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When there are freezing conditions and precipitation, deicing an aircraft is crucial. Frozen contaminants cause critical control surfaces to be rough and uneven disrupting smooth air flow and greatly degrading the ability of the wing to generate lift (force) and increasing drag. This situation can cause a crash. If large pieces of ice separate when the aircraft is in motion, they can be ingested in engines or hit propellers and cause catastrophic failure. Frozen contaminants can jam control surfaces, preventing them from moving properly. Because of this potentially severe consequence, de-icing is performed at airports where temperatures are likely to drop below the freezing point.<br />
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De-icing techniques are also employed to ensure that engine inlets and various sensors on the outside of the aircraft are clear of ice or snow.<br />
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De-icing on the ground is usually done by spraying aircraft with a deicing fluid such as Propylene glycol, similar to ethylene glycol antifreeze used in some automobile engine coolants. Ethylene Glycol (EG) is still in use for aircraft deicing in some parts of the world because it has a lower operational use temperature (LOUT) than PG and is more versatile in application, but Propylene Glycol (PG) 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 the used liquid, so that it cannot seep into the ground and streams. Even though classified as non-toxic, it has negative effects in nature, as it uses oxygen during breakdown, causing 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 recycle used deicing fluid, separating water and solid contaminants, enabling reuse of the fluid in other applications.  Wikipedia