Anti-icing is the process of preventing ice from forming on a surface.
De-icing can be accomplished by mechanical methods (scraping); through the application of heat; by use of chemicals, known as icemelters, designed to lower the freezing point of water (various salts or alcohols); or by a combination of these different techniques.
De-icing techniques are also employed to ensure that engine inlets and various sensors on the outside of the aircraft are clear of contamination caused by ice or snow.
De-icing on the ground is usually done by spraying aircraft with a deicing fluid such as monopropylene glycol, similar to ethylene glycol antifreeze used in some automobile engine coolants. Ethylene glycol is still in use for aircraft deicing in some parts of the world, but Monopropylene glycol is more common because it is classified as non-toxic, unlike ethylene glycol. Nevertheless, it still must be used with a containment system to capture all of the used liquid, so that it cannot seep into the ground and streams. Even if it is classified as non-toxic, it still has negative effects in nature, as it uses oxygen as it breaks down, causing other life to suffocate. (In one case, a significant snow in Atlanta in early January 2002 caused an overflow of such a system, briefly contaminating the Flint River downstream of the Atlanta airport.) Many airports successfully recycle used deicing fluid, separating out water and solid contaminants in order to be able to reuse the fluid.
Though there are several different formulations of deicing fluid, they fall into two basic categories: Heated glycol diluted with water for deicing and snow/frost removal, also referred to as "Newtonian fluids", and unheated, undiluted glycol that has been thickened (imagine half-set gelatin), also referred to as "Non-Newtonian fluids", that is applied as an agent to retard the future development of ice or to prevent falling snow or sleet from accumulating. In some cases both types of fluid are applied, first the heated glycol/water mixture to remove contaminants, followed by the unheated thickened fluid to keep ice from reforming before the aircraft takes off. This is referred to as "a two-step procedure".
Inflight ice buildups are most frequent on the leading edges of the wings, tail and engines (including the propellors or fan blades). Lower speed aircraft frequently use pneumatic boots on the leading edges of wings and tail to affect de-icing in flight. The rubber coverings are periodically inflated, causing ice to crack and flake off in the slipstream. Once the system is activated by the pilot, the inflation/deflation cycle is automatically controlled. In the past, it was thought such systems can be defeated if they are inflated too soon; that the pilot must allow a fairly thick layer of ice to form before inflating the boots. More recent research shows “bridging” does not occur with any modern boots.
Some aircraft may also use electrically heated resistive elements embedded in a rubber sheet cemented to the leading edges of wings and tail surfaces, propeller leading edges, and helicopter rotor blade leading edges. Such systems usually operate continuously. When ice is detected, they first function as de-icing systems, then as anti-icing systems for the duration of flight in icing conditions. Some aircraft use chemical de-icing systems which pump antifreeze such as alcohol or propylene glycol through small holes in the wing surfaces and at the roots of propeller blades, causing the ice to melt and making the surface inhospitable to further ice formation. A fourth system, developed by the National Aeronautics and Space Administration, detects ice on the surface by sensing a change in resonance frequency. Once an electronic control module has determined that ice has formed, a large current spike is pumped into the transducers to generate a sharp mechanical shock, cracking the ice layer and causing it to be peeled off by the slipstream.
Many modern civil fixed-wing transport aircraft use anti-ice systems on the leading edge of wings, engine inlets and air data probes using warm air. This is bled off the powerplants and is ducted into a cavity just under the surface to be anti-iced. The warm air heats the surface up to a few degrees above zero, preventing ice from forming on that surface. The system may operate completely autonomously, switching itself on and off as the aircraft enters and leaves icing conditions.
Infrared is the transmission of energy by means of electromagnetic waves or rays. Infrared is invisible and travels at the speed of light in straight lines from the heat source (the emitter) to all surfaces and objects (the receivers) without significantly heating the space (air) through which they pass. When infrared waves strike an object, they release their energy as heat. This heat is then either absorbed or reflected by the cooler surface. Infrared energy is continually exchanged between "hot" and "cold" surfaces until all surfaces have reached the same temperature (equilibrium). The colder the surfaces, the more effective the infrared transfer from the emitter. This heat transfer mechanism is substantially faster than conventional heat transfer modes used by conventional deicing (convection and conduction) due to the cooling effect of the air on the deicing fluid spray.
More recently, organic compounds have been developed that reduce the environmental issues connected with salts and have longer residual effects when spread on roadways, usually in conjunction with salt brines or solids. These compounds are generated as byproducts of agricultural operations such as sugar beet refining or the distillation process that produces ethanol.
Since the 1990s, use of liquid chemical melters has been increasing, being sprayed on roads by nozzles instead of a spinning spreader. Liquid melters are more effective at preventing the ice from bonding to the surface than melting through existing ice.
In Nagano, Japan, relatively inexpensive hot water bubbles up through holes in the pavement to melt snow, though this solution is only practical within a city or town. Some individual buildings may melt snow and ice with electric heating elements buried in the pavement, or even on a roof to prevent ice dams under the shingles, or to keep massive chunks of snow and dangerous icicles from collapsing on anyone below. Small areas of pavement can be kept ice-free by circulating heated liquids in embedded piping systems.