11.12 Ice and Rain Protection

Ice Formation and Types

• Ice formation occurs rapidly on aircraft surfaces when there's visible moisture, even at altitudes with freezing temperatures.
• Water droplets can remain liquid below freezing until disturbed, at which point they freeze instantly on aircraft surfaces.
• Smooth aerofoil surfaces, free of ice, snow, or frost, are crucial for maintaining lift and preventing stall.

Clear Ice

• Formed by larger supercooled water droplets that freeze gradually.
• Results in a smooth, hard, glossy, and transparent ice layer.
• It's hard, heavy, clings strongly, and is difficult to remove.

Rime Ice

• Formed by small supercooled water droplets that freeze rapidly.
• Trapped air gives it a white appearance.
• Lighter than clear ice, but its irregular shape reduces lift and increases drag.
• It is brittle and easier to remove than clear ice.

Mixed Ice

• A combination of clear and rime ice, forming a rough texture.
• Occurs when water droplets vary in size or mix with snow or ice particles.
• Likely to form when there is visible moisture in the air and the temperature is near or below freezing.

Icing Effects on Aircraft

• Hinders lift and increases drag.
• Causes vibration and false instrument readings.
• Impacts manoeuvrability, radio reception, and engine performance.
• Extra weight and imbalance from ice make control difficult.
• Ice breaking off can cause engine failures and structural damage from Foreign Object Damage (FOD), especially to fuselage aft-mounted engines.
• Aerodynamic drag increases fuel consumption, reducing aircraft range, and hampers the rate of climb

Aircraft Ice Protection Components

• Wing leading edges
• Horizontal and vertical stabilizer leading edges
• Engine cowl leading edges
• Propellers and spinner
• Air data probes
• Flight deck windows
• Water and waste system lines and drains
• Antennas

Ice Detection Systems

• Visual detection
• Hot rod detector
• Accretion or pressure detector
• Rotary ice detector
• Vibrating ice detector

Visual Detection

• Wing Inspection Lights: Located near the fuselage-wing fairing to view ice accretion on wing leading edges during night or low light.
• Visual Ice Indicators: Small posts mounted between windshields, collecting ice to indicate icing conditions.

Hot Rod Detector

• Mounted outside the cockpit for the flight crew to observe, ice builds up on the leading edge of the probe alerting the pilot to icing conditions
• Consists of an aluminum alloy oblong base (called the plinth), on which is mounted a steel tube detector mast of aerofoil section, angled back to approximately 30° from the vertical.
• Includes a light for night use and a heater element for de-icing during observation.

Pressure Operated Ice Detector

• Hollow tube with holes that senses pressure changes due to ice blockage.
• Triggers a warning and automatic anti-icing systems.
• Includes a heating element with a timer to melt ice from the probe.

Rotary Ice Detector

• A serrated rotor is driven by a motor next to a fixed knife-edge cutter
• Ice buildup increases torque, causing the motor to move and trigger a microswitch, activating a warning or anti-icing sequence.

Vibrating Probe Ice Detector

• A probe vibrates at 40 kHz and accumulation of ice causes it to decrease.
• Frequency change triggers a warning or activates ice protection systems.
• An internal heater cycles on and off to clear ice accumulation.

Element Ice Sensing Unit (Sangamo Weston Ice Detector)

• Detects icing conditions by sensing moisture and freezing temperatures using separate components.
• A moisture detector senses temperature differences between "wet" and "dry" bulbs.
• A thermal switch prevents ice warnings above freezing temperatures.

Beta Particle Ice Detection Probe (Sunstrand System)

• Emits beta particles from a Strontium 90 source across a sensing surface.
• Ice accretion reduces detected particles, triggering a warning and activating a heater to remove ice.

Icing Condition Indications

• Ice detectors provide information to the flight crew and ice protection systems.
• Certification types include Primary Automatic, Primary Manual, and Advisory.
• Primary Automatic: Automatically activates ice protection systems.
• Primary Manual: Alerts the crew to manually activate ice protection.
• Advisory: Provides a backup alert, requiring proactive crew activation.

Icing Condition Displays and Warnings

• Ice Detection Alerts: Visual and/or audio alerts notify of icing hazards.
• Warning Lights: Dedicated lights on the instrument panel indicate icing (e.g., "ICE," "ICE DETECTED," “ICING”).
• Master Caution/Warning System: Activates a master caution light to prompt crew action.
• EICAS Messages: Displays icing-related messages on the Enhanced Indication and Crew Alerting System.
• Multi-Function Display (MFD): Shows graphical representations of icing zones, temperature data, and system statuses.
• Audio Alerts: Warning sounds or voice messages provide additional notification.

Anti-icing vs. De-icing

• Anti-icing: Prevents ice formation by keeping surfaces dry, heating them, or preventing freezing.
• De-icing: Removes ice after it has accumulated.

Anti-icing Systems

• Thermal pneumatic anti-icing: Common for large surfaces like wing leading edges, using engine bleed air.
• Thermal electric anti-icing: Used on probes, drains, tanks, and windshields, using electrical elements.
• Chemical anti-icing: Used on smaller aircraft, often automatically operated.

Thermal Pneumatic Anti-icing

• Warm air enters ducts, traveling along the leading edge, and exits through holes.
• Wing Anti-ice (WAI) systems route hot air from engine compressors to wing leading edges through ducts and valves.
• Ducting made from aluminum alloy, titanium, stainless steel, or molded fiberglass is used with expansion bellows.

Thermal Pneumatic Anti-icing - Slat Leading Edges

• If the aircraft wing is fitted with LE slats, these are the only component susceptible to thermal pneumatic WAI air.
• The WAI ducts move hot air from the pneumatic system, through the wing LE, to the LE slats.
• The ducting warms the cavities through which it is routed.

Thermal Pneumatic Anti-icing - WAI Valve

• Electrically controlled and pneumatically actuated valve controlling bleed air flow to WAI ducts.
• Without power, air pressure keeps the valve closed.

Thermal Pneumatic Anti-icing - WAI Pressure Sensor

• The WAI pressure sensor detects the air pressure in the WAI duct after the WAI valve opens.

Thermal Pneumatic Anti-icing - WAI System Control

• ACIPS computer card controls WAI valves based on bleed air temperature and altitude.
• Operational mode inhibited under certain conditions (e.g., takeoff mode, TAT > 10 °C, engine starting).

Thermal Pneumatic Anti-icing - WAI Indication

• The flight crew can monitor the WAI system on the onboard computer maintenance page.
• The following information is shown:
  • WING MANIFOLD PRESS
  • VALVE
  • AIR PRESS
  • AIR FLOW

Thermal Pneumatic Anti-icing - WAI BITE Test

• BITE circuits monitor the system, providing status and maintenance messages for faults, performs automatic power-up and periodic tests

Thermal Pneumatic Anti-icing - Engine Anti-ice (EAI)

• Preventing ice build-up on the engine inlet cowl.
• Can be activated automatically by the ice detection system, which may activate before WAI
• Air is ducted from the engine to the leading edge of the engine cowling; the air then exists the cowl through overboard vents

Electrical Windshield Anti-ice

• Windshields made of laminated glass, polycarbonate, or similar ply material, with clear vinyl plies.
• Heating elements, either resistance wires or transparent conductive material, are included in the glass layers.
• Can have two heat levels: NORMAL and HIGH.

Chemical Anti-icing

• Used on wings, stabilizers, windshields, and propellers, known as weeping wing systems or TKSTM systems.
• Antifreeze solution is pumped through a mesh screen, preventing ice formation.
• Can also de-ice by breaking the bond between ice and the airframe.
• A disadvantage is limited TKS fluid.

De-icing Systems

• Removes ice that has accumulated on aircraft surfaces.
• Used more in smaller turbine-powered and reciprocating aircraft, or in combination with anti-ice.

Pneumatic De-ice Boots

• Inflatable boots on wing and stabilizer leading edges expand with pneumatic pressure, breaking away ice.
• Boots are inflated for six to eight seconds and deflated, and held tightly against the leading edge using vacuum suction
• Outer ply tends to be conductive neoprene

Pneumatic De-ice Boots - Sources of Operating Air

• Reciprocating engine aircraft use a dedicated engine-driven air pump.
• Suction side holds boots tight; pressure side inflates boots.
• Turbine engine aircraft use bleed air from the engine compressor.
  • Valves, controlled by cockpit switches, deliver air to the boots on request

Pneumatic De-ice Boots - Reciprocating System Operation

• De-energized components direct discharge air overboard, the deflate valve is open
• De-ice boots are connected to the suction side of the pump through a check valve manifold and the vacuum regulator.
• The flight crew must manually start this inflation/deflation cycle

Pneumatic De-ice Boots - Turboprop Operation

• Engine bleed air is used as a pneumatic air source for the inboard wing boots, outboard wing boots, and horizontal stabilizer boots
• Wing boots inflate with a 6 second inflation period, electronic timer deflate the wing boots and horizontal stabilizer boots inflate for four seconds

Pneumatic De-ice Boots - Maintenance

• Monitor the following:
  • Do not drag petrol hoses over the de-icers
  • Keep de-icers free of petrol, oil, grease, dirt, and other deteriorating substances
  • Do not lay tools on or lean maintenance equipment against the de-icers
  • Promptly repair or resurface the de-icers when abrasion or deterioration is noted
  • Wrap de-ice boots in paper or canvas when storing

Electric De-ice Boots

• Used on wing sections or horizontal stabilizers.
• Contain electric heating elements, controlled by a sequence timer.
• Operation is initiated by ice detector and temperature probe inputs.

Chemical De-ice Systems

• Used on small aircraft and windshields.
• Liquid chemical is sprayed through a nozzle, preventing ice formation or de-icing after formation.

Chemical De-ice Systems - Propeller De-ice (Chemical)

• Ice formation on the propeller blade, in effect, distorts the blade aerofoil section, compromising propeller efficiency, it usually collects asymmetrically on a propeller blade
• System can be stand-alone or form part of a chemical wing and stabiliser de-icing system
• A tank stores anti-icing fluid; this fluid is forced towards each propeller by a pump

Chemical De-ice Systems - Propeller De-ice (Electrical)

• An electric propeller icing control system consists of an electrical energy source, a resistance heating element, system controls, and necessary wiring
• Power from the aircraft system is transferred to the propeller hub through electrical leads which terminate in slip rings and brushes
• Cycling timers energise heating element circuits

Rain Control Systems

• Chemical rain repellent
• Pneumatic rain removal (jet blast)
• Coating windshields with a hydrophobic surface seal

Rain Control Systems - Chemical Rain Repellent

• The ‘RainBoe’ rain repellent fluid used on most aircraft contains CFC 113 – a type of freon (chlorofluorocarbon)
• When glass is treated with certain chemicals, it forms a transparent film that changes the way water behaves on contact
• The water is easy to remove from the glass
• A rain repellent system uses a switch or push button in the flight deck to apply chemical repellent but should be only used in wet conditions

Rain Control Systems - Windshield Surface Coating

• A hydrophobic coating is used on the external surface of the windshield
• This coating causes raindrops to bead up and roll off, allowing clear visibility through the windshield with minimal distortion

Rain Control Systems - Pneumatic Rain Removal Systems

• The rain removal system shown below controls icing and removes rain by directing a flow of heated air over the windshield with an electric blower or by bleed air
• It breaks the raindrops into small particles to be blown away
• It heats the windshield to stop the moisture from freezing

Probe and Drain Heating

• Electrical power is used to heat various aircraft components to prevent ice from forming.
• Effective thermal electric anti-ice systems are used on most air data probes, such as pitot tubes, static air ports, Total Air Temperature (TAT) and Angle of Attack (AOA) probes, ice detectors, and engine P2/T2 sensors.

Probe and Drain Heating - Probe Anti-ice

• In thermal electric anti-ice systems, the current flows through an integral conductive element that generates heat. The temperature of the system is kept above the freezing point of water so ice cannot form.
• The flight status condition of the aircraft is considered before thermal electrical heaters are activated automatically.

Probe and Drain Heating - Water and Drain Anti-ice

• Water and wastewater tanks use electrical heaters to prevent ice from forming:
  • Water lines that carry water from the potable water tanks to the lavatories and galleys
  • Waste drain lines, waste tank rinse fittings, and drain masts

Wiper Systems

• An aircraft’s windshield rain protection system generally consists of two electrically operated wipers, one on the Captain’s side and one on the First Officer’s side.
• These wipers can be operated independently and at low or high speed, depending on the level of the precipitation.

Wiper Systems - Operation and Maintenance

• Almost all windshield wiper systems use electrical motors but older aircraft types might use hydraulic motors.
• The flight crew’s windshield wipers are operated by separate systems in order to maintain clear vision
• Before performing any maintenance on the windshield wiper system, the window heat circuit breakers must be open. If these circuit breakers are closed, there is a risk of electrical shock when touching the window.