6.3 Structural & Induction Icing

Structural & Induction Icing

Quick Answer: Structural icing requires visible moisture and an aircraft surface temperature at or below 0°C. Clear ice forms from large supercooled water droplets (SLD) at 0°C to -10°C that flow back before freezing into a heavy, hard, transparent glaze. Rime ice forms from small droplets at -15°C to -20°C that freeze instantly, trapping air to create a brittle, milky deposit. Mixed ice occurs at -10°C to -15°C. Icing decreases lift by up to 30%, increases drag by >100%, and increases stall speed at lower angles of attack. Tailplane stall is triggered by flap extension in icing conditions, causing an uncommanded violent pitch-down; recovery requires immediately retracting flaps and pulling back on the yoke. Carburetor ice can form at OATs from -7°C to +21°C (20°F to 70°F) with relative humidity >80%.

Inadvertent flight into icing conditions is a major cause of fatal instrument flight accidents. According to FAA Advisory Circular AC 91-74B, even a minuscule layer of ice—having the roughness of medium-grit sandpaper—on an aircraft's leading edge can reduce wing lift by up to 30% and increase parasite drag by over 100%.

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Environmental Physics of Structural Icing

Structural icing cannot occur unless two physical criteria are met simultaneously:

1. Visible Moisture: The aircraft must be flying through clouds, fog (with visibility under 1 statute mile), rain, drizzle, or snow.
2. Freezing Temperatures: The outside air temperature (OAT) and aircraft skin temperature must be at or below 0°C (32°F).

+-----------------------------------------------------------------------+
|                   STRUCTURAL ICING TEMPERATURE ZONES                  |
|                                                                       |
|   0°C to -10°C:       CLEAR ICE PREDOMINANT                           |
|                       - Large supercooled droplets (cumuliform, SLD)  |
|                       - Slow freezing, runs back, severe weight/drag  |
|                                                                       |
|   -10°C to -15°C:     MIXED ICE PREDOMINANT                           |
|                       - Combination of large and small droplets       |
|                       - Irregular, rough, high drag profile           |
|                                                                       |
|   -15°C to -20°C:     RIME ICE PREDOMINANT                            |
|                       - Small droplets (stratiform clouds)            |
|                       - Instant freezing, milky white, brittle        |
|                                                                       |
|   Below -20°C:        ICE CRYSTALS PREDOMINANT                        |
|                       - Droplets already frozen; structural icing rare|
|                       - Risk of engine core icing / probe blockage    |
+-----------------------------------------------------------------------+

Supercooled Liquid Water (SLW) and Droplet Dynamics

Water in the free atmosphere does not automatically freeze at 0°C. In the absence of freezing nuclei, pure water droplets remain liquid in a supercooled state down to as low as -40°C. When an aircraft collides with these supercooled droplets, the mechanical impact instantly triggers crystallization.

• Droplet Size: Large droplets contain substantial thermal mass. Upon impact, only a portion of the droplet freezes immediately; the remaining liquid flows aft over the airfoil surface before freezing, forming Clear Ice.
• Supercooled Large Droplets (SLD): Droplets with diameters exceeding 50 micrometers (found in freezing drizzle and freezing rain). SLD droplets splash and run aft beyond the active protection zone of de-icing boots, forming hazardous ridges on unprotected wing surfaces that cause uncommanded roll upsets.
• Airfoil Collection Efficiency: Thin airfoils (such as horizontal stabilizers, struts, and antennas) catch ice much more rapidly and efficiently than thick, blunt airfoils (such as main wings). Consequently, tailplanes accumulate ice faster than wings.

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Structural Ice Types & Physical Characteristics

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Aerodynamic Degradation & Aircraft Handling Hazards

Ice accumulation alters the carefully engineered aerodynamic profile of an aircraft's lifting surfaces:

+-----------------------------------------------------------------------+
|                   AERODYNAMIC EFFECTS OF ICING                        |
|                                                                       |
|   [+] Parasite Drag:          Increases by 100% to 200%+              |
|   [-] Maximum Lift:           Decreases by 30% or more                |
|   [+] Stall Speed:            Increases substantially                 |
|   [-] Critical Angle of Attack: Decreases significantly               |
|   [!] Pre-Stall Buffet:       Often absent or occurs simultaneously   |
|                               with full aerodynamic stall             |
+-----------------------------------------------------------------------+

1. Wing Stall in Icing Conditions

Because ice disrupts airflow over the upper wing surface, boundary layer separation occurs at a much lower angle of attack than normal. The aircraft can enter a full aerodynamic stall at normal cruise airspeeds and during standard pitch attitudes.

2. Roll Upset (Aileron Snatch)

When ice forms aft of the de-icing boots or asymmetrically along the wingtips, airflow separation begins near the outer wing sections where ailerons are located. The asymmetric airflow creates an uncommanded aileron deflection (aileron snatch), rolling the aircraft violently into an inverted attitude. Recovery requires reducing the angle of attack (lowering pitch) and maintaining balanced rudder, rather than abruptly overpowering the ailerons.

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Tailplane Stall (ICTS) vs. Wing Stall

One of the most dangerous phenomena in instrument aviation is the Ice-Contaminated Tailplane Stall (ICTS). Because horizontal stabilizers have thinner airfoils, they collect ice 3 to 6 times faster than main wings.

+-----------------------------------------------------------------------+
|                 WING STALL VS. TAILPLANE STALL DYNAMICS               |
|                                                                       |
|   Feature               Wing Stall             Tailplane Stall        |
|   -----------------------------------------------------------------   |
|   Stall Trigger         High AOA / Slow Speed  FLAP EXTENSION         |
|   Pitch Response        Nose pitches down      VIOLENT PITCH DOWN     |
|   Yoke Force            Buffet / Aft force     LIGHTENS / PULLS FWD   |
|   Flap Action           Retract / Leave        IMMEDIATELY RETRACT    |
|   Control Input         PUSH FORWARD (Nose-Dn) PULL BACK (Nose-Up)    |
+-----------------------------------------------------------------------+

Tailplane Stall Mechanics & Recovery

1. The Downwash Trigger: The horizontal stabilizer generates downward aerodynamic lift to balance the aircraft's center of gravity forward of the center of lift. When the pilot extends wing flaps during approach, the downwash angle behind the wing increases sharply. This downwash dramatically increases the negative angle of attack on the horizontal stabilizer.
2. The Stall: If the horizontal stabilizer leading edge is contaminated with ice, this increased negative AOA causes the tailplane to stall. When the tail loses downward lift, the aircraft's nose pitches downward violently and uncontrollably.
3. Immediate Recovery Action:
   • Retract flaps immediately to the previous setting.
   • Apply firm, immediate aft elevator pressure (pull back on the yoke).
   • Reduce power if nose-down pitch trim was applied with high thrust (follow manufacturer POH guidelines).
   • NEVER push forward on the yoke during a suspected tailplane stall—pushing forward exacerbates the negative tail stall and accelerates the dive into the ground.

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Induction & Pitot-Static Icing Hazards

Icing is not limited to airframe surfaces; it attacks engine induction passages and flight instrument sensors.

+-----------------------------------------------------------------------+
|                   CARBURETOR ICING PROBABILITY MATRIX                 |
|                                                                       |
|   Temperature Range:    -7°C to +21°C (20°F to 70°F)                  |
|                         (Can occur up to 38°C / 100°F in extreme RH)  |
|   Relative Humidity:    Above 80% (Clear air or clouds)               |
|   Cooling Mechanism:    1. Fuel Vaporization (Endothermic reaction)   |
|                         2. Venturi Pressure Drop (Bernoulli effect)   |
|   Total Temp Drop:      Up to 20°C (40°F) inside carburetor throat    |
+-----------------------------------------------------------------------+

1. Carburetor Ice Indications

• Fixed-Pitch Propeller: Unexplained, gradual drop in Engine RPM, followed by engine roughness and eventual failure.
• Constant-Speed Propeller: Unexplained, gradual drop in Manifold Pressure (MP), while RPM remains constant due to the governor.
• Carb Heat Application Sequence: Applying full carburetor heat routes unfiltered warm air from around the exhaust shroud into the carburetor. Expect an immediate initial drop in RPM/MP (warm air is less dense), followed by engine roughness as melting water is ingested, and finally a distinct rise in RPM/MP indicating the ice has cleared.

2. Pitot-Static System Icing Failures

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Ice Protection Systems: Anti-Icing vs. De-Icing

+-----------------------------------------------------------------------+
|                 ANTI-ICING VS. DE-ICING TECHNOLOGIES                  |
|                                                                       |
|   ANTI-ICING (PREVENTION)       DE-ICING (REMOVAL)                    |
|   - Turn ON BEFORE icing        - Activate AFTER ice accumulates      |
|   - Pitot Heat                  - Pneumatic De-Ice Boots              |
|   - Heated Windshield           - Expands with bleed air/vacuum       |
|   - TKS "Weeping Wing" Fluid    - Fractures and sheds ice glaze       |
|   - Thermal Bleed Air Slats     - Cycle per AFM/POH procedures        |
+-----------------------------------------------------------------------+

Operational Rules for Ice Encounters

1. Flight Into Known Icing (FIKI): Flight into forecast or reported icing is strictly illegal unless the aircraft is certified with full FIKI-compliant dual systems under 14 CFR Part 23/25.
2. Immediate Tactical Escape: Non-FIKI aircraft entering icing conditions must take immediate corrective action: disconnect the autopilot (to feel control forces), notify ATC, and execute a 180° turn, climb, or descent to exit the icing layer.
3. Approach Configuration: With structural ice on the airframe, fly the approach with a higher airspeed (add 10–20 knots to $V_{REF}$), use a reduced flap setting (or zero flaps), and avoid abrupt control maneuvers.