the accessory drive gear box of the engine. The compression side of each pump supplies air pressure to inflate the wing and tail deicer boots. Suction is supplied from the inlet side of each pump to hold down the boots, when not being inflated, while in flight. One type of pump uses engine oil for lubrication and is mounted so that the drive gear is mated with the drive gear in the accessory drive gear box.
The oil taken in the pump for lubrication and sealing is discharged through the pressure side, to the oil separator. At this point, most of the oil is separated from the air and fed back to the engine oil sump. When installing a new pump, care should be taken to ensure that the oil passages in the gasket, pump, and engine mounting pad are aligned (figure 7-6). If this oil passage does not line up, the new pump will be damaged from lack of lubrication.
Another type of vacuum pump, called a dry pump, depends on specially compounded carbon parts to provide pump lubrication. The pump is constructed with carbon vanes for the rotor. This material is also used for the rotor bearings. The carbon vane material wears at a controlled rate to provide adequate lubrication. This eliminates the need for external lubricants.
When using the dry type of pump, oil, grease, or degreasing fluids must be prevented from entering the system. This is important at installation and during subsequent maintenance.
Maintenance of the engine-driven pump is limited to inspection for loose connections and security of mounting.
Outlet port Intake port Holes in adapter flange for engine pad lubrication.
If this type pump is used, be sure the holes are open and not covered by the flange gasket at installation.
FIcURE 7-6. Lubrication of wet type vacuum pump.
Safety Valves An air pressure safety valve is installed on the pressure side of some types of engine-driven air pumps. Schematically, this valve is placed on the air pressure side of the pump between the primary oil separators and the pump. The safety valve exhausts excessive air at high pump r.p.m. when a predetermined pressure is reached. The valve is preset and is not adjustable.
Oil Separator An oil separator is provided for each wet-type air pump. Each separator has an air inlet port, an air outlet port, and an oil drain line which is routed back to the engine oil sump. Since the air pump is internally lubricated, it is necessary to provide this means of separating oil from the pressurized air. The oil separator removes approximately 75% of the oil from the air.
The only maintenance required on the oil separator is flushing the interior of the unit with a suitable cleaning solvent. This should be done at intervals prescribed by the applicable maintenance manual.
Combination Regulator, Unloading Valve, and Oil Separator The combination regulator, unloading valve, and oil separator consists of a diaphragm-controlled, spring-loaded unloading valve, an oil filter and drain; a diaphragm type air pressure regulator valve with an adjustment screw; and a solenoid selector valve. The assembly has an air pressure inlet port, an exhaust port, an outlet to the solenoid distributor valves, an outlet to the suction side of the engine-driven air pumps, and an oil drain. The combination unit has three functions: (1) To remove all residual oil left in the air by the primary oil separator before it enters the pressure manifold;
(2) to control, direct, and regulate air pressure in the system; and (3) to discharge air to the atmosphere when the deicer system is not in use, thereby allowing the air pump to operate at no pressure load.
Maintenance of this unit consists of changing the filter element as prescribed by the applicable maintenance manual.
The pressure regulator may be adjusted, if the deicer system pressure gage does not register the specified pressure. The adjusting screw should be turned counterclockwise to increase the pressure and clockwise to decrease the system pressure.
Suction Regulating Valve An adjustable suction regulating valve is installed in each engine nacelle. One side of each valve is piped to the inlet (suction) side of the enginedriven air pump and the other side to the main suction manifold line. The purpose of the suction valve is to maintain the deicer system suction automatically.
Maintenance of this valve consists of removing the air intake filter screen and cleaning it with a cleaning solvent as prescribed by the applicable maintenance manual.
This valve may be adjusted to obtain the desired deicing system suction. The deicer system suction is increased by turning the adjusting screw counterclockwise and decreased by turning it clockwise.
Electronic Timer An electronic timer is used to control the operating sequence and the time intervals of the deicing system. When the deicing system is turned on, the electronic timer energizes a solenoid in the unloading valve. The solenoid closes a servo valve, thereby directing air pressure to the unloading valve and closing it until the regulator valve of the combination unit takes over. The regulator valve then tends to keep the entire manifold system at approximately 15 p.s.i. pressure and unloads any surplus air at the separator by dumping it overboard. The pressure manifold line is then routed to the distributor valves. The electronic timer then controls the operating sequence of the distributor valves.
Solenoid Distributor Valve The solenoid distributor valve is normally located near the group of deicer boots which it serves. Each distributor valve incorporates a pressure inlet port, suction outlet port, two ports ("A" and "B") to the boots, and a port piped overboard to a low pressure area. Each distributor also has two solenoids, A and B. The pressure inlet port is integral with the manifold pressure line, thereby making approximately 15
p.s.i. pressure available at all times when the deicer system is operating. The suction port is connected to the main suction line. This allows approximately 4 in. Hg suction available at all times in the distrib.
utor valve. Ports "A" and "B' connect suction and pressure to the boots, as controlled by the distributor valve. The port piped to the low pressure area allows the air under pressure in the boots to be dumped overboard as controlled by the distributor valve servo.
The distributor valve normally allows suction to be supplied to the boots for holddown in flight.
However, when the solenoid in the distributor valve is energized by the electronic timer cycle control, it moves a servo valve, changing the inlet to that section of the boot from suction to pressure. This allows the boot to inflate fully for a predetermined time. This interval is controlled by the electronic timer. When the solenoid is de-energized, the airflow through the valve is cut off. The air then discharges out of the boot through an integral check valve until the pressure reaches approximately 1 in. Hg, the boot is ported to the suction manifold and the remaining air is evacuated, thus again holding the boot down by suction.
PNEUMATIC DEICING SYSTEM MAINTENANCE Maintenance on pneumatic deicing systems varies with each aircraft model. The instructions of the airframe or system components manufacturer should be followed in all cases. Depending on the aircraft, maintenance usually consists of operational checks, adjustments, troubleshooting, and inspection.
Operational Checks An operational check of the system can be made by operating the aircraft engines, or by using an external source of air. Most systems are designed with a test plug to permit ground checking the system without operating the engines. When using an exte:nal air source, make certain that the air pressure does not exceed the test pressure established for the system.
Before turning the deicing system on, observe the vacuum-operated instruments. If any of the gages begin to operate, it is an indication that one or more check valves have failed to close and that reverse flow through the instruments is occurring Correct the difficulty before continuing the test. If no movement of the instrument pointers occurs, turn on the deicing system.
With the deicer system controls in their proper positions, check the suction and pressure gages for proper indications. The pressure gage will fluctuate as the deicer tubes inflate and deflate. A relatively steady reading should be maintained on the vacuum gage. It should be noted that not all systems use a vacuum gage. If the operating pressure and vacuum are satisfactory, observe the deicers for actuation.
With an observer stationed outside the aircraft, check the inflation sequence to be certain that it agrees with the sequence indicated in the aircraft maintenance manual. Check the timing of the system through several complete cycles. If the cycle time varies more than is allowable, determine the difficulty and correct it. Inflation of the deicers must be rapid to provide efficient deicing. Deflation of the boot being observed should be completed before the next inflation cycle.
Adjustments Examples of adjustments that may be required include adjusting the deicing system control cable linkages, adjusting system pressure relief valves and deicing system vacuum (suction) relief valves.
A pressure relief valve acts as a safety device to relieve excess pressure in the event of regulator valve failure. To adjust this valve, operate the aircraft engines and adjust a screw on the valve until the deicing pressure gage indicates the specified pressure at which the valve should relieve.
Vacuum relief valves are installed in a system that uses a vacuum pump to maintain constant suction during varying vacuum pump speeds. To adjust a vacuum relief valve, operate the engines.
While watching the vacuum (suction) gage, an assistant should adjust the suction relief valve adjusting screw to obtain the correct suction specified for the system.
Troubleshooting Not all troubles that occur in a deicer system can be corrected by adjusting system components. Some troubles must be corrected by repair or replacement of system components or by tightening loose connections. Several troubles common to pneumatic deicing systems are shown in the left-hand column of the chart in figure 7-7. Note the probable causes and the remedy of each trouble listed in the chart.
In addition to using troubleshooting charts, operational checks are sometimes necessary to determine the possible cause of trouble.
Inspection During each preflight and scheduled inspection, check the deicer boots for cuts, tears, deterioration, punctures, and security; and during periodic inspections go a little further and check deicer components and hines for cracks. If weather cracking of rubber is noted, apply a coating of conductive cement. The cement in addition to sealing the boots against weather, dissipates static electricity so that it will not puncture the boots by arcing to the metal Deicer Boot Maintenance The life of the deicers can be greatly extended by storing them when they are not needed and by observing these rules when they are in service:
(1) Do not drag gasoline hoses over the deicers.
(2) Keep deicers free of gasoline, oil, grease, dirt and other deteriorating substances.
(3) Do not lay tools on or lean maintenance equipment against the deicers.
(4) Promptly repair or re-surface the deicers when abrasion or deterioration is noted.
(5) Wrap the deicer in paper or canvas when storing it.
Thus far preventive maintenance has been discussed. The actual work on the deicers consists of cleaning, re-surfacing, and repairing. Cleaning should ordinarily be done at the same time the aircraft is washed, using a mild soap and water solution. Grease and oil can be removed with a cleaning agent, such as naptha, followed by soap and water scrubbing.
Whenever the degree of wear is such that it indicates that the electrical conductivity of the deicer surface has been destroyed, it may be necessary to re-surface the deicer. The re-surfacing substance is a black, conductive neoprene cement. Prior to applying the re-surfacing material, the deicer must be cleaned thoroughly and the surface roughened.
Cold patch repairs can be made on a damaged deicer. The deicer must be relieved of its installed tension before applying the patch. The area to be patched must be clean and buffed to roughen the surface slightly.
One disadvantage of a pneumatic deicer system is the disturbance of airflow over the wing and tail caused by the inflated tubes. This unwanted feature 292
Travibla Probablo Carso Komody Pressure gage oscillates. Faulty lines or connections. Repair or replace lines.
Tighten loose connections.
Deicing boots torn or punctured. Repair faulty boots.
Faulty gage: Replace gage, Faulty air relief valve. Adjust or replace relief valve.
Faulty air regulator. Adjust or replace regulator.
Preasure gage oscillates; peaks at a Vacuum check valves installed im- Re-install correctly.
specified pressure while instrument properly.
vacuum gage shows no reading.
Vacuum relief valve improperly Adjust or replace valve as necessary.
adjusted or faulty.
Faulty lines between pump and gage. Tighten, repair, or replace faulty lines or connections.
Pressure gage ahows no pressure Faulty pressure gage line. Repair or replace line.
while vacuum gage shows normal reading. Faulty pressure gage. Replace gage.
Pressure relief valve faulty. Adjust or replace as necessary.
Pressure regulator faulty. Adjust or replace as necessary.
Cycling period irregular. Loose or faulty tubing and connection. Tighten, repair, or replace as necessary.
Boots torn or punctured. Repair faulty boots.
Faulty electronic timer. Replace timer.
FIGURE 7-7. Troubleshooting pneumatic deicing systems.
of the deicer boot system has led to the development of other methods of ice control, one of which is the thermal anti-icing system.
THERMAL ANTI-ICING SYSTEMS Thermal systems used for the purpose of preventing the formation of ice or for deicing airfoil leading edges, usually use heated air ducted spanwise along the inside of the leading edge of the airfoil and distributed around its inner surface. However, electrically heated elements are also used for anti-icing and deicing airfoil leading edges.
There are several methods used to provide heated air, These include bleeding hot air from the turbine compressor, engine exhaust heat exchangers, and ram air heated by a combustion heater.
In installations where protection is provided by preventing the formation of ice, heated air is supplied continuously to the leading edges as long as the anti-icing system is "on." When a system is designed to deice the leading edges, much hotter air is supplied for shorter periods on a cyclic system.
The systems incorporated in some aircraft include an automatic temperature control. The temperature is maintained within a predetermined range by mixing heated air with cold air.
A system of valves is provided in some installations to enable certain parts of the anti-icing system to be shut off. In the event of an engine failure these valves also permit supplying the entire anti-icing system with heated air from one or more of the remaining engines. In other installations the valves are arranged so that when a critical portion of the wing has been deiced, the heated air can be diverted to a less critical area to clear it of ice.
Also, should icing conditions of unusual severity be encountered, the entire flow of air can be directed to the most critical areas.
The portions of the airfoils which must be pro.
tected from ice formation are usually provided with 293
a closely spaced double skin (see figure 7-8). The heated air carried through the ducting is passed into the gap. This provides sufficient heat to the outer skin to melt the layer of ice next to the skin or to prevent its formation. The air is then exhausted to the atmosphere at the wing tip or at points where ice formation could be critical; for example, at the leading edge of control surfaces.
Contour-etched outer skin Inner skin Front spar Deicing Leading edge air duct diaphragm Inner skin Exhaust FIGURE 7-8. A typical heated leading edge.
When the air is heated by combustion heaters, usually one or more heaters are provided for the wings. Another heater is located in the tail area to provide hot air for the leading edges of the vertical and horizontal stabilizers.
When the engine is the source of heat, the air is routed to the empennage through ducting which is usually located under the floor.
Anti-Icing Using Combustion Heaters Anti-icing systems using combustion heaters usually have a separate system for each wing and the empennage. A typical system of this type has the required number of combustion heaters located in each wing and in the empennage. A system of ducting and valves controls the airflow.
The anti-icing system is automatically controlled by overheat switches, thermal cycling switches, a balance control, and a duct pressure safety switch.
The overheat and cycling switches allow the heaters to operate at periodic intervals; and they also stop heater operation completely if overheating occurs.
A complete description of combustion heaters and their operation is discussed in Chapter 14, Cabin Atmosphere Control Systems, of this handbook.
The balance control is used to maintain equal heating in both wings. The duct pressure safety switch interrupts the heater ignition circuits if ram air pressure falls below a specified amount. This protects the heaters from overheating when not enough ram air is passing through.
An airflow diagram of a typical wing and empennage anti-icing system using combustion heaters is shown in figure 7-9.
Anti-Icing Using Exhaust Heaters Anti-icing of the wing and tail leading edges is accomplished by a controlled flow of heated air from heat muffs around a reciprocating engine's tail pipe. In some installations this assembly is called an augmentor. As illustrated in figure 7-10, an adjustable vane in each augmentor aft section can be controlled through a range of positions from closed to open. Partially closing each vane restricts the flow of cooling air and exhaust gases. This causes the temperature to rise in the heat muff forward of the vane. This provides a source of heat for the anti-icing system.
Normally, heated air from either engine supplies the wing leading edge anti-icing system in the same wing section. During single engine operation, a crossover duct system interconnects the left and right wing leading edge ducts. This duct supplies heated air to the wing section normally supplied by the inoperative engine. Check valves in the crossover duct prevent the reverse flow of heated air and also prevent cold air from entering the anti-icing system from the inoperative engine.
Figure 7-11 is a schematic of a typical anti-icing system that uses exhaust heaters. Note that, normally, the wing and tail anti-icing system is controlled electrically by operating the heat anti-ice button. When the button is in the "off" position, the outboard heat source valves and the tail anti-ice valve are closed. While the anti-ice system is off, the inboard heat source valves are controlled by the cabin temperature control system. Furthermore the augmentor vanes can be controlled by the augmentor vane switch.
Pushing the heat anti-ice button to the "on" position opens the heat source valves and the tail antiicing valve. A holding coil keeps the button in the "on" position. In addition, the augmentor vane control circuits are automatically armed. The vanes can be closed by positioning the augmentor vane switch 294
Outboard wing shutoff valve - Wing combustion heater Limited anti-icing valve Exhaust vents Empennage Exhaust vents combustion heater Limited anti-icing valve Outboard wing Wing combustion heater shutoff valve Ram air Heated air Airflow overboard FICURE 7-9. Airflow diagram of a typical anti-icing system.
to "close." This provides for maximum heat from the system. A safety circuit, controlled by thermo.
static limit switches (not shown) in the anti-icing system ducts, releases the anti-ice button to the "off" position whenever a duct becomes overheated.
When overheating occurs, the heat source valves and the tail anti-icing shutoff valve close and the augmentor vanes go to the trail (open) position.
Augmentor muff Exhaust stack Augmentor (engine exhaust heater) Augmentor vane Heat source valve Exhaust gases Heated air Cooling air Exhaust gases and cooling air mixture FIcURE 7-10. Heat source for thermal anti-icing system.
The heat source valves can be closed manually by the manual heat anti-ice shutoff handle. Manual operation may be necessary if the electrical control circuits for the valves fail. In this system, the handle is connected to the valves by a cable system and clutch mechanism. Once the heat source valves have been operated manually, they cannot be operated electrically until the manual override system is re-set: Most systems provide for re-setting of the manual override system in flight.
Anti-Icing Using Engine Bleed Air Heated air for anti-icing is obtained by bleeding air from the engine compressor. The reason for the use of such a system is that relatively large amounts of very hot air can be tapped off the compreasor, providing a satisfactory source of anti-icing and deicing heat.
A typical system of this type is shown in figure
7-12. This system is divided into six sections. Each section includes (1) a shutoff valve, (2) a temperature indicator, and (3) an overheat warning light.
The shutoff valve for each anti-icing section is a pressure regulating type. The valve controls the flow of air from the bleed air system to the ejectors, where it is ejected through small nozzles into mixing chambers. The hot bleed air is mixed with ambient air. The resultant mixed air at approximately 350° F. flows through passages next to the leading edge skin. Each of the shutoff valves is pneumatically actuated and electrically controlled. Each shutoff valve acts to stop anti-icing and to control airflow when anti-icing is required. 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 when the temperature in the leading edge reaches approximately 185° F. When the temperature drops, the valve opens, and hot bleed air enters the leading edge.
The temperature indicator for each anti-icing section is located on the anti-icing control panel. Each indicator is connected to a resistance-type temperature bulb located in the leading edge area. The temperature bulb is placed so that it senses the temperature of the air in the area aft of the leading edge skin, not the hot air passed next to the skin.
Overheat warning systems are provided to protect the aircraft structure from damage due to excessive heat. If the normal cyclic system fails, temperature sensors operate to open the circuit controlling the anti-ice shutoff valves. The valves close pneumatically to shut off the flow of hot air.
PNEUMATIC SYSTEM DUCTING The ducting usually consists of aluminum alloy, titanium, stainless steel, or molded fiber glass tubes.
Augmentor vane position indicator LH Note:
Inboard and outboard LH System shown.
augmentors RH System similar.
CLOSED TRAIL Augmentor vane actuator Augmentor overheat warning bell Trail Augmentor Off vane switch Note: Close The override position permits continued Note:
operation of the system if the circuit Controls inboard has been deenergized by malfunction and outboard of the thermal switches. • LH Valve heat source valves Structural Override overheat limit override switch Normal Manual anti-ice heat Off From RH inboard heat source valves shutoff LH inboard Push on heat source valve LH outboard Pull off heat source Heat valve To RH outboard anti-ice To RH inboard heat source valve button heat source valve To RH wing leading edge Tail anti-ice valve 100 LH wing Tail temperature temperature indicator indicator To LH wing Engine cooling air To tail To leading edge Exhaust gases leading edges air-conditioning system Heating air Electrical actuation Mechanical actuation FIcURE 7-11. Wing and tail anti-icing system schematic.
The tube, or duct, sections are attached to each other by bolted end flanges or by band-type veeclamps. The ducting is lagged with a fire-resistant, heat insulating material, such as fiber glass.
In some installations the ducting is interposed with thin stainless steel expansion bellows. These 297
Bleed air Electrical actuation Anti-icing wing Off Bleed air system Isolation valve Isolation valve Anti-icing empennage Off On Mercury thermostat (closes light circuit on rising temperature) Air valve Temperature bulb Pneumatically-actuated, Thermal switch (opens solenoid-controlled valve valve control circuit (opens when solenoid is energized) on risíng temperature) FIGURE 7-12. Schematic of a typical theral anti-icing system.
bellows are located at strategic positions to absorb any distortion or expansion of the ducting which may occur due to temperature variations.
The joined sections of ducting are hermetically sealed by sealing rings. These seals are fitted into annular recesses in the duct joint faces. When installing a section of duct, make certain that the seal bears evenly against, and is compressed by, the adjacent joint's flange.
When specified, the ducts should be pressure tested at the pressure recommended by the manufacturer of the aircraft concerned. Pressure testing is particularly important with pressurized aircraft, since a leak in the ducting may result in the inability to maintain cabin pressure. However, pressure tests are more often made to detect defects in the duct which would permit the escape of heated air.
The rate of leakage at a given pressure should not exceed that recommended in the aircraft maintenance or service manual.
Air leaks can often be detected audibly, and are sometimes revealed by holes in the lagging or thermal insulation material. However, if difficulty arises in locating leaks, a soap and water solution may be used.
All ducting should be inspected for security, general condition, or distortion. Lagging or insulating blankets must be checked for security and must be free of flammable fluids such as oil or hydraulic
GROUND DEICING OF AIRCRAFT The presence of ice on an aircraft may be the result of direct precipitation, formation of frost on integral fuel tanks after prolonged fight at high altitude, or accumulations on the landing gear following taxiing through snow or slush.
Any deposits of ice, snow, or frost on the external surfaces of an aircraft may drastically affect its performance. This may be due to reduced aerodynamic lift and increased aerodynamic drag resulting from the disturbed airflow over the airfoil surfaces, or it may be due to the weight of the deposit over the whole aircraft. The operation of an aircraft may also be seriously affected by the freezing of moisture in controls, hinges, valves, microswitches, or by the ingestion of ice into the engine.
When aircraft are hangared to melt snow or frost, any melted snow or ice may freeze again if the aircraft is subsequently moved into subzero temperatures. Any measures taken to remove frozen deposits while the aircraft is on the ground must also prevent the possible re-freezing of the liquid.
Frost Removal Frost deposits can be removed by placing the aircraft in a warm hangar or by using a frost remover or deicing fluid. These fluids normally contain ethylene glycol and isopropyl alcohol and can be applied either by spray or by hand. It should be applied within 2 hrs. of fight.
Deicing fluids may adversely affect windows or the exterior finish of the aircraft. Therefore, only the type of fluid recommended by the aircraft manufacturer should be used.
Removing Ice and Snow Deposits Probably the most dificult deposit to deal with is deep, wet snow when ambient temperatures are slightly above the freezing point. This type of deposit should be removed with a brush or squeegee.
Use care to avoid damage to antennas, vents, stall warning devices, vortex generators, etc., which may be concealed by the snow.
Light, dry snow in subzero temperatures should be blown off whenever possible; the use of hot air is not recommended, since this would melt the snow, which would then freeze and require further treat- Moderate or heavy ice and residual snow deposits should be removed with a deicing fluid. No attempt should be made to remove ice deposits or break an ice bond by force.
After completion of deicing operations, inspect the aircraft to ensure that its condition is satisfactory for flight. All external surfaces should be examined for signs of residual snow or ice, particularly in the vicinity of control gaps and hinges.
Check the drain and pressure sensing ports for obstructions. When it becomes necessary to physically remove a layer of snow, all protrusions and vents should be examined for signs of damage.
Control surfaces should be moved to ascertain that they have full and free movement. The landing gear mechanism, doors, and bay, and wheel brakes should be inspected for snow or ice deposits and the operation of uplocks and microswitches checked.
Snow or ice can enter turbine engine intakes and 299
freeze in the compressor. If the compressor cannot be turned by hand for this reason, hot air should be blown through the engine until the rotating parts are free.
WINDSHIELD ICING CONTROL SYSTEMS In order to keep window areas free of ice, frost, etc., window anti-icing, deicing, defogging, and demisting systems are used. The systems vary according to the type of aircraft and its manufacturer.
Some windshields are built with double panels having a space between, which will allow the circulation of heated air between the surfaces to control icing and fogging. Others use windshield wipers and anti-icing fluid which is sprayed on.
One of the more common methods for controlling ice formation and fog on modern aircraft windows is the use of an electrical heating element built into the window. When this method is used with pressurized aircraft, a layer of tempered glass gives strength to withstand pressurization. A layer of transparent conductive material (stannic oxide) is the heating element and a layer of transparent vinyl plastic adds a nonshattering quality to the window.
The vinyl and glass plies (figure 7-13) are bonded by the application of pressure and heat. The bond is achieved without the use of a cement as vinyl has a natural affinity for glass. The conductive coating dissipates static electricity from the windshield in addition to providing the heating element.
- Tempered glass ply, 3 places Vinyl ply, 2 places Conductive coating on the inner face of the outer glass ply FIcURE 7-13. Section through a laminated windshield.
On some aircraft, thermal electric switches automatically turn the system on when the air temperature is low enough for icing or frosting to occur.
The system may stay on all the time during such temperatures, or on some aircraft it may operate with a pulsating on-and-off pattern. Thermal overheat switches automatically turn the systems off in case of an overheating condition which could damage the transparent area.
An electrically heated windshield system includes:
(1) Windshield autotransformers and heat control relays.
(2) Heat control toggle switches.
(3) Indicating lights.
(4) Windshield control units.
(5) Temperature-sensing elements (thermistors) laminated in the panel.
A typical system is shown in figure 7-14. The system receives power from the 115 v. a.c. buses through the windshield heat control circuit breakers. When the windshield heat control switch is set to "high," 115 v., 400 Hz a.c. is supplied to the left and right amplifiers in the windshield control unit.
The windshield heat control relay is energized, thereby applying 200 v., 400 Hz a.c. to the windshield heat autotransformers. These transformers provide 218 v. a.c. power to the windshield heating current bus bars through the windshield control unit relays. The sensing element in each windshield has a positive temperature coefficient of resistance and forms one leg of a bridge circuit. When the windshield temperature is above calibrated value, the sensing element will have a higher resistance value than that needed to balance the bridge. This decreases the flow of current through the amplifiers and the relays of the control unit are de-energized.
As the temperature of the windshield drops, the resistance value of the sensing elements also drops and the current through the amplifiers will again reach sufficient magnitude to operate the relays in the control unit, thus energizing the windshield heaters.
When the windshield heat control switch is set to "low," 115 v., 400 Hz a.c. is supplied to the left and right amplifiers in the windshield control unit and to the windshield heat autotransformers. In this condition, the transformers provide 121 v. a.c.
power to the windshield heating current bus bars through the windshield control unit relays. The sensing elements in the windshield operate in the same manner as described for high-heat operation to maintain proper windshield temperature control.
The temperature control unit contains two hermetically sealed relays and two three-stage electronic amplifiers. The unit is calibrated to maintain a windshield temperature of 40° - 49° C.
(105° - 120° F.). The sensing element in euch windshield panel has a positive temperature coeficient of resistance and forms one leg of a bridge which controls the flow of current in its associated ampli- 300
(238V) R1 Right windshield (228V) R2 heat transformer (218V) R3 Right windshield (200V) L2 heat Com LJ K2 PIOA Right windshield Windshield heat 115V 400~ Amplifier (right) control relay 1ỌV AC - Right windshield heat • Output R3
• Sensing element Input • 115V 400~ 2 115V R2 Left windshield heat CR2 115V 400~ Left windshield heat Pion Com LI 115V 400~ (200v) L2 (218V) R3 Left windshield heat transformer (228V) R2 Left windshield (238V) R1 Amplifier (left) 10V AC •
• Output Sensing element Input Windshield heat control High Windshield heat = 1150 O TA control switch CR2 Off 115V. 400~ Low FIGURE 7-14. Windshield. temperature control circuit.