Aircraft Ice and Rain Protection Systems

Ice and Rain Protection Overview

• Ice and rain protection systems are crucial for maintaining aircraft aerodynamic efficiency and ensuring clear visibility.
• Aircraft are equipped with ice and rain protection on critical surfaces.
• These surfaces include wing leading edges, stabilizers, engine cowls, probes, windows, and waste lines for ice protection.
• Windshield wipers and rain repellent systems are used to maintain clear visibility in rain.

Ice Formation

• Ice forms when water droplets at sub-zero temperatures lose latent heat and freeze upon contact with the aircraft's metal surfaces.
• Icing can occur at 0°C or colder in the presence of liquid water, influenced by cloud type, temperature, and altitude.
• The most severe icing conditions occur near the freezing level in heavy stratified clouds or rain, up to 16,000 ft ASL.
• Catastrophic icing accidents often happen in holding patterns, between 12,000 and 16,000 ft ASL in clouds, icing is rare above this altitude.
• Cumuliform clouds with strong updrafts can carry large water droplets to higher altitudes, enabling structural icing up to 30,000 ft ASL.
• Freezing level distortion in cumuliform clouds increases the potential for severe icing at almost any level.

Types of Ice

• Ice formation is classified into three types based on formation and appearance: glaze, rime, and mixed ice.
• Glaze ice is clear, adheres strongly, and forms when supercooled droplets freeze gradually, accumulating on forward-facing surfaces and is hard to remove
• Rime ice is white, brittle, and rough, formed by rapid freezing of small supercooled droplets, trapping air and is easier to remove than clear ice.
• Mixed ice, a combination of clear and rime ice, is the most common type encountered.
• Pure rime ice is typically found in high altostratus or altocumulus clouds, while pure clear ice is confined to freezing rain.
• Ice can form when there is visible moisture near or below freezing temperatures; carburettor ice is an exception as it can form in warm conditions.

Dangers of Ice Formation on Aircraft

• Ice formation can increase aircraft weight, disturb airflow, and reduce lift.
• It lowers propeller efficiency and cause imbalance and vibration.
• It can restrict airflow to jet engines and cause power loss or overheating.
• Ice breaking off can damage compressor blades
• Blocked ram air intakes and failed systems due to ice on masts and probes can also occur. Ice and rain can obscure vision.

Ice Detection Methods

• Ice detection is primarily visual or electrical.
• Visual ice detection is common on smaller and older aircraft, using leading edge lights and black witness marks.
• Electrical ice detection uses pressure, ice shave, or frequency-based sensors.

Pressure Probe Ice Detection

• A probe detects ice formation by measuring air pressure changes through small inlet holes.
• Ice restricts small holes, causing a diaphragm to move and close contacts.
• Closing the contacts activates a warning light and a heater to melt the ice.
• Once the ice melts, the diaphragm returns to its normal position, deactivating the warning and heater.

Ice Shave System Operation

• Air passes over a rotor driven by an electric motor, and ice formation triggers a warning signal.
• If ice forms, a blade shaves it off, increasing motor torque which activates a micro-switch and turns on a warning light.
• Alternatively, a current meter senses increased current draw as the rotor slows due to ice, activating a warning light above a preset level.
• An ice shave system automatically controls the ice protection system.

Frequency Probe System Operation

• A probe mounted on the aircraft has a natural resonant frequency in the ultrasonic range, induced by an oscillator.
• Ice formation reduces the probe's resonant frequency, triggering electronic circuitry to activate a warning light and a probe heater.
• The probe heater melts the ice within 5 seconds but if it reforms within 55 seconds, the heater reactivates, maintaining the warning light.

Ice Control Systems

• Systems categorized into de-ice systems that remove ice after formation and anti-ice systems that prevent ice formation.

De-icer Boots

• These are common on piston and turbine-powered propeller aircraft, consisting of rubber boots on airfoil leading edges.
• Inflatable boots are constructed with separate air passages or chambers to inflate some portions while deflating others.
• During operation tubes inflate by 18 psi for 6 seconds and then reconnect to the low-pressure line.
• Alternate tubes inflate making them protrude to break the ice which is blown away by airflow.
• A timer/distributor valve directs high-pressure (18 psi) air to de-ice boots in a sequence for aircraft stability during inflation and deflation.
• A ‘Cold’ sequence inflates the boots for 6 seconds and deflates them for 54, while a ‘Warm’ sequence inflates for 6 seconds and deflates for 2 minutes and 54 seconds.

Thermal Pneumatic Systems

• Turbine-engine aircraft use warm compressor bleed air for anti-icing and thermal ice control
• Smaller turbines use bleed air to inflate pneumatic de-icing boots.
• Systems have a pressure regulator to lower pressure, and a venturi to create suction when boots aren't inflated, to hold them deflated against the leading edges.
• Hot air is directed from the engine compressor between an aerofoil's outer and inner skins before being exhausted overboard.
• They tap hot air from the compressor, mixing it with ambient air for anti-icing and de-icing which then flows through passages near the leading edge skin
• Each shutoff valve is pneumatically actuated and electrically controlled. When the wing reaches 185°F, a thermal switch closes the shutoff valve.
• A test plug on most systems permits ground checks without running engines, however, external air pressure must not exceed test limits
• Air for airframe de-icing/anti-icing vents via small holes under the wing; other areas exhaust into the airframe's non-pressurized areas.

Chemical De-icing

• While less common on modern aircraft, chemical de-icing effectively handles all de-icing needs on slower aircraft.
• The de-icing fluid is a mix of isopropyl alcohol and ethylene glycol, emulsifying with water to lower its freezing temperature.
• The system's electric motor powers a pump, delivering fluid from a storage tank to various areas.
• Uses a rheostat (variable resistor) is to adjust pump motor speed, controlling the de-icing fluid flow rate.

Chemical De-Icing Fluid Distribution

• Fluid is sprayed over the windshield for de-icing.
• Fluid is sprayed into the carburettor air intake for de-icing.
• Fluid is sprayed out along the blades for de-icing.
• De-icing liquid slowly releases through a porous boot fixed to the leading edges for wing and empennage de-icing.

Propeller De-Icing – Fluid System

• Anti-icing system consists of a control unit, fluid tank, and a pump.
• A rheostat controls pump output. The fluid is pumped to a nozzle behind the propeller on the engine nose. It then passes through a slinger ring. Once the fluid is in the slinger ring, it is forced through a delivery tube on each blade shank through the centrifugal force.

Electric Propeller Ice Control

• Electrical heating is the preferred method of ice control for propellers.
• Rubber boots with embedded heater wires are bonded to propeller leading edges
• Current heats the rubber, melting ice for centrifugal force and airflow removal.
• Electric elements are set in insulating material on propeller leading edges and spinners for de-icing.
• Piston engines have no air intake de-icing, some do not have propeller de-icing.

Timer Unit Function

• Controls the sequence of current to each of the heater mats
• The sequence of heating is important for loosening ice, for centrifugal force to carry it away.
• Ensures same portion of each blade is heated at the same time.
• If heat is applied for too long, delamination may occur, causing damage.

Load Meter Function

• The meter is an ammeter which monitors the operation of the system and assures the pilot that each heater element is drawing the needed current.
• The system illustrated in Figure 15 would typically use a 28 V DC power supply.
• Each Propeller has two heater elements. Those elements are connected to the power supply
• It is connected via slip rings and brush box assemblies. timer unit is common to all propellers
• When the system is on, power is supplied through the circuit breaker and ammeter to the timer unit.
• The timer then supplies the power to the heater via slip rings; power is delivered to the heater element. The current is then passed back to the slip rings.

Engine Air Inlet Ice Control Systems

• Ice buildup disrupts airflow thus reducing efficiency. Broken ice can cause serious damage to compressor blade
• Turbine inlet ducts are typically equipped with some form of anti-icing.

Engine Air Inlet Ice Control Systems – Hot Bleed Air

• When the engine anti-icing system is activated, a bleed air valve directs hot air to the inlet duct leading edge, nose dome, and inlet guide vanes to prevent ice from forming.
• An indicator light shows in the cockpit confirms the anti-icing that blue is used. Vents in the engine cowl vent air onto the airflow.
• Power loss is a disadvantage with bleed air taken from a turbine engine.
• Taking bleed air is limiting on smaller turbine engines, but not typically a problem on modern turbofans.

Engine Anti-Ice System Operation

• When anti-ice is selected to ‘on’, the motor in the engine anti-icing valve moves to the ‘open’ position.
• Bimetallic spring coil controls the amount of air flow to the engine anti-icing system.
• Pneumatic air flows over the bimetallic strip which increases temperature to a predetermined level with the strip expands and reduces airflow
• This controls airflow/temperature to the surfaces where de-icing is occurring which can cause damage if not.
• After this, it's divided/distributed to spinner and nose cowl anti-ice systems. Engine removes air as per venting holes.

Engine Air Inlet Ice Control Systems - Electric Heating

• Engine power loss may be alleviated by the use of an electric engine air inlet anti-icing system
• Electric heating elements surround the engine air intake.
• This system is only used on turboprop aircraft and not used by high bypass turbofans, with the excessive current required.
• Systems provide heating options for various icing conditions, but continuous use of the high setting in low to medium icing conditions can cause damage to the heater elements.

Electric Windshield Anti-Icing

• Windshields are heated to keep clear visibility in foggy/icing conditions + protects against bird strikes.
• Systems often have a gold film built in with 28V DC through the wire element and thermistor which controls the DC current to be 45°C.
• 3-phase, 200V AC powers and is frequency-wild so The construction for large aircraft is shown in Figure 19 is the standard.
• If there are wire elements, the thermistor is going to embedded in the windshield during manufacture.

Windshield Temperature Control System

• A 3-phase 200 V AC power source connects to an auto transformer, which has two output voltages that are higher than the input.
• The windshield control switch selects either LOW or HIGH heat with elements on.
• Selecting LOW heat, the controller makes a flow path for RL1 if the temperature is below 45°C
• This energises and connects the voltage to the windshield elements. Then, the thermistor will break the flow, by de-energising the power in the circuits
• If there is ice when LOW is selected, then HIGH must be selected so it connects with RL2. Still must be 45°C. If that doesn't happen or overheat the thermistor will break the relays, which would stop the current flow through the circuits.

Magnetic Indicators And Windshield Strength

• A magnetic indicator will show when the system is normal, overheat, and off.
• The aircraft withstand forces up to 1,000 km/hr and 2kg object, and to do so it helps if the heating system is in low. Leaving the windshield heat selected to low is helpful due to increase the resistance.
• Increase the windshield Service Life
• Better Dissipation of Static Electricity

Probe Heaters

• Aircraft approved for flight into known icing conditions must be appropriately equipped.
• Pitot and static probe heaters are provided to eliminate ice formations which would affect the airflow into the tubes, or completely block the openings.

How Pitot Static Probes Work

• Elements electrically heat and prevent ice during flight
• There are probes with multiple systems that are heater independently. -DO NOT TOUCH THE PROBE AFTER A FLIGHT. WILL CAUSE SEVERE BURNS
• Don't operate electrically heated probes on the ground, except for maintenance tests as they can lead to the heated area being damaged
• Always have cotton or canvas fitted over the heated probes as heated ones will burn away. Vinyl/Synthetic materials can stick to the probes which possibly occludes the openings.

Pitot-Static Probe Heaters Circuit & Operations

• Each main probe are anti-iced by 2 electric heaters
• On the ground with engines NOT operating, the probes are not powered
• Strut supplied with 115 volts AC with operating engines
• Reduced power supplied to the head
• In flight, relay R7425 is deenergized, relay R7423 is energized through the engine speed -Head and strut supplied with 115 AC Power
• Display Power is sensed by two current sensors.
• If power not supplied, information gets sent to the engine indicating/crew alert system by the EFIS/EICAS. In regards to the test, can be checked by ground CMC with 115 AC and observed on the control display unit or CDU.

Total Air Temperature (TAT) Heater Circuit

Each air temperature probe is anti-iced by an electric heater.

• Ground mode: Probe not powered up
• Flight : relay R8268 energised through the air/ground relay R7334. The heater is supplied with 115 Volts
• Power gets checked by sensor to get EFIS feed backs, the probes uses ground CMC with 115AC and get results back through the control drive unit CDU.

Galley and Lavatory Drain Heaters

• Serves to heat wash basins, galleys and floor drains that are subjected to freezing temps in flight
• Integrally heated hoses. Ribbon, blanket, wrap and gaskets also used

Designed for constant ops, has air and ground modes, controled by Air-to-Ground Sensor 26V AC on ground. 115V AC during air Anti Icing/ Bleed Air may be used, with Small Line from engine to heat the probe

Drain Mast Heater Circuits

• With ground handlers working/ Ground Handling Bus Powered, Relay 8277 is relaxed. Relay 731 is powered. So heaters are going to be supplied with 42.5 volts from the drain mast heater transformer

• No ground handlers/ power to busses, R7277 and R731 powered. 42.5 volts AC from heater transformers

• In flight , No ground handlers or power, relays are relaxed. Supplied 115V AC from transfer bus.

Windshield Wipers

• Power comes from pneumatic, hydraulic or electric motors -These are helpful with removing rain from windshield
• Aircraft uses independent working wipers. DO NOT OPERATE IN DRY WINDSHIELD

Windshield Wiper Parts

• The Flexible shaft is powered by motor(DC or AC), which turns the torque to rotor movement.
• Converters are attached to the fuselage structure by screws and a pivot stud and change rotary to oscillating motion. It determines the stroke arc

The dive arms gets attached and helps secure wiper blades, so that the pattern and motion is correct Tension uses about 4- and half pounds, the arms can release, to let the wiper blades release

Windshield Wiper Circuits

• Four position selector switch (Off, Park, slow etc)
• Park positions is temporary, that puts wipers out of sight
• Slow and fast ops provides power to the windshield

Windshield Washers

• Switch, electric operated reservoirs and pumps
• Fluid pumps and distributes which can be used for cleaning.

Rain Removal Systems

• Pneumatic.

• Early windshield wipers have issues with blade loading and speed Turbine powered aircraft makes removal feasible as air blasts forms barrier

Air blasts bled air from engine compressor then forced by holes

Rain Repellent System

Rain repellant Fluid reduces surface tension so it forms globules, that air blows away.

• Rainex is used as a Popular fluid. Pressure Container Connects to Reservoir. A line connect reservoir and spray nozzle at bottom each windshield. A valve is there for each nozzle .
• Solenoids will only open for a Quarter second, that sprays 5 cubic centimetres on a windshield. It's time circuited to stop power after 0.25 s Avoid use on dry windshields. Avoid running blades after running fluid, as it will smear the visibility and image