7.2 Structural & Induction Icing

7.2 Structural & Induction Icing

Quick Summary: In-flight structural icing is one of the deadliest meteorological hazards confronting air carrier operations. It requires two concurrent physical conditions: visible moisture (clouds, fog, rain) and an airframe surface temperature at or below 0°C. Ice accretion occurs primarily between 0°C and -20°C, with maximum severity concentrated between 0°C and -10°C. Ice is classified into Clear (large droplets, slow freezing, transparent, dense, most hazardous), Rime (small droplets, instantaneous freezing, milky-white, granular), and Mixed. Supercooled Large Droplets (SLD) present in freezing rain (FZRA) and freezing drizzle (FZDZ) flow aft beyond heated leading edges and deice boots, forming spanwise ridges that induce uncommanded roll upsets. Structural ice can reduce maximum lift by up to 30%, increase drag by over 40%, and elevate stall speeds. An Ice Contaminated Tailplane Stall (ICTS) induces a violent, uncommanded nose-down pitch upon flap extension, requiring immediate flap retraction and aft control column pressure—the exact opposite of a wing stall recovery.

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1. Physics of Structural Icing

Structural icing does not occur simply because the outside air temperature is cold. It requires the physical impingement of liquid water droplets onto an airframe whose skin temperature is at or below freezing:

Supercooled Liquid Water (SLW)

Pure water in the free atmosphere does not freeze automatically at 0°C. For liquid water to transition into ice, it requires microscopic freezing nuclei (dust particles, mineral grains, or bacteria). In the clean upper atmosphere, freezing nuclei are scarce. Consequently, liquid water droplets remain in a metastable, liquid state at temperatures far below freezing—down to -40°C (the homogeneous nucleation point). These droplets are known as Supercooled Liquid Water (SLW). When an aircraft strikes an SLW droplet, the mechanical impact provides the necessary shock to initiate instantaneous crystallization.

Total Air Temperature (TAT) vs. Static Air Temperature (SAT)

At high transport jet cruise speeds, aerodynamic compression of air molecules against the airframe produces ram rise, heating the leading edges:

$\text{TAT} = \text{SAT} \times \left(1 + 0.2 \times K \times M^2\right)$

(where M is Mach number and K is the recovery factor, roughly 0.85–1.0)

At Mach 0.80, ram rise warms the leading edges by approximately +25°C to +30°C above the ambient Static Air Temperature (SAT). Therefore, a jet cruising at FL 350 where SAT is -45°C experiences a TAT of roughly -18°C. Structural icing can occur only when Total Air Temperature (TAT) is at or below 0°C.

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2. Temperature Regimes & Ice Classifications

+-----------------------------------------------------------------------------------------+
|                       STRUCTURAL ICING CLASSIFICATION & CHARACTERISTICS                 |
+-------------+----------------+---------------+-------------------+----------------------+
| Ice Type    | Temp Range     | Droplet Size  | Physical Structure| Aerodynamic Penalty  |
+-------------+----------------+---------------+-------------------+----------------------+
| Clear       | 0°C to -10°C   | Large SLW     | Transparent glaze;| Severe: Disrupts     |
| (Glaze)     |                | droplets      | smooth flow aft;  | airflow, heavy weight|
|             |                | (cumuliform)  | strong adhesion   | high drag, hard break|
+-------------+----------------+---------------+-------------------+----------------------+
| Mixed       | -10°C to -15°C | Mixed droplet | Rough, irregular, | High: Very rough     |
|             |                | sizes         | milky and clear   | surface texture      |
|             |                |               | layers; mushroom  | induces early stall  |
+-------------+----------------+---------------+-------------------+----------------------+
| Rime        | -15°C to -20°C | Small SLW     | Milky-white,      | Moderate: Changes    |
|             |                | droplets      | opaque, brittle,  | airfoil camber;      |
|             |                | (stratiform)  | traps air bubbles | easily deiced        |
+-------------+----------------+---------------+-------------------+----------------------+
| Glaciated   | Below -20°C    | Ice crystals  | Dry crystals;     | Low structural risk; |
| (Crystals)  | (to -40°C)     | (no liquid)   | bounce off skin   | core engine ice (ICI)|
+-------------+----------------+---------------+-------------------+----------------------+

1. Clear Ice (Glaze Ice)

• Environmental Occurrence: Temperatures between 0°C and -10°C in areas of high liquid water content (LWC) and large droplet sizes—typically in convective clouds (cumulus, cumulonimbus) or in freezing rain (FZRA).
• Physical Process: When a large droplet strikes the leading edge, only a fraction of the droplet freezes instantaneously. The release of latent heat of fusion (80 calories per gram) warms the remaining liquid, retarding complete freezing. The unfrozen liquid flows backward along the chord of the airfoil before freezing solid.
• Characteristics: Dense, transparent, glass-like coating. It forms horn-like projections along the leading edge that severely destroy laminar flow. It adheres tenaciously to the airframe, adds immense weight, and is exceptionally difficult for deicing boots to shed.

2. Rime Ice

• Environmental Occurrence: Temperatures between -15°C and -20°C (and down to -30°C in stratiform clouds) with low liquid water content and small droplet sizes.
• Physical Process: Because the ambient air is cold and droplets are small, the latent heat of fusion is rapidly dissipated. The entire droplet freezes instantaneously upon impact, trapping tiny air bubbles within the ice matrix.
• Characteristics: Milky-white, opaque, brittle, granular appearance with a rough, sandpaper-like surface. It accretes primarily along the stagnation line of leading edges, antennas, and probes. Because of trapped air, it has low density and is relatively brittle, making it easier to fracture and remove using pneumatic deicing boots.

3. Mixed Ice

Forms at intermediate temperatures (-10°C to -15°C) where droplets of varying sizes collide simultaneously. Clear and rime ice intermingle, forming an irregular, mushroom-shaped accretion along leading edges that combines the tenacious adhesion of clear ice with the extreme surface roughness of rime ice.

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3. Supercooled Large Droplets (SLD) & Runback Icing

Supercooled Large Droplets (SLD) are droplets of liquid water with diameters greater than 50 micrometers. SLDs occur in two distinct operational regimes:

1. Freezing Drizzle (FZDZ): Droplet diameters between 100 and 500 micrometers.
2. Freezing Rain (FZRA): Droplet diameters greater than 500 micrometers (up to several millimeters).

                 SUPERCOOLED LARGE DROPLET (SLD) RUNBACK DANGER

          Protected Heated Slat / Boot         Unprotected Upper Wing Surface
         /                              /                                    \
    ===[  Deice Boot Area  ]===========[ Aft Ice Ridge Accretion ]============
         \                              \                                    /
          Droplets strike & flow aft      Disrupts airflow over ailerons;
                                          Induces uncommanded roll upset!

The Fatal Physics of SLD

Conventional aircraft ice protection systems (pneumatic boots and thermal bleed air slats) are certified under 14 CFR Part 25 Appendix C, which historically only accounted for droplet sizes up to 50 microns. SLD droplets possess substantial mass and kinetic momentum.

When an SLD droplet strikes an airfoil:

1. Instead of freezing on the protected leading-edge boot or heated slat, the droplet splashes and flows chordwise aft beyond the protected surface.
2. The water freezes on the unprotected upper and lower surfaces of the wing, forming a spanwise ridge of ice immediately ahead of the ailerons.
3. This aft ridge creates severe boundary layer separation, generating an aerodynamic suction peak that violently deflects the ailerons to full deflection—known as aileron snatch—causing an unrecoverable roll upset (the root cause of the 1994 ATR 72 crash at Roselawn, Indiana).

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4. Aerodynamic Penalties & Handling Hazards

Even imperceptible amounts of structural ice cause catastrophic aerodynamic degradation:

1. Lift Reduction: Maximum coefficient of lift ($C_{L,\max}$) decreases by up to 30%. Ice roughness trips the laminar boundary layer into turbulent flow, triggering premature boundary layer separation at lower angles of attack.
2. Drag Increase: Total parasitic drag increases by 40% to 100% or more. Cruising aircraft may find engine thrust insufficient to maintain altitude, resulting in an uncommanded drift-down.
3. Stall Speed Elevation: Stall speed ($V_s$) increases by 10 to 20 knots or more. An aircraft on approach flying at normal clean reference speeds may suddenly stall without warning.
4. Critical Angle of Attack Reduction: The wing's stall angle of attack drops precipitously from normal values of 16°–18° down to 8°–10°. Conventional stall warning vanes and stick shakers, calibrated for clean wings, may fail to trigger before the aircraft stalls.

Wing Stall vs. Ice Contaminated Tailplane Stall (ICTS)

[!WARNING] Critical ADX Exam Distinction: In an Ice Contaminated Tailplane Stall (ICTS), extending flaps directs high-speed downward air (downwash) onto the horizontal stabilizer, pushing the tail beyond its negative stall angle of attack. The tail loses downward lift, causing the nose to pitch violently down. Applying classic wing stall recovery (pushing forward) will accelerate the dive straight into the ground! The flight crew must retract flaps immediately and pull back firmly on the control column.