Aircraft icing is the accumulation of ice on an aircraft’s external surfaces during flight or on the ground. This phenomenon occurs under specific atmospheric conditions and can compromise flight safety. Understanding how and why ice forms on aircraft is important for appreciating the measures taken to ensure safe air travel.
The Science of Aircraft Icing
Aircraft icing occurs when an aircraft encounters supercooled liquid water droplets, which remain liquid even below freezing. These droplets freeze upon impact with the aircraft’s surfaces. Conditions for ice formation include air temperatures between 0°C and -40°C, along with visible moisture such as clouds, rain, or drizzle. Supercooled liquid water droplets are predominantly found between 0°C and -20°C, though smaller amounts can exist down to -40°C.
Three types of ice can form on an aircraft, influenced by temperature and droplet size.
Rime ice, opaque, rough, and brittle, forms when small supercooled water droplets freeze rapidly, trapping air. This occurs in colder temperatures, between -10°C and -20°C, and is common in stratiform clouds.
Clear ice, also known as glaze ice, is transparent, dense, and tenacious, forming when larger supercooled water droplets freeze more slowly. This allows the water to spread across the surface before solidifying, resulting in a smooth, glassy layer, typically in warmer temperatures near 0°C to -10°C, and is often found in cumuliform clouds or freezing rain.
Mixed ice is a combination of both rime and clear ice, resulting from various droplet sizes or a mix of liquid and frozen particles. This type of ice presents a rough, uneven surface.
Impact of Ice on Aircraft Performance
Ice accumulation compromises an aircraft’s ability to fly safely by altering its aerodynamic properties. Even a thin layer of ice, as rough as coarse sandpaper, can disrupt airflow over the wings, reducing lift by up to 30% and increasing drag by 40% or more. This change in the wing’s shape, known as the airfoil, reduces its efficiency and increases the speed at which the aircraft stalls.
The added weight from accumulated ice also demands more engine power and increases fuel consumption. Beyond aerodynamic surfaces, ice can impede the movement of control surfaces like flaps, ailerons, and elevators, making the aircraft difficult to control and leading to uncommanded rolls or pitches. Ice ingested into jet engines can cause damage, reduce thrust, or even lead to engine flameout. Additionally, instruments such as pitot tubes, which measure airspeed, can become blocked by ice, providing inaccurate readings to the pilots.
Protecting Aircraft from Ice
Aviation employs various strategies and systems to prevent or remove ice, ensuring flight safety. Anti-icing systems prevent ice from forming. Many large aircraft use bleed air systems, which route hot, compressed air from the engines to the leading edges of wings, tails, and engine inlets, keeping these surfaces above freezing. Electric heating elements are also used for anti-icing on smaller components like propeller blades, windshields, and sensors such as pitot tubes. On the ground, aircraft are treated with anti-icing fluids, typically glycol-based, applied before takeoff to prevent ice accumulation.
De-icing systems, conversely, remove ice after it has formed. A common method involves inflatable rubber boots on the leading edges of wings and tail surfaces. These boots periodically inflate and deflate, cracking the ice, which then breaks off and is carried away by airflow. De-icing fluids are also used on the ground to melt and remove existing ice from the aircraft’s surfaces. Beyond technological solutions, pilots receive training to recognize and avoid icing conditions, adjusting altitude or flight paths to steer clear of areas where ice may form.