GENERAL Rain, snow, and ice are transportation's ancient enemies. Flying has added a new dimension, particularly with respect to ice. Under certain atmospheric conditions, ice can build rapidly on airfoils and air inlets.
The two types of ice encountered during fight are rime and glaze. Rime ice forms a rough surface on the aircraft leading edges. It is rough because the temperature of the air is very low and freezes the water before it has time to spread.
Glaze ice forms a smooth, thick coating over the leading edges of the aircraft. When the temperature is just slightly below freezing, the water has more time to flow before it freezes.
Ice may be expected to form whenever there is visible moisture in the air and the temperature is CHAPTER 7
ICE AND RAIN PROTECTION near or below freezing. An exception is carburetor icing which can occur during warm weather with no visible moisture present. If ice is allowed to accumulate on the wings and empennage leading edges, it destroys the lift characteristics of the airfoil. Ice or rain accumulations on the windshield interfere with vision.
Icing Effects Ice on an aircraft affects its performance and efficiency in many ways. Ice buildup increases drag and reduces lift. It causes destructive vibration, and hampers true instrument readings. Control surfaces become unbalanced or frozen. Fixed slots are filled and movable slots jammed. Radio reception is hampered and engine performance is affected (figure 7-1).
Effects of Icing are Cumulative
LIFT LESSENS
DRAG INCREASES THRUST FALLS OFF
WEIGHT GROWS Staling Speed Increases FIGURE 7-1. Effects of structural icing.
The methods used to prevent icing (anti-icing) or to eliminate ice that has formed (deicing) vary with the aircraft make and model. In this Chapter ice prevention and ice elimination using pneumatic pressure, application of heat, and the application of fluid will be discussed.
Ice Prevention Several means to prevent or control ice formation are used in aircraft today: (1) Heating surfaces using hot air, (2) heating by electrica.
elements, (3) breaking up ice formations, usually by inflatable boots, and (4) alcohol spray.
surface may be anti-iced either by keeping it dry by heating to a temperature that evaporates water upon impingement; or by heating the surface just enough to prevent freezing, maintaining it running wet; or the surface may be deiced by allowing ice to form and then removing it.
Ice prevention or elimination systems ensure safety of flight when icing conditions exist. Ice may be controlled on aircraft structure by the following methods.
Location of Ice Control
1. Leading edge of the wing Deicer tubes are inflated by an engine-driven air pump (vacuum pump), or by air bled from gas turbine engine compressors. The inflation sequence is controlled by either a centrally located distributor valve or by solenoid operated valves located adjacent to the deicer air inlets.
Deicers are installed in sections along the wing with the different sections operating alternately and symmetrically about the fuselage. This is done so that any disturbance to airflow caused by an inflated tube will be kept to a minimum by inflating only short sections on each wing at a time.
Deicing system not operating. Cells lie close to airfoil section. Ice is permitted to form.
Flexible hose
2. Leading edges of vertical and horizontal stabilizers
3. Windshields, windows, and radomes After deicer system has
4. Heater and Engine air inlets been put into operation, center cell inflates, cracking ice.
5. Stall warning transmitters
6. Pitot tubes
7. Flight controls edges
9. Carburetors Alcohol 10 When center cell deflates, outer cells inflate. This raises cracked ice causing it to PNEUMATIC DEICING SYSTEMS be blown off by air stream.
Pneumatic deicing systems use rubber deicers, called boots or shoes, attached to the leading edge of the wing and stabilizers. The deicers are composed of a series of inflatable tubes. During operation, the tubes are inflated with pressurized air, and deflated in an alternating cycle as shown in figure
7-2. This inflation and deflation causes the ice to crack and break off. The ice is then carried away by the airstream. FIcURE 7-2. Deicer boot inflation cycle.
DEICER BOOT CONSTRUCTION Deicer boots are made of soft, pliable rubber or rubberized fabric and contain tubular air cells. The outer ply of the deicer is of conductive neoprene to provide resistance to deterioration by the elements and many chemicals. The neoprene also provides a conductive surface to dissipate static electricity charges. These charges, if allowed to accumulate, would eventually discharge through the boot to the metal skin beneath, causing static interference with the radio equipment.
Deicer boots are attached to the leading edge of wing and tail surfaces with cement or fairing strips and screws, or a combination of both.
Deicer boots which are secured to the surface with fairing strips and screws or a combination of fairing strips, screws, and cement have a bead and bead wire on each lengthwise edge. On this type installation, screws pass through a fairing strip and the deicer boot just ahead of the bead wire and fit into Rivnuts located permanently in the skin of the aircraft.
The new type deicer boots (figure 7-3) are completely bonded to the surface with cement. The trailing edges of this type boot are tapered to provide a smooth airfoil. By eliminating the fairing strips and screws, this type installation cuts down on the weight of the deicer system.
- Wing skin
- Inflated tube Deflated tube FIGURE 7-3. Deicer boot cross section.
The deicer boot air cells are connected to system pressure and vacuum lines by nonkinking flexible hose.
In addition to the deicer boots, the major components of a representative pneumatic deicing system are a source of pressurized air, an oil separator, air pressure and suction relief valves, a pressure regulator and shutoff valve, an inflation timer, and a distributor valve or a control valve. A schematic of a typical system is shown in figure 7-4.
In this system, air pressure for system operation is supplied by air bled from the engine compressor.
The bleed air from the compressor is ducted to a pressure regulator. The regulator reduces the pressure of the turbine bleed air to the deicer system pressure. An ejector, located downstream of the regulator, provides the vacuum necessary to keep the boots deflated.
The air pressure and suction relief valves and regulators maintain the pneumatic system pressure and suction at the desired settings. The timer is essentially a series of switch circuits actuated successively by a solenoid-operated rotating step switch. The timer is energized when the deicing switch is placed in the "on" position.
When the system is operated, the deicer port in the distributor valve is closed to vacuum and system operating pressure is applied to the deicers connected to that port. At the end of the inflation period the deicer pressure port is shut off, and air in the deicer flows overboard through the exhaust port. When the air flowing from the deicers reaches a low pressure (approximately 1 p.s.i.), the exhaust port is closed. Vacuum is re-applied to exhaust the remaining air from the deicer.
This cycle is repeated as long as the system is operating. If the system is turned off, the system timer automatically returns to its starting position.
A pneumatic deicing system that uses an enginedriven air pump is shown in figure 7-5. The right hand side of the system is illustrated, however, the left hand side is identical. Notice that inflatable deicers are provided for the wing leading edges and the horizontal stabilizer leading edges. Included in the system are two engine-driven air (vacuum) pumps, two primary oil separators, two combination units, six distributor valves, an electronic timer, and the control switches on the deicing control panel.
To indicate system pressure, a suction indicator and a pressure indicator are included in the system.
Pneumatic System Operation As shown in figure 7-5, the deicer boots are arranged in sections. The right-hand wing boots include two sections: (1) An inboard (inner boot 287
Supply air Vertical stabilizer boots Vacuum Distributor valves - Pressure regulator and shutoff valve (numbers indicate inflation sequence Pressure relief valve Pressure switch (vacuum)
- Suction relief Exhaust port valve (typical) Pressure switch
- Ejector Outboard Outboard Center Inboard Inboard R.H. Horizontal stabilizer boots L.H. Horizontal stabilizer boots
• Cooling fins Off (On pneumatic system From wing Press suction Deicing control panel ports of all boots. Note: System not in operation. Vacuum supplied to all FIGURE 7-4. Schematic of a pneumatic deicing system.
Al and outer boot B2) section and (2) an outboard (inner boot A3 and outer boot B4) section.
The right-hand horizontal stabilizer has two boot sections (inner boot A5 and outer boot B6). A distributor valve serves each wing boot section and another distributor valve serves both horizontal stabilizer boot sections. Notice that each distributor valve has a pressure inlet port, a suction outlet port, a dump port, and two additional ports (A and B).
Distributor valve A and B ports are connected to related boot A and B ports. Pressure and suction can be alternated through A and B ports by the movement of a distributor valve solenoid servo valve. Note also that each distributor valve is connected to a common pressure manifold and a common suction manifold. When the pneumatic deicing system is on, pressure or suction is applied by either or both engine-driven air (vacuum) pumps. The suction side of each pump is connected to the suction manifold. The pressure side of each pump is connected through a pressure relief valve to the pressure manifold. The pressure relief valve maintains the pressure in the pressure manifold at 17 p.s.i. The pressurized air then passes to the primary oil separator. The oil separator removes any oil from the air. Oil-free air is then delivered to the combination unit. The combination unit directs, regulates to 15 p.s.i., and filters the air supply to the distributor valves.
When the pneumatic deicing system is off, air pump suction, regulated at 4 in. Hg by an adjustable suction relief valve, holds the deicing boots deflated. Air pump pressure is then directed overboard by the combination unit.
DEICING SYSTEM COMPONENTS Engine-Driven Air Pump The engine-driven air pump is of the rotary; four vane, positive displacement type and is mounted on 288
Right-hand wing deicers Inboard section Outboard section Inner SAI% Outer B2 20 Inner Outer B4 Suction indicator Solenoid Solenoid valve Distributor Dump Distributor Dump Pressure valve indicator Pressure Primary oil separator Combination unit manifold To opposite wing engine air supply, distributor valves, $ Ram air and boots inlet Fressure relief valve - Oil drain Suction Ram air manifold inlet Shroud Air overboard Electrical and electronic control Engine-driven air pump Motor-operated 'Oil cooler shutoff valve Suction relief valve Distributor valve Check valve (solenoid B shown energized, pressurizing boot B6) Horizontal stabilizer boots
• A5 Solenoid Inner ¡ Outer To boots on opposite side of horiz, stabilizer Pressure SE Suction — Electrical connection FicURE 7-5. A pneumatic deicing system using an engine-driven air pump.
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.