Aircraft icing is one of the most insidious weather hazards in aviation. Unlike turbulence or low visibility, ice accumulates silently on surfaces the pilot cannot always see, progressively degrading every aerodynamic surface it touches. A thin layer of ice roughly the texture of sandpaper on a leading edge can reduce lift by 30 percent or more and increase drag dramatically, yet it may be nearly invisible in poor lighting. The FAA Aviation Weather Handbook (FAA-H-8083-28B), Chapter 20, provides the authoritative foundation for understanding structural icing — how it forms, what types it takes, and why certain varieties are far more dangerous than others.
This article examines the physics of supercooled water, the three categories of structural ice, the special threat posed by Supercooled Large Drops (SLD), the key meteorological and aircraft factors that govern icing severity, and the specific aerodynamic effects — including the often-misunderstood tailplane stall — that make airframe ice so operationally dangerous.
The Physics of Supercooled Water
Water does not always freeze at 0 °C. Small, pure water droplets suspended in the atmosphere remain liquid at temperatures well below freezing because the surface tension of the droplet inhibits ice-crystal nucleation. This is called supercooling. Pure water suspended in air will not freeze spontaneously until it reaches −40 °C, at which point freezing occurs without the need for any nuclei. This single fact defines both the upper and lower boundaries of the structural icing threat.
The composition of clouds changes significantly with temperature. Between 0 °C and −10 °C, clouds consist mainly of supercooled liquid water droplets and therefore present the highest icing potential. Between −10 °C and −20 °C, liquid droplets coexist with ice crystals, and icing is still a significant concern. Below −20 °C, clouds are generally composed entirely of ice crystals, which slide off airframes rather than freezing to them, sharply reducing icing potential. Strong updrafts inside cumulonimbus clouds are an exception — they can carry supercooled water to extreme altitudes where temperatures approach −40 °C.
Statistically, about half of all icing reports occur between −8 °C and −12 °C, and approximately half occur between 5,000 and 13,000 feet MSL, with the peak near 10,000 feet. These numbers are highly testable and operationally important for flight planning.
Three Types of Structural Ice
Structural ice is any ice that adheres to the outside of the airframe. The three recognized types differ in appearance, density, and aerodynamic impact.
Rime Ice
Rime ice is rough, milky, and opaque. It forms when small supercooled droplets strike the airframe and freeze almost instantaneously. The rapid freezing traps air bubbles within the ice, producing a porous, brittle structure. Because the droplets freeze on contact, rime ice grows directly into the airstream from the leading edge rather than spreading back along the surface. Rime ice favors colder temperatures (generally below −15 °C), lower liquid water content (LWC), and small droplet sizes. It is the most frequently reported icing type. Although its hazard is somewhat less severe than clear ice, rime ice's jagged, rough texture disrupts the smooth airflow over the wing and can significantly degrade lift and increase drag.
Clear (Glaze) Ice
Clear ice, also called glaze ice, is glossy, dense, and either transparent or translucent. It forms when large supercooled droplets strike the airframe and only a small fraction of each droplet freezes immediately. The remaining liquid portion flows or smears back along the surface before gradually freezing. Because air bubbles are not trapped during this slow process, clear ice is denser and harder than rime ice. Clear icing favors temperatures warmer than −10 °C, higher LWC, and larger droplets.
Clear ice is considered the more dangerous type for several reasons. It tends to form horns — curved protrusions near the top and bottom of the leading edge — that generate a large region of disrupted, turbulent airflow far greater than the physical size of the ice itself would suggest. It is difficult to see, especially in flight, so a pilot may not recognize its accumulation quickly. It also spreads aft beyond deicing boot coverage, limiting the effectiveness of protection systems.
Mixed Ice
Mixed ice is exactly what the name implies: a combination of rime and clear ice that accumulates when an aircraft flies through pockets of varying temperature, LWC, and droplet size over short distances. Viewed from the side, mixed ice shows alternating clear and opaque layers. It poses hazards similar to clear ice — horn formation, aft spreading beyond deicing equipment coverage, and significant disruption of airflow — while being even more difficult to remove because its irregular composition resists uniform shedding.
Supercooled Large Drops (SLD): A Special Threat
Supercooled Large Drops (SLD) are a subset of clear icing that deserves its own discussion because of its exceptional danger. SLDs are defined as supercooled water droplets with diameters larger than 40 microns. Freezing drizzle droplets range from 40 to 200 microns; freezing rain droplets exceed 200 microns. These are orders of magnitude larger than typical cloud droplets.
Because of their size and momentum, SLDs do not follow the airstream around an airfoil. They cross streamlines and impact the wing surface well aft of the leading edge — sometimes far beyond the protected area of deicing boots or anti-icing systems. The resulting ice is lumpy, uneven, and textured, resembling frosted bathroom glass. This residual aft ice acts like an aerodynamic spoiler, generating turbulence and flow separation bubbles across a large portion of the airfoil. In extreme cases, these separation bubbles can travel along the wing and inadvertently deflect ailerons, creating dangerously unstable lateral control. Even a small amount of SLD ice on the lower or upper airfoil surface can severely compromise the aircraft's aerodynamic integrity.
SLD conditions are most commonly associated with freezing rain and freezing drizzle. They are particularly hazardous because conventional deicing and anti-icing equipment certified under older standards was not designed with SLD in mind. Pilots encountering SLD should exit the icing environment immediately.
Key Factors Governing Icing Severity
The FAA handbook ranks the meteorological factors driving icing type and severity in the following order of importance:
- Supercooled Liquid Water Content (SLWC): The more liquid water available, the more ice can accumulate. Cumuliform clouds have the highest SLWC; stratiform clouds have the lowest, though most icing encounters involve relatively low SLWC.
- Temperature (altitude): Icing potential peaks in the 0 °C to −20 °C range; below −40 °C, icing cannot occur. The cold-soak effect means an airframe can remain below 0 °C even after descending into above-freezing air, especially when fuel tanks are mounted flush to the airframe skin.
- Droplet size: Smaller droplets freeze near the leading edge; larger drops (including SLDs) spread aft. Size matters most when drops enter SLD territory.
- Airspeed: Higher airspeed increases the rate of droplet impact but also raises airframe skin temperature through aerodynamic heating. Structural icing is generally negligible above approximately 575 knots.
- Aircraft type and design: Wing sweep, airfoil shape, and the location of deicing equipment all affect vulnerability. Pilots must know the specific limitations of their aircraft.
Aerodynamic Effects: Performance Loss, Control Degradation, and Tailplane Stall
Structural ice degrades aircraft performance through two fundamental mechanisms: reduced lift and increased drag. The rough or irregular surface created by ice disrupts the laminar boundary layer, causing early flow separation over the wing. This lowers the critical angle of attack (the wing stalls at a lower angle), reduces maximum lift coefficient, and can dramatically alter stall characteristics — the stall may occur more abruptly and at a higher airspeed than placarded. Drag increases substantially, reducing climb performance and increasing fuel burn.
Control effectiveness is also compromised. Ice on control surfaces adds weight and can restrict range of motion. More subtly, ice on the wing ahead of ailerons can change local airflow and make the aircraft roll unpredictably.
Perhaps the most dangerous and least understood icing effect is the tailplane (horizontal stabilizer) stall. The horizontal stabilizer typically operates at a negative angle of attack to provide the downward tail force that keeps the nose up. Because the tailplane has a smaller chord than the main wing, it accumulates ice faster and is more sensitive to leading-edge contamination. If ice causes the tailplane to stall, the tail loses its downward force — or produces an upload — and the nose pitches uncontrollably downward. This is the opposite of a main-wing stall and responds to opposite inputs: reducing flaps and increasing speed may relieve a tailplane stall, whereas applying back pressure (which would recover a wing stall) can make a tailplane stall worse. Tailplane stall is most likely to occur or worsen when flaps are extended, because flap deployment increases the negative angle of attack on the tailplane.
Why It Matters for Operations
Icing affects every phase of flight. On the ground, cold-soaked fuel tanks can cause frost or ice to form even when ambient temperatures are slightly above 0 °C. In cruise, encounter with an unexpected icing layer can accumulate ice faster than equipment can shed it. On approach, extending flaps in iced conditions can precipitate a tailplane stall. Understanding the meteorological conditions that favor icing — particularly the temperature, altitude, and cloud-type signatures — allows pilots to anticipate, avoid, or exit icing environments before accumulation becomes critical.
Key Numbers and Rules
- Supercooled water can exist down to −40 °C; below this, spontaneous freezing occurs regardless of nuclei.
- Most icing occurs between 0 °C and −20 °C; peak reports occur between −8 °C and −12 °C.
- Roughly half of icing reports occur between 5,000 and 13,000 feet MSL, peaking near 10,000 feet.
- Rime ice favors temperatures colder than −15 °C; clear ice favors temperatures warmer than −10 °C; mixed ice forms in between.
- SLDs are droplets larger than 40 microns (freezing drizzle: 40–200 µm; freezing rain: >200 µm).
- Airframe icing is generally negligible above approximately 575 knots due to aerodynamic heating.
- Tailplane stall recovery: reduce flap deflection and increase speed — opposite of wing stall recovery.
Common Test Traps
- Confusing rime and clear ice severity: Students often assume rime is more dangerous because it's rough. Clear ice is generally more hazardous — it forms horns, spreads aft, is harder to see, and is more difficult to shed.
- Assuming cold = more icing: Very cold temperatures (below −20 °C) actually reduce icing because clouds consist primarily of ice crystals, not supercooled liquid water. The worst icing is in the moderate temperature range.
- Forgetting the cold-soak effect: An airframe can ice up on the ground or in above-freezing air if it has been cold-soaked — especially aircraft with flush-mounted fuel tanks.
- Tailplane stall inputs: Applying back pressure when the tailplane stalls worsens the pitch-down; the correct recovery involves reducing flaps and increasing airspeed, not pulling back.
- SLD equipment limitation: Deicing boots and anti-icing systems may not protect against SLD because SLD ice forms aft, beyond the protected area — activating boots gives a false sense of security in freezing rain.