PRINCIPLES OF FIRE-DETECTION SYSTEMS For a fire to occur, three conditions must be met. There must be fuel, oxygen, and enough heat to raise the tempe rat ure of the fue l to its igni ti on or kin dli ng point . If any of the se elements is missing or removed, fire will not be sustained. A fire triangle illustrates that a fire requires fuel, oxy gen, and enough heat to cause the fuel and oxygen to ignite. If any of these elements is missing, a fire will not ignite or continue to burn. Chemically, fire is a reaction between oxygen and fuel. This reaction reduces fuel to its basic chemical elements and in the pro cess prod uces tremendous amou nts of heat. Pa pe r, fo rexample, is an organic material composed primarily of carbon and hydrogen. hen the paperis heated to its kindling temperature in the presence of air, the carbon and hydrogen will unite with oxygen to form carbon dioxide !C"#$ and water !%#"$. "ther elements in the paper, and the products of incomplete combustion, show up as ash and black carbon to form smoke. In the case of smoke and fire ha&ards aboard aircraft, the emission of smoke or the presence offlames and heat makes it reasonably easy for a person to physically detect a fire or overheat condition. The smoke produced by combustion produces strong odors and is readily visible in most circumstances, so the crew of an aircraft can physically detect a fire ha&ard in its early stages, provided they are in the same compartment or area of the aircraft where the fire occurs. %owever, many aircraft areas are inaccessible to the crew and, because of the design of the aircraft, airflow around and through various compartments may prevent the ha&ard from being d etected until it is too late to r emedy the problem. To provide a more thorough means of monitoring remote locations of an aircraft for smoke orfire, detection systems are mounted in areas the crew doe s not have ac ces s to in fli gh t. 'ome examp les of areas where these systems may be installed include engi ne nacell es, baggage compartments, ele ctric al or elec tronic e(ui pment bays and passenger lava torie s. )epending on t he types of combustible materials that may smol der or igni te, the systems are designed to activate by various means to provide the most accurate indication of an actual ha&ard. CLASSES OF FIRES To understand how and why different types of fire*detection systems are better suited forcertain applications, you need to be familiar with the classifications of fire as identified by the +ational Fire Protection ssociation. These fires are identified in con-unction with the types of materials consumed by a fire and are assigned different letter classifications as followsClass A fireis one in wh ich so lid co mbu sti ble material burns, such as wood, paper, orcloth. Control cabins or passenger compartments are examples of locations where Class fires are likely to occur. 'ince the interiors of the passenger compartment and of the cockpit are readily accessible to the crew, fire detection in these areas is generally accomplished by visual surveillance. "n the other hand, such fires can also occur in baggage compartments, where crew access is limited or even impossible during flight. In these areas, monitoring is pri* marily accomplished with electrically powered smoke* or flame*detector systems.
For a fire to occur, three conditions must be met. There must be fuel, oxygen, and enoughheat to raise the temperature of the fuel to its ignition or kindling point. If any of theseelements is missing or removed, fire will not be sustained.
A fire triangle illustrates that a fire requires fuel, oxygen, and enough
heat to cause the fuel and oxygen to ignite. If any of these elements is
missing, a fire will not ignite or continue to burn.
Chemically, fire is a reaction between oxygen and fuel. This reaction reduces fuel to its basicchemical elements and in the process produces tremendous amounts of heat. Paper, for
example, is an organic material composed primarily of carbon and hydrogen. hen the paper is heated to its kindling temperature in the presence of air, the carbon and hydrogen will unitewith oxygen to form carbon dioxide !C"#$ and water !%#"$. "ther elements in the paper, and theproducts of incomplete combustion, show up as ash and black carbon to form smoke.
In the case of smoke and fire ha&ards aboard aircraft, the emission of smoke or the presence of flames and heat makes it reasonably easy for a person to physically detect a fire or overheatcondition. The smoke produced by combustion produces strong odors and is readily visible inmost circumstances, so the crew of an aircraft can physically detect a fire ha&ard in its earlystages, provided they are in the same compartment or area of the aircraft where the fireoccurs. %owever, many aircraft areas are inaccessible to the crew and, because of the designof the aircraft, airflow around and through various compartments may prevent the ha&ard from
being detected until it is too late to remedy the problem.
To provide a more thorough means of monitoring remote locations of an aircraft for smoke or fire, detection systems are mounted in areas the crew does not have access to in flight.'ome examples of areas where these systems may be installed include engine nacelles,baggage compartments, electrical or electronic e(uipment bays and passenger lavatories.)epending on the types of combustible materials that may smolder or ignite, the systems aredesigned to activate by various means to provide the most accurate indication of an actualha&ard.
CLASSES OF FIRES
To understand how and why different types of fire*detection systems are better suited for certain applications, you need to be familiar with the classifications of fire as identified by the+ational Fire Protection ssociation. These fires are identified in con-unction with the typesof materials consumed by a fire and are assigned different letter classifications as follows
Class A fire is one in which solid combustible material burns, such as wood, paper, or cloth. Control cabins or passenger compartments are examples of locations where Class fires are likely to occur. 'ince the interiors of the passenger compartment and of the cockpitare readily accessible to the crew, fire detection in these areas is generally accomplished byvisual surveillance. "n the other hand, such fires can also occur in baggage compartments,where crew access is limited or even impossible during flight. In these areas, monitoring is pri*
marily accomplished with electrically powered smoke* or flame*detector systems.
Class B fires are composed of combustible li(uids such as gasoline, oil, -et fuel, and many of the paint thinners and solvents used in aviation maintenance. "n an aircraft, these classes of fires typically occur in engine compartments or nacelles and in compartments that house anauxiliary power unit !P/$. 'ince operating temperatures within these areas can beextreme, overheat detection systems, which sense the rate of temperature rise, are oftenused to monitor the &one for the presence of fire or overheat conditions. ith these types of
monitoring devices, false alarms are less likely than with other types of detection systems.
Class C fires are those that involve energi&ed electrical e(uipment. These fires re(uirespecial care because of the dangers from the electricity, in addition to those from the fire itself.'uch fires are generally confined to electrical and electronic e(uipment bays and to areasbehind electrical control panels. 'ince the initial stages of electrical e(uipment fires areusually preceded by large amounts of smoke, these areas of an aircraft are generally mon*itored by smoke*detection systems.
Class D fires involve burning metals such as magnesium, and are difficult to extinguish./sing the wrong type of extinguishing agent with these fires may not only be ineffective, butmay even cause the fire to spread. lthough these types of fires are not common in aircraftduring flight, they can occur in maintenance shops, where metal shavings may ignite whenexposed to intense heat such as from a welding torch or high*voltage source.
FIRE ZONES
0arious compartments in an aircraft are classified into fire &ones based on the amount andcharacteristics of airflow through them. The airflow through a compartment determines theeffectiveness of fire*detection systems, as well as the effectiveness of suppressant materialsused to extinguish a fire. Fire &ones are primarily classified by the amount of oxygen that isavailable for combustion and are identified as , 1, C, ), or 2 &ones.
Class A zones have large (uantities of air flowing past regular arrangements of similarly
shaped obstructions. The power section of a reciprocating engine is a common example of this &one. For these areas, a fire*extinguishing system is usually installed, but may notprove ade(uate since the suppressant may be carried out into the air*stream beforeextinguishing the fire.
Class B zones have large (uantities of air flowing past aerodynamically clean obstructions.%eat*exchanger ducts and exhaust manifold shrouds are usually of this type, as are &oneswhere the inside of the cowling or other enclosure is smooth, free of pockets, and ade(uatelydrained so that leaking flammables cannot puddle. For example, turbine enginecompartments are in this &one class, if the engine surfaces are aerodynamically clean and afireproof liner is installed to produce a smooth enclosure surface over any ad-acent airframestructure. Class 1 &ones are usually protected by temperature sensing elements or flame andsmoke detection systems as well as extinguishing e(uipment, to provide a means of controlling a fire if one should occur.
Class C zones have relatively low airflow through them. n auxiliary power unit !P/$compartment is a common example of this type of &one. These may be protected by a fire*detection and extinguishing system, or the compartment may have provisions for isolatingflammable materials such as fuel, oil, and hydraulic fluids.
Class D zones have very little or no airflow. These include wing compartments and wheelwells, where little ventilation is provided. )ue to the lack of airflow, fire*extinguishingsystems are usually not necessary since the fire will self*extinguish as it consumes theatmosphere. %owever, fire*detection systems are often installed in Class ) &ones to warn thecrew that damage may have occurred to airframe components, so that corrective actions
may be taken. For example, a fire in a wheel well should self*extinguish due to lack of air, butthe wheels and tires may be damaged. fire*detection system will warn the flight crew, sothat special precautions may be taken during the landing to preclude further ha&ards.
Class X zones have large (uantities of air flowing through them and are of unusualconstruction, making fire detection and uniform distribution of an extinguishing agent verydifficult. 3ones containing deeply recessed spaces and pockets between large structuralformers are of this type. Fires in Class 2 &ones will need twice the amount of extinguishingagent normally used in a Class &one.
REQIREMENTS FOR O!ER"EAT AND FIRE-DETECTION SYSTEMS
4odern detection systems have been proven to be highly reliable when properly maintained.These systems consist of electrical or electronic sensors that are installed in remotelocations. The sensors warn the operator of impending ha&ards by sounding an audiblealarm and illuminating a warning light that indicates the location of the ha&ard. 1efore thesesystems are approved by the F for installation in an aircraft, the manufacturer must provethat the fire*detection system design meets the following criteria
1. The system must be constructed and installed in a manner that prevents false warningsunder all flight and ground operating conditions.
2. There must be a rapid indication of a fire and an accurate indication of the fire5s location.
3. The system must have an accurate indication that a fire has been extinguished.
4. The system must automatically reset once a fire is extinguished, to provide an immediateindication if the fire re*ignites.
5. hen there is a fire, there must be a continuous indication for its duration.
6. The detection system must have a means for electrically testing the integrity of thedetection*system circuitry from the cockpit.
7. The detector or sensing units must be able to resist exposure to oil, water, vibration,
extreme temperatures, and maintenance handling. The units should also be lightweightand easily adaptable to any mounting position and must also operate directly from theaircraft power system, without inverters. In addition, when the detectors are notsensing a ha&ard, there should be minimal re(uirements for electricity to power thesystem.
8. 6ach detection system must actuate a cockpit light indicating the location of the fire, aswell as an audible alarm.
9. In the case of multi*engine aircraft, the detection system must consist of a separatesensing circuit for each engine.
FIRE-DETECTION SYSTEMS
6ngine fire*detection systems generally fall into two categories spot*detection type systemsand continuous*loop type systems. ith a s#o$-%e$e&$ion $'#e s's$e(, individual firedetectors, or switches, are used to detect a fire. 'uch detectors must be placed in locationswhere a fire is likely to occur, because with this type of system a fire warning sounds onlywhen a fire exists in the same location as the detector. The &on$in)o)s-loo# $'#e s's$e(works on the same basic principle as the spot*type fire detectors, except that a single switchin the form of a long inconel tube is used instead of several individual switches. The small*diameter inconel tube is run completely around an engine nacelle or an area that surroundsan auxiliary power unit, thus allowing more complete coverage than spot*type detectionsystems.
FEN*AL SYSTEMSFenwal produces a thermoswitch fire*detection system, a thermocouple fire*detection system,and a continuous*loop fire*detection system.
T"ERMOS*ITC" DETECTOR
thermoswitch fire detection system is a spot*type detection system that uses a number of thermally activated switches. 6ach switch, or sensor, consists of a bimetallic thermoswitch thatcloses when heated to a predetermined temperature.
With a thermoswitch detector, the actual switch is mounted inside a stainless steel housing. If a fire starts the switchhousing heats up and elongates, causing the contact points toclose.
Sin+le-Loo# S's$e(ith a Fenwal single*loop system, all of the thermoswitches are wired in parallel with eachother, and the entire group of switches is connected in series with an indicator light. In thisarrangement, once a thermoswitch closes, the circuit is completed and power flows to thewarning light.
Do),le-Loo# S's$e(In a double*loop system, all of the detectors are connected in parallel between two completeloops of wiring. The system is wired so that one leg of the circuit supplies current to thedetectors while the other leg serves as a path to ground. ith this double*loop arrangementthe detection circuit can withstand one fault, either an open or short circuit, without causing a
false fire warning.
T"ERMOCOPLE DETECTOR thermocouple*type, E%ison fire*detector system is similar to a thermoswitch system in thatthey are both spot*type detection systems. %owever, a thermocouple detector initiates a firewarning when the temperature of the surrounding air rises too rapidly !warms too fast$, rather than responding to a preset temperature as does the thermoswitch detector.
CONTINOS-LOOP DETECTORIn addition to a thermoswitch detection system, Fenwal also produces a continuous*loop
type system that consists of a single fire or overheat*sensing element that varies in length,depending on the si&e of the fire &one. typical sensing element can be anywhere from 7 footto 78 feet long. s mentioned earlier, the sensing element used in a continuous*loop firedetection system consists of a flexible, small*diameter inconel tube.
IDDE SYSTEMThe 9idde system is also a continuous*loop type system consisting of a single overheat*sensing element that varies in length. The sensing element consists of a rigid, preshapedinconel tube with two internal wire conductors. The conductors are embedded in a$.er(is$or/ or $.er(al resis$or (a$erial/ to prevent the two electrodes from touching eachother and the exterior casing. :ike the eutectic salt used in the Fenwal system, the thermistor material has an electrical resistance that decreases as the temperature increases.
A Kidde sensing element consists of a sealed inconel tubecontaining two conductors embedded in a thermistor material.
LINDBER0 SYSTEMThe :indberg fire detection system is a #ne)(a$i& &on$in)o)s-loo# $'#e s's$e( consistingof a stainless steel tube filled with an inert gas and a discrete material that is capable of absorbing a portion of the gas. The amount of gas the material can absorb varies withtemperature. "ne end of the tube is connected to a pneumatic pressure switch called ares#on%er/ which consists of a diaphragm and a set of contacts.
The sensing element used with a Lindberghcontinuousloop system consists of a stainless tubethat is filled with an inert gas and a gas absorbing
material. !ne end of the tube is sealed while the other end is connected to a diaphragm switch.
SYSTRON-DONNER SYSTEM
:ike the :indberg system, the 'ystron*)onner system5s principle of operation is based on thegas law if the volume of a gas is held constant and the temperature increases, gas pressurealso increases. The helium gas surrounding the titanium wire provides the systems averagingor overheat function. t normal temperatures, the helium pressure in the tube exerts aninsufficient amount of force to close the overheat switch. %owever, when the average
temperature along the length of the tube reaches an overheat level, the gas pressureincreases enough to close the diaphragm switch, which activates the alarm. "nce the sourceof the overheat condition is removed, the helium gas pressure drops and the diaphragmswitch opens.
The "ystron#onner fire detection and o$erheat system consists of a heliumfilled sensor tube surrounding a hydrogencharged core. With this system,excessi$e temperatures increase thegas pressure which forces adiaphragm switch closed. !nce
closed, power flows to the warning light and bell.
nother type of fire detection system that is used on an aircraft is a flame detector system.4ost flame detectors consist of a photoelectric sensor that measures the amount of visiblelight or infrared radiation in an enclosed area. The sensor is placed so it can see thesurrounding area, and anytime there is an increase in the amount of light that strikes the
cell, an electrical current is produced. "nce enough current is produced and channeledthrough an amplifier, a fire warning light and bell are activated.
A typical installation of a "ystron#onner system consists of two independent loops attached to a support tube. Thesupport tube establishes the routing of the detector element and pro$ides attach points to the airplane.
FIRE EXTIN0IS"IN0 SYSTEMS
%and*held fire extinguishers and extinguishing systems are installed in many aircraft toprovide the flight crew and maintenance personnel with the ability to fight fires while theaircraft is operating on the ground or in flight. Portable extinguishers are commonly installed inthe cockpit and passenger cabin of many aircraft. 4ore elaborate extinguishing systems areinstalled in transport category and corporate airplanes to extinguish fires in the engine,
auxiliary power unit, baggage, and electronic e(uipment compartments. In addition, manytransport category airplanes have fire*extinguishing systems located in trash receptacles toprotect against fires that may occur in the lavatories of passenger*carrying aircraft.
FIRE-EXTIN0IS"IN0 A0ENTS s previously mentioned, the three elements that are needed to support combustion are acombustible fuel, oxygen, and heat. If any one of these elements is removed, a fire will notburn. The portable and fixed fire*extinguisher systems used in most aircraft are designed todisplace the oxygen with an inert agent that does not support combustion. The most commontypes of aircraft extinguishing agents that are used include carbon dioxide and halogenatedhydrocarbons.
CARBON DIOXIDECarbon dioxide !C"#$ is a colorless, odorless gas that is about one and one*half timesheavier than air. To be used as an extinguishing agent, carbon dioxide must becompressed and cooled until it becomes a li(uid that can be stored in steel cylinders.hen released into the atmosphere, carbon dioxide expands and changes to a gas thatcools to a temperature of about ;7<℉. 1ecause of the cooling effect, the water vapor in theair immediately condenses to form =snow,= which causes the C"# to appear to settle over the flames and smother them. %owever, the fire is actually extinguished by the C"#
displacing the oxygen in the atmosphere, interrupting the chemical reaction between thefuel and the oxygen. "nce the =snow= warms, it evaporates, leaving almost no residue.
Carbon dioxide is effective on both Class 1 and Class C fires. carbon dioxide hand held fireextinguisher can be used on an electrical fire, provided the discharge horn is constructed of a nonmetallic material. metallic horn would tend to transfer an electrical charge back to thefire extinguisher and to ground through the person holding the extinguisher. In addition,since carbon dioxide leaves almost no residue, it is well suited for engine intake andcarburetor fires. Furthermore, carbon dioxide is nontoxic and does not promote corrosion.
%owever, if used improperly, carbon dioxide will dissipate oxygen uptake in the lungs, whichcan cause physiological problems such as mental confusion and suffocation. 1ecause of itsvariation in vapor pressure with temperature, it is necessary to store C"# in stronger containers than re(uired for most other extinguishing agents.
"ALO0ENATED "YDROCARBONS .alo+en element is one of the groups that consists of chlorine, fluorine, bromine, or iodine.'ome hydrocarbons combine with halogens to produce very effective fire*extinguishingagents that work by excluding oxygen from the fire source and by chemically interfering withthe combustion process. %alogenated hydrocarbon fire*extinguishing agents are mosteffective on Class 1 and C fires but can be used on Class and ) fires as well. %owever, their
effectiveness on Class and ) fires is somewhat limited.
%alogenated hydrocarbons are numbered according to their chemical formulas with five*digit%alon numbers, which identify the chemical makeup of the agent. The first digit representsthe 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. If there is no iodinepresent the fifth digit does not appear. For example, bromotrifluoromethane CF?1r is referredto as %alon 7?<7, or sometimes by the trade name Freon 7?;.
%alon 7?<7 is extremely effective for extinguishing fires in engine compartments of both pistonand turbine powered aircraft and is also considered to be one of the best extinguishing agents
for aircraft interior fires. In engine compartment installations, the %alon 7?<7 container ispressuri&ed by compressed nitrogen and is discharged through spray no&&les. %alon 7?<7 isalso widely used as the agent for portable fire extinguishers.
%alogenated hydrocarbon fireextinguishing agents pro$ide effecti$e fire suppression in aircraft.
nother once*popular agent was methyl bromide !%alon 7<<7$. %owever, methyl bromide is toxicto personnel and corrosive to aluminum alloys, magnesium, and &inc. "f all the halogenatedhydrocarbon extinguishing agents, %alon 7?<7 is the safest to use from the standpoint of toxicityand corrosion ha&ards. In small dosage amounts, the gas has a low toxicity, but has similar effects of depriving oxygen from the lungs. 1ecause of changing regulations and developing envi*ronmental impact data, you should keep abreast of current developments pertaining to the useof halogenated hydrocarbons as fire*extinguishing agents. For example, several studiessuggest that chlorofluorocarbons !CFCs$, such as %alon, damage the o&one layer in thestratosphere, allowing higher levels of ultraviolet radiation to reach the earth. To reduce damageto the o&one layer, the 6nvironmental Protection gency banned the production of CFCsafter )ecember ?7, 7@@8. %owever, existing stocks of CFCs are still allowed to be used after this date. 'everal alternatives to CFCs have recently been developed and will most likely findapplications as aviation fire*extinguishing agents. For example, )uPont F6*#8A has proven tobe an acceptable substitute for %alon 7?<7 as an extinguishing agent and has no harmful affecton the earth5s o&one layer. "ther replacement extinguishing agents being researched includewater mist sprays, which have been proven to be effective in combating many , 1, and C classfires.
s an aviation maintenance technician, it is important to be aware of 6P and F regulationsgoverning the use and disposal of CFCs. Improper handling or disposal of halogenatedhydrocarbons can lead to civil and criminal penalties.
FIXED FIRE-EXTIN0IS"IN0 SYSTEMS
In an aircraft, it is important that the type of fire*extinguishing system be appropriate for theclass of fire that is likely to occur. There are two basic categories of fixed fire*extinguishingsystems conventional systems, and high*rate*of*discharge !%B)$ systems. 1oth systemsutili&e one or more containers of extinguishing agent and a distribution system that releases
the extinguishing agent through perforated tubing or discharge no&&les. s a general rule,the type of system installed can be identified by the type of extinguishing agent used. For example, conventional systems usually employ carbon dioxide as the extinguishing agentwhile %B) systems typically utili&e halogenated hydrocarbons.
CON!ENTIONAL SYSTEMSThe fire*extinguishing installations used in most older aircraft are referred to as conventionalsystems. 4any of these systems are still used in some aircraft, and are satisfactory for their intended use. conventional fire*extinguisher system consists of a cylinder that storescarbon dioxide under pressure and a remotely controlled valve assembly that distributes theextinguishing agent.
Carbon dioxide cylinders come in various si&es, are made of stainless steel, and aretypically wrapped with steel wire to make them shatterproof. In addition, the normal gasstorage pressure ranges from ;<< to 7,<<< psi. 'ince the free&ing point of carbon dioxide isso low, a storage cylinder does not have to be protected against cold weather. %owever,cylinders can discharge prematurely in hot climates. To prevent this, manufacturerssometimes charge a cylinder with about #<< psi of dry nitrogen before they fill the cylinder with carbon dioxide. hen treated in this manner, most C"# cylinders are protected againstpremature discharge up to 7<℉. The nitrogen also provides additional pressure duringnormal release of the agent.
Carbon dioxide cylinders are e(uipped internally with one of three types of siphon tubes. The
cylinders used in aircraft typically utili&e either a straight*rigid, or a short*flexible siphon tube.
The type of siphon tube installed in the cylinder is determined by the cylinder5s mountingposition.
"I0"-RATE DISC"AR0E SYSTEMS%igh*rate*of*discharge !%B)$ is the term applied to the fire*extinguishing systems found inmost modern turbine engine aircraft. typical %B) system consists of a container to hold theextinguishing agent, at least one bonnet assembly, and a series of high*pressure feed lines.
The containers used in an %B) system are typically made of steel and spherically shaped.There are four si&es commonly in use today, ranging from ##D cubic inches to @D8 cubicinches. The smaller containers generally have two openings, one for the ,onne$ asse(,l'or o#era$in+ .ea%/ and the other for a f)si,le safe$' #l)+1 The larger containers are usuallye(uipped with two bonnet assemblies.
6ach container is partially filled with an extinguishing agent, such as %alon 7?<7, and sealedwith a frangible disk. "nce sealed, the container is pressuri&ed with dry nitrogen. container
pressure gauge is provided so you can (uickly reference the container pressure. The bonnetassembly contains an electrically ignited discharge cartridge, or s(uib, which fires apro-ectile into the frangible disk. "nce the disk breaks, the pressuri&ed nitrogen forces theextinguishing agent out of the sphere. strainer is installed in the bonnet assembly toprevent the broken disk fragments from getting into the distribution lines.
In a typical %&# container, the extinguishing agent is released by anelectrically actuated explosi$e that ruptures a frangible dis'. !ncebro'en, the dis' fragments collect in a strainer while theextinguishing agent is directed to the engine nacelle.
s a safety feature, each extinguishing container is e(uipped with a thermal fuse that melts
and releases the extinguishing agent if the bottle is sub-ected to high temperatures. If abottle is emptied in this way, the extinguishing agent will blow out a red indicator disk as itvents to the atmosphere. "n the other hand, if the bottle is discharged normally, a yellowindicator disk blows out. :ike a conventional system, the indicator disks are visible from theoutside of the fuselage for easy reference.
Two colored indicator dis's are $isible on the exterior of anaircraft equipped with (!) or %&# extinguisher system bottles.If the red dis' is missing, it indicates that the fire bottles ha$edischarged because the bottle pressure exceeded limits dueto thermal heating. If the yellow dis' is missing, it indicatesthat the bottles were discharged through acti$ation of the
hen installed on a multi*engine aircraft, the fire*extinguishing*agent containers are typically e(uippedwith two firing bonnets. The two discharge ports allowone container to serve both engines.
A typical extinguishingagent container on a multiengine aircraft has two firing bonnets.
"n large, multi*engine aircraft, two extinguishing*agent containers are generally installed,each with two firing bonnets. This allows twin*engine aircraft to have a dedicated container for each engine. In addition, the two discharge ports on each bottle provide a means of discharging both containers into one engine compartment.
A typical highrateof dischargeextinguishing system installed on atwinengine, turbinepowered aircraft utili*es two agent containers, eachwith two discharge ports. This permits two applications of extinguishing agent to any oneengine.
A234 FIRE PROTECTION SYSTEM
The ?#< has
. smoke detection and automatic fire extinguishing system in the lavatories1. smoke detection in the avionics bayC. smoke detection and fire extinguishing in the forward cargo compartment). smoke detection and fire extinguishing in the aft cargo compartment6. fire detection and extinguishing systems for the enginesF. fire detection and extinguishing systems for the P/
E portable fire extinguishers for the flight compartment and the cabin
EN0INE6ach engine is e(uipped with # firebottles. The discharge of the bottlesis controlled by an associated pb sw.The pb switches are located on theFIB6 panel on the overhead panel.
AP
The engines and P/ haveindividual fire detection systems.6ach system consists of
* Two identical detection loops !and 1$ mounted in parallel* fire detection unit !F)/$
For the P/, there is -ust one fireextinguisher bottle and only one dischargebutton.
PB S*ITC"ES 5O!ER"EAD PANEL6
The guarded FIB6 pb switches provide fireindication and the means to isolate thecorresponding system.
The T6'T buttons are used to test therespective fire detection and extinguishingsystem operation.
The ?#< family aircraft are provided witha cooling system for the avionicse(uipment. The cooling system iscontrolled and monitored by the vionics6(uipment 0entilation Controller !60C$.The airis circulated through the system bya blower fan !cool air supply$ workingtogether with an extraction fan !warm air removal$. The extraction airflow isdownstream of the avionics e(uipment, sothe vionics '4"96 detector is fitted inthe extraction duct and willdetect smokecoming from the computers and controlboxes. The detector is monitored by the
60C. The 60C signals the Flightarning Computer to display the 0I"+IC' '4"96 warning in the cockpit.
LA!ATORY
:avatory smoke detection consists of onesmoke detector in each lavatory linked tothe 'moke )etection Control /nit!')C/$.
In addition, each lavatory waste bin has anautomatic fire extinguishing system.
total of six smoke detectors are installed in the cargo compartments. There are twodetectors in the forward compartment, four in the aft compartment. 6ach detector is linkedto one of the two detection loops.
The aft cargo compartment on the ?#< only is ventilated and heated for live animalcarriage. %ot air is admitted through a trim air valve, and the air is exhausted through anisolation valve. ctivation of the smoke detection system causes these valves to closeautomatically. hen activated by the smoke system, they cannot be overridden by the crew.
