Overview

Aviation icing occurs when supercooled liquid water freezes on aircraft surfaces and systems, degrading lift, increasing drag and weight, and potentially causing control and engine problems. Icing severity ranges from light accumulations that require monitoring to rapid buildup that demands immediate avoidance or diversion. Effective risk management combines preflight analysis, rigorous in-flight monitoring, and conservative decision making.

Preflight checklist

• Weather brief: Review AIRMET/SIGMET, TAFs/METARs, freezing levels, and PIREPs for icing — note temperature layers and expected liquid water content.
• Route & altitude: Plan altitudes above/below known icing layers when feasible; identify safe diversion altitudes and alternates with lower freezing levels.
• Aircraft systems: Verify anti-ice and deice system serviceability (pitot heat, windshield heat, prop/wing boots, wing/engine anti-ice) and review POH/AFM procedures for system use and penetration speeds.
• Fuel & alternates: Add margins for diversions and possible increased fuel burn when flying in icing or when using anti-ice systems.
• Crew brief: Assign scan duties for icing detection, review escape options, and set go/no-go criteria for encountering icing.

In-flight monitoring & immediate actions

• Scan pattern: Combine outside visual checks, instrument cross-checks, and tactile awareness (control feel, vibrations) — maintain heightened vigilance in visible moisture and near-freezing temperatures.
• Initial response: Activate pitot/static and windshield heat and engage anti-ice per POH at the first sign of icing; increase airspeed to recommended penetration speeds and avoid configuration changes that reduce margins.
• Exit strategy: Request altitude changes to leave the icing layer or divert to an alternate; prioritize a climb/descend that moves out of visible moisture over continuing in-place mitigation when safe to do so.
• System use: Use deice boots or thermal systems per aircraft-specific procedures; avoid rapid or unnecessary cycling of pneumatic boots unless recommended by the manufacturer.

Reporting & post-flight

• PIREP: File a PIREP for significant icing encounters with location, altitude, and intensity to inform other pilots and ATC.
• Inspection: After flight in icing conditions, inspect for structural or system damage, record anti-ice/deice usage, and report anomalies to maintenance.
• Documentation: Log any system activations, sensor anomalies, or engine indications in the aircraft log and maintenance forms as required.

Formation

Icing forms when supercooled liquid water droplets collide with aircraft surfaces and freeze on impact. Key factors that determine icing intensity are ambient temperature, droplet size, liquid water content (LWC), and the type of cloud or precipitation.

Important parameters

• Temperature range: Icing is most likely between about 0°C and -20°C. The highest risk for large-droplet icing (clear/glaze ice) is typically between 0°C and -10°C; rime ice is more common at colder temperatures (around -10°C to -20°C).
• Droplet size: Large droplets (from convective clouds or warm-frontal drizzle) spread across the surface before freezing and form clear ice. Small droplets (in stratiform clouds) freeze quickly on contact and form rime ice.
• Liquid water content (LWC): Higher LWC increases accumulation rate and severity. Embedded convection and cumuliform clouds can produce localized high LWC.
• Cloud and precipitation types: Stratiform clouds (stratus, nimbostratus) commonly produce widespread rime or mixed icing. Cumuliform clouds and freezing rain/drizzle produce heavier, often clear ice with rapid accumulation.
• Altitude layering: Icing layers can be shallow or extend for thousands of feet; frontal zones often produce continuous layers, while convective activity creates patchy but intense icing regions.

Operational implications

• Rapid accumulation: Clear ice from large droplets can quickly change the aircraft aerodynamics; avoid penetration of known large-droplet environments.
• Temperature transitions: Flying across a shallow temperature layer (e.g., crossing from above-freezing to subfreezing) can initiate icing even when ambient temperatures appear marginal.
• Detection limits: Visual and tactile cues (frost, unusual vibrations) may lag actual ice accretion — monitor performance and instruments closely in suspected conditions.
• Mitigation: When practical, change altitude to exit icing layers, divert, or use certified deice/anti-ice systems; file alternates and plan fuel margins for potential diversions when icing is forecast.

Propeller Icing

Propeller icing occurs when ice accumulates on propeller blades, affecting balance and thrust. It often forms in similar conditions to airframe icing but can have distinct operational impacts.

• Source: Supercooled water droplets in clouds or precipitation freeze on contact with the propeller blades.
• Early Accumulation: Ice often forms on propellers before wings due to their exposure and rotation.
• Conditions: Visible moisture and temperatures at or below freezing.
• Effects: Blade imbalance, vibration, reduced thrust, and potential damage. Ice chunks can break off and be ingested by the engine.
• Detection: Vibration increases, RPM fluctuations, or visible ice on blades.
• Mitigation: Propeller de-ice systems, heaters, and avoiding known icing.

Inspection & Maintenance Tips

• Visually inspect propeller blades for nicks, corrosion, or adhered ice accumulations during preflight and post-flight.
• Check for blade deformation, imbalance, or unusual wear that could indicate repeated icing exposure.
• Ensure propeller de-ice/anti-ice systems (if installed) are operational before flight into potential icing conditions.
• When ice is removed, inspect for erosion or fretting at bolt holes and blade roots; replace or service per manufacturer guidance.
• Follow manufacturer service intervals for propeller balancing and blade inspections after suspected icing events.

Antenna Icing

Antenna icing refers to ice accumulation on aircraft antennas which can degrade radio performance and navigation signals.

• Effects: Signal attenuation, reduced reception, and possible false indications from sensors.
• Detection: Loss of signal quality, dropped comms, or degraded navigation accuracy.
• Mitigation: Antenna heaters, protective boots, or operational procedures to avoid prolonged exposure in icing conditions.

Hazards

Icing creates multiple hazards that affect aircraft performance, handling, systems, and pilot workload. The following items summarize the primary risks and practical implications for flight operations.

Aerodynamic effects

• Lift reduction: Ice changes wing shape and roughens surfaces, reducing maximum lift and increasing stall speed. Expect degraded climb and approach performance.
• Increased drag: Surface roughness and ice accretions significantly increase parasite drag, which raises fuel burn and reduces range/endurance.
• Control issues: Ice on control surfaces and hinges can cause reduced control effectiveness, control surface asymmetry, and potential control lock in severe cases.

Propulsion and systems

• Engine icing: Ingestion of ice or ice shedding can damage engines or affect induction airflow in carbureted and turbine engines. Ice accretion on intakes and sensors can cause power loss or degradation.
• Pitot/static and sensors: Blocked pitot tubes and static ports can cause false airspeed, altitude, and vertical speed indications if pitot heat or anti-ice is not used or fails.
• Electrical/avionics: Ice on antennas and sensors can attenuate signals or give intermittent failures; icing of external connectors may prevent proper system operation.

Flight-deck and operational impacts

• Higher stall speeds: Because stall speed increases, approach and landing margins shrink — plan higher approach speeds and longer landing distances when iced.
• Instrument reliability: Erratic instrument readings can increase pilot workload and cause spatial disorientation risks, especially in IMC at night.
• Weight and balance: Accumulated ice increases weight and can shift the center of gravity, affecting stability and performance calculations.
• Unexpected behavior: Ice shedding or sudden changes in aerodynamic loading may cause sudden buffeting or roll/pitch excursions requiring prompt control inputs and possible diversion.

Examples & mitigations

• Example — approach with rime ice: Expect longer landing roll and reduced flare effectiveness; consider increased approach speed and select longer/runway alternates.
• Example — convective clear ice: Avoid penetration; if inadvertent, exit the conditions via altitude change or diversion and use anti-ice systems per checklist.
• Mitigations: Use certified deice/anti-ice systems, maintain proper airspeeds, brief alternates and fuel, monitor instruments, and avoid known severe icing when possible.

Detection

Early detection of icing is critical. Use a combination of visual, tactile, and instrument cues, and maintain a continuous scan in known or suspected icing environments.

Visual cues

• Visible moisture: Flying in clouds, precipitation, freezing drizzle, or near cloud tops increases icing risk.
• Ice on windshield or leading edges: Frost, rime streaks, or glaze formation visible on the windshield, wing roots, or engine inlets.
• Ice shedding: Small chunks or sloughing from forward surfaces or propellers may be visible.

Tactile and vibration cues

• Control feel changes: Increased stick forces, reduced control responsiveness, or sluggish aileron/elevator response.
• Vibration and vibration changes: Propeller or airframe vibration may indicate ice on rotating components or uneven accumulation.

Instrument and sensor cues

• Pitot/AS indicator anomalies: Erratic or frozen airspeed readings if pitot heat is off or ineffective.
• Altimeter/vertical speed anomalies: Blocked static ports can cause incorrect altitude or VSI readings.
• Autopilot/flight director behavior: Sudden or unexplained autopilot alerts, difficulty maintaining altitude/heading, or unexpected trim changes.
• Engine instrument changes: EGT, RPM, or thrust changes may indicate induction icing or ice ingestion.

Recommended scan pattern & immediate actions

• Continuous scan: Combine outside visual checks with instrument cross-checks and tactile awareness — assign one pilot to monitor for ice in multi-crew operations.
• If icing is suspected:
  • Turn on required pitot and windshield heat and anti-ice systems per the POH/checklist.
  • Change altitude (climb or descend) to exit visible moisture or move out of the icing layer.
  • Increase airspeed to the recommended icing penetration speed if published, or apply a moderate increase in approach/holding speeds to maintain margins.
  • Consider diversion to an alternate with known better conditions and brief passengers and ATC as appropriate.

Prevention & Mitigation

Preventing and mitigating icing requires appropriate preflight planning, effective ground procedures, and timely in‑flight actions. Follow the aircraft POH/AFM and operator SOPs for system usage and speeds.

Preflight planning

• Weather briefing: Check AIRMETs/SIGMETs for icing, freezing levels, and forecasts for freezing precipitation. Review TAF/METARs for cloud bases and precipitation type.
• Route & alternates: Plan routes and alternates that minimize time in known icing layers and select alternates with lower freezing levels or airport deicing resources.
• Equipment check: Verify anti-ice and deice equipment, pitot heat, windshield heat, and prop/engine systems are serviceable and testable on the ground per checklist.
• Fuel & payload: Add contingency fuel for potential diversions and increased fuel burn due to icing or deicing procedures.

Ground and pre-takeoff

• Anti-icing/deicing treatment: When frost or ice is present on critical surfaces, perform proper deicing using approved fluids and follow holdover time guidance before takeoff.
• Preflight inspection: Check control surfaces, pitot/static inlets, and windshields for ice and contamination; remove frost/ice per manufacturer procedures.
• Brief the crew: Discuss expected conditions, anti-ice configuration, and go/no‑go criteria.

In-flight systems & procedures

• Anti-ice vs deice: Anti-ice systems (e.g., heated leading edges, bleed air) prevent ice build-up when used proactively; deice systems (boots) remove accumulated ice after it forms. Use systems as recommended by the POH and operator SOPs.
• System usage: Turn on pitot heat and windshield heat before entering visible moisture and enable wing/engine anti-ice when required by the aircraft manual.
• Airspeed: Fly at recommended icing penetration speeds; increase approach and holding speeds per POH or operator guidance to maintain control margins.
• Altitude strategy: When icing is encountered, consider a climb or descent to exit the icing layer (ensure the new altitude is clear of icing and within aircraft performance limits).
• Avoidance: Avoid convective clouds and known freezing-rain conditions where possible; divert early rather than push into severe icing.

Post-flight

• Inspection: After flights in icing conditions, inspect for structural or system damage and document any de-ice/anti-ice system usage or anomalies.
• Maintenance reporting: Report any signs of ice ingestion, unusual vibrations, or sensor failures to maintenance for inspection.

Types of Aviation Icing

Overview of the common types of airframe icing. See the subsections for details.

Clear / Glaze Ice

Clear ice (also called glaze ice) forms when large supercooled liquid water droplets impact a surface and spread out before freezing. The slow freezing process produces a hard, smooth, and often transparent sheet of ice that adheres tenaciously to the structure.

Formation & conditions

• Droplet size: Large droplets from convective or warm-frontal sources create clear ice because they spread and run back on the surface before freezing.
• Temperature: Most frequently encountered between about 0°C and -10°C, but can form down to -20°C in high-liquid-water environments.
• Environment: Freezing rain, freezing drizzle, and liquid-containing cumulus elements are common sources; large-droplet or warm-front conditions increase severity.

Characteristics

• Appearance: Smooth, glossy, and transparent — often difficult to see from the cockpit until accumulation is substantial.
• Adhesion: Bonds strongly to the surface and can form horns, ridges, or teardrop shapes ahead of control surfaces and on propeller blades.
• Accumulation rate: Can build rapidly in high liquid water content (LWC) environments and on large-droplet encounters.

Hazards & performance impact

• Aerodynamic: Smooth but contoured shapes change camber and disrupt airflow, reducing maximum lift and increasing stall speed significantly.
• Weight & balance: Heavy, concentrated accumulations on leading edges and propellers increase weight and can cause balance issues.
• Sensors & systems: May obscure visual cues and affect pitot/static and angle-of-attack sensors if accretion occurs near inlets.
• Removal difficulty: Difficult to shed with pneumatic boots alone; anti-ice and thermal methods are more effective if available and approved for the aircraft.

Pilot detection & actions

• Detection: Watch for rapid performance degradation, increased stall buffet, heavy glazing on the windshield, and changes in control feel; clear ice may be less obvious visually at first.
• Immediate actions: Activate anti-ice systems if available, increase airspeed to published penetration speeds, and consider an immediate climb/descent or diversion to escape the liquid water layer.
• Avoidance: Avoid flying in freezing rain or freezing drizzle — these conditions are prone to severe clear ice that can quickly exceed aircraft system capabilities.

Rime Ice

Rime ice forms when small supercooled droplets freeze quickly on impact, trapping air and producing a brittle, rough, opaque deposit. It tends to build in colder temperatures within stratiform cloud layers and is more common with small-drop clouds.

Formation & conditions

• Droplet size: Small droplets freeze almost immediately on contact, producing porous, white deposits.
• Temperature: Typical formation temperatures are colder than clear ice, often around -10°C to -20°C.
• Environment: Stratiform clouds, widespread layered clouds, and light precipitation with low liquid water content.

Characteristics

• Appearance: Opaque, rough, and milky white; easy to spot visually compared with clear ice in many cases.
• Adhesion: Less tenacious than clear ice — often flakes or chips away under aerodynamic loads or when deice boots deploy.
• Accumulation: Generally slower than clear ice but can still degrade performance over time.

Hazards & performance impact

• Aerodynamic: Rough surface increases drag and decreases lift, but the effect is usually more gradual than with clear ice.
• Handling: Control effectiveness can be reduced, and, on some aircraft, asymmetric accretions can cause roll or yaw tendencies.
• Removal: Deice boots are often effective at breaking rime ice loose; however, repeated cycling can stress the boots and airframe if used excessively.

Pilot detection & actions

• Detection: Look for white, crusty deposits on windshields and leading edges; feel for steady, predictable increases in control forces and drag.
• Immediate actions: Use pneumatic boots or other deice equipment as approved, maintain increased speeds as recommended, and consider altitude changes to escape the cloud layer if safe.
• Operational note: Rime ice is often an indicator of widespread icing in the layer — brief ATC and consider diversion planning early.

Mixed Ice

Mixed ice contains characteristics of both clear and rime ice: partially transparent patches of dense glaze intermingled with opaque rime deposits. It usually forms when droplet sizes and temperatures vary within the icing layer, producing complex accretions that can be particularly hazardous.

Formation & conditions

• Transition zones: Common in frontal boundaries or when flying through layers where droplet sizes and temperatures change, such as the interface between cumulus and stratiform clouds.
• Temperature: Often occurs in the -5°C to -15°C range where both freezing behaviors can coexist.

Characteristics

• Heterogeneous surface: Patches of hard, transparent glaze next to porous, white rime make the surface unpredictable aerodynamically.
• Shedding behavior: Mixed ice can create large chunks that shed irregularly, causing buffeting or ingestion risks.

Hazards & pilot actions

• Hazards: Rapid changes in handling as different ice types build; more difficult for deice boots and may require combined system usage (anti-ice plus boots).
• Pilot actions: Activate all approved anti-ice/deice systems, increase airspeed to recommended penetration values, and prefer early diversion or altitude change to leave the icing environment.
• Operational planning: If mixed icing is forecast or encountered, plan for increased fuel burn, potential missed approaches/diversions, and notify dispatch/ATC early.