the cabin at constant-pressure altitude during flight at various levels. It is used until the aircraft reaches the altitude at which the difference between the pressure inside and outside the cabin is equal to the highest differential pressure for which the fuselage structure is designed. Differential control is used to prevent the maximum differential pressure, for which the fuselage was designed, from being exceeded. This differential pressure is determined by the structural strength of the cabin and often by the relationship of the cabin size to the probable areas of rupture, such as window areas and doors.
The cabin pressure regulator is designed to control cabin pressure by regulating the position of the outflow valve. The regulator usually provides either fully automatic or manual control of pressure within the aircraft. Normal operation is automatic, requiring only the selection of the desired cabin altitude and rate-of-change of cabin pressure.
Atmospheric vent ports Dump solenoid The cabin pressure regulator may be constructed integral with the outflow valve or may be mounted remote from the outflow valve and connected to it by external plumbing. In either instance the principle of operation is similar.
The regulator illustrated in figure 14-12 is integral with the outflow valve. This regulator is a differential pressure type, normally closed, pneumatically controlled and operated. This type regulator consists of two principal sections: (1) The head and reference chamber section, and (2) the outflow valve and diaphragm section.
The outflow valve and diaphragm section contains a base, a spring-loaded outflow valve, an actuator diaphragm, a balance diaphragm, and a baffle plate. The baffle plate is attached to the end of a pilot which extends from the center of the cover assembly. The outflow valve rides on the pilot Ground test valve Cabin air inlet True static atmosphere Isobaric control system Differential. connection control system . Filter
- Cabin air orifice Head -
• Solenoid Reference chamber_ air valve Cover -
- Flight Pilot Differential "on" Outflow - diaphragm Actuator valve — All "off" Base - C Baffle plate Balance diaphragm FIcURE 14-12. Cabin air pressure regulator.
between the cover and the baffle plate and is spring loaded to a closed position against the base.
The balance diaphragm extends outward from the baffle plate to the outflow valve, creating a pneumatic chamber between the fixed baffle plate and the inner face of the outflow valve. Cabin air flows into this chamber through holes in the side of the outflow valve to exert a force against the inner face, opposing spring tension, to open the valve. The actuator diaphragm extends outward from the outflow valve to the cover assembly, creating a pneumatic chamber between the cover and the outer face of the outflow valve. Air from the head and reference chamber section flows through holes in the cover, filling this chamber and exerting a force against the outflow valve's outer face to aid the spring tension in holding the valve closed. The position of the outflow valve controls the flow of cabin air to atmosphere for cabin pressure control. The action of components in the head and reference chamber section controls the movements of the outfow valve by varying the pressure of reference chamber air being exerted against the outer face of the valve.
The head and reference chamber section includes an isobaric control system, a differential control system, a filter, a ground test valve, a true static atmosphere connection, and a solenoid air valve.
The area inside the head is called the reference chamber.
The isobaric control system incorporates an evacuated bellows, a rocker arm, a follower spring, and an isobaric metering valve. One end of the rocker arm is connected to the head by the evacuated bellows. The other end of the arm positions the metering valve to a normally closed position against a passage in the head. A follower spring between the metering valve seat and a retainer on the valve causes the valve to move away from its seat as the rocker arm permits.
Whenever the reference chamber air pressure is great enough to compress the bellows the rocker arm pivots about its fulcrum. This allows the metering valve to move from its seat an amount proportionate to the amount of compression in the bellows.
When the metering valve is open, reference chamber air flows to atmosphere through the true static atmosphere connection.
The differential control system incorporates a diaphragm, a rocker arm, a metering valve, and a follower spring. One end of the rocker arm is attached to the head by the diaphragm. The diaphragm forms a pressure-sensitive face between the reference chamber and a small chamber in the head.
This small chamber is opened to atmosphere through a passage to the true static atmosphere connection. Atmospheric pressure acts on one side of the diaphragm and reference chamber pressure acts on the other. The opposite end of the rocker arm positions the metering valve to a normally closed position against a passage in the head. A follower spring between the metering valve seat and a retainer on the valve causes the valve to move away from its seat as the rocker arm permits.
When reference chamber pressure exceeds atmospheric pressure sufficiently to move the diaphragm, the metering valve is allowed to move from its seat an amount proportionate to the movement of the diaphragm. When the metering valve is open, reference chamber air flows to atmosphere through the true static atmosphere connection.
By regulating reference chamber air pressure, the isobaric and differential control systems control the actions of the outflow valve to provide for three modes of operation called unpressurized, isobaric, and differential.
During unpressurized operation, figure 14-13, reference chamber pressure is sufficient to compress the isobaric bellows and open the metering valve.
Cabin air entering the reference chamber through the cabin air orifice flows to the atmosphere through the isobaric metering valve. Since the cabin air orifice is smaller than the orifice formed by the metering valve, reference chamber pressure is maintained at a value slightly less than cabin pressure.
As pressure increases in the cabin, the differential pressure between the outflow valve inner and outer face increases. This unseats the outflow valve and allows cabin air to flow to the atmosphere.
As the isobaric range (figure 14-14) is approached reference chamber pressure, which has been decreasing at the same rate as atmospheric pressure, will have decreased enough to allow the isobaric bellows to expand and move the metering valve toward its seat. As a result, the flow of reference chamber air through the metering valve is reduced, preventing further decrease in reference pressure. In response to slight changes in reference chamber pressure, the isobaric control system modulates to maintain a substantially constant reference pressure in the chamber throughout the isobaric range of operation. Responding to the differential 553
Isobaric metering As the differential range is approached, the presvalve and bellows sure differential between the constant reference Cabin air pressure and the decreasing atmospheric pressure becomes sufficient to move the diaphragm and open the differential metering valve. As a result, reference chamber air flows to atmosphere through the differential metering valve, reducing the reference pressure. Responding to the decreased reference pressure, the isobaric bellows expands and closes the isobaric metering valve completely. Reference chamber pressure is now controlled, through the differential metering valve, by atmospheric pressure being reflected against the differential diaphragm.
Outlet As atmospheric pressure decreases, the metering valve valve opens more and allows reference pressure to decrease proportionately. Responding to the pressure differential between cabin and reference pressures, the outflow valve opens or closes as required Cabin air pressure to meter air from the cabin and maintain a prede- Control pressure termined differential pressure value.
- Atmospheric pressure In addition to the automatic control features just described, the regulator incorporates a ground test FIGURE 14-13. Cabin air pressure regulator in valve and a solenoid air valve, both of which are the unpressurized mode. located in the head and reference chamber section.
The solenoid air valve is an electrically activated Differential diaphragm valve spring-loaded to a normally closed position and metering valve True atmospheric vent against a passage through the head that opens the Cabin air Orifice reference chamber to atmosphere. When the cockpit Atmospheric pressure switch is positioned to "ram" the regulator vent solenoid opens, causing the regulator to dump cabin air to the atmosphere.
The ground test valve (see figure 14-12) is a three-position, manually operated control that allows for performance checks of the regulator and cabin pressurization system. In the test only-all off" position the valve renders the regulator completely inoperative. In the test only-differential on" position, the valve renders the isobaric control system inoperative so that the operation of the differential control system can be checked. In the "flight" position, the valve allows the regulator to a702? Cabin air pressure function normally. The ground test valve should Control pressure always be lockwired in the "flight" position unless Atmospheric pressure being tested.
FIGURE 14-14. Cabin air pressure regulator in the isobaric range.
between the constant reference chamber pressure and the variable cabin pressure, the outflow valve opens or closes, metering air from the cabin, as required, to maintain a constant cabin pressure.
Cabin Air Pressure Safety Valve The cabin air pressure safety valve (figure 14-15) is a combination pressure relief, vacuum relief, and dump valve. The pressure relief valve prevents cabin pressure from exceeding a predetermined differential pressure above ambient pressure.
The vacuum relief prevents ambient pressure from 554
Head
- Base Diaphragm P Middle compartment Inner compartment Outer compartment. 00000 Outflow valve Metering valve
- Pilot
• Filter Calibration screw Calibration spring
• Diaphragms Filter- Reference chamber P Solenoid valve Ball check valve Control chamber 190 Cabin pressure Decreasing control chamber pressure Atmospheric pressure Pressure bulkhead Diaphragm Pressure relief adjust
• Filter Vent port Bleed Cabin pressure Pressure relief being dumped Normal flight above (5.4 p.s.i.) (solenoid energized) FIcURE 14-15. Cabin air pressure safety valve.
exceeding cabin pressure by allowing external air to enter the cabin when ambient pressure exceeds cabin pressure. The dump valve illustrated is actuated by the cockpit control switch. When this switch is positioned to "ram," a solenoid valve opens, causing the valve to dump cabin air to atmosphere. On some installations a manual system, using cables and bellcranks, is provided to actuate the dump valve.
The safety valve consists of an outflow valve section and a control chamber. The outflow valve section and the control chamber are separated by a flexible pressure-tight diaphragm. The diaphragm is exposed to cabin pressure on the outflow valve side 555
and control chamber pressure on the opposite side.
Movement of the diaphragm causes the outflow valve to open or close. A filtered opening in the outflow valve allows cabin air to enter the reference chamber. The outflow valve pilot extends into this opening to limit the flow of air into the chamber.
Air pressure inside the reference chamber exerts a force against the inner face of the outflow valve to aid spring tension in holding the valve closed. The pressure of cabin air against the outer face of the outflow valve provides a force opposing spring tension to open the valve. Under normal conditions, the combined forces within the reference chamber are able to hold the outflow valve in the "closed" position. The movement of the outflow valve from closed to open allows cabin air to escape to atmosphere.
The head incorporates an inner chamber, called the pressure relief control chamber. Within the con trol chamber are located the two pressure relief diaphragms, the calibration spring, the calibration screw, and the spring-loaded metering valve. The action of these components within the chamber controls the movement of the outflow valve during normal operation.
The two diaphragms form three pneumatic compartments within the control chamber. The inner compartment is open to cabin pressure through a passage in the outflow valve pilot. The middle compartment is open to the reference chamber and is vented to the outer compartment through a bleed hole in the metering valve. The flow of reference chamber air from the middle compartment to the outer compartment is controlled by the position of the metering valve, which is spring-loaded to a normally closed position. The outer compartment, in which the calibration spring and screw are located, is opened to atmosphere through a passage in the head. Atmospheric pressure, reflected against the diaphragms, aids the calibration spring in keeping the metering valve closed. Cabin pressure, acting, on the diaphragms through the inner compartment, tries to open the metering valve by moving it back against the calibration screw. Under normal conditions, the combined forces of atmospheric pressure and the calibration spring hold the metering valve away from the calibration screw, keeping it closed.
Pressure relief occurs when the cabin pressure exceeds atmospheric pressure by a predetermined value. At this point, cabin pressure overcomes the combined forces of atmospheric pressure and spring tension in the control chamber, moving the metering valve back against the calibration screw, opening the metering valve. With the valve open, reference chamber air can escape through the outer compartment to the atmosphere. As the reference chamber air pressure is reduced, the force of cabin pressure against the outflow valve overcomes spring tension and opens the valve, allowing cabin air to flow to atmosphere. The rate-of-flow of cabin air to atmosphere is determined by the amount the cabinto-atmosphere pressure differential exceeded the calibration point. As cabin pressure is reduced, the forces opening the valve will be proportionately reduced, allowing the valve to return to the normally closed position as the forces become balanced.
In addition to the sutomatic operating provisions just described, the valve includes provisions for electrical activation to the dump position. This is accomplished by a passage in the head that allows reference chamber air to vent directly to atmosphere. The flow of air through the passage is controlled by a ball-check valve and an air solenoid valve. The solenoid valve is spring-loaded to a normally closed position. When the solenoid valve is opened by positioning the cockpit pressure switch to "ram," air flows from the reference chamber, decreasing the reference pressure and allowing the outflow valve to open and dump cabin air.
It should be remembered that the foregoing description of a pressure control system is for illustrative purposes and should not be construed to represent any particular make or model aircraft. Always refer to the applicable manufacturer's manual for the system details and limitations for the aircraft with which you are concerned.
AIR DISTRIBUTION The cabin air distribution system includes: (1) Air ducts, (2) filters, (3) heat exchangers, (4) silencers, (5) nonreturn (check) valves, (6) humidifiers, (7) mass flow control sensors, and (8) mass flow meters. The distribution system shown in figure 14-16 is typical of the system used on small turboprop aircraft.
Air enters the cabin supercharger through a screen-covered opening in the left engine oil cooler airscoop. If the air inlet screen ices over, a springloaded door beside the screen opens allowing air to bypass the screen. From the cabin supercharger, the air passes through a firewall shutoff valve, a pressure relief valve, and a silencer which dampens the supercharger noise and pulsations. The air then 556
Combustion heater Main duct Condenser • Valve Controller Electric motor Refrigerant compressor Fan Blower Evaporator Ram air- Return air Venturi Check valve Check and shutoff valve Air-to-air heat exchanger Hydraulic pump. Ram air-
- Overboard Ram air
- Flow control valve
- Silencer - Dump to ambient air Hydraulic motor gear box Pressure relief valve Supercharger firewall shutoff valve FICURE 14-16. Typical air distribution system.
passes through a flow control valve which governs the airflow rate to maintain the maximum pounds per minute airflow.
Air Ducts Ducts having circular or rectangular cross sections are most frequently used in air distribution systems. Circular ducts are used wherever possible.
Rectangular ducts are generally used where circular ducts cannot be used because of installation or space limitations. Rectangular ducts may be used in the cabin where a more pleasing appearance is desired.
Distribution ducts for various cabin zones, individual air outlets for passengers, and window demisters can have various shapes. Examples of circular, rectangular, elliptical and profiled ducts are illustrated in figure 14-17.
Cabin air supply ducts are usually made from aluminum alloys, stainless steel, or plastic. Main ducts for air temperatures over 200° C. are made from stainless steel. Those parts of the ducting where the air temperature does not exceed 100° C.
are usually constructed from soft aluminum. Plastic ducts, both rigid and flexible are used as outlet ducts to distribute the conditioned air.
Since heated air is routed throughout the duct system, it is important that the ducts be permitted to grow (expand through heating) and to shrink FIGURE 14-17. Cross sections of air distribution ducts.
again when the air cools down. This expansion and contraction must take place without loss of the pressure-tight integrity of the ducts. Expansion bellows (figure 14-18) are incorporated at various places throughout the duct system to permit the ducts to expand or contract.
In general, supports are necessary on both sides of a connecting bellows, a fixed support on one side to prevent duct movement, and a sliding support plus a fixed support on the other side. The sliding 557
Solid supports Fixed clamp Bellows Roller-fitted sliding clamp Fitting with fixed clamp and roller clamp (or sliding support)
- Swinging support FIcURE 14-18. Expansion bellows and duct supports.
support permits movement of the bellows while the duct section is under pressure. Typical duct support systems are illustrated in figure 14-18.
Whenever a duct is angled, means are provided to take care of the end forces which tend to push the duct sections apart. This can be accomplished with external swinging supports which attach the duct to rigid airframe structure (figure 14-19).
In some instances a connecting link is incorporated within the duct itself to transmit end loads.
The tension link within the bellows resembles a single link of chain that joins two segments of ducts. Figure 14-20 illustrates one such connecting link.
Filters The air delivered to a pressurized cabin from a supercharger or engine compressor may contain dust particles, oil mist, or other impurities. Unfiltered air which contains a considerable amount of impurities usually has an offensive odor and causes headache and nausea. Filters are generally incorporated into the ducting to clean the air.
AIR CONDITIONING SYSTEM The function of an air conditioning system is to maintain a comfortable air temperature within the aircraft fuselage. The system will increase or decrease the temperature of the air as needed to obtain the desired value. Most systems are capable of producing an air temperature of 70° to 80° F. with normally anticipated outside air temperatures. This temperature-conditioned air is then distributed so that there is a minimum of stratification (hot and cold layers). The system, in addition, must provide for the control of humidity, it must prevent the fogging of windows, and it must maintain the temperature of wall panels and floors at a comfortable level.
In a typical system the air temperature is measured and compared to the desired setting of the temperature controls. Then, if the temperature is 558
L- Rigid support Clamp Swinging support Combination solid & swinging support FIGURE 14-19. Typical supports for angled ducts.
FIGURE 14-20. Connecting link inside of bellows.
not correct, heaters or coolers are set into operation to change the air temperature, and the air is mixed together to create a uniform temperature in the cabin. In summary, an air conditioning system is designed to perform any or all of the following functions: (1) Supply ventilation air, (2) supply heated air, and (3) supply cooling air.
Ventilation Air Ventilation air is obtained through ram air ducts installed in the leading, lower, or upper surfaces of the aircraft or through other vents in the aircraft skin. Air entering these openings usually passes into and through the same duct system that is used for heating and cooling. On some aircraft, recirculating fans or blowers are present in the system to assist in circulating the air. Many aircraft have ground connections for receiving heated, cooled, or ventilating air from ground servicing equipment.
HEATING SYSTEMS A large part of the heating requirements for the conditioned air is accomplished automatically when the air is compressed by the cabin superchargers.
In many cases additional heat need not be added.
Compression of the air often provides more than the necessary heating. Consequently, cooling, to some degree, is required even when the outside air temperature is not high.
When a degree of heating in addition to that obtained from the "heat of compression" is needed, one of the following types of systems is put into operation: (1) Gasoline combustion heaters, (2) electric heaters, (3) re-cycling of compressed air, and (4) exhaust gas air-to-air heat exchanger.
Combustion Heater Combustion heaters operate similarly to the burner section of a turbojet engine. Gasoline is injected into the burner area under a pressure which breaks up the fuel into a fine mist. Combustion air is supplied to the burner by means of a ram air scoop or an electric motor driven fan. Ignition is supplied by continuous sparking of a special 559
spark plug. The combustion of fuel and air takes place continuously. The temperature output of the heater is controlled by a cycling process whereby combustion is turned on and off for short periods of time depending upon the heating required. The air which eventually mixes with the cabin air is routed around the burner section in a separate air passage. This ventilating air picks up heat from the burner by convection through the metal walls of the burner. The burner combustion gasses are exhausted overboard to prevent carbon monoxide contamination of the cabin.
Various automatic combustion heater controls prevent operation of the heater when dangerous conditions exist. As examples, the flow of fuel is cut off if there is insufficient combustion air, insuffcient ventilating air, and in some cases if the ignition system is not operating. Other controls prevent too rapid heating of the combustion chamber and prevent exceeding a maximum output temperature.
Electric heaters may be in the form of air duct heaters or electric radiant panels. The duct heater incorporates a series of high-resistant wire coils located in an air supply duct. When electric power is applied to the coils, they become hot. The air flowing through the duct carries the heat to the area where it is needed. Most duct heaters require the use of a fan to ensure sufficient airflow over the coils. Without the aid of the fan-produced airflow, the coils would fail due to overheating. Usually an electrical circuit is used which prevents heater operation unless the fan is in operation.
Air vent Conditioned air inlet Air vent Windshield defrosting blower Blower control Emergency spill valve Cold air scoop Main cabin heat control Main cabin air scoop Air scoop Heat exchanger Critical temperature waming lights.
FIGURE 14-21. Engine exhaust heater system.
Radiant Panels Radiant panels consist of wall and floor surfaces which have electric wires embedded in the panel material. When electric power is spplied to the wires, the wires and panel surface become hot. This type panel supplies heat to the cabin air principally by radiation.
Electric Heaters Electric heating systems require the expenditure of large amounts of electrical power. They cannot be used if the electrical system has limited capacity.
Electric heating systems, however, are quick acting and can be used to preheat the aircraft on the ground before the engines are started if an adequate electric ground power source is available.
Compressed Air Heating Some turbojet aircraft we a heating system in which the hot compressed air output of the cabin compressor is re-routed back into the compressor inlet. This double compression raises the temperature of the air to a sufficiently high degree so that other types of heating are usually not necessary.
Exhaust Gas Heaters A relatively simple heating system used on a few large aircraft utilizes the engine exhaust gases, figure 14-21, as a heat source. This system is particularly effective on aircraft where the engine exhaust is ejected through a long tailpipe. A hot air muff or jacket is installed around the tailpipe. Air Aircraft skin routed through the hot air muff picke up heat by convection through the tailpipe material. This heated air is then routed to an air-to-air heat exchanger, where its heat is given up to the air going to the cabin. By using the air-to-air heat exchanger in addition to the hot sir muff, the danger of carbon monoxide entering the cabin is minimized.
Regardless of the type, heating systems provide heated air for comfort and furnish heat for defrosting, deicing, and anti-icing of aircraft components and equipment. Nearly all types of heating systems use the forward motion of the aircraft to force conditioned air to various points on the aircraft. A heating system consists of a heating unit and the necessary ducting and controls. The units, ducts, and controls used will vary considerably from sys.
tem to system.
COMBUSTION HEATERS The number and size of combustion heaters used in a particular aircraft depend upon its size and its heating demands. These heaters are installed singly or in combination to fit the heating requirements of specific aircraft. A large single heater or several smaller heaters may be used. Regardless of size, every combustion heater needs four things for operation: (1) Fuel to burn, (2) ignition to ignite the fuel, (3) combustion air to provide the oxygen required to support the flame, and (4) ventilating air to carry the heat to the places where it is needed.
Aircraft fuel tank Combustion air scoop - Fuel fülter Sealed fuel control assy. Pamp Solenoid valve Regulator assembly Combustion air ground blower Combustion chamber Relief Thermal switches valve overheat and cycling Differential air pressure switch Filter Damper Line of flight -Spray nozzle Plenum Heated Damper air Exhaust Ignition unit • shroud Ventilating Drainair scoop Ventilating air Drain Aircraft skin Exhaust ground blower FIcuRE. 14-22. Heater installation schematic.
All combustion heaters are similar in operation and construction features. The major differences are in the methods of introducing fuel and in the location of units and accessories. The various components that comprise a complete aircraft heating system are shown schematically in figure 14-22.
Heater Fuel System The fuel used in the heaters is supplied in most cases from the same fuel tanks which supply the engines. Fuel flows from the tank to the heater by gravity or is pumped there by a fuel pump. Like the fuel which flows to the aircraft engine, heater fuel must first pass through a filter to remove impurities.
If foreign particles are not removed, they may eventually clog heater system units and prevent heater operation.
After the fuel is filtered, it flows through a fuel solenoid valve and metering nozzle. There are seyeral types of these valves and metering nozzles.
tained in a vertical standpipe. The latter type is illustrated in figure 14-24.
A preheater, in the form of an electrical wire coiled around the fuel line, Is used with some heaters having a vapor wick. It warms the fuel to speed vaporization and aids ignition when the outside temperature is below zero. Its use is usually limited to 2 min., because longer operation would damage the wire coil.
Fuel nozzle retainer Fuel screen Fuel line Air line Regardless of type, they usually have the same purpose, to maintain a constant volume at the fuel outlet to the combustion chamber. This uniform volume, in combination with a fixed combustion airflow, ensures a relatively constant fuel/air ratio to the heater. The result is a steady heater output. Fuel nozzle Tip core To increase or decrease the cabin temperature, the heaters are permitted to operate longer when Whirling Combustion more heat is needed and for shorter periods of time fuel spray chamber head when less heat is desired. On most heater systems this is accomplished automatically by an amplifier FIcURE 14-23. Typical heater spray nozzle assembly.
connected to temperature-sensing devices or by cycling switches which open and close the circuit to the fuel solenoid valve. Thus the heater cycles on Standpipe.
and off to maintain the temperature selected on a temperature control rheostat located in the cabin.
Most heater systems also include overheat Resistor (heater) switches in each heater outlet to automatically turn off the heater fuel supply when the temperature reaches about 350° F. It can be seen that control of the heater fuel supply is necessary, not only for normal operation of the heater but also for stopping it when overheated.
Another essential unit of the heater fuel system is the one that feeds fuel into the combustion chamber. Depending on the installation, it may be either Stainless steel a spray nozzle or a vapor wick. The spray nozzle wick steady spray into the stream of combustion air, (figure 14-23) is shaped so as to inject a fine, • Igniter assembly where it is ignited by a spark plug.
The vapor wick is made of asbestos contained in a circular hanged casting or of stainless steel con- FICURE 14-24. Stainless steel vapor wick.
Ignition System High voltage for heaters using spark plug igniters is supplied either by a high-potential ignition unit operating from the 28-v. d.c. aircraft supply or by ignition transformers operating from a 115-v.
a.c. aircraft source. The 28-v. d.c. ignition unit consists chiefly of a vibrator and step-up coil which produces a high-voltage spark at high frequency. A shielded lead is used to connect the step-up coil to the spark plug. The spark is produced between the center spark plug electrode and a ground electrode.
About the same result is obtained where ignition transformers are used. Here, however, power is supplied by the 115.v., 400-Hz main inverter a.c. sys.
tem. This is routed to the transformers, where it is stepped up to the very high voltage required to jump the spark gap at the spark plug. But whether a d.c. or an a.c. source is used to fire the spark plug, ignition is continuous during heater operation. This continuous operation prevents fouling of the spark plug electrodes.
It is the arrangement of the electrodes that makes the difference in the types of spark plugs used in aircraft combustion heaters. One type of spark plug is shown in figure 14-25A. This is known as a dual-electrode spark plug. Another type of plug to be found on combustion heaters is the shielded elec- 1/16" (A) Ground electrode Electrode (B) Shield (C) FIGURE 14-25. Heater ignition plugs.
trode plug (figure 14-25B). In this plug, the ground electrode forms a shield around the center electrode.
Although spark plug igniters differ somewhat in appearance, most glow coil igniters look similar to that shown in figure 14-25C. They consist of a resistance wire wound into a coil around a pin extending from the body of the igniter. The outer end of the coil is connected to the pin, thus providing both support and electrical continuity. The body of the igniter is fitted with two terminals, which are connected across the coil, and is threaded to provide for installation. The glow coil operates from the 24 or 28.v. d.c. power supply on the aircraft. The direct current causes the coil to become red hot, thereby igniting the fuel/air mixture until the heater is operating at a temperature sufficient to maintain the flame after the glow coil is turned off. A thermal cutout switch breaks the circuit to 'the' glow coil when this temperature is reached. This prolongs the life of the igniter.
Another type of plug used is a single electrode type (not shown). The ground electrode used with this type of plug is a separate installation attached to the heater at an angle that will provide an airgap between the plug's electrode and ground.
Combustion Air System Combustion air for each cabin heater is received through either the main air intake or through a separate outside scoop. On both pressurized and unpressurized aircraft it is provided by ram pressure during flight and by ground blowers during ground operation. To prevent too much air from entering the heaters as air pressure increases, either a combustion air relief valve or a differential pressure regulator is provided. The air relief valve is located in the line leading from the ram-air intake duct and is spring-loaded to dump excess air into the heater exhaust gas stream. The differential pressure regulator is also located in the combustion air intake line, but it controls the amount of air reaching the combustion chamber in a slightly different manner.
While the relief valve takes a large volume of air and bypasses the amount not needed, the pressure regulator allows only the needed amount of air to enter its intake in the first place. It does this by the use of a diaphragm and spring type control mechanism. One side of the diaphragm is vented to the heater intake air line and the other side to the heater exhaust gas line. Any change in the pressure 563