Ice crystals & high ice water content

What does ice water content mean?

Keep crystal number, size and mass concentration separate.

IWC and ice-crystal icing

Ice water content (IWC) is the mass of frozen cloud particles per volume of air, commonly expressed in g/m³. Liquid water content counts the liquid phase; total condensed water content includes liquid plus ice when both are present. A cloud of many small crystals can have a substantial IWC even if individual particles are small. High ice water content describes a high mass concentration; it is not simply a count of crystals and has no single pilot-visible brightness threshold.

Ice-crystal icing is accretion or obstruction associated with ingestion of ice particles, notably within turbine-engine compression-system flow paths or air-data probes. It differs from ordinary wing icing caused by external supercooled droplets freezing on impact. Fully frozen particles may strike a cold external surface without leaving the classic leading-edge deposit, yet ingestion into a warmer component can create local melting, sticking and subsequent accretion.

IWC must refer to the air volume sampled at the relevant conditions; it is not automatically grams of ice per square metre of wing. For example, 2 g/m³ distributed through 100 m³ of air represents 200 g of ice within that air, not a prediction that all 200 g will be collected or frozen on a component. Ingestion, impingement, heat transfer and residence time determine the component response.

Where can a high crystal concentration develop?

Follow deep convective ascent into the upper cloud and surrounding outflow.

Atmospheric production and likely areas

Deep convection carries large quantities of condensed water upward. Droplets freeze, ice grows by deposition and collection, and cloud microphysical processes can generate or multiply numerous crystals. Updraughts sustain the ice population and upper outflow spreads it into anvils and surrounding stratiform cloud. Crystal size, aggregation, sedimentation, regeneration and the age of the system change the local IWC; a simple temperature or cloud-top height does not diagnose it.

High concentrations have been observed in deep tropical and subtropical convective systems, including large mesoscale cloud systems, and can also occur in other deep convection. Relevant regions include near active convective updraughts and adjacent or downwind upper/anvil and stratiform areas. The highest ice concentration does not have to coincide with the strongest weather-radar echo. Some hazardous regions can look relatively smooth or have little turbulence compared with the core.

At very cold cruise temperatures, the absence of expected supercooled liquid airframe icing can therefore coexist with engine or probe risk. A weak external icing detector response, little visible wing ice or lack of strong lightning is not a reliable all-clear for ice crystals. Use the weather structure and multiple sources rather than interpreting the absence of one familiar structural-icing cue as proof of safety.

How can frozen particles create a deposit in a warm compression flow?

Track ingestion, partial melting, surface cooling and shedding.

Component mechanisms and flight hazards

Ingested crystals pass into engine regions where the air and components are warmer than the external cloud. Some particles can partially melt and create a mixed ice–water film on surfaces, allowing further crystals to adhere. Heat transfer and evaporation/melting can cool the wetted component region and sustain accretion. The exact regime depends on engine design, power and the particle/thermal conditions; a hot engine as a whole does not mean every internal surface remains free of accretion.

Accumulation can obstruct or disturb compressor flow, leading to loss of thrust or thrust control, rollback, surge/stall or flameout. Shed ice can damage downstream components and produce vibration. More than one engine may share the same atmospheric exposure, so the weather is not necessarily a single-engine isolated fault. Air-data probes can also become obstructed or affected, causing unreliable speed or other indications.

Possible cues can include unusual precipitation-like streaks on a heated windshield in very cold cloud or engine/air-data anomalies near deep convection, but none is a universal diagnostic signature. Do not wait for a visible external wing deposit to recognise the possibility. Apply the aircraft’s approved procedures to the actual indications and remove the weather exposure as appropriate; this conceptual explanation does not prescribe a common engine-control action for all types.

Which sources help identify the wider convective ice cloud?

Combine current forecasts, imagery, radar limitations and aircraft-specific guidance.

Planning and in-flight assessment

Before flight, identify deep convection and extensive upper cloud systems along the route using significant-weather forecasts, current satellite imagery, relevant SIGMETs, reports and other available products. Assess evolving storm organisation and outflow, not just the centre or one forecast cloud-top symbol. Temperature alone does not specify a crystal-free cruise level, and moving higher can place the aircraft deeper in an anvil.

Airborne weather radar is valuable for assessing convection and precipitation, but many conventional reflectivity displays respond much less strongly to small ice crystals than to rain. Weak returns or a region above the strongest echo can still contain a substantial ice mass. Radar attenuation, tilt, beam geometry and display processing add limitations. Use the installed system’s guidance and any specific ice-crystal awareness functions without assuming every aircraft has the same detection capability.

Maintain the prescribed aircraft/operator margins from convective weather and consider the extent and movement of the surrounding ice cloud. There is no single universal distance from a core that guarantees absence of ice crystals. If an encounter is suspected, coordinate an appropriate weather deviation and apply the flight manual/operator procedures for the actual engine or air-data indications. Report position, time, level, aircraft type and relevant conditions so subsequent flights can assess the exposure.

Can you apply the ideas?

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