Short Notes 12 : Ice Accretion
Effects of Structural Icing
- Hazard: Severe aerodynamic degradation.
- Lift ↓ , Drag ↑ , Weight ↑
- Stalling Speed ↑ (by 5–10% even with frost).
- Result: Aircraft more prone to stall during approach & departure.
Types of Ice Formation
Clear Ice (Glaze Ice)
- Cause: Large Supercooled Water Droplets (SWD) – slow freezing, smooth transparent coating.
- Clouds: CU, CB, NS.
- Temp Range: 0°C to −20°C (most severe at 0°C to −7°C).
- Appearance: Hard, glossy, heavy.
- Hazard: Most dangerous; rapid buildup; reduces lift, increases drag & stall speed.
Rime Ice
- Cause: Small SWDs – instant freezing, trapping air.
- Clouds: ST, SC, AS, AC (also cold parts of CB/CU below −20°C).
- Temp Range: -15°C to −40°C (mainly below −20°C).
- Appearance: White, opaque, granular, brittle.
- Severity: Light to moderate.
Mixed Ice
- Cause: Both large & small SWDs (transition zone).
- Temp Range: Around −10°C to −20°C (CU/CB/NS).
- Composition: Combination of Clear (glaze) and Rime ice.
- Hazard: Worst of both — heavy, uneven, rapid accretion.
Cloud Type & Icing Severity
| Cloud Type | Severity | Remarks |
|---|---|---|
| AS (Altostratus) | Light–Moderate | Mix of droplets & ice crystals |
| NS (Nimbostratus) | Moderate–Severe | Large SWDs, heavy precipitation |
| AC / ST / SC | Light–Moderate | Small droplets → Rime Ice |
| ACL (Altocumulus Lenticularis) | Moderate–Severe | Orographic lifting increases droplet size |
| CB / CU / TCU | Severe (0°C to −20°C) | Large SWDs → Clear or Mixed Ice |
| High Clouds (Ice Crystal Only) | Negligible | Contain ice crystals only |
Temperature Influence
- Max Icing Zone: 0°C to −20°C (esp. around −15°C).
- Below −20°C: Large droplets freeze → only small SWDs → Rime Ice.
- Below −30°C to −40°C: Negligible icing risk.
- SWDs exist down to −40°C but rarely hazardous.
Carburetor Icing (Engine Hazard)
- Occurs: +30°C to −10°C (most dangerous −10°C to +25°C).
- Cause: Adiabatic + Evaporative cooling in venturi → ice constricts airflow.
- Result: Power loss, possible engine failure.
- Condition: High humidity (mist/fog/precipitation).
Hoar Frost
- Formation: Sublimation (vapor → ice) in clear air when airframe < 0°C & air saturated.
- Appearance: White, feathery crystalline layer.
- Condition: Clear night cooling or descent from cold to moist layer.
- Hazard: Mild but must be removed before takeoff (spoils smooth surface).
- Note: Not a cloud icing type (forms without liquid droplets).
Key Icing Zones
- CB / CU (0°C to −20°C): Clear or Mixed Ice → Severe
- Layer Clouds (< −20°C): Rime Ice → Light to Moderate
- Mountain Wave (ACL): Moderate to Severe
- Upper Clouds (Ice Crystals): Negligible Icing
Quick Recall Points
- Clear Ice: Large SWD | 0°C to −20°C | Severe
- Rime Ice: Small SWD | -15°C to −40°C | Light–Moderate
- Mixed Ice: Both SWD types | −10°C to −20°C | Worst Combination
- Hoar Frost: Vapor → Ice | Clear Air | Remove before flight
- Max Icing Risk: 0°C to −20°C (esp. −15°C)
- Below −30°C: Icing risk negligible.