11.icing-conditions. Structural Icing and Icing Conditions
Structural icing is the accumulation of ice on the exterior surfaces of an aircraft in flight. It forms when an aircraft passes through visible moisture—clouds, rain, drizzle, or wet snow—at a temperature at or below freezing on the airframe. Icing is one of aviation's most insidious hazards because it simultaneously degrades lift, increases weight, increases drag, and reduces thrust, while also potentially blocking pitot-static instruments, control surfaces, and antennas.
Conditions Required for Structural Icing
Two conditions must exist simultaneously for structural ice to form:
- Visible moisture (clouds, rain, drizzle, freezing rain, wet snow, fog with visibility < 1 sm)
- Aircraft surface temperature at or below 0 °C (32 °F)
Note that the outside air temperature (OAT) can be slightly above freezing and ice can still form if the airframe itself is colder—for example, after a rapid descent from a cold-soaked altitude.
Supercooled Water Droplets
Liquid water can exist in clouds at temperatures well below 0 °C as supercooled droplets. These droplets freeze on contact with an airframe. The most severe icing typically occurs between 0 °C and −20 °C, with the greatest concentration of supercooled liquid water generally found between −10 °C and −15 °C. Below about −20 °C, most cloud moisture has already crystallized into ice, and ice generally does not adhere readily to the airframe.
Types of Structural Icing
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Clear (glaze) ice — Forms when large supercooled droplets strike the airframe and flow back before freezing, creating a smooth, transparent, glassy sheet. It is the most dangerous type: heavy, hard to see, hard to remove, and it conforms to airfoil shape in ways that destroy lift. Clear ice is typical in freezing rain and in cumuliform clouds with large droplets, generally between 0 °C and −10 °C.
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Rime ice — Forms when small supercooled droplets freeze instantly on impact, trapping air and producing a rough, opaque, milky deposit. It is brittle and easier to remove than clear ice. Rime is common in stratiform clouds at colder temperatures, typically between −10 °C and −20 °C.
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Mixed ice — A combination of clear and rime, having the rough surface of rime with the density of clear ice. It often forms between −10 °C and −15 °C and can build rapidly into irregular, lift-destroying shapes.
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Frost — Ice crystal deposits formed by sublimation when an airframe is below freezing and below the dew point. Frost on wings can reduce lift by up to 30% and increase drag by up to 40%. All frost must be removed before flight.
Intensity Reporting
Pilot reports (PIREPs) classify icing intensity:
- Trace — Ice becomes perceptible; rate of accumulation slightly greater than sublimation. Anti-ice/de-ice not utilized unless encountered for over an hour.
- Light — Rate of accumulation may create a problem if flight is prolonged (over one hour). Occasional use of anti-ice/de-ice removes/prevents accumulation.
- Moderate — Rate of accumulation is such that even short encounters become potentially hazardous; use of anti-ice/de-ice or diversion is necessary.
- Severe — Rate of accumulation exceeds the capability of anti-ice/de-ice equipment to reduce or control the hazard. Immediate diversion is required.
Freezing Rain and Freezing Drizzle
The most dangerous icing environment is freezing rain (FZRA), which occurs when rain falls through a sub-freezing layer near the surface. The droplets are very large and produce extremely rapid clear-ice accumulation. Freezing rain aloft indicates an inversion with warmer air above—climbing into the warm layer is often the safest escape, though descending into above-freezing air may also work if terrain allows.
Effects on the Aircraft
- Lift decreases as ice disrupts the smooth airflow over the wing; stall speed rises and stall AOA decreases.
- Weight increases, sometimes by hundreds of pounds.
- Drag increases sharply, requiring more power and reducing range.
- Thrust decreases if induction or carburetor systems ice over.
- Control surfaces may jam or experience aileron snatch and tail stall.
- Pitot-static instruments can fail if pitot heat is not used.
Induction and Carburetor Icing
Carburetor icing can occur in OATs from 20 °F to 70 °F (−7 °C to 21 °C) with visible moisture or high humidity, due to the temperature drop from fuel vaporization and venturi pressure drop. Symptoms include power loss and a drop in RPM (fixed-pitch) or manifold pressure (constant-speed). Apply full carburetor heat at the first indication.
Pilot Actions
- Check icing AIRMETs (AIRMET ZULU), PIREPs, and forecast freezing levels during preflight.
- A non-known-icing-certified airplane must exit icing conditions immediately: change altitude to find above-freezing or colder-than-−20 °C air, or reverse course.
- Increase approach speed to compensate for the higher stall speed; do not extend full flaps if tail-stall is suspected.
- Use pitot heat anytime icing is possible.