ATMOSPHERE - As you're sitting on the runway, watching snow or sleet or freezing rain come down and thinking about how it might be covering your car or your house or the sidewalk, consider that it's also covering the airplane you're about to take off in. While ice on your personal belongings or your sidewalk may cause some consternation, ice on an airplane is an enormous safety consideration for a large number of people. It's not that the ice is heavy or will freeze some essential part of the plane, which would make it unsafe. It's that a coating of ice on an airplane will disrupt airflow around an airplane's wings and affect the lift, which is the ability of the plane to fly. That's not something you want ice, or anything else, to mess with.
Deicing is the process of removing ice from the plane. A second step, called anti-icing, may also be required to keep ice from forming again once it's been removed. Here's how these two processes work to keep airplanes — and their crews and passengers — safe in the skies.
Deicing agents are compounds made up of the chemical glycol and water. Glycol lowers the freezing point of the solvent, water [source: Ritter]. The agents are used in different formulations for different weather conditions, but whatever the formulation, the compound is applied in the same way. It is heated and sprayed through a hose over an airplane to remove snow, ice or frost. Pilots make the call for deicing when necessary and oversee the process, which must be done in accordance with detailed Federal Aviation Administration (FAA) rules. In fact, the FAA rules chapter on deicing runs more than 30 pages and covers everything from makeup and handling of the deicing compound to its application, how quickly it must be applied and the documentation required following each deicing occurrence.
Speed and thoroughness are essential in applying deicer. As such, applications must be performed in a timely and well-ordered fashion, with an eye on what's called holdover time, the amount of time following deicing before the plane has iced-over again and must be re-treated. Holdover time varies based on the makeup of the compound and is set by the fluid manufacturer. With deicing compounds costing upwards of $5 per gallon, the procedure can cost thousands of dollars (factoring in handling and storage costs) , so it's essential that holdover times not be exceeded [source: Smith]. Holdover time also explains why deicing is performed after everyone is on board and the plane is otherwise ready to take off. Holdover time may only be a few minutes, so the plane must be ready to taxi onto the runway and get into the air soon after deicers are applied.
Deicing agents are generally not designed to keep ice or frost from re-forming on the plane. So, if snow, sleet or freezing rain is actively falling as an airplane is being deiced, the plane may also need an anti-icing application to keep ice from re-forming before the plane takes off. Anti-icing fluids are also made up of glycol and water, but with a higher concentration of glycol than deicing fluids. A thickening agent is added to help the compound adhere to the plane as it takes off.
A thin layer of anti-icing agent is sprayed uniformly over the plane when required. The FAA recommends that anti-icing agents be applied within three minutes of deicing and not at all if the deicing fluid has frozen or if frost has reformed on the plane. If either of those situations occur, deicing must be performed again before the anti-icing fluid is applied.
If the plane is coated with just a dusting of frost, rather than ice, it may be possible to deice and anti-ice at the same time. In this instance, the anti-icing fluid is used and acts as both a deicer, to melt the frost, and an anti-icer, which protects the airplane from further icing.[HWS]