If you are a frequent flyer, you’ve probably been at the airport waiting to jet somewhere on a winter trip when the voice of an airline employee announces over the intercom that there will be a slight delay while the plane gets deiced. But how does this process actually work, and why is it needed?

As a mechanical engineer who studies frost growth and water droplets on surfaces, I have come to appreciate the importance of deicing planes. Indeed, deicing is an important safety step performed by the airlines on wintry days because of how snow and ice can affect the physics of flying.

Why deice?

In short, deicing is necessary because snow and ice on airplane wings can decrease lift by as much as 30%. Lift is the vertical upward force that keeps a plane in the sky. It is generated when air flows over the wings of a plane.

Ice and snow can alter how air flows over the wings, which can affect a pilot’s ability to maneuver and control the aircraft. It can also increase the stall speed, which is not good either. Stall speed is the minimum speed needed by an aircraft to generate enough lift to keep it aloft.

Additionally, ice on the wings can break off in flight, potentially damaging one or more of the flaps on the wings or an engine. Needless to say, deicing has become an indispensable part of flying, especially in the winter months.

Deicing chemicals

Most people are familiar with the chemical deicers that are used on roads during the winter months. However, the salts in these products can be corrosive, so they’re not used on aircraft.

Aircraft deicers consist of a water-based solution of glycol – a colorless, odorless organic liquid – mixed with various additives. These additives might include a thickening agent; a substance that prevents corrosion; a surfactant, which decreases the surface tension; a flame retardant, and a dye.

Glycols are very good at lowering the freezing point of water, which makes it harder for water to freeze or stay frozen on surfaces. Propylene glycol and ethylene glycol are the two most common types used, typically making up 30% to 70% of the deicing solution.

For years, only ethylene glycol was used in deicers because of its low cost. However, because propylene glycol is less toxic to wildlife and humans, its adoption by commercial airlines has grown steadily since the 1980s.

How does the deicing process work?

Airlines use four standard fluid types when deicing aircraft. These fluids have different viscosities – viscosity is a measure of a fluid’s resistance to flow – and holdover times, which is the length of time the fluids are expected to protect the plane during snow or icing conditions.

In the United States, airlines typically use a two-step process before flying. First, they perform deicing using either a heated Type I fluid or a heated solution of Type I fluid and water.

Deicing removes existing ice and snow from the wings of the plane, which is why airlines often heat the deicing fluid to around 140 to 150 degrees Fahrenheit (60 to 66 degrees Celsius) before application.

Type I fluids are the thinnest of the deicing fluids, and they’re often red or orange. They spread the easiest on a plane’s surface because they have the lowest viscosity. Since they’re thin enough to flow off a plane when it’s not moving – or moving slowly – they can be applied to any aircraft.

But as a result, they also have the shortest holdover times, often less than 20 minutes depending on the weather conditions. These holdover times vary, though, and can be less than five minutes for snow if the outside air temperature is below 14 F (minus 10 C).

Next, the ground crews will typically apply an anti-icing fluid to the aircraft – often Type II or Type IV. Anti-icing solutions are used to help prevent the future accumulation of snow and ice on the wings of planes.

Type II and Type IV fluids contain thickening agents that increase their viscosity. These thickeners allow the fluid to remain on the aircraft longer to help melt newly forming frost or ice. This translates to longer holdover times – often more than 30 minutes for snow – but it also means the plane needs to reach a higher speed to shear, or blow off, the fluid.

Once applied, Type II and IV fluids will generally stay on the aircraft until the plane is taxiing down the runaway during takeoff. By then, it has gained enough speed to produce the shear force necessary to remove the fluid from the plane. Type II fluids are a clear or pale straw color, while Type IV fluids are generally green. Including a colored dye helps the ground crew clearly see what parts of the plane have been coated and which areas still need application.

Type III fluids are not as common anymore. They are formulated to shear off at lower speeds and thus are sometimes used on small commuter aircraft since these planes typically don’t go as fast as commercial jetliners.

Environmental impact of deicing

Environmental considerations are also an important part of deicing. Glycols require a lot of oxygen to biodegrade, which can deplete dissolved oxygen in streams or lakes. This, in turn, can threaten aquatic life, like fish and other organisms, that need dissolved oxygen to breathe.

In addition, ethylene glycol is toxic to wildlife, so the Environmental Protection Agency requires airports to monitor their stormwater runoff. For this reason, most airports collect and treat stormwater runoff on-site or send it to a municipal wastewater treatment facility.

Airports are also increasingly starting to use fluid recovery systems to recycle the glycols and capture the additives in these fluids, which are often toxic, too. They’ll often use designated areas outside for deicing planes so they can collect and store the fluids after they run off the plane in holding tanks underground until they can be recycled.

Atmospheric icing

During flight, planes use other technologies to mitigate the icing risks. For example, most modern aircraft use bleed air systems, which channel hot air from the engine’s compressor through interior ducts to the leading edges of the wings and other critical areas to help prevent ice buildup while the plane is in the sky.

Some planes also use electrically heated panels embedded in the aircraft’s wings to generate heat. These control systems typically cannot be used while the plane is on the ground, since they rely on cold air flowing across the wing’s surface. This airflow is usually achieved at cruising altitude and is necessary to prevent the plane’s surface from getting too hot.

Airlines may sometimes also use icephobic coatings to help keep new ice from forming and sticking on the outside surfaces of planes. These coatings delay how soon new ice can form. They can also reduce how strongly the ice adheres to the surface.

Smaller planes may also use inflatable rubber strips called pneumatic boots on the wings that can be inflated as needed to break off accumulated ice on the leading edge of the wings.

Flying is truly a modern scientific marvel. A lot of engineering goes into not only getting planes off the ground but also keeping them ice-free during flight. So the next time you experience a weather-related delay at the airport, just remember that deicing is needed to ensure both a safe flight and a truly uplifting one.

Andrew Sommers does not work for, consult, own shares in or receive funding from any company or organization that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.