Structural icing is one of the most insidious weather hazards in aviation. Unlike turbulence or low visibility, ice can accumulate silently on an airframe, degrading lift, increasing drag, and altering handling characteristics before the flight crew fully appreciates what is happening. The FAA Aviation Weather Handbook (FAA-H-8083-28B), Chapter 20, provides a thorough framework for understanding how and why ice forms, what the three structural icing types look like, and which conditions favor each. A solid grasp of this material is essential for any pilot flying in IMC or near-freezing temperatures.
At the heart of all structural icing is a single physical phenomenon: supercooled water. Pure water suspended as tiny droplets can remain liquid well below 0 °C because surface tension inhibits the formation of ice crystals. In fact, pure water droplets will not freeze spontaneously until they reach -40 °C. When these supercooled droplets are disturbed — for example, by the leading edge of a wing slicing through a cloud — they freeze almost immediately on contact with the airframe surface. That freezing process, fast or slow, determines which type of ice accumulates.
Supercooled Water and Cloud Composition
Understanding where supercooled water exists helps pilots anticipate icing altitude and intensity. Between 0 °C and -10 °C, clouds consist mainly of supercooled liquid water droplets. Between -10 °C and -20 °C, liquid droplets coexist with ice crystals. Below -20 °C, clouds are generally composed entirely of ice crystals, which pose little icing hazard by themselves. This means the most significant icing potential occurs between 0 °C and -20 °C. In altitude terms, the peak icing occurrence is near 10,000 feet MSL, with approximately half of all incidents between 5,000 and 13,000 feet.
There is an important exception: strong vertical currents inside cumulonimbus clouds can loft supercooled water to altitudes where temperatures approach -40 °C, so convective clouds carry icing hazards far higher than the general rule suggests. Supercooled Large Drops (SLDs) — defined as droplets with diameters greater than 40 microns — are particularly common in freezing rain (droplets exceeding 200 microns) and freezing drizzle (40 to 200 microns) situations, and they introduce a category of hazard beyond what standard cloud droplets produce.
The Three Types of Structural Icing
Rime Ice
Rime ice is the most frequently reported structural icing type. It forms when small supercooled water droplets strike the airframe and freeze almost instantaneously. Because freezing happens so rapidly, air becomes trapped within the ice structure, producing a rough, milky, opaque, and porous deposit. Rime ice is brittle compared to clear ice and grows forward into the airstream from leading edges, antennas, and other exposed surfaces.
Rime icing favors colder temperatures (generally below -15 °C), lower liquid water content (LWC), and smaller droplets. The combination of cold air and tiny droplets means the entire droplet solidifies almost before it can spread. While rime ice is somewhat easier to see than clear ice because of its white color, its jagged and irregular texture disrupts airflow over the airfoil and can meaningfully degrade aerodynamic performance. It is commonly encountered in stratiform clouds, where LWC is relatively low.
Clear (Glaze) Ice
Clear ice — also called glaze ice — is considered the more hazardous type for most aircraft. It forms when large supercooled droplets strike the airframe and only a small fraction of each drop freezes on initial contact. The remaining unfrozen water flows or smears rearward and along the surface before gradually freezing. Because this process is slow and few air bubbles are trapped, clear ice is dense, glossy, translucent, and heavier than rime ice for the same volume.
Clear icing conditions favor warmer temperatures (generally above -10 °C), higher LWC, and larger droplets. The most operationally significant feature of clear ice is its tendency to form horns — protruding ridges near the top and bottom of the airfoil's leading edge. These horns generate a zone of disrupted, turbulent airflow that is considerably larger than the disruption caused by rime ice. Clear ice is also difficult to spot in flight because its translucency makes it blend with the airframe surface, meaning pilots may not recognize accumulation quickly.
Deicing equipment can remove most clear ice from protected surfaces, but the ice can spread to unprotected areas aft of the boots or heated panels, where it persists and continues to degrade lift.
Supercooled Large Drop (SLD) Icing — A Special Clear-Ice Hazard
SLD conditions deserve specific attention because they represent a particularly dangerous subset of clear icing. Because SLD droplets are so large (freezing drizzle: 40–200 microns; freezing rain: >200 microns), they have enough momentum to cross the airfoil's boundary layer streamlines and impact well aft of the leading edge — beyond the protected area of most deicing systems. The resulting ice is lumpy, textured, and uneven, described in the handbook as resembling the frosted glass of a bathroom window.
Residual SLD ice aft of the deice boots acts like a spoiler, separating airflow over a large portion of the wing. In extreme cases, flow-separation bubbles can travel along the wing surface and inadvertently deflect ailerons, creating unpredictable roll behavior. Even small amounts of SLD-formed ice on the upper or lower wing surface can seriously compromise aerodynamic integrity. Pilots should be aware that standard Pilot Reports (PIREPs) may not always indicate SLD conditions explicitly; freezing rain or freezing drizzle reported at altitude is a strong indicator of SLD exposure.
Mixed Ice
Mixed ice is exactly what the name implies: a combination of rime and clear ice accumulating simultaneously or in rapid alternation. It forms when an aircraft passes through regions where LWC, temperature, and droplet size vary over short distances — sometimes tens of kilometers or less. Viewed from the side, mixed ice shows alternating layers of relatively clear and opaque ice.
Mixed ice hazards are similar to those of clear ice. It can form horns, spread beyond protected surfaces, and create widespread flow separation and turbulence across the airfoil. Because it contains dense, adhering clear-ice components, it is more difficult to remove than rime ice and can resist deicing equipment more stubbornly than either pure type alone. Mixed icing occurs where rime and clear icing conditions overlap, generally in the temperature range of approximately -10 °C to -15 °C, where droplet sizes and liquid water content vary enough for both ice types to form on the same surfaces.
Key Factors Governing Icing Type and Severity
The FAA identifies the following factors, roughly in order of importance, as determining icing type and severity:
- Supercooled Liquid Water Content (SLWC): The most critical factor. More available liquid water means more ice. Cumuliform clouds have the highest SLWC; stratiform clouds have lower SLWC but are far more extensive horizontally. Most icing cases involve relatively low SLWC.
- Temperature (OAT): Must be at or below 0 °C for structural icing. As temperature drops below -20 °C, SLWC falls rapidly; below -40 °C, spontaneous freezing eliminates supercooled droplets entirely. Rime tends to occur colder than -15 °C; clear ice warmer than -10 °C; mixed ice between those values. These are general guidelines — actual type depends on multiple variables.
- Droplet size: Smaller droplets favor rime; larger droplets favor clear ice. Droplet size becomes especially critical when drops exceed the cloud-droplet range and become SLDs, which impact farther aft and overwhelm standard deice systems.
- Airspeed: Higher airspeeds increase the rate of droplet impact and thus ice accumulation, but aerodynamic heating of the skin at higher speeds can partially counteract this effect at the upper end of an aircraft's performance envelope.
- Aircraft type and design: Airfoil shape, sweep angle, and the location of fuel tanks all influence ice accumulation. Cold-soaked airframes — those chilled by extended flight in cold air — can collect ice even when ambient air temperature is slightly above 0 °C, because the skin temperature remains below freezing. Aircraft with fuel tanks mounted flush to the wing skin are particularly susceptible.
Why It Matters Operationally
Ice affects an airframe in multiple simultaneous ways: it increases weight, increases drag, decreases lift, and can alter stall characteristics unpredictably — an ice-contaminated wing can stall at a higher airspeed and lower angle of attack than the clean aircraft would. The ice-contaminated stall may also occur without the buffet cues a pilot expects. SLD and clear-ice horn formations create flow separation large enough to activate control surfaces, making the aircraft difficult or impossible to control. Even a thin, rough coating of ice can meaningfully reduce lift and increase drag on the affected surfaces. Many light aircraft are prohibited by their aircraft flight manual limitations from flight into known icing conditions unless equipped with an approved ice-protection system, and turbine-powered and large airplanes are subject to specific operating restrictions under 14 CFR 91.527.
Pilots must also appreciate the cold-soak effect: an aircraft that has been cruising at altitude in very cold air will have a chilled airframe. Upon descent into a layer where OAT is slightly above 0 °C, the airframe skin can remain below freezing long enough to collect ice even though thermometers show above-freezing air. This is particularly relevant during approach and landing, where a pilot might assume the icing threat has ended with a rise in OAT.
Key Numbers and Rules
- Supercooled water possible from 0 °C down to -40 °C; most icing between 0 °C and -20 °C.
- Peak icing occurrence near 10,000 ft MSL; roughly half of all reports between 5,000 and 13,000 ft.
- Rime ice general temperature range: colder than -15 °C; clear ice: warmer than -10 °C; mixed: -10 °C to -15 °C.
- SLD threshold: droplets larger than 40 microns; freezing drizzle 40–200 µm; freezing rain greater than 200 µm.
- Clouds predominantly supercooled liquid: 0 to -10 °C; mixed liquid/crystal: -10 to -20 °C; predominantly ice crystals: below -20 °C.
Common Test Traps
- Confusing rime and clear ice hazard levels: Students sometimes assume the rougher-looking rime ice is more dangerous. In most circumstances, clear ice is the greater hazard because of its horn formation, greater density, difficulty in detection, and spread beyond protected surfaces.
- Assuming ice cannot form above 0 °C OAT: A cold-soaked airframe can collect ice even in slightly above-freezing ambient air; OAT alone does not tell the whole story.
- Ignoring SLD as just another clear-ice variant: SLD ice is distinct because it forms aft of deicing equipment, making standard protection systems insufficient. Freezing rain and freezing drizzle are SLD indicators.
- Thinking ice crystals cause structural icing: Pure ice crystals in very cold clouds (below -20 °C) generally do not adhere to airframes the way supercooled liquid droplets do. The hazard comes from liquid water that freezes on contact, not from solid ice crystals already formed in the atmosphere.
- Misidentifying the most-reported type: Exam questions sometimes probe whether students know that rime ice is the most frequently reported structural icing type, even though clear ice is generally the most hazardous type.