingement limits: The end of the water impingement region on both upper and lower surfaces of an airfoil.

Intermittent maximum icing: Icing category defined in Appendix C, based on cumuliform clouds having high liquid water content for short times.

Langmuir drop size distribution: A seven-bin drop size distribution defining at representative drop size for each bin, and a fraction of the total water content of a cloud, something like a normal or "bell" curve. Named after Iring Langmuir, who proposed several distributions circa 1945. See figure.

Latent heat: The exchange of heat required for water to change from liquid to solid ice (latent heat of fusion or freezing), or from liquid to vapor (latent heat of evaporation).

Liquid water content: Water contained as free moisture in a volume of air (does not include water in vapor form). Typically measured in g/m^3 (grams per cubic meter).

Local collection rate: Mass of impinging water per unit time and per unit area at a given location.

Mean effective drop diameter (MED): Half the volume of water in a given sample of air is contained in larger drops, half in smaller drops. Functionally interchangeable with MVD.

Mixed Ice: Simultaneous appearance of rime and clear ice or an ice formation that has the characteristics of both rime and glaze (or clear) ice. See figure.

Median volumetric diameter (MVD): drop diameter defined by the condition that half the volume of water in a given sample of air is contained in large drops, half in small drops. Functionally interchangeable with MED.

Melting point of ice: The temperature at which ice melts at normal pressures, 0°C or 32°F. Sometimes termed "freezing point", but that does not account for supercooled liquid water.

Messinger heat balance: A thermodynamic model of the freezing of water. Named after Bernard Messinger, who published it circa 1951. See figure.

Micrometer (μm): One millionth of a meter. Sometimes called "micron" in older literature.

Rime ice: Milky white, low-density ice formed at low temperatures with a relatively streamlined shape; also referred to as spearhead and knife-edge ice. See figure.

Runback ice: Ice accumulated aft of the protected region, resulting from the water running back area freezing (may be found with partially evaporative anti-icing systems and with de-icing systems). See figure.

Running wet: A condition at which an airframe surface is maintained above 32°F (by a heating anti-icing system) so that impinging water drops will not freeze.

Stratus clouds: Layer-type clouds, usually at low altitudes. Also termed "Stratiform".

Supercooled water drops: Small water drops suspended in the air at temperatures below the melting point of ice (0°C or 32°F).

Water catch: The mass of water impinging on an airframe surface (per unit length of span and per unit time for an airfoil).

Water vapor: The gaseous state of water.

Related

This is part of The Basics series.