Deicing

An American Airlines MD-80 aircraft being de-iced at Syracuse Hancock International Airport

De-icing is the process of removing ice from a surface. Most commonly referring to liquids applied to the wings of aircraft, de-icing can also be done by other means and on other surfaces.

Aircraft

When there are freezing conditions and precipitation, it is critical that an airplane be de-iced. Failure to do so means the airplane's wings will be too rough to be aerodynamic, thereby causing a loss of lift, and resulting in a crash. Because of this potentially severe consequence, de-icing is performed at airports where temperatures are likely to dip below the freezing point.

De-icing on the ground is usually done by spraying aircraft with a liquid such as propylene glycol, similar to the ethylene glycol antifreeze used in automobile engine coolants. While less toxic than its automotive counterpart, 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. (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.)

Airplanes frequently use pneumatic boots on the leading edges of wings to effect 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. Such systems can be defeated if they are inflated too soon; the pilot must allow a fairly thick layer of ice to form before inflating the boots. Another type of system uses 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 [[National Aeronautics and ice on the surface, sensing a change in resonant 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.

However, many modern civil fixed-wing transport aircraft tend not to use de-ice systems in flight, but opt for anti-ice systems on the leading edge of wings, engine inlets and air data probes (anti-ice prevents ice from forming, de-ice removes it once it has accreted). Warm air 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 often operates completely autonomously, switching itself on and off as the aircraft enters and leaves icing conditions.

Roads

De-icing of roads has traditionally been done with salt, spread by snowplows or other dump trucks designed to spread it, along with sand and gravel, on slick roads. Sodium chloride (table salt) is normally used, as it is inexpensive and readily available in large quantities. However, since salt water still freezes at -18°C or 0°F (the basis for Fahrenheit's thermometer scale), it is of no help when the temperature falls below this point. It also has a strong tendency to cause corrosion, rusting the steel used in most vehicles and the rebar used in concrete bridges, one reason leading the northern U.S. to be called the "Rust Belt". More recent snowmelters use other salts, such as calcium chloride and potassium chloride, which not only lower the freezing point of water much lower, but also produce an exothermic reaction. They are somewhat safer for concrete sidewalks, but excess should still be removed.

Since the 1990s, use of liquid chemical melters has been increasing, being sprayed on roads by nozzles instead of a spinning spreader. These have the advantage of soaking down through snow and spreading evenly across ice. While these are environmentally safer and also more effective, they are also much more expensive. Brine has also been used in a limited basis.

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.