the aircraft power system to the coil of the slave relay which closes and completes the circuit to the fire-warning light.
The total number of thermocouples used in individual detector circuits depends on the size of the fire zone and the total circuit resistance. The total resistance usually does not exceed 5 ohms. As shown in figure 10-4, the circuit has two resistors.
The resistor connected across the terminals of the slave relay absorbs the coil's self-induced voltage.
This is to prevent arcing across the points of the sensitive relay, since the contacts of the sensitive relay are so fragile they would burn or weld if arcing were permitted.
When the sensitive relay opens, the circuit to the slave relay is interrupted and the magnetic field around its coil collapses. When this happens, the coil gets a voltage through self-induction, but with the resistor across the coil terminals, there is a path for any current flow as a result of this voltage.
Thus, arcing at the sensitive relay contacts is eliminated.
Continuous-Loop Detector Systems A continuous-loop detector or sensing system permits more complete coverage of a fire hazard area than any type of spot-type temperature detectors.
Continuous-loop systems are versions of the thermal switch system. They are overheat systems, heat-sensitive units that complete electrical circuits at a certain temperature. There is no rate-of-heat-rise sensitivity in a continuous-loop system. Two widely used types of continuous-loop systems are the Kidde and the Fenwal systems.
In the Kidde continuous-loop system (figure 10-5), two wires are imbedded in a special ceramic core within an Inconel tube.
FIGURE 10-5. Kidde sensing element.
FICURE 10-6. Fenwal sensing element.
One of the two wires in the Kidde sensing system is welded to the case at each end and acts as an internal ground. The second wire is a hot lead (above ground potential) that provides a current signal when the ceramic core material changes its resistance with a change in temperature.
Another continuous-loop system, the Fenwal system (figure 10-6), uses a single wire surrounded by a continuous string of ceramic beads in an Inconel tube.
The beads in the Fenwal detector are wetted with a eutectic salt which possesses the characteristic of suddenly lowering its electrical resistance as the sensing element reaches its alarm temperature. In both the Kidde and the Fenwal systems, the resistance of the ceramic or eutectic salt core material prevents electrical current from flowing at normal temperatures. In case of a fire or overheat condition, the core resistance drops and current flows between the signal wire and ground, energizing the alarm system.
The Kidde sensing elements are connected to a relay control unit. This unit constantly measures the total resistance of the full sensing loop. The system senses the average temperature, as well as any single hot spot.
The Fenwal system uses a magnetic amplifier control unit. This system is non-averaging but will sound an alarm when any portion of its sensing element reaches the alarm temperature.
Both systems continuously monitor temperatures in the affected compartments, and both will automatically reset following a fire or overheat alarm after the overheat condition is removed or the fire extinguished.
• Averaging section Discrete element Operating power unit Responder 115v.a.c.
400~ Light Bell Teflon sheath Diaphragm switch 115v.a.c. c- Fire test switch FIcURE 10-7. Lindberg fire detection system schematic.
Continuous Element System The Lindberg fire detection system (figure 10-7) is a continuous-element type detector consisting of a stainless steel tube containing a discrete element.
This element has been processed to absorb gas in proportion to the operating temperature set point.
When the temperature rises (due to a fire or overheat condition) to the operating temperature set point, the heat generated causes the gas to be released from the element. Release of the gas causes the pressure in the stainless steel tube to increase.
This pressure rise mechanically actuates the diaphragm switch in the responder unit, activating the warning lights and an alarm bell.
A fire test switch is used to heat the sensors, expanding the trapped gas. The pressure generated closes the diaphragm switch, activating the warring system.
Overheat Warning Systems Overheat warning systems are used on some aircraft to indicate high area temperatures that may lead to a fire. The number of overheat warning systems varies with the aircraft. On some aircraft, they are provided for each engine turbine and each nacelle, on others they are provided for wheel well areas and for the pneumatic manifoid.
When an overheat condition occurs in the detector area, the system causes a light on the fire control pane's to flash.
in most systems the detector is a type of thermal switch. Each detector is operated when the heat rises to a specified temperature. This temperature depends upon the system and the type and model of the aircraft. The switch contacts of the detector are on spring struts, which close whenever the meter case is expanded by heat. One contact of each detector is grounded through the detector mounting bracket. The other contacts of all detectors connect in parallel to the closed loop of the warning light circuit. Thus, the closed contacts of any one detector can cause the warning lights to burn.
When the detector contacts close, a ground is provided for the warning light circuit. Current then flows from an electrical bus through the warning lights and a flasher or keyer to ground. Because of the flasher in the circuit, the lights flash on and off to indicate an overheat condition.
TYPES OF FIRES The National Fire Protection Association has classified fires in threc basic types:
a. Class A fires, defined as fires in ordinary combustible materials such as wood, cloth, paper, upholstery materials, etc.
b. Class B fires, defined as fires in flammable petroleum products or other flammable or combustible liquids, greases, solvents, paints,
c. Class C fires, defined as fires involving energized electrical equipment where the electrical non-conductivity of the extinguishing media is of importance. In most cases where electrical equipnent is de-energized, extinguishers suitable for use on Class A or B fires may be employed effectively.
Aircraft fires, in flight or on the ground, may encompass either or all of these type fires. There- 411
fore, detection systems, extinguishing systems and extinguishing agents as applied to each type fire must be considered. Each type fire has characteristics that require special handling.
usable on Class A fires are not acceptable on Class B or C fires. Agents effective on Class B or C fires will have some effect on Class A fires but are not the most efficient.
FIRE ZONE CLASSIFICATION Powerplant compartments are classified into zones based on the air flow through them.
a. Class A Zone. Zones having large quantities of air flowing past regular arrangements of similarly shaped obstructions. The power section of a reciprocating engine is usually of this type.
b. Class B Zone. Zones having large quantities of air flowing past aerodynamically clean obstructions. Heat exchanger ducts and exhaust manifold shrouds are usually of this type.
Also, zones where the inside of the enclosing cowling or other closure is smooth, free of pockets, and adequately drained so leaking flammables cannot puddle are of this type.
Turbine engine compartments may be considered in this class if engine surfaces are aerodynamically clean and all airframe structural formers are covered by a fireproof liner to produce an acrodynamically clean enclosure surface.
c. Class C Zone. Zones having relatively small air flow. An engine accessory compartment separated from the power section is an example of this type of zone.
d. Class D Zone. Zones having very little or no These include wing compartments compation and wheel wells where little ventilation is
e.
N of this type.
EXTIN Airc common characteristics which patible agents dverse quality. Agents in current use will not freeze at normally expected atmospheric temperatures. The nature of the devices inside a powerplant compartment require agents that are not only uscful against flammable fluid fires but also effective on electrically caused fires. The various agents' characteristics are narratively described and then summarized in tabular form in Figures 10-8, 10-9, and 10-10.
Agents are classified into two general categories based on the mechanics of extinguishing action:
the halogenated hydrocarbon agents and the inert cold gas agents.
a. The Halogenated Hydrocarbon Agents.
(1) The most effective agents are the compounds formed by replacement of one or more of the hydrogen atoms in the simple hydrocarbons methane and ethane by halogen atoms.
(a) The halogens used to form extinguishing compounds are fluorine, chlorine, and bromine. Iodine may be used but is more expensive with no compensating advantage. The extinguishing compounds are made up of the element carbon in all cases, along with different combinations of hydrogen, fluorine, chlorine, and bromine. Completely halogenated agents contain no hydrogen atoms in the compound, are thus more stable in the heat associated with fire, and are considered safer. Incompletely halogenated compounds, those with one or more hydrogen atoms, are classed as fire extinguishing agents but under certain conditions may become flammable.
(b) The probable extinguishing mechanism of halogenated agents is a chemical interference in the combustion process between fuel and oxidizer. Experimental evidence indicates that the most likely method of transferring energy in the combustion process is by molecule fragments" resulting from the chemical reaction of the constituents. If these fragments are blocked from transferring their energy to the unburned fuel mole cules the combustion process may be slowed or stopped completely (extinguished). It is believed that the halogenated agents react with the molecular fragments, thus preventing the energy transfer. This may be termed "chemical cooling" or "energy transfer blocking." This extinguishing mechanism is much more effective than oxygen dilution and cooling.
AGENT ADVANTAGES DISADVANTAGES Bromotrifluoromethane Exte Cat extinguishant, about 4x as effective Moderately high cost CBr,F Heavier storage containers required "BT" Nontoxic at normal temperatures Halon 1301 Noncorrosive Compatible with conventional system, excellent with HRD Bromochlorodi- Very effective extinguishant Low relative toxicity fluoromethane Lightweight storage containers Requires N, pressure for expellant CBrCIF, "BCF" Halon 1211 Bromochloromethane Very effective extinguishant when used in Relatively toxic at normal temperatures CH,BrC1 Noncorrosive to steel and brass conventional systems Requires mechanical vaporization during dis- Very toxic when pyrolized "CB" Lightweight storage containers Halon 1011 Corrosive to aluminum and magnesium Methyl bromide More effective than CO, Relatively toxic CH,Br Lightweight storage containers Rapidly corrodes aluminum, zinc, and mag- ""MB" Readily available nesium Halon 1001 Low cost Compatible with conventional and HRD systems Carbon tetrachloride Liquid at normal temperatures Relatively toxic CCI, Readily available Severely toxic when pyrolized "CTC" Low cost Corrosive to iron and other metals Halon 104 Requires propellant charge Dibromodifluoromethane Very effective extinguishant Relatively toxic at noma, temperatures CBr‚F2 Noncorrosive to aluminum, steel and brass Very toxic when pyrolized Halon 1202 Lightweight storage containers High cost.
Conventional or HRD system Carbon dioxide Conventional or HRD system Can cause suffocation of personnel from cO, Relatively nontoxic lengthy exposure Noncorrosive Requires heavy storage containers Readily available Requires N, booster in cold climates Low cost Under normal temperatures it provides its own propellant Nitrogen Very effective extinguishant Can cause suffocation from lengthly ex- Noncorrosive posure Basically nontoxic Requires dewar to maintain liquid System may provide large quantities of extinguishant N, provides greatest 0, dilution FICURE 10-8. Extinguishing Agent Comparison.
(c) Since halogenated agents react with the molecular fragments, new compounds are formed which in some cases present hazards much greater than the agents themselves. Carbon tetrachloride, example, may form phosgene, used in warfare as a poison gas. However.
413 most agents generate relatively harmless halogen acids. This chemical reaction caused by heat (pyrolysis) makes some of these agents quite toxic in extinguishing use while being essentially nontoxic under normal room conditions.
evaluate the relative toxic hazard for
GROUP DEFINITION EXAMPLES (Highest) by volume for duration of exposure of the order of 5 minutes are lethal Gases or vapors which in concentration of the order of 1/2 to 1 per cent Sulfur dioxide or produce serious injury.
Gases or vapors which in concentrations of the order of 1/2 to 1 per Ammonia cent by volume for durations of exposure of the order of 1/2 hour are Methyl bromide lethal or produce serious injury.
sases or vapors which in concentrations of the order of 2 to 22 per cent Carbon tetrachloride oy volume for durations of exposure of the order of 1 hour are lethal oi Chloroform produce serious injury.
4 Gases or vapors which in concentrations of the order of 2 to 2% per cent Methyl chloride by volume for durations of exposure of the order of 2 hours are lethal or Ethyl bromide produce serious injury.
5 Gases or vapors less toxic than Group 4 but more toxic than Group 6. Methylene chloride Carbon dioxide Ethane, Propane, Butane 6 Gases or vapors which in concentrations up to at least about 20 per cent Bromotrifluoromethane (Lowest) by volume for durations of exposure of the order of 2 hours do not pro.
duce injury.
Ficure 10-9. Comparative life hazard of various refrigerants and other vaporizing liquids and gases. (Classified by the Underwriters' Laboratories, Inc.).
each agent, some consideration must be given to the effectiveness of the individual agent. The more effective the agent. the less quantity of agent required and the quicker will be the extinguishment, with less generation of decomposition products.
(d) These agents are classified through a system of "halon numbers" which describes the several chemical compounds making up this family of agents. The first digit represents the number of carbon atoms in the compound molecule; the second digit, the number of fluorine atoms; the third digit, the number of chlorine atoms; the fourth digit. the number of bromine atoms;
and the fifth digit, the number of iodine atoms, if any. Terminal zeroes are not expressed. For example, bromotrifluoromethane (CBrFs), is referred to as Halon 1301.
room temperatures some are referred to as "vaporizing liquid" extinguishing agents. Other agents are gaseous at normal room temperature but may be liquefied by compression and cooling, enabling them to be stored under pressure as liquids; these are called "liquefied gas" extinguishing agents. Both types of agents may be expelled from extinguishing system storage vessels by using nitrogen gas as a propellant.
(2) Characteristics of some halogenated agents
(a) Bromotrifluoromethane, CBrF., was developed by the research laboratories of of E. I. DuPont de Nemours & Co. in a program sponsored by the U.S. Armed Forces for the development of an aircraft fire extinguishing agent. It is very effective as an extinguishant, is relatively non-toxic and requires no pressurizing agent. This recently developed agent is gaining in usage because of its obvious advantages.
(b) Bromochlorodifluoromethane, CBrCIF another very effective agent that has been extensively tested by the U.S. Air Force. It has relatively low toxicity but it requires pressurization by nitrogen to expel it from storage at a satisfactory rate for extinguishment.
(c) Chlorobromomethane, CH.BrCL, was originally developed in Germany in World War Il for military aircraft. It is a more effective extinguishing agent than carbon tetrachloride and is somewhat less toxic although it is classed in the same hazard group.
(d) Methyl bromide, CH Br, has been used in the extinguishing systems in Britishbuilt aircraft engine installations for many years. Its natural vapor is more toxic than carbon tetrachloride and this characteristic hinders its use. Methyl bromide, as an incompletely halogenated compound with three hydrogen atoms per molecule, is a "borderline" material which may be flammable in itself at elevated temperatures. Tests indicate, however, that it is quite effective in its flame quenching power. Under the conditions found in an aircraft engine nacelle, the explosion suppression characteristic is dominant.
(e) Dibromodifluoromethane, generally considered more effective than methyl bromide and at least twice as effective as carbon tetrachloride as a flame suppressant. However, its rela-.
tive toxicity limits its use where it may enter inhabited compartments.
(1) Carbon tetrachloride, CCI,, is described in this manual primarily because of its historical interest and to provide a comparison with the other agents. CC1, is seldom used in aircraft extinguishing systems. It was the first generally accepted agent of the halogenated family and has been used commercially during the past 60 years, particularly for electrical hazards. in recent years, however, use of CC1, has declined due principally to the development of more effective agents and in part to the growing concern about the toxic nature of the CCI, vapors, especially when decomposed by heat.
b. Inert Cold Gas Agents.
Both carbon dioxide (CO:) and nitrogen (N,) are effective extinguishing agents.
ily available in gaseous and liquid forms; their main difference is in the temperatures and pressures required to store them in their compact liquid
(1) Carbon dioxide, CO2, has been used for many years to extinguish flammable fluid 415 fires and fires involving electrical equipment. It is noncombustible and does not react with most substances. It provides its own pressure for discharge from the storage vessel except in extremely cold climates where a booster charge of nitro.
gen may be added to "winterize" the system. Normally, CO, is a gas, but it is easily liquefied by compression and cooling. After liquefication, CO. will remain in a closed container as both liquid and gas. When CO. is then discharged to the atmosphere, most of the liquid expands to gas. Heat absorbed by the gas during vaporization cools the remaining liquid to -110° F. and it becomes a finely divided white solid. dry ice "snow."
CO. is about 1½ times as heavy as air which gives it the ability to replace air above burning surfaces and maintain a smothering atmosphere. CO: is effective as an extinguishant primarily because it dilutes the air and reduces the oxygen content so that the air will no longer support combustion. Under certain conditions some cooling effect is also realized.
CO. is considered only mildly toxic, but it can cause unconsciousness and death by suffocation if the victim is allowed to breath CO. in fire extinguishing concentrations for 20 to 30 minutes.
CO, is not effective as an extinguishant on fires involving chemicals containing their own oxygen supply, such as cellulose nitrate (some aircraft paints). Also fires involving magnesium and titanium (used in aircraft structures and assemblies) cannot be extinguished by CO.
(2) Nitrogen, N2, is an even more effective extinguishing agent. Like CO2, Ne is an inert gas of low toxicity. N, extinguishes by oxygen dilution and smothering. It is hazardous to humans in the same way as But more cooling is provided by N2 and pound for pound, N, provides almost twice the volume of inert gas to the fire as CO: resulting in greater dilu- The main disadvantage of No is that it must be stored as a cyrogenic liquid which requires a dewar and associated plumbing to maintain the -320° F.
temperature of liquid nitrogen (LN.).
Some large Air Force aircraft already in service use LN. in several ways. IN, systems are primarily utilized to inert the
Vapor HALON NUMBER UL TOXICITY (3) SPECIFIC GRAVITY OF LIQUID @ 65-F SPECIFIC WEIGHT BOILING POINT, FREEZING POINT, HEAT OF VAPORI- ZATION (BTU/ib.) APPROX. LETHAL Pressure, psi AGENT CHEMICAL FORMULA TYPE OF AGENT GROUPING Ib./in.s ppra CON 70°F 16001 Carbon dioxide cO, Cas 0.1234 - 110 112.5 658,000 750 Liquid (2) 658,000 Carbon tetrachloride CTC CCI, Liquid 170 — 8 83.5 28,000 1.9 12.5 300 Methyl bromide MB CH,Br Liquid 1001 2 1.73 0.0625 39 - 139 108.2 5,900 27 120 9,600 Bromochloromethane CB CH, BrCI Liquid 1011 3 1.94 0.069/ 149 - 124 99.8 65,000 2.7 17.0 BCM, 0.070 4,000 CMB Bromochlorodifluoro- BCF CBrCIF, Liquefied 1211 5 1.83 0.0663 25 -257 57.6 324,000 35 135 methane Gas 7,650 Dibromodifluoro- CBr,F¿ Liquid 1202 4 2.28 0.0822 76 - 112 52.4 54,000 13 58 methane 1,850 Bromotrifluoro- BT CBrF, Liquefied 1301 6 1.57 0.057 -72 -270.4 47.7 800,000 312 550 methane (Bromotri) Gas 20,000 Nitrogen N, 5 0.97 0.078 - 320 85 (I) (2)
(1) Dry gas compared to dry air at same temperature and pressure.
(2) Specific weight, Ib/fts @ 1 atmosphere pressure and 0°C.
(3) See Figure 10-9 for definitions.
(4) Ist value represents unheated agent.
2nd value represents agent heated to 1475°F.
FIGURE 10-10. Characteristics of extinguishing agents.
atmosphere in the fuel tank ullage by replacing most of the air with dry gaseous nitrogen, thereby diluting the oxygen content. With the large quantities of LN.
thus available, N. is also being used for aircraft fire control and is feasible as a practical powerplant fire extinguishing agent.
A long-duration LN system discharge can provide greater safety than conventional short-duration system by cooling potential reignition sources and reducing the vaporization rate of any flammable fluids remaining after extinguishment. Liquid nitrogen systems are expected to see commercial usage in the near future.
FIRE EXTINGUISHING SYSTEMS
a. High-Rute-of-Discharge Systems. This term, abbreviated HRD, is applied to the highly effective systems most currently in use. Such HRD systems provide high discharge rates through high pressurization, short feed lines, large discharge valves and outlets. The extinguishing agent is usually one of the halogenated hydrocarbons (halons) sometimes boosted by high-pressure dry nitrogen (N.). Because the agent and pressurizing gas of an HRD system are released into the zone in one second or less, the zone is temporarily pressurized, and interrupts the ventilating air flow. The few.
large sized outlets are carefully located to produce high velocity swirl effects for best distribution.
b. Conventional Systems. This term is applied to those fire extinguishing installations first used in aircraft. Still used in some older aircraft, the systems are satisfactory for their intended use but are not as efficient as newer designs. Typically these systems utilize the perforated ring and the so-called distributor-nozzle discharge arrangement.
One application is that of a perforated ring in the accessory section of a reciprocating engine where the air fow is low and distribution requirements are not severe. The distributor-nozzle arrangements are used in the power section of reciprocating engine installations with nozzles placed behind each cylinder and in other areas necessary to provide adequate distribution.
uses carbon dioxide (COs) for extinguishant but may use any other adequate agent.
RECIPROCATING ENGINE CONVENTIONAL CO: SYSTEM CO, is one of the earliest types of fire extinguisher systems for transport aircraft and is still used on many older aircraft.
Fire extinguisher operating valve Main distribution line
- Cable conduit Cylinder assembly FIGURE 10-11. Carbon dioxide (CO:) cylinder installation.
This fire extinguisher system is designed around a cylinder (figure 10-11) that stores the flame-smothering CO; under pressure and a remote control valve assembly in the cockpit to distribute the extinguishing agent to the engines. The gas is distributed through tubing from the CO, cylinder valve to the control valve assembly in the cockpit, and then to the engines via tubing installed in the fuselage and wing tunnels. The tubing terminates in perforated loops which encircle the engines (figure 10-12).
To operate this type of engine fire extinguisher system, the selector valve must be set for the engine which is on fire. An upward pull on the T-shaped control handle located adjacent to the engine selector valve actuates the release lever in the CO, cylinder valve. The compressed liquid in the CO, cylinder flows in one rapid burst to the outlets in the distribution line (figure 10-12) of the affected engine. Contact with the air converts the liquid into gas and "snow" which smothers the flame.
A more sophisticated type of CO, fire protection system is used on many four-engine aircraft. This system is capable of delivering CO, twice to any one of the four engines. Fire warning systems are installed at all fire hazardous locations of the aircraft to provide an alarm in case of fire.
Vent line CO, cylinder Distribution lines Perforated loop FIcURE 10-12. CO: fire extinguisher system on a twin-engine transport aircraft.
The various warning systems operate fire warning lights on the cockpit fire control panels and also energize a cockpit warning bell.
In one type of aircraft the CO; system consists of six cylinders, mounted three to a row in each side of the nose wheel well with flood valves installed on each COz bottle. The flood valves of each row are connected with gas interconnect lines. The valves on the two aft bottles in each bank are designed to be opened mechanically by means of a cable connected to the discharge control handles on the main fire control panel in the cockpit. In case of discharge by thermosafety-discharge indicator disk set to rupture at or above a pressure of 2,650 p.s.i. Discharge overboard will occur at temperatures above 74° C.
Each bank of bottles is also equipped with a yellow system-discharge indicator disk. Mounted adjacent to the red disk, the yellow disk indicates which bank of bottles has been emptied by a normal discharge.
This type of CO, fire protection system includes a fire warning system. It is a continuous-loop, lowimpedance, automatic re-setting type for the engine and nacelle areas.
mechanical means, the forward bottle flood valve in A single fire detector circuit is provided for each each bank is operated by the released CO, pressure engine and nacelle area. Each complete circuit confrom the two aft bottles through the interconnect sists of a control unit, sensing elements, a test relay, lines. The flood valve on the forward bottle of each a fire warning signal light, and a fire warning sig.
bank contains a solenoid. It is designed to be oper- nal circuit relay. Associated equipment such as flexiated electrically by energizing the solenoid by de- ble connector assemblies, wire, grommets, mounting pressing a button on the control panel. In case of a brackets, and mounting clamps are used in various discharge by electrical means, the valves on the two quantities, depending upon individual installation aft bottles of each bank are operated by the re- requirements. On a four-engine aircraft, four warnleased CO pressure from the forward bottle ing light assemblies, one for each engine and nathrough the interconnector lines. celle area, give corresponding warning indications Each bank of CO, bottles is equipped with a red when an alarm is initiated by a respective engine 418
fire warning circuit. Warning light assemblies in the CO, manual release handles are connected into all four engine fire detector circuits, along with a fire warning bell with its guarded cutoff switch and indicating light.
The insulated wire of the detector circuit runs from the control unit in the radio compartment, through the fuselage and wing, to the test relay.
The wire is then routed through the nacelle and engine sections and back to the test relay, where it is joined to itself to form a loop.
The control units are normally located on a radio compartment rack. Each unit contains tubes or transistors, transformers, resistors, capacitors, and a potentiometer. It also contains an integrated circuit which introduces a time delay that desensitizes the warning system to short-duration transient signals that would otherwise cause momentary false alarms.
When a fire or overheat condition exists in an engine or nacelle area, the resistance of the sensing loop decreases below a preset value determined by the setting of the control unit potentiometer which is in the bias circuit of the control unit detector and amplifer circuit. The output of this circuit is used to energize the fire warning bell and fire warning light.
TURBOJET FIRE PROTECTION SYSTEM A fire protection system for a large multi-engine turbojet aircraft is described in detail in the following paragraphs. This system is typical of most turbojet transport aircraft and includes components and systems typically encountered on all such aircrait.
The fire protection system of most large turbine engine aircraft consists of two subsystems: (1) A fire detection system and (2) a fire extinguishing system. These two subsystems provide fire protection not only to the engine and nacelle areas but also to such areas as the baggage compartments and wheel wells.
Each turbine engine installed in a pod and pylon configuration contains an automatic heat-sensing fire detection circuit. This circuit consists of a heat-sensing unit, a control unit, a relay, and warning devices. The warning devices normally include a warning light in the cockpit for each circuit and a common alarm bell used with all such circuits.
Extinguishing agent container (Reserve agent container for No. 1 pod) Overheat control unit (Typ.)
Fire control unit Test relay Test relay Diffuser orifice To outboard pod Pylon discharge Continuous overheat Continuous fire detection (sensing) detection (sensing) loop-aft compartment loop-fwd compartment • Fire extinguisher discharge tube outlets FIcURE 10-13. Typical pod and pylon fire protection installation.
The heat-sensing unit of each circuit is a continuous loop routed around the areas to be protected.
These areas are the burner and tailpipe areas. Also included in most turbine engine aircraft are the compressor and accessory areas, which in some installations may be protected by a separate fire protection circuit. Figure 10-13 illustrates the typical routing of a continuous-loop fire detection circuit.
A typical continuous loop is made up of sensing elements joined to each other by moistureproof connectors, which are attached to the aircraft structure. In most installations, the loop is supported by attachments or clamps every 10 to 12 in. of its length. Too great a distance between supports may permit vibration or chafing of the unsupported section and become a source of false alarms.
In a typical turbine engine fire detection system, a separate control unit is provided for each sensing circuit. The control unit contains an amplifier, usually a transistorized or magnetic amplifier, which produces an output when a predetermined input current flow is detected from the sensing loop.
Each control unit also contains a test relay, which is used to simulate a fire or overheat condition to test the circuit. All the control units are mounted in a relay shield or junction box located in a radio compartment or in a special area of the cockpit.
The output of the control unit amplifer is used to energize a warning relay, often called a fire relay.
Usually located near the control units, these fire relays, when energized, complete the circuit to appropriate warning devices.
The warning devices for engine and nacelle fires and overheat conditions are located in the cockpit.
A fire warning light for each engine is usually located in a special fire switch handle on the instrument panel, light shield, or fire control panel. These fire switches are sometimes referred to as fire-pull T-handles. As illustrated in figure 10-14, the T-handle contains the fire detection warning. light. In some models of this fire-pull switch, pulling the T-handle exposes a previously inacceasible extinguishing agent switch and also actuates microswitches which energize the emergency fuel shutoff valve and other pertinent shutoff valves.
TURBINE ENGINE FIRE EXTINGUISHING SYSTEM The typical fire extinguishing portion of a complete fire protection system includes a cylinder or container of extinguishing agent for each engine and nacelle area. One type of installation provides for a container in each of four pylons on a multi- Extinguishing agent
- discharge switch Fire-pull T-handle Microswitches T-handle shaft Fire and overheat warning lights FICERE 10-14. Fire-pull T-handle switch.
engine aircraft. This type of system uses an extinguishing agent container similar to the type shown in figure 10-15. This type of container is equipped with two discharge valves which are operated by electrically discharged cartridges. These two valves are the main and the reserve controls which release and route the agent to the pod and pylon in which the container is located or to the other engine on the same wing. This type of two-shot, crossieed configuration permits the release of a second charge of fire extinguishing agent to the same engine it another fire breaks out, without providing two containers for each engine area.
Another type of four-engine installation uses two independent fire extinguisher systems. The two en.
gines on one side of the aircraft are equipped with two fire extinguisher containers, but they are located together in the inboard pylon (figure 10-16).
A pressure gage, a discharge plug, and a safety discharge connection are provided for each container. The discharge plug is sealed with a breakable disk combined with an explosive charge which is electrically detonated to discharge the contents of the bottle. The safety discharge connection is capped at the inboard side of the strut with a red indicating disk. If the temperature rises beyond a predetermined safe value, the disk will rupture, dumping the agent overboard, and the discharge will be indicated in the cockpit.
The manifold connecting the two containers of the dual installation (figure 10-16) includes a double check valve and a tee-fitting from which tubing connects to the discharge indicator. This indicator is capped at the inboard side of the strut with a 420
To No. 1 engine fire-pull switch Second shot to No. 1 engine To cockpit light Pressure switch Second shot for No. 2 engine To No. 2 engine fire-pull switch agent out Pressure gage (2) To overboard vent No. 2 engine No. 1 engine container container Explosive cartridge Double check tee valve (2)- To No. 2 engine pod To No. 1 engine pod FIcURE 10-15. Fire extinguisher system for a multi-engine aircraft.
yellow disk, which is blown out when the manifold is pressurized from either container. The discharge line has two branches (figure 10-16), a short line to the inboard engine and a long one extending along the wing leading edge to the outboard engine.
Both of the branches terminate in a tee-fitting near the forward engine mount.
Discharge tube configuration may vary with the type and size of turbine engine installations. In figure 10-17, a semicircular discharge tube with Y-outlet terminations encircles the top forward area of both the forward and aft engine compartments.
Diffuser orifices are spaced along the diffuser tubes.
A pylon discharge tube is incorporated in the inlet line to discharge the fire extinguishing agent into the pylon area.
Another type of fire extinguisher discharge configuration is shown in figure 10-18. The inlet discharge line terminates in a discharge nozzle, which is a tee-fitting near the forward engine mount. The tee-fitting contains diffuser holes which allow the fire extinguishing agent to be released along the top of the engine and travel downward along both sides of the engine.
When any section of the continuous-loop circuit is exposed to an overheat condition or fire, the detector warning lights in the cockpit illuminate and the fire warning bell sounds. The warning light may be located in the fire-pull T-handle, or in some installations the fire switch may incorporate the associated fire warning light for a particular engine under a translucent plastic cover, as shown in figure
10-19. In this system, a transfer switch is provided for the left and right fire extinguisher system. Lach transfer switch has two positions: "TRANS" and "NORMAL." If a fire occurs in the No. 4 engine, the warning light in the No. 4 fire switch will illuminate; and with the transfer switch in the "NORMAL" position, the No. 4 fire switch is pulled and the No. 4 push button discharge switch located directly under the fire switch will be accessible.
Activating the discharge switch will discharge a container of fire extinguishing agent into the No. 4 engine area.
If more than one shot of the agent is required, the transfer switch is placed in the "TRANS" position so that the second container can be discharged into the same engine.
An alarm bell control permits any one of the engine fire detection circuits to energize the 421