Accumulation of frozen moisture on an aircraft.
When an aircraft encounters freezing temperatures and visible moisture, icing, the accumulation of frozen moisture, is possible. Although icing is a serious hazard to the safety of any flight, light aircraft are particularly susceptible to aircraft icing, as such craft have few, if any, anti-icing or deicing systems. Icing can destroy an aircraft’s ability to create lift and engine power when ice builds up on the structure and within the engine-induction system.
There are two types of aircraft icing, structural and induction. Structural ice may form on aircraft lifting surfaces, such as the wing and horizontal stabilizer, and on the windshield and protruding devices, such as the propellers, engine air intakes, antennas, struts, and landing gear. Ice adds additional weight to the aircraft. More critical, though, is the additional drag that the ice causes by disrupting the smooth flow of air over the lift-producing surfaces. Moderate to severe accumulations of structural ice can greatly affect aircraft controllability. Both wind-tunnel and flight tests have proven that ice accumulations no thicker or rougher than a piece of coarse sandpaper can reduce lift by 30 percent and increase drag by as much as 40 percent.
Structural icing may be present as rime, clear, or mixed ice. Rime ice has a rough, milky-white appearance. Rime ice forms when relatively small drops of moisture strike freezing aircraft surfaces and adhere to the surface rapidly. The milky white appearance is caused by the presence of air trapped in the rapidly freezing ice. Deicing systems are generally effective in removing rime ice, because rime ice is less tenacious than other forms of ice.
Clear ice forms when large drops of moisture strike aircraft surfaces and freeze at a slower rate. The slower freezing process displaces air from the accreting ice, allowing the formation of a clear, very tenacious coating of ice on the aircraft’s surfaces. Because of the lack of aeration in the ice, clear ice is difficult to remove and quite heavy. Mixed ice is a combination of rime and clear ice and exhibits characteristics of both.
Induction icing can reduce engine performance and may result in complete engine stoppage. Aircraft equipped with carburetors may experience ice buildup in a restricted air passage, called a venturi, that is located in the carburetor. An increase in air velocity and a resultant decrease in pressure within the venturi results in a reduction in air temperature. This lowering of air temperature creates the potential for moisture within the air to freeze and create ice accumulations along the sides of the venturi. The ice buildup reduces the flow of air and fuel through the venturi, resulting in decreased engine performance. In severe instances, ice may completely occlude the venturi, resulting in complete loss of engine power. In the case of aircraft equipped with fuel-injection systems, ice can accumulate in air intakes, reducing the flow of air to the engine. Ice occluding engine air intakes can cause reductions in available engine power as well as complete engine failure.
Although all clouds are the manifestation of water in its gaseous state, the moisture content of clouds can vary greatly. Very-cold-winter states such as Montana, North Dakota, and Minnesota often have relatively dry clouds. In contrast, states such as Pennsylvania and New York often produce very wet winter clouds that, when temperatures drop below freezing, have a high potential for icing. Clouds with temperatures at or just below the freezing point, 32 degrees Fahrenheit, or 0 degrees Celsius, are the most likely to result in aircraft icing. Moisture in air that is well below the freezing point is already frozen and therefore will not adhere to aircraft. Wind can move moisture-laden air between regions. Wind moving across large bodies of water, such as oceans or the Great Lakes, will result in greater moisture content within the air. Mountains can cause a lifting phenomenon that may force moisture-laden air upward in the atmosphere where the natural temperature lapse rate cools the surrounding air to the freezing point.
Areas of low-pressure and fronts are the greatest producers of ice. Although in some instances, isolated air-mass instabilities may also produce sufficient moisture and temperatures capable of producing ice-generating conditions.
Freezing rain and drizzle are the most hazardous ice-producing conditions. Freezing rain occurs when temperature inversions exist. Rain falling from clouds in warmer air aloft begins to freeze as it enters freezing air at lower altitudes. Freezing rain and drizzle can produce severe ice accumulations that rapidly overwhelm the ice-shedding capabilities of even the best anti-icing and deicing equipment.
In the United States, the U.S. National Weather Service (NWS) is the government agency responsible for weather forecasting. Utilizing NWS and other weather forecasting sources, the Federal Aviation Administration (FAA) disseminates weather information to the aviation community through a network of Flight Service Stations (FSS). FSS specialists provide comprehensive weather briefings to pilots. These briefings are usually conducted over the telephone but may also be accomplished in person at the Flight Service Station. Pilots may also obtain icing and other weather information on the Direct User Access Terminal System (DUATS), utilizing a personal computer and Internet connection. Graphic weather charts available to FSS specialists and DUATS users include predictions of areas of potential icing. In addition, special meteorological notices called AIRMETS and SIGMETS are issued when potentially hazardous icing conditions exist. These notices provide pilots with an additional warning of potential icing. Pilots experiencing icing conditions report these conditions to the nearest FSS. Pilot reports (PIREPS) are usually conveyed directly to FSS specialists via the aircraft radio. PIREPS are an important component of the weather reporting system, because they describe actual conditions and not merely forecasts.
Dondzila, Kathy, and John Steuernagle, eds. Aircraft Icing. Frederick, Md.: Aircraft Owners and Pilots Association Air Safety Foundation, 1999. An excellent safety pamphlet addressing all aspects of aircraft icing. Peters, Lestor. Aviation Weather. Englewood, Colo.: Jeppesen-Sanderson, 1998. A comprehensive overview of meteorology, including basic and advanced weather theory as well as interpretation of weather observations and forecasts. Schlachter, Kathleen. Aviation Weather Services AC 00-45E. Oklahoma City, Okla.: Federal Aviation Administration, 1999. The official Federal Aviation Administration guide to coded weather observations and forecasts. Willits, Pat, ed. Private Pilot Manual. Englewood, Colo.: Jeppesen Sanderson, 1997. An excellent basic flight training text with two chapters devoted to meteorology and the interpretation of weather data.
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The accumulation of ice on the exterior structures of an aircraft can affect a plane’s lift and drag. NASA has conducted extensive research on icing in its icing research tunnel at the Glenn Research Center.