Ice and Rain Protection PDF

Ice and Rain Protection Ice Protection The ice protection systems prevent icing on critical areas on the aircraft and helps maintain aerodynamic efficiency by protecting the following areas: Wing leading edges. Horizontal and vertical stabiliser leading edges. Engine nose cowls. Pitot static probes....

Ice and Rain Protection Ice Protection The ice protection systems prevent icing on critical areas on the aircraft and helps maintain aerodynamic efficiency by protecting the following areas: Wing leading edges. Horizontal and vertical stabiliser leading edges. Engine nose cowls. Pitot static probes. Angle of attack (AOA) probes. Total air temperature (TAT) probes. Flight compartment windows. Water and waste lines. Engine probes. Ice and rain protection block diagram 2023-08-15 B1-11e Turbine Aeroplane Aerodynamics, Structures and Systems CASA Part Part 66 - Training Materials Only Page 77 of 140 Rain Protection The rain protection systems used on commercial aircraft provide clear forward visibility through use of: Windshield wipers Rain repellent. Ice Formation The formation of ice on aircraft is caused by water droplets in the atmosphere which are at a temperature below zero degrees celsius but which have to lose their latent heat before they will freeze. As the droplets strike the metal surfaces of the aircraft, the metal conducts the latent heat away from the water and the droplets freeze on the metal. In general, ice forms on aircraft surfaces at 0 °C or colder when liquid water is present. However, icing can also occur in various conditions depending on cloud type, ambient temperature and altitude. Generally, the worst continuous icing conditions are found near the freezing level in heavy stratified clouds or in rain, with icing possible up to 4 900 m (16 000 ft.) Typically, most catastrophic inflight icing accidents occurred in the ‘holding pattern’ prior to landing, usually between 3 700 m (12 000 ft) and 4 900 m (16 000 ft) ASL (Above Sea Level) in clouds. Icing is rare above this higher altitude as the droplets in the clouds are already frozen. However, in cumuliform (cumulus) clouds with strong updrafts, large water droplets may be carried to higher altitudes and structural icing is possible up to very high altitudes of 9 100 m (30 000 ft) ASL or more. Further, in cumuliform clouds, the freezing level may be distorted upwards in updrafts and downwards in downdrafts, often by many thousands of feet. This leads to the potential for severe icing to occur at almost any level. The foregoing may vary according to the local conditions and whether the aircraft is being operated in temperate or in tropical climates. Cumulus Clouds Stratiform Clouds Rain and Drizzle 0 °C to -20 °C High 0 °C to -15 °C 0 °C and below -20 °C to -40 °C Medium -15 °C to -30 °C < -40 °C Low < -30 °C 2023-08-15 B1-11e Turbine Aeroplane Aerodynamics, Structures and Systems CASA Part Part 66 - Training Materials Only Page 78 of 140 Types of Ice Icing is classified by its formation and appearance. There are three standard classifications for ice: Glaze or clear ice Rime ice Mixed ice. Glaze or Clear Ice After the initial impact of supercooled droplets from large raindrops strike the surface, the remaining liquefied portion flows out over the surface and freezes gradually. This freezes as a smooth sheet of solid ice. This is a glassy transparent or whitish form of ice that adheres tenaciously to exposed surfaces. It accumulates most heavily on all forward-facing surfaces, including the leading edges of the wings, empennage and propellers. It often forms in successive smooth strong layers and is difficult to remove except by breaking the seal between it and the underlying surface or by melting. Glaze ice formation 2023-08-15 B1-11e Turbine Aeroplane Aerodynamics, Structures and Systems CASA Part Part 66 - Training Materials Only Page 79 of 140 Rime Ice Rime ice is formed from small supercooled droplets such as those in stratified cloud or light drizzle. The liquefied portion remaining after initial impact freezes rapidly before the drop has time to spread over the surface. This traps air between the droplets and gives the ice a white appearance. It is lighter in weight than clear ice, and its formation is irregular and its surface is rough. It is brittle and more easily removed than clear ice. Rime ice formation Mixed Ice Different moisture droplet sizes are commonly encountered in clouds, and this variation produces a mixture of clear ice (from large drops) and rime (from small droplets). Known as mixed ice, most ice encounters take this form. Pure rime ice is usually confined to high altostratus or altocumulus, while pure clear ice is confined to freezing rain. Mixed ice formation Ice may be expected to form whenever there is visible moisture in the air and the temperature is near or below freezing. The exception to this is carburettor ice, which can occur during warm weather with no visible moisture present. 2023-08-15 B1-11e Turbine Aeroplane Aerodynamics, Structures and Systems CASA Part Part 66 - Training Materials Only Page 80 of 140 Dangers of Ice Forming on an Aeroplane Ice forming on an aircraft is dangerous and can cause an accident for the following reasons: Ice formation on the wings and stabiliser increases the aircraft weight and disturbs the smooth aerofoil shape, reducing lift. Ice formation on a propeller of either a piston or jet engine will lower the efficiency of the propeller, making it ineffective or affecting its balance and causing vibration. Ice formation on jet engine air intakes restricts the airflow, causing loss of power or overheating. Ice breaking off the intake can cause compressor blade damage. Ice formation can block ram air intakes. Ice formed on masts and probes such as pitot tubes, stall warning vanes and antennas can cause the associated system to fail, as well as increase drag. Ice formed on windshields can obscure vision. Rain on windshields can also obscure vision. 2023-08-15 B1-11e Turbine Aeroplane Aerodynamics, Structures and Systems CASA Part Part 66 - Training Materials Only Page 81 of 140 Ice Detection Ice Detection Methods Ice detection is carried out by two methods: Visual Electrical. Visual Ice Detection Visual detection is the main form of ice detection on smaller and older aircraft and the reason for the ice detection (leading edge) lights. Black witness marks are used on some aircraft as a visual aid of detecting ice build-up. As ice builds up on the leading edge of the wing, the black witness marks become more reflective and harder to see when the light from the leading-edge lights are shined on them. This indicates to the crew that icing conditions are present. Ice will usually build up on windshield wipers (very visual) in the event of flying in icing conditions. Leading edge witness mark 2023-08-15 B1-11e Turbine Aeroplane Aerodynamics, Structures and Systems CASA Part Part 66 - Training Materials Only Page 82 of 140 Electrical Ice Detection Electrical and electronic detection methods are also used, of which there are three types: Pressure. Ice shave. Frequency. This is a probe positioned in the airflow which has several small air inlet holes acting to hold the diaphragm up, while a large air inlet hole acts to push the diaphragm down. Normally, the air pressure acting on the small holes exceeds that of the large holes, keeping the diaphragm in the up position. Should ice form, it will restrict the small holes, reducing upward pressure on the diaphragm and allowing air entering the large hole to push the diaphragm down, causing a set of contacts to close. The contacts supply current to an ice warning light and a heater in the probe to melt the ice formed on it. The crew takes the necessary de-icing action while the heater in the probe melts the ice formed on it. Pressure probe ice detector Once the probe heater melts the ice, upward pressure on the diaphragm increases, opening the contacts and switching off the detection probe heater and associated warning light. The warning is repeated if ice forms again. 2023-08-15 B1-11e Turbine Aeroplane Aerodynamics, Structures and Systems CASA Part Part 66 - Training Materials Only Page 83 of 140 A sample of the air passes over a rotor driven by an electric motor. Two methods are used to supply a warning light signal: If enough ice forms on the rotor, a blade in close proximity will shave it off. The shaving requires more motor torque, causing the motor to twist slightly in its mount. This action activates a micro-switch to turn on a warning light. If icing ceases, the shaving stops and the motor torque decreases. This deactivates the micro-switch, turning off the warning light. It can also be sensed by the current draw; as the drum slows, it draws more current to maintain its rpm, which is sensed by a current metre. When the current draw level exceeds a pre-set point, a warning light is activated in the cockpit. This system is also used to automatically control the ice protection system. Ice shave system This probe is mounted in a suitable place on the aircraft and is often duplicated. This is the most common form of ice detector used on modern commercial aircraft. The probe is designed to have a natural resonant frequency in the ultrasonic range. An oscillator induces this frequency in a coil around the probe. If ice forms on the probe, its resonant frequency reduces due to the extra weight of the ice. When this reduction reaches a pre-set level, electronic circuitry switches on a crew warning light and a probe heater. 2023-08-15 B1-11e Turbine Aeroplane Aerodynamics, Structures and Systems CASA Part Part 66 - Training Materials Only Page 84 of 140 The probe heater is kept on by the 5-second time delay and should melt all ice formed and then will switch off after 5 seconds. If ice re-forms within the remaining 55 seconds that the warning light remains on, the process will be repeated. The warning light delay will continue to keep the light on while ice continues to form. Frequency probe system Frequency probe system schematic 2023-08-15 B1-11e Turbine Aeroplane Aerodynamics, Structures and Systems CASA Part Part 66 - Training Materials Only Page 85 of 140 Anti-Icing and De-Icing Systems Types of Ice Control Systems Ice control systems can be categorised as: De-ice systems, which remove ice after it forms Anti-ice systems, which prevent the formation of ice. De-Icer Boots This system is in common use on piston-powered and turbine-powered propeller aircraft. The diagram shows a rubber boot fixed to the leading edge of an aerofoil. For many years, aircraft have used de-icing systems consisting of inflatable boots on leading edges and stabilisers. The inflatable boots are usually constructed with several separate air passages or chambers, enabling some to be inflated while the others are deflated. These chambers or tubes in the boot are attached to plumbing from an air control valve. The valve will apply low pressure or vacuum to all the tubes when the de-icing system is not in use, so the boot is held hard against the aerofoil, presenting a smooth surface to the airflow. This vacuum pressure can be achieved by either engine driven pumps or by a simple ejector operating on the venturi principle. When in use, tubes will be inflated with high-pressure air (approximately 18 psi) for a specified time (usually 6 seconds) then reconnected to the low-pressure line. The tubes are inflated for the same period and reconnected to the low pressure line. The inflation of alternate tubes makes them protrude out and break the ice, which is blown away by the airflow. De-icer boots 2023-08-15 B1-11e Turbine Aeroplane Aerodynamics, Structures and Systems CASA Part Part 66 - Training Materials Only Page 86 of 140 The image illustrates a typical airframe de-icing system. A timer/distributor valve directs highpressure (18 psi) air to the selected de-ice boots in turn in a definite sequence so as to ensure aircraft stability during de-ice boot inflation and deflation. For example, Left and right outboard wing leading edges Left and right stabilisers and vertical fin Left and right inboard wing leading edges. There typically two sequences: cold and warm. In cold, the boots inflate in sequence for 6 seconds and deflate for 54 seconds before commencing a new cycle. In warm, the boots inflate in sequence for 6 seconds and deflate for 2 minutes and 54 seconds before commencing a new cycle. De-icer Boot Inspection, Maintenance and Repair Inspection and Maintenance The most important part of de-icer boot maintenance is keeping the boots clean. Wash the boots with a mild soap and water solution. Remove any cleaning compounds used on the aircraft from the boots using clean water. Remove oil and grease by scrubbing the surface of the boot lightly with a rag that is damp with white spirit or lead-free gasoline. Wipe dry before the solvent has a chance to soak into the rubber. Boots are often sprayed with silicon to give the rubber an extremely smooth surface to which ice cannot adhere. During inspection, check the surface of the boots for condition and security. Also, inspect the condition of plumbing fittings and lines/hoses. Conclude the inspection with a thorough operational check of the system. Repair Schemes A de-icer boot repair method referred to as a ‘cold patch repair,’ and it includes refurbishing scuff damage and repairing damage to the tube area, tears in the fillet area and ‘pin-hole’ damage from the abrasive nature of the air striking the leading edge in flight. Scuff damage is the most common type of damage that engineers encounter when maintaining de-icer boots. For any type of de-icer boot damage, always refer to the manufacturer’s maintenance manual for guidance in making appropriate repairs and follow the approved repair processes explicitly. Never use tyre patches on aircraft de-icer boots, even as a temporary repair, because tyre patches contain a fabric which will not stretch enough and will tear away after about the second boot inflation cycle, probably causing more damage to the boot. Always use the approved parts supplied by BF Goodrich, the principal manufacturer of de-icer boots and accessories. Generally, patch repairs can be considered permanent for the life of the de-icer boot. The kit consists of a scuffing pad, glue, a roller, a variety of large and small patches and a tin of talcum powder. The following is a guide to installing a patch on a de-icer boot tube; however, always use the maintenance manual for proper guidance. The aircraft should be hangared and ground power and shop air made available to activate the airframe pneumatic de-icer systems. 2023-08-15 B1-11e Turbine Aeroplane Aerodynamics, Structures and Systems CASA Part Part 66 - Training Materials Only Page 87 of 140 1. Enter the results of the de-icer boots test in the aircraft maintenance log as a defect. 2. Spread a mild soap-and-water solution over the entire surface of the de-icer boots. Only do one leading edge at a time; start from the left wing, then right wing, then right horizontal stabiliser, vertical stabiliser and finally left horizontal stabiliser. 3. Using shop air through the proper regulator, operate the de-icer boots through a full cycle and watch for bubbles on the tubes during boot inflation. 4. Mark the leaky areas with a white chinagraph pencil. 5. Dry the boot thoroughly by dabbing with a dry, clean, lint-free cloth so as not to remove the previously marked leaky areas. 6. Select a suitable patch for the size of the damage and remove the chinagraph pencil mark by rubbing gently with the cloth. Lay the patch on the boot and draw around it with a white chinagraph pencil. Ensure the damage is at the centre of the patch. 7. Using the scuffing pad, gently abrade the areas for at least 1/2 an inch (12 mm) beyond the edges of the patch. Clean with a dry, clean, lint-free cloth. 8. Apply an even film of glue to the scuffed area so it extends beyond the edges of the patch. Allow to dry for 15 minutes. 9. Peel the backing strip off the patch and apply to the glued area. Press firmly to ensure a good bond. 10. Using the rubber roller supplied, roll across the patch, back and forth, up and down and diagonally in both directions for at least five minutes to ensure a final and lasting bond. 11. Dust the patched area with a light film of talcum powder. 12. Continue this sequence of events with all other leaks on all boots. 13. Finally, cover each entire de-icer boot with a liberal coating of ‘Icex II,’ ‘Age Master’ or ‘Shine Master’ to give the boots a good shine and assist in protecting from scuff damage. 14. Perform a final functional test of the system and certify for the repairs in the aircraft maintenance log. 2023-08-15 B1-11e Turbine Aeroplane Aerodynamics, Structures and Systems CASA Part Part 66 - Training Materials Only Page 88 of 140 A well-maintained wing de-icer boot on a commuter aircraft Chemical De-icing Although it is not often used on modern aircraft, chemical de-icing can carry out all de-icing requirements on slower aircraft. The common de-icing fluid used is a mixture of isopropyl alcohol and ethylene glycol. These substances emulsify with water and lower its freezing temperature so the ice will melt. The de-ice fluid also makes the surfaces slick so ice will have trouble reforming on them. Therefore, the de-ice fluid performs both a de-ice and an anti-ice function. Chemical de-icing wiring diagram 2023-08-15 B1-11e Turbine Aeroplane Aerodynamics, Structures and Systems CASA Part Part 66 - Training Materials Only Page 89 of 140 System Operation Each system being chemically de-iced has an electric motor driving a pump. The pump supplies the de-icing fluid from a storage tank through plumbing to the area to be de-iced as follows: Windshield de-ice: the fluid is sprayed over the windshield. Carburettor de-ice: the fluid is sprayed into the carburettor air intake. Propeller de-ice: the fluid is sprayed out along the blades. Wing and empennage: the fluid is slowly released through a porous boot fixed to the leading edges. A rheostat (variable resistor) is used to control the speed of each pump motor, which controls the fluid flow rate. Propeller De-icing – Fluid System A typical anti-icing system consists of a control unit, an anti-icing fluid tank a pump to deliver fluid to the propeller and nozzles. The control unit contains a rheostat which is adjusted to control pump output. Fluid is pumped from the tank to a stationary nozzle installed just behind the propeller on the engine nose case. As fluid passes through the nozzle, it enters a circular U-shaped channel called a slinger ring. A typical slinger ring is designed with a delivery tube for each blade. Once fluid is in the slinger ring, centrifugal force sends the anti-icing fluid out through the delivery tube to each blade shank. Chemical de-icing system to propeller blades 2023-08-15 B1-11e Turbine Aeroplane Aerodynamics, Structures and Systems CASA Part Part 66 - Training Materials Only Page 90 of 140 Electrical Propeller De-Ice Control Electrical heating is the preferred method of ice control for propellers due to the available supply of electrical power during flight. Rubber boots with heater wires embedded in the rubber are bonded to the leading edges of the propeller blades. Electrical current is passed through these wires to heat the rubber and melt any ice that has formed, allowing centrifugal force and airflow to carry the ice away. Electric elements are set in insulating material on the propeller leading edges and in the propeller spinner for the purpose of de-icing. There is no air intake de-icing on piston engines, and some do not have propeller de-icing. Aviation Australia Electrical propeller ice control wiring diagram Timer Unit The timer controls the sequence of current to each of the heater mats. The sequence of heating is important to provide the best loosening of the ice so it can be carried away by the centrifugal force. It is also important that the same portion of each blade be heated at the same time. Also, if heat is applied for too long duration to the heating mat, delamination could occur, damaging the system. 2023-08-15 B1-11e Turbine Aeroplane Aerodynamics, Structures and Systems CASA Part Part 66 - Training Materials Only Page 91 of 140 Load Meter The load meter is a simple ammeter which monitors the operation of the system and assures the pilot that each heater element is drawing the required amount of current. This system would typically use a 28-V DC power supply. Two heater elements are fitted to each propeller: an outboard and inboard heater element. Each of the elements is connected to the power supply via slip rings and brush box assemblies. The timer unit is common to all propellers used on the aircraft. With the system selected on, power is supplied through the circuit breaker and ammeter to the timer unit. The timer then supplies the power to the heater in the cycle sequence and, through the brush block and slip rings, power is delivered to the heater element. After this, the current is then passed back to the slip rings and through to earth. Thermal Pneumatic Anti-Icing Systems Turbine-engine aircraft have a ready source of warm compressor bleed air for anti-icing, and they normally use thermal ice control. Thermal pneumatic system Thermal pneumatic systems take hot air from the engine compressor and direct it between the aerofoil leading edge outer skin and an inner skin before exhausting it overboard. 2023-08-15 B1-11e Turbine Aeroplane Aerodynamics, Structures and Systems CASA Part Part 66 - Training Materials Only Page 92 of 140 Heated air for anti-icing is obtained by bleeding air from the engine compressor. The reason for the use of this type of system is that large amounts of very hot air can be tapped off the compressor, providing a source of anti-icing and de-icing heat. The hot bleed air is mixed with ambient air and at approximately 177 °C (350 °F) and flows through passages next to the leading edge skin. Each of the shutoff valves is pneumatically actuated and electrically controlled. When the temperature in the leading edge reaches approximately 85 °C (185 °F), a thermal switch connected to the control solenoid of the shutoff valve causes the valve to close and shut off the flow of bleed air. An operational system check can be carried out by using an external source of air. Most systems are designed with a test plug to permit ground checking of the system without operating the aircraft engines. When using an external air source, make certain that the air pressure does not exceed the test pressures established for the system. The air used for airframe leading edge de-icing/anti-icing is vented back out in most aircraft via small holes located under the wing. For other areas of the aircraft where thermal pneumatic air is used, this is exhausted via the rear of the leading-edge surface into the non-pressurised area of the airframe (e.g., vertical stabiliser, horizontal stabiliser). Sectional view of the wing showing the ducting and skins 2023-08-15 B1-11e Turbine Aeroplane Aerodynamics, Structures and Systems CASA Part Part 66 - Training Materials Only Page 93 of 140 Example of wing thermal anti-ice outlet holes 2023-08-15 B1-11e Turbine Aeroplane Aerodynamics, Structures and Systems CASA Part Part 66 - Training Materials Only Page 94 of 140 Engine Air Inlet Anti-Ice Systems When an aircraft flies through icing conditions, ice can build up in the engine’s inlet duct and on its inlet guide vanes. This disrupts airflow and reduces efficiency. Furthermore, large pieces of ice could break off and enter the engine, causing serious damage to the compressor blades. To prevent ice formation and ingestion, turbine inlet ducts are typically equipped with some form of anti-icing system. Engine Air Inlet Ice Control Systems – Hot Bleed Air When the engine anti-icing system is switched on, a bleed air valve directs hot air to the inlet duct leading edge, nose dome and inlet guide vanes to prevent the ice from forming. An indicator light illuminates (usually blue) in the cockpit to show anti-icing is on. Blue is used as an advisory display as this system is not normally an operating system. Once the air has been used, it is vented out into the airflow by venting ports located in the engine cowl. A disadvantage with this type of system is that whenever bleed air is taken from a turbine engine, the power output is decreased. This is not normally a problem on larger modern turbofans but can be a limiting factor on smaller turbine-engine aircraft. Engine air-inlet ice control system When the anti-ice is selected ‘on,’ this drives the motor in the engine anti-icing valve to the ‘open’ position. 2023-08-15 B1-11e Turbine Aeroplane Aerodynamics, Structures and Systems CASA Part Part 66 - Training Materials Only Page 95 of 140 With the valve open, pneumatic system air is allowed to pass through the air regulator. The air regulator is fitted with a bimetallic spring coil. This controls the amount of airflow to the engine antiicing system by the temperature of the air being delivered. As the pneumatic system air flows over the bimetallic strip, it increases the temperature of the bimetallic strip. Once the temperature increases to a predetermined level, the bimetallic strip will expand and reduce the airflow to the deicing system. This will then control the airflow and the temperature of the de-icing surfaces. Excessive temperature can cause damage to these areas. By using this type of air regulator, airflow and temperature can be controlled from the one device. From this point on, it is divided up and distributed to the spinner and nose cowl anti-ice systems. Air is then removed from the engine via venting holes located around the nose cowl and the spinner of the engine. Engine anti-ice system operation 2023-08-15 B1-11e Turbine Aeroplane Aerodynamics, Structures and Systems CASA Part Part 66 - Training Materials Only Page 96 of 140 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. This system utilises electric heating elements surrounding the engine air intake. Electric engine air inlet heaters This system is only used on turboprop aircraft. This system is not used by high bypass turbofans, as the current required would be too great. Some systems have two selections: low and high. This provides heating over a range of icing conditions. However, continuous use of the high setting in low to medium icing conditions could cause damage to the air intake heater elements. Electric Windshield Anti-Icing Windshield construction for anti-icing 2023-08-15 B1-11e Turbine Aeroplane Aerodynamics, Structures and Systems CASA Part Part 66 - Training Materials Only Page 97 of 140 Windshields, also called windscreens, are heated to keep clear visibility in foggy and icing conditions; heating also improves the windshield’s ability to resist bird strike damage. Some windshields have fine wire elements embedded in an inner layer of vinyl; the vinyl is sandwiched between two layers of glass. 28-V DC current is passed through the wire element, and a thermistor, also embedded in the windshield, controls the DC current to maintain a temperature of approximately 45 °C. The conductive film can be stannic oxide or gold, rolled so thin that it is transparent. The temperature of each windshield is monitored by the temperature-sensing elements and controlled by the temperature controller unit, which turns the electric power to relays RL1 and RL2 on and off to regulate the temperature. Windshield heat should not be operated on the ground for extended periods – only a quick test is approved by the aircraft manufacturer. Always test in accordance with the aircrafts maintenance manual to prevent possible damage to the aircraft’s windshield or heating element. Windshield Temperature Control The diagram illustrates a typical system powered by a three-phase 200-V AC source which can be of constant frequency or be frequency wild. The three elements, one for each phase, and the normal and overheat temperature sensing thermistors are imbedded in the windshield during manufacture. Windshield temperature control wiring diagram 2023-08-15 B1-11e Turbine Aeroplane Aerodynamics, Structures and Systems CASA Part Part 66 - Training Materials Only Page 98 of 140 The three-phase, 200-V AC power source goes to an auto transformer. The transformer has two output voltages, both of which are higher than the input voltage. The lower voltage is applied to the windshield element when low heat is selected, and the high voltage is applied when high heat is selected. Assume the windshield control switch is set to low. If the windshield temperature is below 45 °C, the temperature controller will complete a current path for the coil of relay RL1. When energised, RL1 connects the lower voltage from the autotransformer to the windshield elements. The windshield elements heat the windshield until it reaches 45 °C. Then the normal heat thermistor will cause the temperature controller to break the current path to the coil of RL1, which de-energises, breaking the current path to the heating elements. As the windshield cools, the thermistor causes the controller to switch the heat back on. If ice still forms on the windshield when low heat is selected, high will have to be selected. This will cause the temperature controller to operate relay RL2 instead of relay RL1. Relay RL2 will switch the higher voltage output from the autotransformer to the heating elements, so more current will flow through the elements, producing more heat. The temperature controller will still control the windshield temperature so that it does not exceed 45 °C. If the temperature controller or the normal thermistor fails, the temperature of the windshield can rise above 45 °C. Should the temperature reach 55 °C, the overheat thermistor will cause the other windshield temperature controller to break the earth circuits of relays RL1 and RL2. This will stop current flow through the windshield elements. The windshield can continue to operate in this condition but must be fixed when the aircraft lands. A magnetic indicator for each windshield will show NORM when the system is operating normally, OH if it is operating in the overheat condition or OFF when off. Transport Aircraft Windshield Strength The windshield of a jet aircraft has to take all the forces of the air acting on it at speeds up to 1000 kilometres per hour. It must also be able to withstand the impact of a two-kilogram bird at the cruise speed of the aircraft. Leaving the windshield heat set on low even when there are no icing conditions will have the following results: Increase the windshield’s impact resistance Increase the windshield’s service life Better dissipate static electricity. 2023-08-15 B1-11e Turbine Aeroplane Aerodynamics, Structures and Systems CASA Part Part 66 - Training Materials Only Page 99 of 140 Probe and Drain Heating Pitot Static Probe Heaters Aircraft approved for flight into known icing conditions must be appropriately equipped. Pitot static probe heaters are provided to eliminate ice formations which would affect the airflow into the tubes or completely block the openings, thereby producing erroneous airspeed and altitude readings. Pitot static probe heating The probe is electrically heated by elements which heat the probe to a very high temperature to prevent ice forming during flight. Where more than one pitot-static system is fitted, independent heaters are fitted to each system. CAUTION: Do not touch the probe immediately after flight; the probe gets very hot and can cause severe burns. Allow the probe to cool before attempting maintenance. Electrically heated probes should not to be operated on the ground, except for maintenance tests. Without the cooling effect of airflow over the heater, it is possible that the temperature of the heater can rise sufficiently to cause severe damage to the heated area. 2023-08-15 B1-11e Turbine Aeroplane Aerodynamics, Structures and Systems CASA Part Part 66 - Training Materials Only Page 100 of 140 Pitot static probe cover Where heated probes are fitted, the covers should be manufactured from cotton or canvas rather than vinyl or synthetic materials so that in the event of the heater probes being turned on with the covers still fitted, they will burn away rather than stick to the probes, possibly plugging up the openings. Pitot Static Probe Heater Circuit Each main pitot-static probe is anti-iced by two electric heaters. Operation on Ground On the ground, with engines not operating, the probe heaters are not powered. With any engine operating, relays R7423 and R7425 are energised. The strut of the probe is supplied with 115-V AC power. Reduced power is supplied to the head of the probe. Operation in Flight In flight, relay R7425 is deenergised, and relay R7423 remains energised through the engine speed sense card or relay R7334. The head and strut of the probe are supplied with 115-V AC power. 2023-08-15 B1-11e Turbine Aeroplane Aerodynamics, Structures and Systems CASA Part Part 66 - Training Materials Only Page 101 of 140 Display The power supply to the heaters is sensed by two current sensors. If power is not supplied through the heater, the current sensor provides this information to Engine Indicating and Crew Alerting System (EICAS) through the Electronic Flight Instrument System (EFIS)/EICAS interface units. Test The heater’s operation can be checked on the ground by the Central Maintenance Computer (CMC). During testing, relays R7421 and R7423 are energised. The head and strut of the probe are supplied with 115-V AC power. The test results are observed on the Control Display Unit (CDU). Example of a pitot-static probe heaters circuit 2023-08-15 B1-11e Turbine Aeroplane Aerodynamics, Structures and Systems CASA Part Part 66 - Training Materials Only Page 102 of 140 Total Air Temperature Probe Heater Circuit Each total air temperature probe is anti-iced by an electric heater. Operation on Ground On the ground, the probe heater is not powered. Operation in Flight In flight, relay R8268 is energised through air/ground relay R7334. The heater is supplied with 115-V AC power. Display The power supply to the heater is sensed by a current sensor. If power is not supplied through the heater, the current sensor provides this information to EICAS through the EFIS/EICAS interface units. Test The heater’s operation can be checked on the ground by the central maintenance computer. During testing, relays R7421 and R8268 are energised. The probe heater is supplied with 115-V AC power. The test results are observed on the CDU. Example of a total air temperature probe heater circuit 2023-08-15 B1-11e Turbine Aeroplane Aerodynamics, Structures and Systems CASA Part Part 66 - Training Materials Only Page 103 of 140 Galley and Lavatory Drain Heaters Heaters are provided for lavatory wash basins and galley sinks and floor drains where they are located in an area that is subjected to freezing temperatures in flight. Typical heater types used are: Integrally heated hoses Ribbon Blanket Wrap around patch Gasket. These heaters are designed for continuous operation. They usually operate in two modes: air and ground. This is controlled by the air-to-ground sensor system of the aircraft. Most systems operate at 26-V AC on ground mode and 115-V AC in air mode. Anti-icing may also use bleed air. A small line from the aircraft bleed air manifold is directed onto the probe. Drain heating 2023-08-15 B1-11e Turbine Aeroplane Aerodynamics, Structures and Systems CASA Part Part 66 - Training Materials Only Page 104 of 140 Drain Mast Heater Circuit With the ground handling bus powered, relay R8277 is relaxed, relay R731 is energised and the heaters are supplied with 42.5 volts. AC comes from the drain mast heater transformer. No Ground Handling Bus Power When power is supplied to the main airplane buses, relays R8277 and R731 are energised and the heaters are supplied with 42.5 volts. AC comes from the drain mast heater transformer. Air Operation In flight, the ground handling bus is not powered and relays R8277 and R731 are relaxed. The heaters are supplied with 115-V AC from the ground service transfer bus. Example of a drain mast heater circuit 2023-08-15 B1-11e Turbine Aeroplane Aerodynamics, Structures and Systems CASA Part Part 66 - Training Materials Only Page 105 of 140 Rain Removal and Repellent Systems Windshield Wipers Windshield wipers Do not operate on dry windshields and ensure the windshield is clear of foreign matter, as this will damage the windshield. 2023-08-15 B1-11e Turbine Aeroplane Aerodynamics, Structures and Systems CASA Part Part 66 - Training Materials Only Page 106 of 140 Windshield wiper parts breakdown Aircraft windshield wipers can be powered by a pneumatic motor, a hydraulic motor or electric motor. The latter can be a DC or AC motor. This is the most common form of rain removal from aircraft windshields. Most aircraft wipers can be operated independently of each other. Converters A converter is attached to the fuselage structure by screws and a pivot stud. A flexible drive shaft connects the DC motor to each converter. The converters change the rotary motion of the motor to the oscillating motion required by the wiper blades. The converter determines the stroke arc. The pivot stud, which passes through one of the mounting holes, supports the end of the guide arm Drive Arms, Guide Arms and Wiper Blades The drive arm is attached to the serrated shaft of the converter and is the attachment point for the wiper blade. The guide arm, connected to the pivot stud and wiper blade, keeps the wiper blade vertical and governs wiper pattern. Spring tension of about 4-1/2 pounds against the windshield is obtained by an adjustment screw near the bottom of the drive arm. 2023-08-15 B1-11e Turbine Aeroplane Aerodynamics, Structures and Systems CASA Part Part 66 - Training Materials Only Page 107 of 140 To Remove Pull on the top of the arm and compress the pressure mechanism until a 2-1/2- to 3-inch long, 1/8inch-diameter steel pin (or 1/8-inch drill) can be inserted through the pinhole in the arm. This supports the spring tension for easy removal/installation. Removal and installation of wiper blade 2023-08-15 B1-11e Turbine Aeroplane Aerodynamics, Structures and Systems CASA Part Part 66 - Training Materials Only Page 108 of 140 Windshield Wiper Circuit Usually, a four-position selector switch is used (off, park, slow and fast). Park is a momentary switch, which is used to park the wiper out of view once the wipers are no longer required. This operates against spring tension, and once released, the switch goes back to the off position. This forces the cam to operate the wiper until the internal park switch is activated, which stops the motor and puts the wiper in the park position. The slow and fast operation provides power to the wiper motor. During the slow operation, a higher resistance value is used. Windshield wiper electrical circuit 2023-08-15 B1-11e Turbine Aeroplane Aerodynamics, Structures and Systems CASA Part Part 66 - Training Materials Only Page 109 of 140 Windshield Washers An electrically operated windshield washer system consists of a switch, a reservoir and a pump. Washer fluid for the system is supplied by the reservoir through a tube to the pump. When the windshield washer switch is closed, the pump will operate. Washer fluid is pressurised by the pump and distributed by tubes to a fluid dispenser on the windshield wiper blades. The windshield wipers may be used for cleaning. Windshield washer 2023-08-15 B1-11e Turbine Aeroplane Aerodynamics, Structures and Systems CASA Part Part 66 - Training Materials Only Page 110 of 140 Pneumatic Rain Removal System Early windshield wipers had problems with blade loading and speed. With the advent of turbinepowered aircraft, the pneumatic rain removal system became feasible. The air blast forms a barrier that prevents raindrops from striking the windshield surface. The air blast was bled from the engine compressor bleed air system. This air was then forced through small ducts and over the windshield. Example of a pneumatic rain removal system 2023-08-15 B1-11e Turbine Aeroplane Aerodynamics, Structures and Systems CASA Part Part 66 - Training Materials Only Page 111 of 140 Rain Repellent System Rain repellent fluid reduces the rainwater surface tension so that it forms into globules and is blown away by the airflow. ‘Rainex’ is the most popular fluid in use today. The rain repellent fluid is in a pressurised container connected to a reservoir. Plumbing connects the reservoir via a solenoid valve to a spray nozzle at the bottom of each windshield; there is a solenoid valve for each spray nozzle. When operated, the solenoid valve only opens for approximately 0.25 seconds so that approximately 5 cubic centimetres of fluid is metered out on the windshield. This is achieved by a time circuit, stopping power to the solenoid valve after 0.25 seconds. Rain repellent should not be applied to a dry windshield, as undiluted repellent will restrict visibility. Windshield wipers should not be operated immediately after having applied repellent, smearing will occur and reduce visibility. Rain repellent system 2023-08-15 B1-11e Turbine Aeroplane Aerodynamics, Structures and Systems CASA Part Part 66 - Training Materials Only Page 112 of 140