Icing

Icing is one of the most significant hazards to aviation. It occurs when ice forms on the aircraft structure, altering its aerodynamic characteristics and potentially affecting the operation of engines and instruments.

Conditions for Ice Formation

For structural ice to form on an aircraft, three simultaneous conditions are generally required:

1. Presence of visible liquid water: Clouds, fog, rain, or drizzle.
2. Ambient temperature below freezing: Typically between 0°C and -40°C.
3. Structure temperature below freezing: The aircraft surface must be at or below freezing temperature.

Water in the atmosphere can remain in a liquid state below 0°C; this is known as supercooled water droplets. When these droplets impact a cold structure, they freeze.

Types of Ice

1. Rime Ice

• Formation: Formed by the instantaneous freezing of small supercooled water droplets upon impact with the leading edge.
• Appearance: White, opaque, and rough (like cauliflower). The opacity is due to air trapped between ice particles during rapid freezing.
• Occurrence: Typical of stratiform clouds (Stratus, Nimbostratus) and lower temperatures (typically -10°C to -30°C).
• Characteristics: It is brittle and easier to remove with de-icing systems. It tends to accumulate on leading edges and does not spread backwards.

2. Clear Ice / Glaze Ice

• Formation: Formed by the impact of large supercooled water droplets. Upon impact, only a portion freezes instantly; the rest flows backwards over the surface before freezing due to the release of latent heat.
• Appearance: Transparent, smooth, and solid. Does not trap air.
• Occurrence: Typical of cumuliform clouds (Cumulus, Cumulonimbus) with high water content and temperatures between 0°C and -10°C (although it can occur at lower temperatures in Cb). It is also characteristic of freezing rain.
• Hazard: It is the most dangerous form of ice.
  • It is heavy and difficult to see.
  • It adheres strongly to the surface and is difficult to break.
  • By flowing backwards (runback), it can freeze on surfaces not protected by de-icing systems, severely altering the wing profile and affecting control surfaces.

3. Mixed Ice

• A combination of clear and rime ice.
• Occurs when there are water droplets of various sizes mixed with snow particles or ice crystals.
• It is the most common form of icing in clouds.
• Whitish, irregular, and rough appearance.

4. Hoar Frost

• Formation: Occurs by deposition (reverse sublimation): water vapor transforms directly into ice crystals without passing through the liquid state.
• Conditions:
  • On the ground: On clear, cold nights (radiation), similar to frost on cars.
  • In flight: When descending rapidly from a very cold zone into a layer of warm, moist air. The aircraft structure is "cold-soaked," and water vapor freezes on contact.
• Effects: Although it looks harmless, a thin layer (like sandpaper) can increase drag and reduce lift significantly (up to 30% loss of lift and 40% increase in drag). It must be removed before takeoff.

Freezing Rain (FZRA)

It is an extremely dangerous condition that produces rapid accumulation of clear ice.

• Typically occurs ahead of a warm front in winter.
• Rain falls from a layer of warm air (temperature inversion) into a layer of sub-freezing air near the surface.
• Raindrops become supercooled as they fall and freeze upon impact with the aircraft.
• Action: The best maneuver is usually to climb into the warmer air (if the existence of the inversion is known) or turn to exit the area.

Ice Crystal Icing (ICI)

• Location: High altitudes, near intense convective systems (thunderstorms), especially in the anvil region.
• Mechanism: Ice crystals are tiny and do not adhere to the cold aircraft structure (they bounce off). However, they can enter the engines.
• Engine Risk: Crystals partially melt upon entering the hot compressor, adhering to internal surfaces. This can cause power loss, surge, compressor stall, or flameout.
• Indications: Erroneous temperature readings (anomalous TAT) due to crystals blocking or cooling heated probes. Usually not detectable by conventional weather radar (low reflectivity).

Effects of Icing

Ice accumulated on the structure has cumulative and detrimental effects:

1. Aerodynamics:
   • Decreases Lift: Alters the wing profile.
   • Increases Drag: Increases friction and form drag.
   • Increases Stall Speed: The aircraft stalls at a higher speed.
   • Reduces Critical Angle of Attack: The aircraft stalls at a lower angle of attack.
2. Weight: Increases the total weight of the aircraft.
3. Control: Can block control surfaces (ailerons, elevators) or cause severe vibrations.
4. Instruments: Blockage of Pitot tubes and static ports, giving erroneous speed and altitude readings.
5. Engines: Carburetor icing (in piston engines) or air intake icing (in turbines), reducing power.

Intensity Factors

• Droplet Size: Larger droplets = more severe icing (Clear Ice).
• Water Concentration: Higher Liquid Water Content (LWC) = faster accumulation.
• Object Shape: Thin objects with sharp edges (antennas, thin leading edges) are more efficient ice collectors than blunt, thick objects.
• Speed: Higher speed means higher accumulation rate (although kinetic heating at very high speeds can mitigate ice).

Intensity Classification (ICAO)

• Light: The rate of accumulation may create a problem if flight is prolonged (over 1 hour). No immediate changes required.
• Moderate: The rate of accumulation is such that even short encounters are potentially hazardous. Use of de-icing equipment is necessary. A change of heading or altitude may be desirable.
• Severe: The rate of accumulation is so high that de-icing equipment cannot control the hazard. Immediate change of heading and/or altitude is essential.