A Pilot's Guide to Inflight Icing

Module I - Before You Fly

Know the Situation

Section: Basic Icing Physics

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Ice typically forms on an aircraft in flight when the aircraft surface collides with water droplets that have remained liquid although they are below the freezing point (supercooled water). You can evaluate the risk of an icing encounter by considering three factors:

Temperature
Moisture
Droplet Size

Wing model (vertical) in NASA Glenn's Icing Research Tunnel (IRT)

Wing model (vertical) in NASA Glenn's Icing Research Tunnel (IRT)

Time-lapse of ice forming on leading edge of research airfoil in the IRT

Temperature

Icing is most frequent when the static air temperature (SAT) is between +2°C and -20°C, although ice can accrete outside this range.

The more hazardous ice shapes tend to form at temperatures closer to freezing. Warmer conditions support the mechanism whereby the supercooled water droplet impacts, then flows aft before freezing. This process usually forms horns which can substantially disrupt the airflow over the wing. These shapes are called clear, or glaze ice.

At colder temperatures, the supercooled water droplets tend to freeze on impact. This process tends to form conformal or wedge-shaped accretions. These shapes are called rime ice.

'Warm', SAT= -5 degrees C

"Warm", SAT= -5 degrees C

'Cold', SAT= -10 degrees C

"Cold", SAT= -10 degrees C

Related Information

Ice Accretion Temperatures

Moisture can exist as a supercooled liquid until about -40 degrees C, the theoretical limit. Contaminates in the atmosphere, however, set the practical limit to approximately -20 degrees C.

Ice can form on an aircraft when the SAT is above 0°C if the aircraft surface is below freezing. This situation can occur when the aircraft descends from subfreezing temperatures. It can also occur on areas where the local temperature is reduced to below freezing due to local flow acceleration.

Temperature gauge

Moisture

For ice to accrete on an aircraft in flight, there must be sufficient liquid water in the air. Water in the form of vapor, snow, or ice will generally not stick to an airplane's external surfaces and contributes little or nothing to the overall ice buildup.

If there is sufficient liquid water in the air to pose an icing threat, it will be visible in the form of cloud or liquid precipitation.

The amount of water in the air (measured in mass of water per volume of air, g/m3) can also affect the ice shape. In general, the more water, the greater the accumulation rate.

Wing ice formation on model in the Icing Research Tunnel Tracing of ice shape from prior image

Moisture effect in ice accretion: 'Dense cloud', 1.25 g/m3

Wing ice formation on model in the Icing Research Tunnel Tracing of ice shape from prior image

Moisture effect in ice accretion: 'Thin cloud', 0.31 g/m3

Droplet Size

As an aircraft moves through supercooled cloud or precipitation, droplets will impact the wing and tail along a narrow band near the leading edge (the stagnation line) and form a slender line of ice.

The larger the water droplet is, the further aft it is able to strike the aircraft. Their greater mass allows the larger droplets to cross the flow lines of the air stream and strike the airfoil further aft. Ice accretion from larger droplets is more likely to form into shapes that can interrupt the flow of air over the wing or tail and cause performance and handling problems.

Droplet size effect on ice accretion: 'Large', 40 microns

Droplet size effect on ice accretion: "Large", 40 microns

Droplet size effect on ice accretion: 'Small', 15 microns

Droplet size effect on ice accretion: "Small", 15 microns