Airframe & induction icing

Which combination creates structural accretion?

Compare the ambient air with the aircraft surface and the particles actually encountered.

Liquid freezing and direct vapour deposition

The usual in-flight structural-icing pathway is interception of supercooled liquid water by an aircraft surface on which it freezes. Supercooled means still liquid below the normal freezing point. Droplets in cloud, freezing fog, freezing rain or freezing drizzle can remain liquid because freezing requires nucleation; they do not all freeze at 0°C simply because the ambient air is sub-zero. Contact and the local heat balance allow accretion on the surface.

Supercooled liquid is often abundant in the warmer sub-zero part of a cloud, while the ice fraction commonly increases as temperature decreases. Mixed-phase cloud can contain both droplets and crystals; ice growth can consume vapour and reduce the liquid reservoir. Nevertheless, liquid can persist below −20°C, especially in strong convective ascent and some mixed-phase layers. Around −38 to −40°C, homogeneous freezing strongly limits ordinary liquid cloud droplets. These are phase tendencies, not rigid temperature bins assigning one icing type or guaranteeing a liquid-free cloud at −20°C.

Ice can also form by direct deposition of vapour onto a sub-freezing surface when conditions exceed saturation over ice: hoar frost. Some older aviation wording calls this a sublimation process; the precise phase-change term for vapour to ice is deposition. Ambient air temperature, aircraft surface temperature, water phase and moisture content must all be considered. A cold-soaked aircraft can collect ice even in slightly above-zero air if its surface remains cold and moisture is supplied.

Why can droplet size and airspeed change collection?

Separate available liquid mass from the fraction intercepted by the aircraft.

Cloud water and aircraft-specific factors

Liquid water content is the mass of liquid droplets per volume of air, often expressed in g/m³. Cumuliform cloud can have high local liquid water content, strong vertical motion and a broad distribution including large droplets; stratiform cloud often has smaller droplets and a shallower liquid layer but can extend a great horizontal distance. Neither cloud name fixes droplet number or guarantees a low icing rate. Aerosols, growth, precipitation and entrainment change the population.

For a fixed liquid water content, smaller equal-sized droplets require a greater number than larger ones because drop mass varies with diameter cubed. Doubling every drop’s diameter while conserving the same total liquid mass reduces the number by a factor of eight. Real cloud has a size distribution, not one uniform diameter. Large drops can cross streamlines and strike farther aft, while very small drops can follow the airflow around some surfaces more readily.

Interception depends on liquid water content, relative speed, effective projected collection area and collection efficiency. Thin leading edges, antennas and other small-radius parts can collect droplets efficiently; a large blunt shape can deflect some small droplets around it. Higher speed supplies more water per second but also changes surface heating and collection, so actual ice growth is not a simple speed-only rule. Air temperature, surface heat transfer, freezing/runback, exposure time and the mixture of ice and liquid all matter. Pure dry crystals do not usually accrete on a cold external wing like liquid droplets, but can cause distinct engine/probe hazards.

Why can a wing ice on a mild wet apron?

Follow the recent thermal history, cold fuel and night-time radiation.

Ground and flight pathways

An aircraft arriving from a cold cruise can retain sub-zero fuel and structure after descending into warmer humid air. A wing skin in contact with cold fuel can remain below freezing even if the ambient air is above 0°C. Rain or condensed moisture can then freeze on it. Cold fuel uplifted during refuelling can have a similar effect. A positive outside-air-temperature report does not prove that every aircraft surface is above freezing.

On a clear calm night, exposed surfaces radiate heat to the sky and can cool below the air temperature. If a surface becomes sub-freezing and sufficiently humid relative to ice, hoar frost can grow; dew that forms first can also freeze if the surface subsequently cools. Freezing fog deposits supercooled droplets, while freezing rain, drizzle, wet snow, slush and melt/refreeze processes can contaminate surfaces on the ground. Humidity, precipitation and actual surface temperature determine the pathway.

In flight, supercooled cloud and freezing precipitation provide liquid accretion. Outside cloud or active precipitation, a cold aircraft entering warmer moist air can develop frost or frozen condensate; moisture is still required. The surface history therefore matters during descent as well as before departure. Small frozen contamination can have substantial aerodynamic effects. Ground de-icing removes contamination and anti-icing provides limited protection against further accumulation under applicable procedures; it is not permanent protection or proof of the current state of every surface.

Why can carburettor icing occur in warm clear air?

Add pressure-drop cooling and fuel evaporation before comparing with freezing.

Induction icing weather

In a conventional venturi carburettor, reduced pressure and fuel evaporation lower the temperature of the air/fuel flow. Moisture can condense and freeze near the throat or throttle even when the outside air is well above freezing. Ice restricts the flow and can reduce power or stop the engine. It is a different local cooling mechanism from a wing intercepting sub-zero cloud droplets.

Humid air and temperatures in a broad cool-to-warm range are favourable. A commonly illustrated higher-risk environment is below about 20–21°C with relative humidity above about 80 percent, but icing can also occur at higher temperatures and lower humidities under suitable carburettor and power conditions. It does not require visible cloud or sub-zero outside air. Partial throttle/low power can be important, including during descent and approach.

Use the applicable aircraft/engine guidance and its carburettor-icing chart, rather than treating a generic temperature/humidity pair as a universal limit. Recognise the weather exposure during planning and throughout changes of power and altitude. Carburettor heat and the response to power loss are aircraft-specific procedures; the meteorological point is that the internal temperature can be below freezing while the reported outside temperature is positive.

Why can liquid spread before it finally freezes?

Include the latent heat released as incoming water freezes.

Formation, structure and appearance

Clear ice is a relatively dense, hard, transparent or translucent accretion formed when intercepted liquid freezes comparatively slowly, allowing some water to spread before final freezing. Larger supercooled drops, relatively warm sub-zero conditions and high liquid supply favour this wet-growth behaviour. Cumuliform cloud, freezing drizzle and freezing rain are important settings, but a simple temperature band alone cannot diagnose the ice type.

Freezing releases latent heat into the surface and droplet environment. If heat cannot be removed rapidly enough to freeze all the arriving water immediately, only part freezes at impact; liquid can run back and later freeze elsewhere. Fewer trapped air bubbles make the deposit clearer and denser than porous rime for a comparable volume. It can be smooth initially, or develop irregular ridges, rivulets, bumps and horns as the heat balance and airflow evolve.

Clear ice can adhere strongly, be difficult to see and extend aft beyond the normally protected leading-edge area, especially with supercooled large drops. Its mass can be substantial, but aerodynamic disruption and unprotected accretion are often more urgent than weight alone. A glossy appearance does not mean a smooth original aerofoil is preserved. The affected surface, shape and aircraft response determine the hazard.

How do rapid freezing and changing conditions alter the deposit?

Compare trapped air, density and roughness with the clear-ice process.

Rime and mixed structure

Rime ice is a rough opaque white or milky deposit formed when supercooled droplets freeze rapidly with little spreading, trapping air between frozen elements. Small droplets, lower liquid supply and colder sub-zero surfaces favour it; it is common in stratiform liquid cloud but not confined to one cloud genus. For a comparable bulk volume it is generally less dense and more porous than clear ice, often brittle, and tends to project into the airflow from exposed leading parts.

Its lower density or white appearance does not make it safe. Roughness and changed leading-edge geometry disturb the boundary layer, increase drag and reduce lift and stall margin; probes, intakes and antennas can also be affected. Thin sensitive surfaces may show accretion before a large blunt surface. Actual intensity depends on the environment and aircraft, not the colour of the ice.

Mixed ice combines clear and rime characteristics as droplet size, liquid supply, surface temperature and freezing fraction vary in space or time. It can appear as alternating clear and opaque layers or a hard rough irregular conglomerate, sometimes with horns and runback. Mixed-phase cloud and wet snow can contribute to the conditions, but the name mixed ice describes the accretion structure, not merely the presence of both crystals and droplets in the undisturbed cloud. Density, adhesion and weight vary; mixed ice can be difficult to remove and aerodynamically serious.

Which frozen contamination can occur without freezing rain?

Distinguish dry crystals, wet snow and direct growth from vapour.

Snow-related accretion and hoar frost

Dry snow crystals often bounce from a cold external airframe more readily than liquid droplets, but snow is not universally harmless. Wet or partly melted snow can adhere and refreeze on colder surfaces, and snow mixed with supercooled liquid can contribute to irregular accretion. On the ground, melting followed by refreezing can create hard bonded contamination. Intakes and other components can ingest, retain or compact snow even when an exposed wing shows little classic liquid-droplet ice.

Hoar frost is a white crystalline or feathery deposit grown by direct vapour deposition onto a surface below freezing when the surrounding moisture is sufficient for saturation over ice. It commonly forms on exposed parked aircraft after radiative cooling, or on very cold surfaces entering warmer moist air. No rain or cloud-droplet impact is needed. It is often fragile and can appear thin or light, unlike dense clear ice, but it changes surface roughness.

Even a thin frost layer on a critical aerofoil can disrupt airflow and degrade lift and drag characteristics; its mass is not a useful safety test. Frost or frozen condensate on a windshield can obscure vision. Follow the aircraft’s approved surface-contamination limitations and ground procedures; the general study rule is to recognise and assess frozen contamination rather than assuming feather-like ice or dry-looking snow can be ignored.

How do fronts and mountains concentrate the exposure?

Combine the sloping thermal structure, lift and route constraints.

Dangerous weather zones

In layered frontal cloud, rising moist air can remain saturated through a broad sub-zero layer, producing prolonged exposure. Embedded convection can add higher local liquid content and larger droplets. Cumuliform cloud often has a deeper and more variable icing region above the freezing level; strong updraughts can maintain supercooled liquid to very cold temperatures. Neither a stratiform label nor absence of visible CB guarantees mild icing.

Below an elevated warm layer, rain can become supercooled in colder air and produce freezing-rain accretion outside the main cloud. Freezing drizzle can also develop by collision–coalescence in an entirely sub-zero liquid cloud without a warm nose. Large drops can strike or run back beyond protected areas. Snow or ice pellets at the surface may indicate melting/refreezing processes and a different hazard above; surface precipitation alone does not locate the complete icing layer.

Windward mountain lift cools moist air, increases condensation and can sustain an enhanced liquid-water supply in stratiform orographic cloud. Liquid concentrations can be high near windward slopes and crests where lift continues, even without tall convective towers. Frontal and orographic lift can combine, while waves produce alternating cloudy lifting and clearer descending regions. Terrain can prevent descent into warmer air or below cloud, and accretion can reduce the climb performance needed to clear the ridge. There is no fixed universal distance or height above a ridge delimiting the icing zone.

What does moderate or severe tell the receiving pilot?

Report what the icing does in the encountered aircraft and environment.

Intensity, effects and relative dangers

The main ICAO intensity qualifiers used for airframe icing are light, moderate and severe. They are distinct from the structural types clear, rime and mixed. Light describes comparatively limited accretion or effects in the encountered circumstances; prolonged exposure can still become hazardous. For special air-report interpretation, ICAO/SERA moderate conditions can make a course or altitude change desirable, while severe conditions make an immediate course and/or altitude change essential. Do not replace these operational descriptions with a universal millimetres-per-minute scale.

Accretion can reduce lift, increase drag and stall speed, change stall behaviour and control forces, cause roll or pitch problems and reduce climb or ceiling. Propeller/intake icing can reduce propulsion; probes and ports can give unreliable indications; windshield ice reduces visibility. Weight adds a penalty, but small roughness or a harmful shape can cause major aerodynamic changes before the total ice mass looks large.

Clear and mixed deposits can be dense, adherent, hard to see or extend aft; rime can be rough and aerodynamically damaging; frost can alter critical surfaces despite little weight. There is no universally harmless type or fixed ranking valid for every shape and aircraft. A report should identify aircraft type, position, time, level, observed intensity and other relevant conditions. Reports help locate a variable hazard but cannot guarantee the same result in a different aircraft or at a later time.

Which change removes the exposure without creating a different problem?

Use the vertical and horizontal weather structure together with terrain and aircraft limits.

Planning, recognition and avoidance

Before flight, combine temperature/freezing-level forecasts, cloud and precipitation structure, icing forecasts, SIGMET/AIRMET where applicable, recent air-reports and terrain. Identify alternative tracks or levels that can leave the liquid-water environment, with adequate terrain clearance, airspace and performance. Check the aircraft’s certified capability and limitations, including whether the likely large-drop environment is covered. Icing protection is not approval to remain in every icing condition.

During flight, monitor visible accretion, indications, handling/performance and the weather around the aircraft; some harmful runback or clear ice may be hard to see. Recognition of freezing rain/drizzle, unexpectedly rapid accretion or effects beyond the protection capability calls for prompt action under the approved aircraft/operator procedure. EU Air Operations requires departure from icing without delay if the exposure exceeds the certified intensity or an aircraft without known-icing certification encounters it: select an appropriate level and/or route change and declare an emergency to ATC if necessary. Coordinate with ATC and report the encounter so others can assess it.

Changing track to leave cloud or precipitation, or changing altitude to a genuinely warmer or liquid-free layer, can remove exposure when safe and authorised. A blanket rule to descend is wrong over high terrain, beneath freezing rain or where lower cloud persists; climbing may also be impractical in deep convection or with degraded climb performance. Choose from the actual profile, available escape route and aircraft capability. This meteorology lesson identifies the decision factors; specific equipment use and manoeuvre sequences follow the approved flight manual and operating procedures.

Can you apply the ideas?

Four original questions. Decide from the facts, then review the explanation.