Aviation Icing: Structural & Carburettor
Ice is insidious — it forms quickly, degrades performance dramatically, and can be fatal. This reference covers the two main types of aviation icing, the conditions that cause each, recognition signs, and escape procedures.
Structural Icing
Structural icing is the accretion of ice on airframe surfaces including wing leading edges, tail surfaces, propeller blades, fuselage, antennae, and pitot-static probes. Unlike carburettor icing, structural ice is directly visible from the cockpit (on wings, pitot tubes, and windshield).
Two conditions MUST coexist for structural icing: (1) Visible moisture — cloud, fog, drizzle, freezing rain, or snow. (2) Airframe surface temperature between approximately -20°C and 0°C. Most severe icing occurs between -10°C and 0°C. Note: Outside Air Temperature (OAT) may be warmer than the surface temperature due to kinetic heating effects.
Types of Structural Ice
Clear Ice (Glaze Ice)
Formed in freezing rain or large water droplets (such as in cumuliform cloud). Water does not freeze instantly on impact — it flows back along the surface before freezing, creating a smooth, transparent, and dense layer of ice.
- Appearance: Clear or translucent, smooth surface
- Formation: Freezing rain, drizzle, large supercooled water droplets
- Weight: Heavy — denser than rime ice
- Hazard level: Most hazardous type. Hard to see, conforms to and extends beyond leading edge protection areas, creates significant aerodynamic penalty and weight addition
- Temperature range: Most common near 0°C
Rime Ice
Formed when small supercooled water droplets (as found in stratiform cloud or freezing fog) freeze immediately on contact with the airframe surface. Air becomes trapped between droplets, giving the ice its characteristic white, opaque, rough appearance.
- Appearance: White, opaque, rough, brittle
- Formation: Stratiform cloud, freezing fog, small droplets
- Weight: Lighter than clear ice (trapped air)
- Hazard level: Significant — rough surface disrupts airflow over wing, increasing drag and reducing lift. More easily detected visually than clear ice
- Temperature range: More common at lower temperatures (-10°C to -20°C)
Mixed Ice
A combination of clear and rime ice occurring when conditions vary through a cloud layer, or when both large and small droplets are present simultaneously. Most common in complex cloud systems and cumuliform cloud with embedded stratiform layers.
- Appearance: Rough and irregular, combination of clear and white areas
- Formation: Complex cloud systems, cumuliform with embedded stratiform
- Hazard level: High — rough surface plus weight penalty
Effects on Aircraft Performance
Even small amounts of structural ice have dramatic effects on aircraft performance. These effects compound rapidly as ice accumulates:
| Parameter | Effect of Icing | Magnitude |
|---|---|---|
| Drag | Significantly increased due to disrupted airflow over rough ice surface | Up to 200% increase |
| Lift | Reduced as wing camber and leading edge shape are altered | Up to 30% reduction |
| Stall speed | Increased — aircraft stalls at a lower angle of attack than normal | Stall AoA reduced |
| Weight | Increased by ice accumulation on airframe, propeller, and landing gear | Can be significant |
| Propeller efficiency | Reduced thrust, possible propeller imbalance from uneven ice shedding | Significant thrust loss |
| Pitot/static system | Blocked pitot tube gives false/zero airspeed; blocked static gives incorrect altitude and VSI | Instrument unreliable |
| Stall warning | May activate later or not at all due to changed airflow at sensor | Warning may not function |
De-icing vs Anti-icing: De-icing removes ice that has already formed (e.g., inflatable boots cycling). Anti-icing prevents ice from forming (e.g., heated leading edges, TKS fluid). Know which system your aircraft is equipped with and follow the POH precisely. Using de-ice boots too early on clear ice can cause a permanent ice bridge that makes the boots ineffective.
PIREP Icing Intensity Definitions
Icing intensity is reported in PIREPs using standardised definitions. Understanding these categories is essential for route planning and go/no-go decisions.
| Intensity | PIREP Code | Definition | Action |
|---|---|---|---|
| Trace | TRACE | Ice barely perceptible; rate of accumulation slightly greater than sublimation. No hazard unless exposure exceeds 1 hour. | Monitor |
| Light | LGT | Slight problem if flight is prolonged. Occasional use of de-icing equipment removes/prevents accumulation. | Use de-ice; monitor |
| Moderate | MOD | Potential hazard; use de-icing equipment or divert. Short exposure can become hazardous. | Exit conditions; consider divert |
| Severe | SEV | Rate of accumulation is such that de-icing equipment fails to reduce or control hazard. Immediate divert essential. | IMMEDIATE exit / declare emergency |
Escape Procedure — Inadvertent Structural Icing
If you encounter unforecast or worsening structural icing, the following steps apply. Always follow your specific aircraft's POH/AFM, which takes precedence.
-
1
Reduce altitude (if terrain and airspace allow)
Descending below the freezing level removes the temperature condition for icing. Check MEA/MORA for terrain clearance before descending. Alert ATC early to request a block altitude or lower level.
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2
Turn back or divert
Reverse course to exit the icing area. Check the weather behind you — do not turn back into worse conditions. Alternatively, divert to alternate routing outside the icing region.
-
3
Activate de-ice and anti-ice systems per POH
Apply carburettor heat, pitot heat, and any structural de-ice/anti-ice equipment available. Follow the POH checklist exactly — including timing and cycling instructions for pneumatic boots.
-
4
Declare emergency if control is affected
If ice accumulation is beyond the aircraft's ability to cope, or handling is degraded, declare MAYDAY on the current frequency. Request priority handling and the nearest suitable aerodrome. Do not delay this decision.
Carburettor Icing
Carburettor icing occurs in the carburettor venturi due to two simultaneous cooling effects: fuel evaporation (absorbs heat) and pressure drop (air cools as it expands). Combined, these can reduce carburettor temperature by 25–30°C below OAT — sufficient to freeze moisture out of the air even on a warm, sunny day.
Critical point: Carburettor icing does NOT require below-freezing outside air temperature. It is most likely at OAT between +5°C and +25°C with high relative humidity. A warm, muggy summer day with OAT of +20°C and dewpoint spread <5°C is a high-risk scenario, especially during descent with reduced power settings.
Recognition Signs
- Fixed-pitch propeller aircraft: Unexplained RPM drop (without throttle input)
- Constant-speed propeller aircraft: Unexplained manifold pressure drop
- Rough engine running, vibration
- Loss of power; aircraft sinking unexpectedly
- Often first noticed during descent with reduced power — low power settings increase ice risk
Prevention and Treatment — Carburettor Heat
Carburettor heat (carb heat) raises the temperature of air entering the carburettor by routing it over the exhaust manifold. This prevents ice formation or melts existing ice.
- When carb heat is applied to existing ice: expect a temporary RPM drop (ice melting partially blocks airflow) followed by rough running (water passing through), then smooth running and RPM restoration
- If no ice was present, applying carb heat may cause a slight RPM drop due to the less-dense warm air — this is normal
- Carb heat should generally NOT be used at full power (takeoff/go-around) — follow POH limitations
- In conditions of high ice risk, carb heat may need to be left on continuously, accepting the slight power reduction
Fuel-Injected Engines
Fuel-injected engines do not have a carburettor and are not susceptible to carburettor icing. However, they may experience induction icing — ice forming at the air filter/induction system when operating in visible moisture. An alternate air source is fitted to bypass a blocked air filter.
Ground Frost and Pre-Flight Contamination
Never take off with frost, ice, or snow on the wing surfaces. Even a thin layer of frost (1mm) dramatically disrupts the boundary layer airflow over the wing, reducing lift and increasing stall speed. The aircraft may not achieve normal rotate speed behaviour and may fail to climb away safely. Remove all contamination with approved de-icing fluid; verify the critical surfaces are clean before takeoff — the "clean aircraft concept."
Even a thin layer of frost (1mm) on wing surfaces can reduce lift by up to 30% and increase stall speed significantly. Never take off with any frost, snow, or ice on critical surfaces.
Structural Icing Risk — OAT vs Cloud Type
The following matrix gives a general icing risk assessment by combining OAT with cloud type. Always obtain actual PIREPs for the route — this is a planning guide only.
| OAT | Cirrus/Cs | As/Ns | St/Sc | Cu (flat) | TCu/Cb | Freezing Rain |
|---|---|---|---|---|---|---|
| Above 0°C | None | None | None | None | None | None |
| 0°C to -5°C | Trace | Moderate | Moderate | Light | Moderate | Severe |
| -5°C to -10°C | Trace | Moderate | Moderate | Light | Severe | Severe |
| -10°C to -20°C | Trace | Light | Light | Trace | Moderate | Moderate |
| Below -20°C | None | Trace | Trace | None | Light | None |
Note: Below -20°C most moisture is ice crystal rather than supercooled water. Structural icing risk decreases but is not zero in cumuliform cloud. Obtain current PIREPs before flight.