This week's background information is available in Chapters
Types of Icing Formations
Icing refers to any deposit or coating of ice on an aircraft. There are several different types of icing ranging from icing critical in the operation of aircraft, to aircraft stability and takeoff and landing hazards.
- Induction icing - The formation of ice on aircraft air induction ports and air filters. Induction icing's main effect is the result of the loss of power due to ice blocking the air before it enters the engine.
- Carburetor icing - The formation of ice in the throat of a carburetor when moist air drawn into the carburetor is cooled to the frost point. Often detrimental to engine operation, carburetor icing can result in a partial or full loss of power.
- Structural icing - The formation of ice on the exterior of an aircraft during flight through clouds or liquid precipitation when the skin temperature is equal or less to 0 degrees C. The main concern of structural icing is the loss of aerodynamic efficiency due to an increase in drag and a decrease in lift. Icing can interfere with several parts of the aircraft, such as propeller balance, jam landing gear, cover antennas, and reduced visibility through the windshield. Additionally, problems can occur with instruments such as the airspeed indicator, altimeter, and vertical speed indicator.
- Ground icing - Ground icing can include freezing rain, freezing drizzle, wet snow, as well as frost. It can be a concern during takeoff, as it could prevent an aircraft from becoming airborne at normal takeoff speed.
Observing and Reporting Structural Icing
Identification and assessment of icing are discussed below.
- Icing types
- Clear ice - The formation of a layer of hard, smooth, glossy ice on an aircraft. It is relatively transparent or translucent. Clear ice is heavy and difficult to remove.
- Rime ice - The formation of a white or milky and opaque granular deposit of ice on an aircraft. Rime ice is the most common type of icing.
- Mixed ice - A combination of clear ice and rime ice.
- Icing Intensity is related to the rate of accumulation of ice on the aircraft. The different intensities are listed below:
- Trace - Ice becomes perceptible. Rate of accumulation of ice is slightly greater than the rate of loss due to sublimation.
- Light - The rate of accumulation may create a problem for flight in this environment for one hour.
- Moderate - The rate of accumulation is such that even short encounters become potentially hazardous.
- Severe - The rate of accumulation is such that de-icing/anti icing equipment fails to reduce or control the hazard.
Use Appendix D-13, the examples on page 13-8, and the map on 13-9 to cover the icing part of PIREPS.
Microscale Icing Processes
- TemperatureThe possibility of icing ocurrs as temperatures reach just below zero. At just below zero, there is the highest threat of severe icing. As temperatures continued to become colder, the threat diminished. The types of icing fall between different temperature ranges.
- Clear: 0 to -5 C
- Clear or Mixed: -5 to -10 C
- Mixed or Rime: -10 to -15 C
- Rime: -15 to -20 C
- Liquid Water Content (LWC) - A measure of the liquid water due to all the supercooled droplets in that portion of the cloud where your aircraft happens to be. High LWC values induce the possibility for potentially severe icing conditions.
- Droplet size - Supercooled small droplets will freeze more rapidly on impact with the wind of an aircraft than supercooled large droplets (SLD). Supercooled water droplets are liquid cloud or precipitation droplets at subfreezing temperatures. SLD are supercooled water droplets with diameteres larger than 0.04 mm. These droplets contribute to some of the worst aircraft structural icing conditions. There are two basic formation processes of supercooled water droplets that will increase can increase the size of the droplets to SLD.
- Collision/coalescence - The growth of cloud droplets by collision/coalescence was covered in Chapter 6 of the book.
- Warm layer process - When snow falls into a warm layer (T > 0 C), ice crystals can melt. If they melt and fall into a cold layer (T < 0 C) and become supercooled, they will freeze upon contact. If they freeze beforehand, ice pellets will be produced.
Icing and Macroscale Weather Patterns
- Cyclones and Fronts - Winter cyclones and the associated fronts provide the most optimum conditions for widespread icing. Extratropical cyclones contain a variety of mechanisms that create widespread, upward vertical motions, such as convergence of surface winds, frontal lifting, and convection. The favored locations for icing in a developing cyclone are behind the center of the surface low position (usually north and west), and ahead of the warm front (usually northeast of the low pressure center).
- Influence of Mountains - When winds force moist air up the windward slope of mountans, the upward motions can supply moisture for the production of substantial liquid water in subfreezing regions. In terrain with mountains, the worst icing zone is primarily above mountain ridges and on their windward side. Additionally, standing lenticular clouds downwind of ridges and peaks are also a suspect for icing when temperatures are in the critical subfreezing range.