Module 11A.12 — Ice and rain protection (ATA 30)

Levels

All of 11.12 is level 1 for A1 and level 3 for B1.1: ice formation, classification and detection; de-icing (electrical, hot air, pneumatic, chemical); anti-icing (electrical, hot air, chemical); probe and drain heating; wipers; rain repellent.

11.12(a) Ice formation, classification and detection

Why ice matters

Ice on wings and tail reduces lift, increases drag and weight, lowers the stalling angle of attack, can jam controls, block pitot/static ports and engine intakes, and shed into engines (FOD). Propeller ice causes vibration and thrust loss.

Conditions

Airframe icing needs visible moisture (cloud, rain, drizzle) and a surface temperature at or below 0 °C. Most severe from about 0 °C to −15 °C (supercooled water droplets). Supercooled large droplets (SLD) — freezing drizzle/rain — can freeze behind protected areas.

Ice types

Type	Formation	Characteristics
Rime ice	Small supercooled droplets freeze instantly on impact (colder temperatures, stratiform cloud)	White, opaque, brittle, rough; traps air; forms on leading edge; easier to remove
Clear (glaze) ice	Large droplets freeze slowly, flowing back before freezing (near 0 °C, cumuliform cloud, freezing rain)	Transparent, hard, heavy, smooth; spreads back over surfaces; hardest to remove
Mixed ice	Combination of rime and clear	Rough, hard
Hoar frost	Water vapour sublimates directly to ice on a cold surface (clear air, aircraft parked overnight or descending from cold altitude into moist air)	White crystalline; must be removed before take-off

Key fact: Clear ice forms from large supercooled droplets near 0 °C and is the most dangerous; rime ice forms from small droplets at lower temperatures.

Ice detection

Method	Principle
Visual	Ice-detector probe/rod or wiper post visible to the crew; wing inspection lights at night
Vibrating probe (magnetostrictive)	Probe vibrates at a resonant frequency (about 40 kHz); ice accretion lowers the frequency → signal; probe is then heated to shed ice and the cycle repeats
Obstruction / rotor type	Serrated rotor turning near a knife edge; ice build-up increases torque → switch
Pressure (differential) type	Small holes in a probe face; ice blocks holes → pressure change
Optical / ultrasonic	Newer sensors

Ice detection systems may give a crew alert ("ICE DETECTED") or automatically switch on anti-ice systems (primary vs advisory systems).

11.12(b) De-icing systems

De-icing removes ice after it has formed (cyclic). Anti-icing prevents ice forming (continuous).

Pneumatic de-icing boots

• Rubber boots bonded to leading edges (wings, tail, struts) with spanwise or chordwise tubes.
• Inflated in sequence by regulated engine bleed air or pump pressure (e.g. about 15–20 psi) to crack the ice; airflow removes it. Deflated and held down by vacuum (ejector) at other times to maintain aerofoil shape.
• Timer/sequencing inflates sections alternately (e.g. tail then wings) to avoid asymmetric shedding.
• Maintenance: clean with mild soap/water; treat with approved conductive coating/protectant; repair with cold patches; check for cracks and ozone deterioration; electrically conductive coating to dissipate static.

Electrical de-icing

• Heater elements bonded to propellers (blade boots), helicopter rotor blades, some intakes; current supplied via slip rings and brushes on propellers; cyclic timers heat elements in sequence to save power.
• Electro-impulse and electro-mechanical expulsion systems on some aircraft.

Hot-air de-icing

Cyclic use of hot air is uncommon; most hot-air systems are anti-icing (below).

Chemical (fluid) de-icing

• TKS / "weeping wing": glycol-based fluid pumped through porous titanium/stainless panels on leading edges; can anti-ice or de-ice; also propeller slinger rings and windshield spray bars. Fluid tank quantity limits endurance.
• Ground de-icing with Type I (heated, de-icing) and Type II/IV (thickened, anti-icing with holdover time) fluids — not an aircraft system but an operational procedure; the clean aircraft concept applies.

Probe and drain heating

• Pitot probes, static ports (on some types), AoA vanes, TAT probes, ice detectors are electrically heated; on the ground heating is reduced or inhibited to avoid overheating (and burns to personnel) — air/ground logic.
• Waste-water drain masts and potable water lines/drains are heated to prevent freezing.
• Monitoring: heater current sensing gives "probe heat fail" warnings.