De-icing (also written as "deicing" without the
hyphen) is the process of removing ice buildup from a surface. In
commercial and
civil applications, it typically refers to either
paved surfaces or to vehicles, these being the two areas where it is important enough to warrant an
industry of its own. This writeup is concerned with de-icing vehicles.
In the vehicular realm, deicing is normally most often done to aircraft where it is a critical safety and performance procedure. As ice forms on the wing surfaces of an aircraft, the shape of the airfoil is changed, making it less efficient. This, coupled with the added weight of the ice the wing is carrying, will lead to an increasingly narrow flight envelope and, if there is not enough reserve performance, the airplane will eventually stall. Besides the wings, there are other areas on the modern aircraft where ice will cause problems - Pitot tubes, engine inlets, windscreens, control surfaces and so on.
There are two different times deicing can be done - in flight and on the ground. The difference is that while on the ground, the airplane can be deiced by ground-based systems, whereas in flight it must rely on its own resources - which must be carried at all times, and hence exact a weight and space penalty. Typically, modern airports will have either deicing stations near active runways or will have deicing vehicles available to deice aircraft waiting on the ramp or taxiway in icing conditions.
There are numerous methods of deicing an airplane. However, there are several criteria which limit those which can be safely and cheaply used. First of all, the aircraft is fairly fragile, and sensitive to anything which increases its wear or load cycles. Hence, simply using very hot liquids is usually not a great idea, since it can result in thermal shock on very cold airframes and aluminum skin. Furthermore, salt, although an effective ice melter, will corrode aluminum, and is therefore a bad plan as well. The standard method of de-icing grounded airplanes is a heated mix of water and glycol-based de-icing fluid, similar to the antifreeze used in autombile radiators. Ethylene glycol, which is normally used in antifreeze, used to be common and is still used in places, but it is a fairly toxic substance. Environmental concerns have driven a shift over to the less toxic propylene glycol in recent years. Even so, glycol-based de-icing fluids are usually used at a fixed de-icing station so that they can be recovered, filtered and re-used for both environmental and cost reasons.
Mobile vehicles can and do spray not only glycol de-icer mix but steam as well. Steam is a much better choice than hot water. It is light enough that it can be used to mechanically blast ice from the surface of the aircraft without damage (airplanes are, after all, designed to handle high-speed gas flows). The steam contains enough thermal energy to disrupt the ice, but not enough per unit volume to shock the aircraft's structure.
Once the ice has been removed from the airplane, it is usually treated with a thicker glycol mix as an anti-icing procedure, to allow it to remain standing in icing conditions without ice forming, until it can take off.
In-flight de-icing is usually an ongoing process. Systems built into the airplane are activated whenever the airplane enters icing conditions. There are several means of de-icing an airplane in flight. Along the same line as ground de-icing, some aircraft are equipped to spray glycol de-icing fluid from nozzles in their wing leading edges in order to disrupt and prevent wing ice. The disadvantage with this method is that it requires the use of a consumable deicing fluid, which the aircraft can find itself without and which represents weight that it must sacrifice from paying load.
Some aircraft have inflatable rubber 'boots' on their wings which can be inflated using pressurized air. If you can see black rubber along the front edge of your airliner wing, this is likely what you're looking at. As ice forms along the edge, the boots are inflated. Their expansion cracks the ice, and it is torn away from the wing by the slipstream. The boots are then left in a slow expansion/contraction cycle, which prevents ice from remaining on the wing for long.
Some new airplanes use electric resonance to remove ice, although this is less popular on passenger aircraft due to its method of operation. When sensors on the wings detect a change in the wing's resonant frequency indicating ice buildup, a sudden large electrical charge is sent through panels on the leading edges of the wing. They will expand sharply, cracking the ice just as the rubber boot did and throwing it off. The charge, however, will usually produce a loud CRACK or thumping sound - which isn't usually reassuring to those inside the cabin. More modern versions of this system minimize this side-effect, however, expanding the surface more slowly.
Finally, on turbine engine aircraft (turbo-props and turbofans), ducting can be used to route hot air from the engine exhaust through the leading edges and around any vulnerable instruments or inlet surfaces, ensuring that they remain a few degrees above freezing temperature and thus preventing ice from forming or melting away any that has accumulated. Although this involves a price in complexity and weight for the duct systems, it obviates the need for additional electrical load or carrying consumables.