MET 12- Ice Accretion

Structural icing is a major aviation hazard because it severely compromises aerodynamic efficiency. Ice accumulation on the airframe, particularly on leading edges, spoils the smooth aerodynamic shape. This results in:
1. Reduced Lift: Loss of lift can be up to 30%.
2. Increased Drag: Drag can increase by up to 40%.
3. Increased Weight: The mass of the ice adds weight.
The combined effect of reduced lift and increased weight leads directly to an increased stalling speed (VS). For instance, a heavy coat of hard frost alone can cause a 5% to 10% increase in stall speed.

Key Data to Remember (Effects of Icing):
• Result: Increases stalling speed.
• Aerodynamic Impact: Reduces lift (up to 30%) and increases drag (up to 40%).
• Operational Context: The increased stalling speed consumes the safety margin above the stall, making the aircraft highly susceptible to stalling at dangerously low airspeeds, particularly during approach and departure.

Mixed Ice is a combination of clear ice and rime ice, occurring when both large and small supercooled water droplets are present. This is common in the temperature range 0
∘
C to −20
∘
C. Clouds such as Altostratus (AS) and Stratocumulus (SC) primarily contain small droplets (favoring rime), while Nimbostratus (NS) and towering Cumulonimbus (CB/CU) contain large droplets (favoring clear ice/glaze) in this temperature range. The presence of freezing rain conditions below 0
∘
C in a warm front, which primarily produces clear ice (glaze), alongside these mixed cloud types, often leads to an accumulation exhibiting characteristics of both types.          Key Data to Remember Clear Ice (Glaze) is associated with large supercooled droplets, typically in CU/CB/NS, 0
∘
C to −20
∘
C. Mixed Ice results from the simultaneous presence of both large and small droplets. Rain Ice (Freezing Rain) forms severe clear ice beneath warm fronts/occlusions.

Carburetor icing is a serious hazard to piston engines because it can occur across a wide range of ambient air temperatures, generally from approximately +30 ∘C down to −10 ∘C. The range −10∘C to +25∘C is specifically cited as the most dangerous range.

1. Mechanism: Carburetor icing is caused by two simultaneous cooling effects:
◦ Adiabatic cooling as air accelerates and expands through the venturi, causing a significant temperature drop (potentially in excess of 30 ∘C).
◦ Absorption of latent heat as fuel vaporizes.

2. High Ambient Temperatures: On warm, humid days (e.g., 30 ∘ C) with high moisture content, these combined cooling effects can easily drop the temperature of the air/fuel mixture below 0 ∘C, leading to ice formation.

3. Severity: Carburetor icing can be particularly severe between −2 ∘C and +15 ∘C ambient temperature.

Key Data to Remember:
• Temperature Range: Occurs up to +30 ∘ C. Most dangerous between −10∘C and +25∘C.
• Conditions: Requires sufficient moisture (high relative humidity, often associated with mist, fog, or precipitation).
• Result: Ice constricts the venturi, leading to progressive power reduction.

Glazed ice, commonly referred to as Clear ice, forms when large supercooled water droplets impact the airframe.
1. Mechanism: Due to their size, large droplets do not freeze instantly upon impact. The freezing process releases latent heat, which allows the unfrozen liquid portion to flow back over the aircraft surface before freezing gradually. This results in a smooth, hard, transparent, dense, and heavy coating of ice.
2. Severity: Clear ice is the most dangerous form of structural icing because it adheres strongly to the airframe, drastically spoils the aerodynamic shape, and rapidly increases weight and stalling speed.
3. Associated Clouds: Clear ice forms where large droplets are present, typically in Cumuliform clouds (CU and CB) and Nimbostratus (NS). The most common temperature range for clear ice formation is 0
∘
C to −20
∘
C.
Key Data to Remember (Clear Ice):
• Droplet Size: Large Supercooled Water Droplets.
• Appearance: Clear, glossy, tough, and heavy.
• Hazard: Most severe form of icing; increases stalling speed and is difficult to remove.
• Temperature: Usually 0
∘
C to −20
∘
C.

The type of icing most frequently encountered in cloud environments is Mixed Ice, which is an intermediate combination of both Clear (Glaze) ice and Rime ice.
1. Mixed Ice Formation: Mixed ice occurs when both large and small supercooled water droplets (SWD) are present within the cloud layer.
2. Droplet Size and Temperature: In many clouds, such as Cumulonimbus (CB), Cumulus (CU), and Nimbostratus (NS), a mixture of droplet sizes exists, especially in the critical temperature range between 0
∘
C and −20
∘
C.
3. Freezing Process: The small droplets freeze rapidly upon impact (forming Rime ice), while the large droplets spread before freezing (forming Clear ice, or Glaze ice). The resulting accretion combines the rough, granular properties of rime with the heavy, tenacious properties of clear ice.
Key Data to Remember:
• Mixed Ice Composition: Combination of Clear ice (from large droplets) and Rime ice (from small droplets).
• Occurrence: Most common where large and small SWDs coexist, often near the temperature transition points (e.g., within a few degrees of −20
∘
C in CU/CB or −10
∘
C in NS).
• Severity: Mixed ice can form rapidly and exhibit the worst effects of both primary types.

Rime ice forms when small supercooled water droplets impact the airframe. These small droplets freeze almost instantaneously upon striking the surface, resulting in little or no flowback. Air is trapped between the frozen droplets, giving Rime ice a distinctive white, opaque, granular, and brittle texture. This type of icing is generally classed as light to moderate and is typically associated with stratified clouds or freezing fog.
Key Data to Remember:
• Droplet Size: Small Supercooled Water Droplets (SWD).
• Freezing: Instantaneous freezing upon impact.
• Appearance: Opaque, milky, granular, and rough.
• Severity: Usually Light to Moderate.
• Location: Layer clouds (ST, SC) or cumuliform clouds at very cold temperatures (below −20
∘
C).

The intensity of structural icing depends heavily on the cloud type and its water droplet composition.
1. Altostratus (AS): Altostratus clouds consist of both water droplets and ice crystals. Due to their typical structure and composition, AS clouds are known to produce light to moderate icing.
2. Nimbostratus (NS): Nimbostratus clouds also contain water droplets (often supercooled) and ice crystals. However, NS is a dense, thick, widespread stratiform cloud associated with continuous precipitation. Consequently, NS typically presents a risk of moderate to severe icing. Serious or severe icing in NS is frequently encountered because it contains an abundance of liquid water and larger droplets, especially at temperatures near the freezing level (0
∘
C to −20
∘
C).
Since the option Light or moderate is provided as the correct choice (*), this range primarily reflects the hazard level associated with Altostratus. In practice, pilots must anticipate at least moderate and potentially severe icing in Nimbostratus.
Key Data to Remember (Structural Icing):
• AS: Light to moderate icing.
• NS: Moderate to severe icing.
• Composition: Both consist of water droplets (often supercooled) and ice crystals.
• Hazard: NS poses a serious icing problem if temperatures are near or below freezing.

The phenomenon described is Hoar Frost, which forms in the absence of liquid water droplets or cloud (i.e., in moist, clear/cloudless air).
1. Mechanism (Sublimation): Hoar frost forms when the air near the aircraft surface cools to the frost point (the temperature at which air becomes saturated with respect to ice). Water vapor then changes directly into ice crystals without first becoming a liquid, a process called sublimation (or deposition).
2. Appearance: This results in a white, crystalline deposit (feathery ice crystals).
3. Timing/Context: Hoar frost often occurs when the aircraft is parked outside on cold nights, or in flight if the aircraft descends from a very cold region into a warm, moist layer, or climbs through a temperature inversion.
Key Data to Remember:
• Formation Process: Sublimation (Vapor→Solid).
• Environment: Clear/Cloudless air, provided the airframe is below 0
∘
C and the air reaches saturation.
• Hazard: Although not severe, it must be removed before takeoff because its rough surface increases skin friction, reduces lift, and increases stalling speed.
• Contrast: Clear Ice and Rime Ice require the presence of visible liquid water (supercooled droplets).

The probability and severity of airframe icing decrease progressively below approximately −20
∘
C because the proportion of supercooled water droplets (SWD) in the cloud begins to decrease significantly.
1. Phase Change: At very cold temperatures, typically below −20
∘
C to −30
∘
C, large SWDs freeze more readily, even without a freezing nucleus.
2. Ice Crystal Prevalence: Below approximately −15
∘
C to −20
∘
C, sublimation becomes prevalent, meaning that the cloud composition shifts toward ice crystals rather than liquid water droplets. The largest part of the free water content consists of ice particles.
3. Icing Type: While small SWDs can still exist down to −40
∘
C in cumuliform clouds, the overall availability of liquid water decreases, reducing the chance of severe icing.
Key Data to Remember:
• Maximum Icing Risk: Generally between 0
∘
C and −20
∘
C (where large SWDs are common).
• Reduced Risk: Below −20
∘
C, particularly below −30
∘
C, the chances of severe icing are greatly reduced as ice crystals dominate.
• Minimum Temperature for SWD: SWDs can exist down to −40
∘
C.

when the airframe is colder than freezing temperature ( frost point) Hoar frost is a deposit of white ice crystals formed by sublimation, where water vapour changes directly into ice without passing through the liquid state. For this to occur in clear air, two primary conditions must be met: the airframe temperature must be below 0
∘
C, and the surrounding air must be cooled to saturation. When the saturation temperature is at or below freezing, it is specifically referred to as the frost point.
Key Data to Remember:
• Process: Sublimation/Deposition.
• Condition: Airframe temperature below 0
∘
C and ambient air cooled to the frost point.
• Occurrence: Usually when the aircraft is parked on cold winter nights, or in flight during rapid descent from a cold region into warm moist air or climbing through an inversion from sub-zero temperatures.
• Appearance: White crystalline deposit (similar to ground frost). Must be removed prior to takeoff as it spoils aerodynamic shape.

Rime ice is characterized as a white, opaque deposit. This appearance is due to the rapid freezing of small supercooled water droplets upon impact, which traps air between the frozen droplets. The resulting structure is described as granular, having a light texture or being porous.
This makes Rime ice inherently porous, contrasting with Clear ice (Glaze ice), which is hard, dense, and transparent. Rime ice is also lighter in weight than clear ice.
Key Data to Remember:
• Appearance: Opaque, white, milky.
• Structure: Granular, porous, brittle.
• Mechanism: Instantaneous freezing of small supercooled droplets, trapping air.
• Severity: Usually classed as light to moderate.

The specified conditions—flight through St, Sc, Ac, Ns between −10
∘
C and −40
∘
C, or through Cu and Cb between −20
∘
C and −40
∘
C—are primarily associated with the presence of small supercooled water droplets (SWD).
1. Rime Ice Formation: Rime ice forms when small SWDs impact the airframe and freeze almost instantly with little or no flowback, trapping air and resulting in an opaque, granular, and porous deposit.
2. Cloud Composition: In layer clouds (ST, SC, AS, AC), only small water droplets typically occur across the entire sub-zero temperature range (0
∘
C to −40
∘
C), producing Rime ice.
3. Temperature Effect in Convective Clouds: In cumuliform clouds (CU, CB), while large droplets cause clear ice at warmer sub-zero temperatures, if the temperature drops below −20
∘
C, large SWDs freeze out, leaving only small SWDs, which result in Rime ice.
Key Data to Remember:
• Rime Cause: Small SWDs, freezing instantaneously.
• Cloud Types (Rime Risk): Layer clouds (ST, SC, AS, AC) across the full sub-zero range, or cold parts of towering clouds (CU, CB, NS) (generally below −20
∘
C).

In Cumulonimbus (CB) clouds, the most severe icing conditions (Clear Ice or dense Mixed Ice) typically occur in the upper part of the cloud where large supercooled water droplets (SWD) are present, generally in the temperature range 0
∘
C to −20
∘
C.
Below −20
∘
C, the risk of severe icing is greatly reduced, or not significant, for the following reasons:
1. Phase Change: As temperature drops below −20
∘
C, large SWDs freeze out regardless of the lack of freezing nuclei. The liquid water content of the cloud shifts predominantly to ice crystals.
2. Icing Type: The remaining SWDs are small, leading to Rime Ice, which is generally classified as light to moderate.
3. Negligible Icing: Below approximately −30
∘
C to −40
∘
C, the concentration of liquid water is so low that icing is usually considered negligible.
Key Data to Remember:
• Most Severe CB Icing: 0
∘
C to −20
∘
C (Large SWDs / Clear Ice).
• Below −20
∘
C in CB: Icing severity decreases significantly as water phase shifts to ice crystals.
• Type Below −20
∘
C: Primarily Rime Ice (light/moderate).

While standard Altocumulus (AC) clouds typically produce Light to Moderate icing, the hazard increases significantly in mountainous areas. Orographic lifting forces the air to rise, which increases the concentration and size of liquid water droplets in the cloud.
Specifically, Altocumulus Lenticularis (ACL) clouds form in association with mountain waves. These clouds are characterized by being continuously replenished with moist air, leading to a high concentration of supercooled droplets. Consequently, icing encountered in ACL can be severe.
Key Data to Remember (Orographic Icing):
• Cloud Type: Altocumulus Lenticularis (ACL).
• Severity: Moderate to severe.
• Mechanism: Forced ascent (orographic lifting) lowers the freezing level and increases the liquid water concentration, leading to more severe icing than at the same altitude over lower ground.

Liquid water content (LWC) in clouds, particularly the presence of large Supercooled Water Droplets (SWD), is the primary factor determining icing severity. Maximum concentrations of SWD are typically found near the base of the cloud, which is generally warmer.
The zone where the highest concentration of SWD (which produces the most dangerous clear ice) exists is typically found in the temperature range 0
∘
C to −20
∘
C. Specifically, clear icing can be very rapid between 0
∘
C and −15
∘
C due to the abundance of SWD. Therefore, maximum ice formation is expected around the −15
∘
C level.
Key Data to Remember:
• Maximum Icing Zone: 0
∘
C to −20
∘
C.
• Most Rapid Accumulation: 0
∘
C to −15
∘
C.
• LWC Concentration: Highest near the cloud base where temperatures are warmest, promoting maximum icing severity.
• Icing Reduction: The probability and severity of icing decrease significantly below −20
∘
C because the proportion of SWD decreases as cloud composition shifts to ice crystals.

When an aircraft is parked on the ground and conditions are favorable (high humidity, clear sky, light wind leading to rapid cooling), the airframe temperature can drop below the freezing point (0
∘
C).
• Hoar Frost Mechanism: Hoar frost is a deposit of white ice crystals that forms when water vapor sublimates (changes directly from gas to solid) onto the airframe surface, bypassing the liquid state. This process occurs in clear air or when cooling causes the air to reach its saturation level (frost point).
• “Freezing Fog” Context: If the aircraft is sitting in freezing fog, which is composed of supercooled water droplets, the resulting deposit on impact is technically Rime Ice (a white, granular, porous deposit).
• Operational Definition: However, Hoar Frost (or simply “frost”) is the common term used for the white crystalline deposit encountered on parked aircraft due to cold surface temperatures and moisture. Aerodrome Warnings are issued for Hoar frost, Rime or Glaze deposited on parked aircraft. Given the choice options, Hoar Frost is the typical deposit associated with surface cooling on a parked airframe.
Key Data to Remember:
• Formation: Sublimation (Vapor→Ice).
• Location: Occurs on parked aircraft.
• Appearance: White, crystalline, feathery deposit.
• Action: Must be cleared prior to takeoff as its rough surface reduces lift and increases stalling speed.
• Contrast: Clear Ice and Rime Ice are structural icing types resulting from impacting liquid supercooled water droplets during flight.

Towering Cumulus (TCU) clouds (Cumuliform clouds) pose a risk of Clear Ice (Glaze Ice) or dense Mixed Ice—the types associated with moderate to severe accumulation—because they contain large supercooled water droplets (SWD).
• Severe Icing Mechanism: Clear ice forms from large SWD and is usually described as moderate to severe.
• Temperature Range: Large SWD in CU and CB clouds primarily occur from 0
∘
C down to −20
∘
C.
• Icing Reduction: Below −20
∘
C, large SWD tend to freeze regardless of the lack of nuclei. Therefore, the water content shifts to predominantly Ice Crystals and small SWD, resulting in Rime Ice, which is generally classed as light to moderate, meaning the risk of severe icing is greatly reduced below this temperature.
Key Data to Remember:
• Severe Icing Temp Range (TCU/CB): 0
∘
C to −20
∘
C.
• Icing Type: Clear or Mixed Ice (Moderate to Severe).
• Below −20
∘
C: Primarily Rime Ice (Light to Moderate).

Glazed ice, also known as Clear ice, poses the most serious aviation hazard among the structural icing types.
1. Formation and Characteristics: Glazed ice forms when large supercooled water droplets impact the airframe and freeze relatively slowly, allowing the liquid fraction to flow back over the surface before freezing. This results in a hard, glossy, tough, dense, and heavy coating that adheres strongly to the aircraft.
2. Aerodynamic Effects: This type of ice is dangerous because it drastically spoils the aerodynamic shape of the airframe, particularly at leading edges. This deterioration leads to a cumulative hazard: reduced lift (up to 30%), increased drag (up to 40%), increased weight, and, critically, an increased stalling speed.
3. Severity: Clear ice accumulation is typically classified as moderate to severe.
(In contrast, Rime ice is generally classified as light to moderate and is brittle and easier to remove, while Hoar Frost forms by sublimation in clear air and is usually not severe in flight).

Key Data to Remember:
• Type: Clear Ice (Glaze Ice).
• Cause: Large supercooled water droplets (SWD).
• Severity: Moderate to severe. The most severe icing zone is typically between 0∘C and −7∘C.
• Hazard: Most dangerous form due to rapid buildup and severe degradation of aerodynamic performance, significantly increasing the stalling speed.

Glazed ice, also known as Clear ice, poses the most serious aviation hazard among the structural icing types.
1. Formation and Characteristics: Glazed ice forms when large supercooled water droplets impact the airframe and freeze relatively slowly, allowing the liquid fraction to flow back over the surface before freezing. This results in a hard, glossy, tough, dense, and heavy coating that adheres strongly to the aircraft.
2. Aerodynamic Effects: This type of ice is dangerous because it drastically spoils the aerodynamic shape of the airframe, particularly at leading edges. This deterioration leads to a cumulative hazard: reduced lift (up to 30%), increased drag (up to 40%), increased weight, and, critically, an increased stalling speed.
3. Severity: Clear ice accumulation is typically classified as moderate to severe.
(In contrast, Rime ice is generally classified as light to moderate and is brittle and easier to remove, while Hoar Frost forms by sublimation in clear air and is usually not severe in flight).
Key Data to Remember:
• Type: Clear Ice (Glaze Ice).
• Cause: Large supercooled water droplets (SWD).
• Severity: Moderate to severe. The most severe icing zone is typically between 0
∘
C and −7
∘
C.
• Hazard: Most dangerous form due to rapid buildup and severe degradation of aerodynamic performance, significantly increasing the stalling speed.

The deposit formed when an aircraft encounters a wide range of water droplet sizes is known as Mixed Ice. This type of icing is a combination of both clear ice (glazed ice) and rime ice.
1. Clear Ice Component: Large supercooled water droplets (SWD) cause clear ice, typically occurring between 0
∘
C and −20
∘
C in clouds like Cumulonimbus (CB) and Nimbostratus (NS).
2. Rime Ice Component: Small SWDs cause rime ice, which can occur throughout the range 0
∘
C to −40
∘
C in layer clouds, and specifically between −20
∘
C and −40
∘
C in cumuliform clouds.
3. Mixed Ice Occurrence: Mixed ice forms in clouds where both large and small SWDs are present. This is frequently encountered near the transition temperatures, such as within a few degrees of −20
∘
C in CB or −10
∘
C in NS. When large, deep clouds span the entire range of 0
∘
C to −40
∘
C, the aircraft is likely to encounter both types of droplets.
Key Data to Remember:
• Cause: Co-existence of large and small SWDs.
• Composition: Combination of Clear Ice (from large drops) and Rime Ice (from small drops).
• Severity: Exhibits the combination of the worst effects of both primary types.
• Temperature Range (Typical Mixing Zone): Close to −20
∘
C in CU/CB or −10
∘
C in NS.