Summary Ice creates drag, disrupts airflow on wings, and damages engines, affecting aircraft performance significantly. Supercooled water droplets cause icing; different types of ice, like dangerous clear ice and less dangerous rime ice, have various impacts. Pilots must classify icing conditions based on accumulation rates, from trace to severe, to determine safety and necessary actions.
Aircraft icing is the accretion of ice on an aircraft's surface. It can lead to loss of performance, increased fuel consumption, and, in the worst-case scenario, loss of control. Therefore, anti-icing systems are a critical part of an aircraft that enables it to fly safely.
This threat was evident this week when an ATR 72 crashed near São Paulo after flying through an area with a severe ice warning, leading to the death of 61 souls onboard. How does icing occur? Icing is caused by the presence of Supercooled Water Droplets (SWD). SWD can remain in liquid form at very low temperatures because they lack a freezing nucleus.
Pure water droplets turn into ice when either they are cooled to very low temperatures or when they meet a freezing nucleus. A freezing nucleus can be a speck of dust or any other form of impurity around which the water freezes. Supercooled water droplets are thus pure water droplets, and this allows them to remain as a liquid.
It is these droplets that are most dangerous to aircraft - when supercooled water droplets make contact with an aircraft, they freeze instantly because the surface of the aircraft sort of becomes the freezing nuclei. The supercooled water droplets can exist at temperatures ranging from 0°C to -40°C. Between 0°C and -20°C, large, supercooled water droplets can be found, and these droplets cause most of the trouble for aircraft.
Between -20°C and -40°C, only small, supercooled water droplets exist. Below -40°C, the temperature becomes too low for supercooled water droplets to remain as a liquid, causing them to freeze. Thus, icing usually does not exist unless in precipitation at temperatures below -40°C.
What are the dangers of icing on aircraft? When ice builds up on the leading edges of the wings, it disrupts the smooth airflow that is required to generate the required lift. This causes a loss of lift and an increase in drag, causing the aircraft's performance to become highly inefficient. This also increases the stall speed of the aircraft by reducing the stall angle of attack.
In aircraft with conventional unpowered flight controls, ice can build up between the aerodynamic balances, such as control horns, potentially leading to control jams. Icing also increases the weight of the aircraft. This, together with the extra drag, can increase the overall fuel consumption of the aircraft.
Ice can also be a danger to engines. Icing generally occurs in the intakes of the engine. The formed ice can, at times, separate off the intake and may enter the engine.
These ice shards can then hit the engine's fan blades, causing significant damage. The damage to the blades may also disrupt the airflow inside the engine, causing it to stall. One other danger of icing is known ice ridges.
Ice ridges are formed when water freezes at falls off the fuselage. Ridges that form near the aircraft sensors, such as pitot tubes, can cause airflow disruptions, which can make the sensors give out false data. Types of ice Clear ice This is by far the most dangerous type of ice.
Clear ice is formed when supercooled water droplets hit the aircraft. As the droplets freeze, heat is released, slowing the freezing process. This causes some of the water droplets to flow back over the surface and ice up.
So, clear ice tends to cover a large area of the aircraft. The clear ice is transparent and thus is harder to notice once it forms over aircraft surfaces. Because they have the capability to freeze over a good part of the aircraft, they can significantly increase the weight of the aircraft, and at the same time, cause balance and control problems.
Rime ice Rime ice is formed by small, supercooled water droplets. When rime ice forms, there is no flow back, and the droplets freeze instantly. This usually causes rime ice to form layer by layer.
Compared to clear ice, rime ice is not very strongly attached to the aircraft surfaces and thus can easily break off - rime ice is also easier to see than clear ice. This type of icing can also disrupt the airflow over the wings and cause loss of lift. Mixed ice This is the most common type of icing.
Mixed ice is a combination of rime and clear ice. It is caused by the presence of both small and large supercooled water droplets. There are specific situations where ice crystals can pose safety issues for general aviation aircraft.
Freezing rain Freezing rain, also known as rain ice, is formed when supercooled water droplets fall from an inversion (a layer of the atmosphere where temperature increases with altitude) into the air below 0 degrees Celsius. Freezing rain has a very high accretion rate - thus, pilots should immediately get out of this condition as soon as possible. Hoar frost Hoar frost occurs in clear air and causes water vapor in contact with the skin of the aircraft to change into ice crystals without becoming a liquid, a process called sublimation.
For sublimation to occur, a sublimation nucleus is required. This nucleus can be dust, volcanic ash, or any other impurities. Hoar frost can occur on both the ground and in-flight.
On the ground, it occurs when the aircraft is parked outside during a cold night. The hoar frost must be cleared from the aircraft before departure because even a very thin layer of ice can disrupt the airflow and increase the drag. Inflight, hoar frost usually occurs when the aircraft is put on an accelerated descent from a very cold air mass into a layer of warm moist air.
It can also form when a climb is made from a temperature below 0 degrees Celsius to an inversion where warm moist air may sublimate into ice. Get the latest aviation news straight to your inbox: Sign up for our newsletters today. How do pilots classify icing conditions? There are four levels of icing - Trace, Light, Moderate and Severe.
Trace icing: When icing first becomes visible, it is said to be trace icing. The rate of accumulation of ice is usually greater than the rate of sublimation. This type of icing is not very dangerous, even without the use of de-icing anti-icing systems, unless encountered for a prolonged period of time.
Light icing: In light icing conditions, the rate of accumulation can create a problem if the flight continues in this condition for more than one hour. The use of de-icing and anti-icing systems can easily remove light icing or prevent its formation. Moderate icing: Moderate icing can be dangerous even when accumulated for brief periods of time.
Moderate icing may require a change of altitude, diversions, and the continuous use of anti-icing and de-icing equipment. Severe icing: When the de-icing and anti-icing systems fail to reduce the ice accumulation, it is known as severe icing. This requires the pilots to get out of icing conditions immediately without delay to ensure the safety of the flight.
Ice contamination can severely affect engine performance..
