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Why do airlines de-ice planes?

Because even a thin layer of ice, snow or frost can ruin a wing’s lift. Planes must take off with clean surfaces, so crews spray them with fluid, and thickened fluid thins out and blows off during takeoff.

Quick Answer

Airlines de-ice planes because ice, snow or frost on a wing changes how air flows over it, and that can cut lift and add drag. FAA guidance says contamination as thick and rough as sandpaper on a wing’s leading edge and upper surface can reduce lift by as much as 30 percent and increase drag by 40 percent. That’s why U.S. rules follow a “clean aircraft concept”: wings, tail and other critical surfaces must be free of frozen contaminants at takeoff. Crews first spray fluid to remove ice and snow already on the plane (de-icing), usually a thin Type I fluid that’s orange if dyed. They may then apply a thicker anti-icing fluid, Type IV, emerald green, which helps keep new ice from forming for a limited time called holdover time. Thickened fluids are designed to thin out as the plane speeds up, so most of the fluid shears off during the takeoff roll and the wing’s airflow is restored.

By the Numbers

Up to 30%Lift lost to sandpaper-like ice, snow or frost on a wing

40%Added drag, from the same FAA test data

2 stepsDe-icing, then (when needed) anti-icing

Key Facts

The rule: Wings, tail and other critical surfaces must be free of frozen contamination at takeoff
The danger: Sandpaper-like ice can cut lift by up to 30% and raise drag by 40%, per FAA test data
Two jobs: De-icing removes ice that’s already there; anti-icing helps stop new ice forming
Fluid colors: Type I is orange if dyed; Type IV is emerald green

How a winter de-icing works

Clean it off
Crews spray a thin Type I fluid to remove ice, snow and frost from the wings, tail and other critical surfaces.
Protect it
In some conditions, a second, thicker anti-icing fluid goes on to slow new ice from forming.
Watch the clock
The protection lasts a limited holdover time that depends on the fluid and the weather. Pilots treat it as a guideline.
Shed it on takeoff
As the plane speeds up, the thickened fluid thins out and most of it flows off the wing.

Why a thin layer matters

Ice doesn’t have to be thick to cause trouble. The FAA says snow, ice or frost with the thickness and roughness of medium or coarse sandpaper on the leading edge and top of a wing can reduce lift by as much as 30 percent and increase drag by 40 percent. Because the effects vary by aircraft design and are hard to predict, the rule is simple: take off with clean surfaces. NASA’s icing researchers describe the same basic problem: ice stops air from flowing evenly over the wing, which raises drag and lowers lift.

De-icing versus anti-icing

De-icing removes frozen contamination that’s already on the plane. Type I fluid is thin, runs off quickly and is orange if dyed. Anti-icing uses thickened fluids, such as Type IV, which are emerald green and cling to the surface to slow new ice and snow from sticking. In FAA guidance, thickened fluids behave like “shear-thinning” fluids: they stay thick while the plane is parked, then thin out as airflow speeds up during the takeoff roll, so most of the fluid is shed.

Holdover time

Anti-icing fluid protects only for a while. The time, called holdover time, depends on the fluid and on conditions such as temperature and the type of precipitation, and the FAA publishes tables of estimated times. The FAA says these times are guidelines and shouldn’t be the only basis for a decision to take off.

Myth vs. Fact

MythPlanes are de-iced mainly because ice makes them heavy.

FactThe FAA and NASA describe the main hazard as disrupted airflow: ice and frost cut lift and add drag, even in thin, rough layers.

MythThe colored fluid stays on the wings all flight.

FactThickened anti-icing fluid is designed to thin out and flow off as the plane speeds up on the runway.

The Bottom Line

Airlines de-ice planes because even thin ice or frost can cut a wing’s lift and add drag. Crews spray fluid so the plane takes off with clean surfaces, and thickened fluid then blows off during the takeoff roll.