Ch 13 - Icing

Ch 13 - Icing Introduction Aircraft icing can have serious negative effects on both the powerplant and the aerodynamic performance of your aircraft As a pilot, your life and the lives of your passengers depend on your ability to understand icing and to take the proper preflight and inflight steps to deal with it safely

Ch 13 - Icing In this chapter, you will learn to identify and report the various types of icing, understand its causes, and become familiar with the meteorological conditions under which it is most likely to occur When you complete this chapter, you should have a basic understanding of the icing threat and the knowledge of how to avoid it or at least minimize the problem

Ch 13 - Icing Section A – Aircraft Icing Hazards Induction Icing Structural Icing Ground Icing Section B – Observing and Reporting Structural Icing Observations of Icing Type and Severity Icing PIREPs

Ch 13 - Icing Section C – Microscale Icing Processes Temperature Liquid Water Content Droplet Size Section D – Icing and Macroscale Weather Patterns Cyclones Influence of Mountains Icing Climatology Section E – Minimizing Icing Encounters

Ch 13 - Icing Section A: Aircraft Icing Hazards Icing – refers to any deposit or coating of ice on an aircraft. Two types of icing are critical in the operation of aircraft: induction icing and structural icing.

Ch 13 - Icing Induction Icing Induction icing – a general term which applies to all icing that affects the power plant operation. The main effect of induction icing is power loss due to ice blocking the air before it enters the engine, thereby interfering with the fuel/air mixture. Induction icing includes carburetor icing and icing on air intakes such as screens and air scoops. Carburetor icing – occurs when moist air drawn into the carburetor is cooled to a temperature less than 0 degrees Celsius by adiabatic expansion and fuel vaporization.

Ch 13 - Icing Structural icing Structural icing – Airframe or structural icing refers to the accumulation of ice on the exterior of the aircraft during flight through clouds or liquid precipitation when the skin temperature of the aircraft is equal to, or less than 0 degrees Celsius. The primary concern over even the slightest amount of structural icing is the loss of aerodynamic efficiency via an increase in drag and a decrease in lift.

Ch 13 - Icing Ground icing – Another important form of structural icing to be considered is that which may occur prior to take off. An aircraft that is ice-free is as critical for takeoff as it is in other phases of flight, if not more so. Causes of ground icing include freezing rain, freezing drizzle and wet snow. Also, frost can be a significant hazard.

Ch 13 - Icing ***Test data indicate that ice, snow, or frost having a thickness and roughness similar to medium or coarse sandpaper on the leading edge and upper surface of a wing can reduce lift by as much as 30 percent and increase drag by 40 percent

Ch 13 - Icing ***A hard frost can increase the stalling speed by as much as 5 or 10 percent. An aircraft carrying a coating of frost is particularly vulnerable at low levels if it also experiences turbulence or wind shear, especially at slow speeds and in turns. Frost may prevent an airplane from becoming airborne at normal takeoff speed

Ch 13 - Icing Section B: Observing and Reporting Structural Icing Observations of Icing Type and Severity Rime ice – Structural icing occurs when super cooled cloud or precipitation droplets freeze on contact with an aircraft. The freezing process produces three different icing types: clear, rime, and mixed ice.

Ch 13 - Icing Rime ice is the most common icing type. It forms when water droplets freeze on impact, trapping air bubbles in the ice. This type of ice usually forms at temperatures below -15 degrees Celsius. Rime ice appears opaque and milky white with a rough, porous texture.

Ch 13 - Icing Although rime icing has serious effects on the aerodynamics of the aircraft wing, it is regarded as the least serious type of icing because it is lighter, easier to remove, and tends to form on the part of the aircraft where, if available, anti-icing and/or deicing equipment is located.

Ch 13 - Icing Clear ice – forms when droplets impacting an airplane freeze slowly, spreading over the aircraft components. Air temperatures are usually between 0 degrees Celsius and – 5 degrees Celsius. These conditions create a smooth, glossy surface of streaks and bumps of hard ice. Clear ice is less opaque than rime ice. It may actually be clear but often is simply translucent (clear ice is also called “glaze”).

Ch 13 - Icing Clear ice is the most dangerous form of structural icing because it is heavy and hard it adheres strongly to the aircraft surface it greatly disrupts the airflow over the wing and it can spread beyond the location of de-icing or anti-icing equipment.

Ch 13 - Icing Runback icing – when ice spreads beyond the ice protection equipment. Mixed ice – a combination of rime and clear ice forms at intermediate temperatures (about -5 degrees Celsius to -15 degrees Celsius) and has characteristics of both types. The variation in liquid water content in this temperature range causes an aircraft that is flying in these conditions to collect layers of both less opaque (clear) and more opaque (rime) ice.

Ch 13 - Icing Icing intensity – The severity of icing is determined by its operational effect on the aircraft. Icing intensity is classified as trace, light, moderate and severe and is related to rate of accumulation of ice on the aircraft the effectiveness of available de-icing/anti-icing equipment and the actions you must take to combat the accumulation of ice.

Ch 13 - Icing Icing PIREPs Icing PIREPs – Pilot reports of structural icing are often the only direct observations of that hazard and, as such, are of extreme importance to all pilots and aviation forecasters. The critical information that an icing PIREP should contain includes location, time, flight level, aircraft type, temperature, icing intensity, and icing type. Excellent aids to pilots in the diagnosis of icing conditions are graphical presentations of recent icing PIREPs from the Aviation Digital Data Service (ADDS).

Ch 13 - Icing Section C: Micro scale Icing Processes – icing occurrence, type, and severity depend on three basic parameters: Temperature Liquid water content Droplet size

Ch 13 - Icing Temperature – icing types and critical outside air temperatures include Clear (0 to -5 degrees Celsius Clear or mixed (-5 to -10 degrees Celsius) Mixed or rime (-10 to -15 degrees Celsius) Rime (-15 to -20 degrees Celsius)

Ch 13 - Icing Liquid Water Content (LWC) – simply a measure of the liquid water due to all the super cooled droplets in that portion of the cloud where your aircraft happens to be

Ch 13 - Icing Droplet Size Super-cooled large droplets (SLD) – associated with heavy icing and especially with runback icing problems Collision/coalescence – small water droplets can grow into large super cooled droplets through this process, water droplets are super cooled and they initially formed in subfreezing surroundings

Ch 13 - Icing Warm layer process - small water droplets can grow into large super cooled droplets through this process, when snow falls into a warm layer (temperature greater than 0 degrees Celsius) where ice crystals melt, and then fall into a cold layer (temperature less than 0 degrees Celsius) where the rain droplets become super cooled.

Ch 13 - Icing ***The presence of ice pellets (PL) at the surface is evidence that there is freezing rain at a higher altitude

Ch 13 - Icing Section D: Icing and Macro scale Weather Patterns Cyclones and Fronts – extra tropical cyclones provide a variety of mechanisms to produce widespread, upward motions. These include convergence of surface winds, frontal lifting and convection. Influence of Mountains – mountainous terrain should always be considered a source of icing hazards when subfreezing clouds are present. Icing Climatology – refers to the average distribution of icing for a given area

Ch 13 - Icing Section E: Minimizing Icing Encounters – know capabilities of your aircraft, decision tree Freezing level – analyzed on the freezing level chart and appears on some aviation forecast charts Freezing level chart – solid lines on this chart indicate the position of particular freezing levels. The dashed lines indicate where the freezing level intersects the ground. The open circles indicate the location of sounding stations where freezing levels are reported in hundreds of feet MSL.

Ch 13 - Icing Summary Icing can affect an aircraft in many ways, including the degradation of aerodynamics, and causing difficulties with control surfaces, powerplant operation, propeller balance, operation of landing gear, communications, instrument accuracy, and ground handling

Ch 13 - Icing Summary An icing encounter does not leave much room for error This is especially true when it is combined with the additional complications of turbulence, wind shear, and IMC In this chapter, you have learned how induction and structural icing can form

Ch 13 - Icing Summary You are now aware of the types and severity classifications of structural icing, and how temperature, liquid water content, and droplet size contribute to icing type and severity You now understand that the production of supercooled large droplets, such as found in freezing precipitation, is of particular importance for severe icing

Ch 13 - Icing Summary - In addition, your brief examination of an icing climatology has demonstrated how extratropical cyclones, airmasses, and fronts interact with moisture sources and mountains to make some geographical areas more conducive to icing events than others

Ch 13 - Icing Summary Finally, on the basis of icing causes and characteristics, a number of practical rules of thumb have been established to help you avoid or at least minimize icing effects Keep in mind that these are general guidelines; they have not directly addressed the capabilities of your aircraft to handle icing situations

Ch 13 - Icing Summary - More details with regard to tools and procedures for the general assessment of all weather conditions, including icing, in the preflight phase of flight will be presented in Part IV of this text