Ice and Rain Protection System
The presence of supercooled (temperature below which water freezes) water droplets in the atmosphere causes the icing on an aircraft. For the water to freeze, it must lose its heat to the surrounding, thus when the water droplets strike the aircraft wing or an engine intake or a propeller, which are metallic surfaces conduct away the head from the water droplets and it freezes instantly.
The formation of ice can cause the change in aerodynamic shape of the surface, loss of engine power, create loss of forward vision which can occur due to ice formation on the windshield panel and block pitot probes which can lead to faulty indication of airspeed and altitudes. Hence Ice protection method are required.
Flying in heavy rain conditions cause decrease in visibility through cockpit windshield, hence protection against it is done by the means of windshield wiper and rain repellent system.
Methods of Protection
All the methods of protection are based on two techniques
In de-icing method the ice is allowed to build up to an extent, which will not seriously affect the aerodynamic shape and this is then removed by operating a system. This cycle is then continuously repeated by a timing system.
In anti-icing the system is in continuous operation so that the ice cannot be formed at all.
There are three main types of protection methods
Pneumatic de-icing systems are employed in certain types of piston-engined aircraft and twin turbo-propeller aircraft on the leading edges of wings, horizontal and vertical stabilizer. In this system, section of rubber boots along the leading edge are inflated and deflated causing the ice to break up and with the aid of airstream crack up.
The number of components comprising a system vary together with the method of applying the operating principle.
The de-icer boots, or overshoes, consist of layers of natural rubber and rubberised fabric between which are disposed flat inflatable tubes closed at the ends. The tubes are made of rubberised fabric and are vulcanised inside the rubber layers. In some boots the tubes are so arranged that when the boots are in position on a wing or tail plane leading edge the tubes run parallel to the span; in others they run parallel to the chord.
The tubes are connected to the air supply pipelines from the distribution valves system by short lengths of flexible hose secured to connectors on the boots and to the pipelines by hose clips. The external surfaces of boots are coated with a film of conductive material to bleed off accumulations of static electricity. Depending on the type specified, a boot may be attached to a leading edge either by screw fasteners (rivnuts) or by cementing them directly to the leading edge.
Metal fairing strips are fitted to cover the edges of screw-fastened type boots, both on the upper and lower surfaces of an airfoil, and also at the ends of the boots. These latter strips serve to secure the ends of the boots and prevent inward ‘creep’. The strips are secured by the same screws used for securing the edges of the boots to the rivnuts.
PNEUMATIC DE-ICING BOOTS
Air Supplies and Distribution
The tubes in the boot sections are inflated by air from the pressure side of an engine-driven vacuum pump, from a high-pressure reservoir or in the case of some types of turbo-propeller aircraft, from a tapping at an engine compressor stage. At the end of an inflation stage of the operating sequence, and whenever the system is switched off, the boots are deflated by vacuum derived from the vacuum pump or, in systems utilizing an engine compressor tapping, from the venturi section or an ejector nozzle.
The method of distributing air supplies to the boots depends on the de-icing systems required for a particular type of aircraft but, in general, three methods are in use.
One method employs shuttle valves which are controlled by a separate solenoid valve.
The second method air is distributed to each boot by individual solenoid-controlled valves.
The third method distribution is effected by a motor-driven valve.
TYPICAL PNEUMATIC DEICING DISTRIBUTION SYSTEM
Controls and Indicators
The controls and indicators required for the operation of a de-icing system depend on the type of aircraft and on the particular arrangement of its de-icing system. In the basic arrangement, a main on-off switch, pressure and vacuum gauges or indicating lights form part of the controlling section. Pressure and vacuum is applied to the boots in an alternating timed sequence and the methods adopted usually vary with the methods or air distribution. In most installations, however, timing control is effected by means of an electronic device. Reference should always be made to the relevant aircraft Maintenance Manual for details of the appropriate controlling system and time cycles.
When the system is switched on, pressure is admitted to the boot sections to inflate the tubes. The inflation weakens the bond between ice and the boot surfaces, causing the ice to break away. At the end of the inflation stage of the operating sequence, the air in the tubes is dumped to atmosphere through automatic opening valves and the tubes are fully deflated by the vacuum supply. This inflation and deflation cycle is repeated during the period the system is in operation. When the system is switched off vacuum is supplied continually to all tubes of the boot sections to hold the sections flat against the wing and tail unit leading edges thus minimising aerodynamic drag.
Fluid/ Chemical System
In systems of this type, a de-icing fluid is drawn from a storage tank by an electrically driven pump and fed through micro filters to a number of porous metal distributor panels. The panels are formed to the profiles of the wing and tail unit leading edges into which they are fitted. At each panel the fluid passes into a cavity, and then through a porous plastic sheet to a porous stainless steel outer skin.
As the fluid escapes it breaks the bond between ice and the outer skin and the fluid and ice together are directed rearward, by the air valve is fitted in some types of aircraft to correct for variations in system pressure (head effect) due to differences in level between the wings, horizontal and vertical stabilizers.
The non-return valves prevent back flow when the system is inoperative. Nylon pipelines are usually used throughout the system; those for the main fluid supply being of 8mm (5/16 in) inch outside diameter and those for connections to individual distributor panels of 4.7 mm (3/16 in) outside diameter.
The connector contains a metering tube which is accurately calibrated to provide the required rate of fluid flow through the distributor. In some aircraft the metering of fluid to the distributor panels is done via proportioning units containing the corresponding number of metering tubes.
To prevent electrolytic corrosion, plastic sealing strips are interposed between the stainless steel panel and the metal used in the airfoil structure. In some installations an epoxy resin sealing compound is used, and to facilitate the removal of a panel it is sprayed along its edges with a thin coating of polytetrafluorethylene (p.t.f.e.) to act as a release agent. In addition, a strip of p.t.f.e. tape may be laid along the mating surfaces of the aero foil structure.
TYPICAL CHEMICAL DEICING DISTRIBUTION SYSTEM
Thermal (Hot Gas) System
In systems of this type, the leading edge sections of wings and tail units are usually provided with a second inner skin positioned to form a small gap between and the inside of the leading edge section. Heated air is ducted to the wings and tail units and passes into the gap, providing sufficient heat in the outer skin of the leading edge to melt ice already formed and prevent further ice formation.
The air is exhausted to atmosphere through outlets in the skin surfaces and also, in some cases, in the tips of wings and tail units. The temperature of the air within the ducting and leading edge sections is controlled by a shutter or butterfly type valve system the operation of which depends on the type of heating system employed.
HEATED WING LEADING
There are several methods by which the heated air can be supplied and these include bleeding of air from a turbine engine compressor, heating of ram air by passing it through a heat exchanger located in an engine exhaust gas system and combustion heating of ram air.
In a compressor bleed system the hot air is tapped directly from a compressor stage and after mixing with a supply of cool air in a mixing chamber it passes into the main ducting. In some systems, equipment, e.g. safety shut-off valves, is provided to ensure that an air mass flow sufficient for all de-icing requirements is supplied within pressure limits acceptable to duct and structural limitations.
The heat exchanger method of supplying warm air is employed in some types of aircraft powered by turbo-propeller engines. The heat exchanger unit is positioned so that exhaust gases can be diverted to pass between tubes through which outside air enters the main supply ducts. The supply of exhaust gases is usually regulated by a device such as a thermostatically controlled flap fitted in the ducting between the exhaust unit and the heat exchanger.
In a combustion heating system ram air is passed through a cylindrical jacket enclosing a sealed chamber in which a fuel/air mixture is burned, and is heated by contact with the chamber walls. Air for combustion is derived from a separate air intake and is supplied to the chamber by means of a blower.
TYPICAL THERMAL DEICING DISTRIBUTION SYSTEM
The type of ducting, materials used, methods of inter connection and disposition in an aircraft vary between deicing systems, and reference should therefore always be made to the relevant Aircraft Maintenance Manual for details.
Light alloy and stainless steel are materials normally used in construction, stainless steel being adopted principally in compressor bleed systems. Flanged and bolted end fittings, or band-type vee-clamps with interposed sealing rings are common methods of connecting duct sections together, and in some cases an additional means of sliding duct sections one end into the other and securing by adjustable clamps may be adopted.
In some installations in which ducting passes through the fuselage, joints between duct sections are sealed to prevent loss of cabin air pressure. Fuselage ducting may, in some types of aircraft, comprise an inner stainless steel duct surrounded by an outer fibreglass duct. The two ducts are approximately 13 mm (1/2 in) apart and the interspace is filled with glass wool to provide thermal insulation. The purpose of this ducting arrangement is to serve as a leak warning system by venting interspace air through venturis which operate pressure switches and a warning light.
Expansion and contraction of ducting is catered for by bellows or gimbal type expansion joints and in aircraft having variable incidence tailplanes and other moveable aerofoil surfaces such as leading edge slats and Kruger flaps, swivel joints and telescopic joints are fitted in the ducts supplying air to these surfaces.
In some installations, ducting in certain areas is lagged with a fire-resisting, heat-insulating material, normally fibreglass held in place by glass-cloth bound with glass cord.
DUCTING OF THERMAL ANTI ICE DISTRIBUTION SYSTEM
The control of the air temperature within ducting and leading edge sections is an important aspect of thermal deicing system operation and the methods adopted depend on the type of system.
In a typical compressor bleed system, control is effected by temperature sensing units which are located at various points in the leading edge ducting and by valves in the main air supply ducting. The sensing units and valves are electrically interconnected so that the valves are automatically positioned to regulate the flow of heated air to the system thus maintaining the temperature within a predetermined range. Indications of air temperature conditions are provided by resistance type temperature sensing elements and indicators, temperature sensitive switches and overheat warning lights. On some aircraft the electrical supplies to the valves are interrupted by landing gear controlled relays when the aircraft is on the ground. Under these conditions, valve operation is accomplished by holding the system control switch to a ‘TEST’ position.
When heat exchangers are employed, temperature control is usually obtained by the use of adjustable flaps and valves to decrease or increase the supply of heating and cooling air passed across the exchangers. The method of controlling the flaps and valves varies with different aircraft but a typical system incorporates an electric actuator, which is operated automatically by an inching device controlled by a temperature sensing element fitted in the duct on the warm air outlet side of the heat exchanger.
In some systems, actuators are directly controlled by thermal switches, so that the flaps or valves are automatically closed when a predetermined temperature is reached. Indications of air temperature conditions are provided by resistance type temperature sensing elements and indicators, temperature sensitive switches and overheat warning lights.
In systems incorporating combustion heaters, the temperature is usually controlled by thermal cyclic switches located in the heater outlet ducts, so that when the temperature reaches a predetermined maximum the fuel supply to the heaters is automatically switched off.
Wind Screen De-Ice and Anti-icing System
There are two methods of De-icing and Anti-icing system of wind screen, they are:
Fluid De-icing system
Electrical Anti-icing system
Fluid De-Icing System
The method employed in this system is to spray the windscreen panel with a methyl-alcohol based fluid. The principal components of the system are a fluid storage tank, a pump which may be a hand-operated or electrically-operated type, supply pipe lines and spray tube unit.
The fluid is supplied to the spray tubes by two electrically-operated pumps. The system may be operated using either of the pumps or both, according to the severity of icing.
WINDSHIELD FLUID DE ICING SYSTEM
Electrical Anti Ice System
This system employs a windscreen of special laminated construction heated electrically to prevent, not only the formation of ice and mist, but also to improve the impact resistance of the windscreen at low temperatures.
The film-type resistance element is heated by alternating current supplied from the aircraft’s electrical system. The power required for heating varies according to the size of the panel and the heat required to suit the operating conditions.
Details of these requirements are given in the relevant aircraft Maintenance Manual.
The circuit embodies a controlling device, the function of which is to maintain a constant temperature at the windscreen and also to prevent overheating of the vinyl inter layer which would cause such permanent damage is vinyl ‘bubbling’ and discoloration.
In a typical anti-icing system shown schematically in Figure, the controlling device is connected to two temperature sensing elements laminated into the windscreen. The elements are usually in the form of a fine wire grid, the electrical resistance of which varies directly with the windscreen temperature. One sensing element is used for controlling the temperature at a normal setting and the other is used for overheat protection. A system of warning lights and, in some cases, magnetic indicators, also forms part of the control circuit and provides visual indications of circuit operating conditions, e.g. ‘normal’, ‘off’ or ‘overheat’.
WINDSHIELD ELECTRICAL ANTI ICE SYSTEM
When the power is applied via the system control switch and power relay, the resistance element heats the glass.
When it attains a temperature pre-determined for normal operation the change in resistance of the control element causes the control device or circuit to isolate, or in some cases to reduce, the power supply to the heater element. When the glass has cooled through a certain range of temperature, power is again applied and the cycle is repeated. In the event of a failure of the controller, the glass temperature will rise until the setting of the overheat sensing element is attained.
At this setting an overheat control circuit cuts off the heating power supply and illuminates a warning light. The power is restored and the warning light extinguished when the glass has cooled through a specific temperature range. In some systems a lock out circuit may be incorporated, in which case the warning light will remain illuminated and power will only be re-applied by cycling the system control switch to ‘OFF’ and back to ‘ON'.
In addition to the normal temperature control circuit it is usual to incorporate a circuit which supplies more heating power under severe icing conditions when heat losses are high. When the high power setting is selected, the supply is switched to higher voltage output tappings of an auto transformer which also forms part of an anti-icing system circuit thus maintaining the normal operating temperature. The temperature is controlled in a manner similar to that of the normal control temperature circuit.
For ground testing purposes, the heating power supply circuit may also be controlled by landing gear shock strut microswitches in such a way that the voltage applied to the resistance elements is lower than that normally available in flight.
Rain-removal systems are designed to allow the pilot to have a clear view of the airport when taxing and to allow him or her to see the approach and departure paths and runway environment when taking off and landing during rain.
The systems are not commonly used during flight at altitude.
Rain may be removed by the use of wind shield wipers, chemical repellents in combination with wind shield wipers, or by a flow of air.
Wind shield-Wiper Systems
Wind shield-wiper systems may be operated electrically, hydraulically, or pneumatically.
A typical electrically operated windshield-wiper system is illustrated in Fig. This drawing shows the components of the wipers installed on an airliner. Each wiper on the airplane is operated by a separate system to ensure that clear vision through one of the windows will be maintained in the event of a system failure. The wiper blades clear a path 15 in [38.1 cm] wide through an arc of 40º.
Both wiper systems are electrically operated and controlled by a common gang switch located on the pilot's overhead panel. The switch provides a selection of four wiper-action speeds ranging from 190 to 275 strokes per minute and controls the stowing of the wiper blades in a PARK position when the system is not in use.
Each windshield -wiper system consists of a drive motor, a control switch, a resistor box, a flexible drive shaft, a torque converter, and a windshield-wiper assembly.
Speed control for the windshield wipers is accomplished by changing the voltage applied to the windshield-wiper motor by means of resistance arranged in the resistor box. The required resistance is connected into the motor circuit by turning the windshield-wiper switch to a selected speed. The rotary motion of the windshield-wiper motor is transmitted by the flexible shaft to the converter. The converter reduces the shaft speed and changes the rotary motion to an oscillating motion of the windshield-wiper arm.
Hydraulically and pneumatically operated wiper systems are similar in that each requires a pressure supply to be directed to an actuator. A control unit alternately connects a pressure or return line to opposite sides of the actuator, causing the piston to move back and forth. The actuator piston incorporates a rack that operates a pinion gear at the base of the wiper and causes the side-to side motion of the wiper.
A speed-control valve allows the pilot to select the speed at which the wipers will operate.
WINDSHIELD WIPER SYSTEM
To help maintain the clarity of vision through the windshield during rain conditions, a rain-repellent system is provided for the wind shields of many modern aircraft. This system consists of pressurized fluid containers, a selector valve, solenoid-actuated valves, spray nozzles, push-button switches, a control switch, a time-delay relay, and necessary plumbing.
During rain conditions, the wind shield wipers are turned on, and the repellent is sprayed on the wind shield. The repellent is sprayed evenly by the wiper blades. The rain repellent should not be sprayed on the wind shield unless the windshield is wet and the wipers are operated, nor should the windshield wipers be operated on a dry windshield.
The effect of the rain repellent is to cause the water to form small globules, which are quickly blown away by the rush of air over the windshield in flight.
RAIN REPELLANT SYSTEM
Hydrophobic Seal Coating
Hydrophobic seal coating of the windshield provides an alternative to chemical repellents. The coating is applied on the outside of the windshield and cause raindrops to bead up and roll off, allowing the ﬂight crew to see through with very little distortion. The hydrophobic windshield coating reduces the need for wipers and gives the ﬂight crew better visibility during heavy rain.
Most new aircraft windshields are treated with surface seal coating. The manufacturer’s coating process deeply penetrates the windshield surface providing hydrophobic action for a relative long period of time. When effectiveness declines, the coating must be reapplied.
The effect is similar to the chemical repellent, the differences being that hydrophobic coating last longer (while repellents usually need to be applied more than once during a flight) but coating restoration requires more effort (compared to refilling the repellent tanks).
HYDROPHOBIC SEAL COATING
Pneumatic Rain-Removal system
Some turbine-powered aircraft use engine bleed air to prevent rain from striking the windshield. When the pilot turns on the rain-removal system, bleed air at a high temperature and pressure is directed to an outlet at the base of the wind-shield. This flow of air over the windshield carries away the rain drops before they can strike the windshield. Any raindrops on the windshield when the system is turned on are also blown away.