‍DALLAS — During the winter season, you may have noticed a truck spraying liquid over planes. What's that liquid, and what does it do to the plane?

In this article, we want to explain the de-icing process, its purpose, the fluid types, and everything you need to know about this critical process in aviation.

De-icing 101

We refer to de-icing as removing the ice from the plane. Usually, this process has to be done before the plane takes off because ice or snow on the principal flight surfaces can affect the aerodynamics of the aircraft, resulting in accidents like SAS Flight 751, Continental Airlines Flight 1713, and Air Ontario Flight 1363, all those accidents happened due to icing on the flight surfaces.

The surfaces of a plane are designed to be aerodynamic, allowing the wind to travel along the fuselage or wings without any effort. This means less drag and more speed and lift. However, when ice or snow covers these surfaces, the aircraft's performance is affected due to the drag it creates; in other words, the snow makes the flight surfaces less aerodynamic.

The International Civil Aviation Organization (ICAO) says that snow on the wings can reduce lift by approximately 30% and increase drag by 40%. For example, snow can interrupt the wind flow over the wing, causing the aircraft to stall and end, like Arrow Air (JW) Flight 1825R.

There are different ways to prevent icing formation on flight surfaces: fluids or plane de-icing systems. Small planes can be de-icing manually. However, we will only discuss the de-icing fluids here.

Where, When, and How Is De-icing Done?

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Types of De-icing Fluids

Before explaining the types, we need to know that all de-icing fluids are typically composed of ethylene glycol (EG) or propylene glycol (PG), with thickening agents, wetting agents, corrosion inhibitors, UV-sensitive dyes, and colors for their identification. All of them have to be applied hot.

The principal purpose of using these chemicals is to lower the freezing point; the water has a 32ºF freezing point, and the de-icing fluids have a freezing point between -18 and -67 ºF.

Type I

Type I fluids have a low viscosity. 45% is the de-icing mixture, and the other 55% is water. The fluid must be sprayed hot at high pressure to remove the ice and snow. The fluid's temperature must be between 130 and 180 °F. The Type I fluids are dyed orange for easy identification during application. The freezing point of this fluid is -19 ºF.

Lufthansa Airbus A340-313 during de-icing with Type I. Photo: Fabrizio Spicuglia/Airways

Type II

Type II fluids are pseudoplastic liquids, meaning they have a polymeric thickening agent to prevent them from flowing off the plane quickly. Due to this, the Type II fluid is mixed in 75% water and 25% mixture, making it feel like jelly. This one can be used to de-icing the plane or prevent the formation of ice on it. It does not have any color for identification.

Type III

Type III fluids were created as a middle ground between Type I and Type II. They are usually yellow and used on slower planes with a rotation speed of less than 100 knots.

Type IV

Type IV fluids were created as more potent Type II fluids. For this reason, Type IV is not diluted in water, resulting in a highly viscous substance. This fluid is usually applicable for ice prevention. The type IV fluids are tinted a light green.

Airbus A320 during de-icing with Type II fluid and a second layer of Type IV as anti-icing. Photo: Fabrizio Spicuglia/Airways

After applying the de-icing fluid, another fluid layer must be sprayed to prevent the formation of new ice or snow during the waiting time or the takeoff; Type II or Type IV fluids can be used for this.

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Where, When, and How Is De-icing Done?

Where

All airports in cold countries have a specific zone reserved during the winter for this procedure. These zones are called de-icing pads and are located near the runways or in the taxi routes from the gate to the runway. The de-icing can also be done at the gate or stand. For this procedure, the aircraft has to be closed, cleared of any handling equipment, and the engines off.

Let's take Frankfurt Airport (FRA) as an example. This airport has two de-icing pads, one located at the side of runway 18 and another in front of Terminal 1. FRA is also permitted to do a de-icing process at the gates.

When

De-icing must be done before the plane takes off. As said in the previous part, it can be done at the gate or the de-icing pads. Ice or snow on the aircraft's surfaces can affect takeoff due to increased drag and lift reduction.

EASA and FAA check the maximum time to take off after the de-icing fluid application every year to prevent any accidents.

How

Special trucks are used to apply the de-icing fluid. These trucks have a hydraulic boom with an open bucket or a cabin attached to it where an operator is located. The trucks have a heated tank where the fluids are stored and are already heated for use.

These tanks are equipped with a pumping system powered by the truck engine. This lets the operator correctly spray all the fluid over the plane with the help of the adjustable nozzle located outside the cabin or the bucket.

The de-icing process takes some time, at least 10 minutes, and usually, more than one plane is waiting for the de-icing pad to be available; this can cause delays, but remember, this was created to make aviation safer.

In a situation like this, you have to wait patiently and keep in mind that those 10 minutes will make you arrive safely at home.