Fire Protection PDF

# Fire Protection (11.8) ## Learning Objectives - Explain the purpose and operation of fire and smoke detection and warning systems (Level 3). - Explain the purpose and operation of fire extinguishing systems (Level 3). - Explain the purpose and operation of fire protection system tests (Level 3)....

# Fire Protection (11.8) ## Learning Objectives - Explain the purpose and operation of fire and smoke detection and warning systems (Level 3). - Explain the purpose and operation of fire extinguishing systems (Level 3). - Explain the purpose and operation of fire protection system tests (Level 3). - Describe portable fire extinguishers (Level 2). # Aircraft Fire Protection ## Introduction to Aircraft Fire Protection An aircraft carries large amounts of fuel and has several sources of ignition: electrical devices, hot engines, hot exhaust gases and hot wheels and brakes. An in-flight fire is extremely dangerous; the most likely place an aircraft fire will occur is in an engine and engine nacelle. If the fire is not detected early and extinguished, the consequences could be catastrophic. ## Fire and Smoke Detection and Warning Systems ### Fire/Overheat Detection Systems Fire and overheat detection systems must be capable of providing rapid detection of a localised fire or overheat condition and indicate the area in which corrective action is required. Detectors should not automatically operate fire extinguishing units in flight although they may be used to shut down electrical power or fuel to certain areas or components. Fire detection systems must: - Be reliable - Give immediate indication of a fire or overheat condition and when that condition has been removed - If the system fails, it is more likely that system will be inoperative than to cause erroneous readings - Operate independently of other systems - Be able to operate when main power systems are no longer operational. Most systems operate of the aircrafts 28-V DC or the 28-V DC hot battery bus. A diagram of an overheat/fire protection panel shows the following: - Two indicators labeled "OVHT DET" - One is labeled "NORMAL" and the other is labeled "FAULT - INOP" - An indicator labeled "APU DET" - One is labeled "NORMAL" and the other is labeled "FAULT - INOP" - A "FAULT" indicator - Two indicators indicating whether "L BOTTLE" or "B BOTTLE" is "DISCHARGED" - "ENG I" and "ENG 2" overheat indicators - An APU bottle discharge indicator - Fire switches (fuel shutoff) for the "ENG I" and "ENG 2" - A lock override press button under the handle ### Overheat/fire protection panel Aircraft fire warning systems are installed to alert the flight crew of a fire or of an overheat condition that could lead to a fire. Detector units are installed in locations where there are greater risks of a fire. A fire will be indicated on the flight deck by simultaneous visual and aural warnings, a red master warning light and a local red warning light showing the location of the fire and a ringing warning bell. ## Overheat Warning Overheat warning systems are used in areas of high temperature that may lead to a fire. The number of overheat warning systems will vary with the type of aircraft. They may be used in engine turbine areas, nacelles, wheel well areas and hot pneumatic manifold. When an overheat condition is sensed in the detector area, the systems automatically illuminate both an amber master caution light and a local caution light. ## Fire Detection Units The complete aircraft fire protection system will often incorporate several different detection methods. There are a number of detectors or sensing devices, the most common types include: - Spot-type detectors, e.g. thermal switches and thermocouples - Continuous-loop sensing elements. ### Spot-Type Fire/Overheat Detection Systems Many older model aircraft are equipped with some type of thermal switch system or thermocouple system. More modern aircraft use various continuous-loop fire and overheat detection systems, where greater, more complete coverage of a fire hazard can be achieved. #### Thermal Switch Detector Thermal switches are also known as thermostat switches or spot detectors. The actual switch is mounted inside a stainless-steel housing. If a fire begins, the switch housing heats up and elongates, causing the contact points to close. To adjust a thermal switch, the housing must be heated to a specified temperature and then a tension adjustment is turned in or out until the contacts just close. In most cases, this adjustment is set by the detector manufacturer and is not adjusted in the field. A diagram shows a "LOOP 'B'" connected to 28VDC. It shows the location of the "CONTACT POINTS", the "TENSION ADJUSTMENT", the "SLIDING PISTON", "LOOP 'A'" and the "THERMAL SWITCH" #### Thermal Switch System Operation All the thermal switches are bolted to the aircraft structure which connects one of the contacts of the switch to ground. The illustration shows that the other contact of each of the switches is connected in parallel to the coil of relay RL1. Should any switch close, a current path is made for the coil of RL1. When energised, the contacts of RL1 complete a current path for the bell and red light. When the crew has been alerted, they can stop the bell from ringing by pressing the bell cancel button. This action energises RL2, breaking the bell's current path and completing its own coil's current path. The bell cancel button can be released, but RL2 will remain energised until the fire warning condition disappears. This system normally operates of the 28-V DC battery bus. The diagram shows how a "BUS" is connected to a "BELL CANCEL" which is connected to a "BELL". There is a "LIGHT" connected to "RL1" which is connected to both a "RED" light and a "RL2" which is connected to a "TEST SWITCH". #### Testing The test switch completes a ground connection to R1 via all the wiring connecting the thermal switches. This checks the wiring for continuity. The bell should ring and the red light illuminate. If the bell cancel switch is pressed, the bell should stop, but the red light remains illuminated. **Warning:** Installation of a switch of a different operating temperature could cause a false warning or no warning in the event of a fire. Thermal switches have to be tested with a Jet-Cal, or similar tester, at specified periods. The switches are heated and the temperature indicated by the tester. A test light is connected across the switch contacts and the temperature recorded when the light illuminates. The temperature at which a particular switch operates will be detailed in the aircraft manual. ### Thermocouple-Fire Detection System The below image illustrates a typical thermocouple fire detector. The hot junction is in the open and therefore is exposed to the air around it although it does have a protective frame. The cold junction is in the body of the detector, which insulates it to some extent from the surrounding air. When the hot junction is exposed to heat from an external source, the current increases. The increase in current is directly proportional to the rate of temperature rise at the hot junction. When the current reaches a preset level, a warning indication is activated. This system is used in engines and aircraft pneumatic systems. For redundancy, the slave relay, warning lamp and bell operate from the aircraft 28-V DC system. The diagram shows a "CHROMEL" connected to a "HOT JUNCTION" which is connected to a "CONSTANTAN" which is connected to a "COLD JUNCTION". #### Thermocouple detector A thermocouple system may consist of one or more active thermocouples placed in fire zones around an engine, while a separate thermocouple, called a reference thermocouple, is placed in an air space between two insulated blocks. Under normal operating conditions, the temperature of the air surrounding the reference and active thermocouples is relatively even and no current flow is produced. However, when a fire occurs, the difference in temperature produces a current in the circuit and activates a warning. The diagram shows a "REFERENCE JUNCTION", "THERMAL INSULATION", "MEASURING JUNCTION", "BELL", "ELECTRICAL BUS", "TEST SWITCH", "TEST THERMOCOUPLE", "SENSITIVE RELAY", "WARNING LAMP" and a "SLAVE RELAY". ### Continuous-Loop (Fire Wire) Fire/Overheat System A continuous-loop detector or sensing system permits more complete coverage of a fire hazard area than any type of spot-type temperature detectors. They are overheat detection systems where heat-sensitive units will complete an electrical circuit at a certain temperature. There is no rate-of-heat-rise sensitivity in a continuous-loop system. Fire wire systems are manufactured by the Fenwal and Walter Kidde companies. The Fenwal system uses a single conductor element, and the Walter Kidde system uses a dual conductor element. Both are used primarily for engine fire detection and overheating detection in bleed air systems. #### Fenwal and Kidde continuous elements and their systems The diagram shows a "FENWAL SENSING ELEMENT" and a "KIDDE SENSING ELEMENT" with "EUTECTIC SALT" and a "THERMISTER INSULATOR" at each end. Both are connected to "ALARM" and 28VDC. #### Fenwal Single Conductor Element A Fenwal single conductor element construction consists of a thin-walled tube of approximately 2 mm diameter made from Inconel filled with compound. Embedded in the centre of the filling is a single nickel conductor. The filling compound can be in different forms. Two examples are: - Ceramic beads impregnated with a eutectic salts. The melting point of the salts is low and its resistance decreases when it heats; the resistance will increase again when the salts cool. - Aluminum oxide suspended in fibrous glass. The resistance of this compound lowers with heat; also, its dielectric qualities improve so the capacitance of the fire wire is higher as temperature increases. #### Kidde Two Conductor Element The Kidde fire wire element is made from an Inconel conductor element tube similar in appearance to the Fenwal system. In the Kidde system, two nickel conductors are embedded in a thermistor material, having a negative temperature coefficient of resistance. That is, an increase in temperature decreases resistance. 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. #### Inspection and Maintenance Damage to the element can cause the system to fail operate or give a false warning. Damage can be caused by poor installation, tight bends, being stood on or having tools or other items placed or dropped on them. ### Pneumatic Continuous-Loop System A pneumatic continuous-loop system works on the principle of a contained gas expanding due to the application of heat. These systems are used for engine overheat and fire detection on transport-type aircraft. False alarms are a major concern with any fire detection system. The current generation of pneumatic systems are designed so that mechanical damage to the sensor tube cannot result in a false alarm. Any severe damage to the unit will provide a 'no test' indication, not a false alarm. It is a requirement for flight crew and maintenance crews to perform a fire detection system integrity test before each and every engine start to confirm system serviceability before operating the aircraft engines. * The pneumatic detector has two sensing functions allowing it to respond to: - An overall average temperature threshold - A localized discrete temperature increase caused by impinging flame or hot gasses. Both the average and discrete temperature are factory set and are not field adjustable. #### Average Temperature Sensing The averaging function is activated by the expansion of a fixed volume of helium gas inside the detector. The pressure inside the detector will increase in proportion to the absolute temperature and will activate the alarm switch at a pre-set average temperature within the range of 93 °C (200 °F) to 371 °C (700 °F). #### Discrete Temperature Sensing Discrete sensing is accomplished using a hydrogen filled core material in the sensor tube. Hydrogen gas is released from the detector core whenever a small section of the tube is heated to a pre-set discrete temperature within the range of 1100° Celsius (2000° Fahrenheit) for 5 seconds. This outgassing in the core will increase the pressure inside the detector and activate the alarm switch. Both the averaging and discrete functions are reversible. When the sensor tube cools, the average gas pressure is reduced, and the discrete hydrogen gas returns to the core material. To distinguish between the various methods of construction and operation, the systems are known by their manufacturers' names, which include Lindberg and Systron-Donner. #### The Lindberg System The Lindberg fire detection system consists of a stainless-steel tube with an inert gas inside and a discrete material capable of absorbing a portion of the gas. One end of the tube is sealed, and *the other is connected to* a diaphragm switch unit. When the temperatures surrounding the sensing element rises because of a fire or overheat condition, the discrete material within the tube also heats up and releases the absorbed gas. The gas pressure in the stainless-steel tube increases, forcing the closure of the electrical contacts and activating a warning system on the flight deck. The diagram shows a "METAL TUBING" connected to a "TEFLON OUTER SHEATH" which is connected to a "DIAPHRAGM SWITCH". Inside the metal tubing is a "GAS ABSORBING MATERIAL". ##### Lindberg fire detector element To test a Lindberg system, a test unit supplies a low-voltage AC current to the outer stainless-steel casing. The heat generated releases the gas from the discrete material, eventually resulting in the warning system activating. When the test switch is released, the sensing element cools and the discrete material reabsorbs the inert gas, decreasing the pressure and opening the switch contacts in the detector element. The test verifies the integrity of the control unit, the interconnecting wiring and the detector element. The diagram shows a "TEST UNIT" connected to a "CONTROL UNIT". It shows both a "TEST SWITCH" and a "BELL DISABLE SWITCH" connected to the "WARNING LIGHT". It also shows a "BELL". ##### Lindberg test circuit #### The Systron-Donner System The Systron-Donner pneumatic continuous system uses a stainless-steel tube filled with helium gas and a length of titanium wire running through the centre. The titanium wire acts as the gas absorption material because it contains a quantity of hydrogen. The tube is sealed at one end and connected to a responder at the other end. The responder contains two electrical switches operated by a diaphragm assembly. One of the switches, the integrity switch, is held closed, provided there is sufficient gas pressure in the tube. When the test switch is operated, the warning light will illuminate and the bell sounds, which indicates the detector is serviceable. If the gas has leaked from the detector, the integrity switch breaks the supply from the test switch to the light and bell. The diagram shows a "RESPONDER UNIT" connected to "DIAPHRAGM" and a "SENSING ELEMENT". It shows a "ALARM SWITCH" and a "TEST SWITCH" connected to a "WARNING LIGHT" which is also connected to a "BELL" and a "BELL SILENCE SWITCH". It also shows an "INTEGRITY SWITCH". ##### Systron-Donner system At normal temperatures, the helium gas pressure has insufficient force to close the alarm switch. However, when the average temperature along the length of the tube exceeds a pre-set level, the gas pressure increases enough to close the alarm switch contacts, activating the alarm. The fire warning function is provided by the titanium wire. When the titanium wire is exposed to a discrete heat source such as a fire or a bleed air leak, it releases hydrogen gas, which again increases the overall pressure within the stainless-steel tube, triggering the warning system. After the fire is extinguished, the hydrogen gas is reabsorbed by the titanium wire and the responder contacts break, resetting the alarm (switching it off). The systems are normally used in a dual-loop configuration to increase reliability of the system. Both loops are required to sense a fire or overheat before an alarm will sound; however, if one loop fails (integrity switch opens), the system control box will isolate the defective loop and reconfigure to a single loop operation using the good loop. In the case of this occurring, the system control box would also set a maintenance code indicating the fire loop failure and the need for maintenance at the next possible opportunity. The diagram shows two "SUPPORT TUBES" with "LOOP 1" and "LOOP 2" going into a "RESPONDER". The "TEFLON SPACER", a "STAINLESS STEEL TUBE" and a "INERT GAS (HELIUM)" are inside one of the "SUPPORT TUBES" and a "HEAT SENSING ELEMENT" is inside the other. ##### Systron-Donner pneumatic continuous-loop systems #### Continuous-Loop Installation Inspection and Maintenance Fire elements come in different lengths and are identified by their part number. There may be several sections joined together to make up the one element for the system it protects. These sections are joined by small plugs and sockets. The element is positioned around the engine or compartment and attached by clamps, sleeves or grommets. When replacing fire wire, ensure wire is installed in the same path as the wire being replaced. The diagram shows an example of this setup showing a "LOOSE CLAMP", a "GROMMET" and a "CLAMP SCREW". It also shows a "CLAMP HINGE", a "HEAT SENSING ELEMENT" and a "BRACKET". When installing an element, the following points must be complied with: - Bends can be no sharper than 1 inch in radius. - The grommet must protect the element from the clamp. The clamp should not touch the wire. - The split in the grommet must be placed so that the element will not pull through. - The element must not rub or touch anything. - The element connectors must be free of contamination, as oil and dirt can give erroneous readings. #### Fire element installation Any fire indication, real or false, should result in a comprehensive inspection of the aircraft and engine; the inspection must include the fire detection system to determine system integrity and continuing airworthiness. Damage to the element can cause the system to fail to operate or give a false warning. Damage can be caused by poor installation, being stood on or having tools or other items placed or dropped on them. Ensure not to kink or overbend fire wires. This will cause internal breakage of wire, making the system inoperative. When inspecting an element, you must also ensure that: - The element is not broken, there are no cracks in the surface and no part of the surface is chaffed. - Indentations in the surface do not exceed parameters detailed in the aircraft maintenance manual. - Grommets are in good condition. - All plugs are tight and lock wired. If not, inspect for cleanliness and internal damage. The diagram shows how a "CRUSHED SECTION", a "SHARP BEND", a "LONG UNSUPPORTED LOOP" and a "KINK" can all cause damage to the fire wire. #### Fire wire installation ## Continuous Element Control Units There are several types of control units that can go with fire elements. Some work using resistance only, some use capacitance only and some use both. ### Controller Using Resistance Only The twin conductor element works on the resistance property of its thermistor filler material. At a pre-set level of temperature, the element passes enough current to bias the solid state controller to switch the warning devices. A diagram shows how a "CARGO FRE" is connected to a "MAINTENANCE UNIT" which is connected to "LOOP" which is connected to a "SINGLE-WIRE SENSING ELEMENT". The "SINGLE-WIRE SENSING ELEMENT" is also connect to a "TEST SWITCH", a "WARNING LIGHT", and a "BELL DISABLE SWITCH" which is also connected to a "CONTROL UNITY". #### Fire detection control units ### Controller Using Capacitance The capacitance element acts as a capacitor, storing the positive half of each cycle and releasing it during the negative half of the cycle. As the temperature rises, the capacitance of the element increases, and at a pre-set temperature, the energy released by the element will cause the solid-state controller to conduct and energise the warning relay. #### Testing Testing is carried out by pressing the test switch which completes a DC current path for the test relay coil. When energised, the test relay disconnects the centre conductor from the bridge rectifier and connects it to earth. This causes a warning to occur. Notice the test checks the continuity of the system. Should there be a break in the element, the test will not be successful. When the test relay is de-energised, it reconnects the central conductor to the bridge rectifier so that the power source is connected to both ends of the centre conductor. If a break in the element occurs, power will still be applied to the entire element, so a warning would be given in the event of a fire, but a test would not work. The diagram shows how a "115-V a.c" power source is connected to a "WARNING RELAY" and a "28-V d.c" power source. #### Fire detector circuit ## Overheat Detection System Operation The below image is a typical duct and compartment overheat detection system. The top circuit is for a compartment area of the aircraft. It has several detector sensors arranged in a parallel circuit. This is connected to a warning light and to the Master Warning System (MWS). No test function is supplied to this circuit. The reset temperature of this circuit is 100°C. The bottom circuit is for duct overheat detection. Two detectors are located in separate areas of the aircraft (typically the pneumatic ducting) and are tested from the one test switch. The test switch does not test the function of the detectors, just the circuit. Both detectors are again connected to a warning lamp and to the MWS. The diagram shows how "PE" is connected to "3A" which is connected to a "HP AIR O/HEAT TEST SWITCH". It also shows how "PE", "A", "BCD", and "3A" are connected to "RESETTING DETECTOR SWITCHES 100° ±5°C". It also shows a "HP AIR DUCT O/HEAT DETECTOR SWITCH 350° ±15°C" connected to "3A", and "PE" connected to another "HP AIR DUCT O/HEAT DETECTOR SWITCH 350° ±15°C". #### Overheat detection system schematic ## Smoke Detection Systems The aircraft smoke-detection system samples the cabin air for the presence of smoke, which can be an indication of an impending fire condition. These may include cargo and cargo compartments, equipment bays and the lavatories of transport category aircraft. A smoke-detection system is used where the type of fire anticipated is expected to generate a substantial amount of smoke before temperature changes are sufficient to actuate an overheat-detection system. The presence of carbon monoxide gas (CO) or nitrous oxides are dangerous to flight crews and passengers and may indicate a fire condition. Detection of the presence of either or both of these gases could be the earliest warning of a dangerous situation. Air is continually sampled by the smoke detection system. Any smoke found in the sample causes a change of electric current flow within the system. The change in current flow will trigger a warning indication. The image shows how "FORWARD CARGO COMPARTMENT FLOW THROUGH SMOKE DETECTORS" and "AFT AND BULK CARGO COMPARTMENT FLOW THROUGH SMOKE DETECTORS" are both connected to "VENTURI EJECTOR". #### Smoke detection system ## Smoke Detector Systems There are two main types of smoke detector systems: - Photoelectric (comparison and refraction types) - Ionisation type. ### Refraction Photoelectric System The diagram below illustrates an air sample passing through a chamber. The pilot lamp in the smoke detector is supplied with regulated power. When light is reflected onto the photocell, the circuit resistance decreases, allowing output to the Automatic Fire/Overheat Logic Test Systems (AFOLTS) card and indicating circuits. A photoelectric cell is placed at 90° to a light beam. If smoke comes into the chamber and reaches a concentration level of about 10%, it reflects some of the light onto the photo diode. Some of the light beam is refracted to the photoelectric cell. The output from the photoelectric cell is amplified to switch a warning device such as a light in the cockpit. A small test light is aimed at the photoelectric cell, and when the test is selected, it illuminates, triggering the system. (A photo diode is a device that will conduct electricity when light is directed onto it. Conversely, when no light is present, it blocks current flow.) The light emitting diode (LED) forms part of a test circuit which activates the photo diode to simulate a smoke condition. The diagram shows a "BEACON LAMP", "SMOKE PARTICLES", "LIGHT TRAP", "TEST LAMP," and "PHOTOELECTRIC CELL". #### Refraction photoelectric smoke detector The test switch permits checking the operation of the smoke detector. Closing the switch connects 28 V DC to the test relay. When relay energises, voltage is applied through the beacon lamp and test lamp in series to ground. A fire indication will be observed only if the beacon and test lamp, photoelectric cell, smoke detector amplifiers and associated circuits are all operable. The diagram shows "28V DC", "PILOT LIGHT", "PHOTO DIODE", "COMPARTMENT AIR", "TEST LED", "LIGHT TRAP", and "SMOKE". It also shows how "28V DC" is connected to the "PILOT LIGHT" and the "PHOTO DIODE" which is then connected to "SMOKE". It also shows "BLOWER MOTOR 115V AC". #### Refraction photoelectric smoke detector system The diagram shows a "SMOKE DETECTOR LED INDICATORS" with "28V DC BUS", "LAV SMOKE", "CB PANEL" and "SMOKE". It also shows how "POWER ON (GREEN)", "ALARM (RED)" and "TIMING CIRCUIT" are connected to the "SMOKE DETECTOR LED INDICATORS". It also shows "HORN INTERMITTENT" and a "CALL LIGHT" and a "SMOKE" indicator. It also shows "SENSING CHAMBER", a "SMOKE DETECT CIRCUIT", a "TEST ALARM INTERRUPT", "LAVATORY SMOKE DETECTOR" and a "LAVATORY" indicator. It also shows "MASTER CAUTION", a "CHIME PASSENGER ADDRESS SYSTEM" and a "MASTER LAVATORY CALL LIGHT". #### Refraction photoelectric smoke detector diagram ### Comparison Photoelectric Smoke Detection System The comparison photoelectric smoke detection system has two photoelectric cells. One is in a sealed compartment, the other in a compartment through which a sample of air passes. Both cells receive the same amount of light from one projector lamp. If smoke gets into the sampling compartment, the amount of light to that photoelectric cell will be less than the light to the one in the sealed compartment. This causes an imbalance in the photoelectric cells, which are connected in a bridge circuit or differential amplifier. When the imbalance reaches a pre-set level, a sensitive relay energises, and its contacts pass 28-V DC current to energise a relay which passes current for a warning light in the cockpit. The diagram shows "WARNING SYSTEM SMOKE" and a "DIFFERENTIAL AMPLIFIER" connected to a "REFERENCE CELL PHOTOELECTRIC CELL" which is connected to a "LENS" which is connected to "LIGHT". It also shows the "MEASURING CELL PHOTOELECTRIC CELL" and a "LENS". #### Comparison photoelectric smoke detection system ### Ionisation-Type Smoke Detector The ionisation-type smoke detector has an electrode with a very small amount of radioactive material in one side of the smoke detection chamber. The diagram shows a "RADIOACTIVE MATERIAL" connected to an "ELECTRODE". It also shows how "AIR IN" is connected to "IONISED FLOW" which is connected to a "ELECTRODE" and how "AIR OUT" is connected to "IONISED FLOW". #### Ionisation type smoke detector There is an electrode opposite it. The radioactivity causes ionisation of the oxygen and nitrogen gases in the air, so current is able to pass across the electrodes via the ionised gases. If smoke comes into the chamber, it affects the amount of ionisation and will reduce the current flow between the electrodes. When the current flow falls below a pre-set value, a solid-state sensor turns on the warning device in the cockpit. Again, testing is carried out through the use of a source of smoke generation. ## Cargo Compartment Smoke Detection System The cargo smoke detection system provides a cockpit warning of smoke in cargo compartments. The system is connected directly to a master warning system or through a fire detector module to the master caution system. Sensors are installed in the cargo hold ceiling; this avoids damage as well as being able to detect rising smoke. The image shows a "CAUTION" sign. #### Cargo hold smoke detector The smoke detection system continuously samples the cargo compartment air for smoke. The system consists of a venturi ejector, smoke sampling ports and smoke detectors. #### Venturi Ejector The venturi ejector provides a vacuum to draw air through the smoke detectors. The image shows how "FORWARD CARGO COMPARTMENT FLOW THROUGH SMOKE DETECTORS", "AFT AND BULK CARGO COMPARTMENT FLOW THROUGH SMOKE DETECTORS", and "VENTURI EJECTOR" are connected to "SMOKE SAMPLING PORT". #### Cargo hold smoke detection system ## Smoke Detector Operation The pilot lamp in the smoke detector is supplied with regulated power. When light is reflected onto the photocell, the circuit resistance decreases, allowing output to the Automatic Fire/Overheat Logic Test Systems (AFOLTS) Card and indicating circuits. Smoke detectors are most commonly used in aircraft cargo and toilet compartments. All run on the 28-V DC system to allow for protection during periods when aircraft AC power is not available. ## Cargo Smoke Detection System Operation If smoke is detected by one or more of the smoke detectors located in the aircraft's cargo hold, the signal from the smoke detector is sent to the local control unit which processes the signal. The processed signal is sent to the control display unit, which sends the signal to the cockpit warning. The crew takes the required action to extinguish the fire in the hold. There is usually one bottle per hold; Extended Range (ER) aircraft are required to be fitted with two. The diagram shows how "FWD", "EXT", "AFT", "A and NORM", "B and NORM", and "DET SELECT" are connected to the "DETECTOR FAULT". It also shows how "FWD" and "AFT" are connected to "ARM" and "TEST" which are then connected to the "ARMED" indicator. It also shows how "FWD" and "AFT" are connected to "DISCH" and "FIRE". It also shows "MASTER ALARM 2" and "CONTROL DISPLAY UNIT" connected to "AIRCRAFT 28-V DC BUS". It also shows "FWD CARGO ELECTRONICS UNIT", and "AFT CARGO ELECTRONICS UNIT", which are both are connected to "SMOKE SENSORS" and "DISCHARGE NOZZLES" which are also connected to a "HALON SUPPRESSANT BOTTLES AND METERING SYSTEM". It also shows a "FWD CARGO COMPARTMENT" and an "AFT CARGO COMPARTMENT". #### Cargo smoke detection system ## Lavatory Smoke Detection Systems The lavatory smoke detection system operates with an internal warning system which normally consists of a warning tone and a small LED. On some aircraft, they are also connected to the master caution system, which alerts flight crew. Not classified as a master warning system input, so no warning bell is activated. In most cases a chime and the appropriate caution lamp will illuminate. The diagram shows how "SMOKE DETECTOR", "ELECTRICAL CONNECTOR", "MOUNTING BRACKET", "ALARM HORN (INTERNAL)", "INTERRUPT SWITCH", "POWER-ON LED (GREEN)", "ALARM LED (RED)" and "SMOKE SENSOR" are all connected. #### Lavatory smoke detection system ## Fire Extinguishing Systems ### Fire Extinguishing Systems Three things must be present at the same time in order to produce fire: - Oxygen to sustain combustion - Fuel to burn - Heat to raise the fuel to its ignition temperature. Take any of these elements away, and the fire will be extinguished. Fire safety is based upon the principle of keeping fuel sources and ignition sources separate. The diagram shows a triangle with "OXYGEN", "HEAT", and "FUEL". #### Fire triangle ## Aircraft fire Aircraft fire extinguisher systems are used for the purpose of removing fire from the aircraft. Most aircraft extinguisher systems will remove either heat or oxygen from the fire triangle to extinguisher fire from the aircraft. Most aircraft have fire extinguisher systems for various areas of the aircraft: - Engines - Auxiliary Power Unit (APU) - Cargo holds - Lavatories. ## Engine Fixed Fire Extinguishers Fixed fire-extinguishing systems used in turbine engine aircraft today are typically High-Rate-of-Discharge (HRD) systems. HRD systems typically utilise halogenated hydrocarbons in one or more containers and a distribution system that releases the extinguishing agent through perforated tubing or discharge nozzles. The containers used in an HRD system are typically made of steel and are spherically shaped. They have at least one operating head assembly called a bonnet. There are four sizes commonly in use today, ranging from 224 cubic inches to 945 cubic inches. The smaller containers generally have two openings: one for the bonnet assembly and the other for a fusible safety plug. The larger containers are usually equipped with two bonnet assemblies. Each container is partially filled with an extinguishing agent, such as Halon 1301, and sealed with a frangible disc. Once sealed, the container is pressurised with dry nitrogen. A container pressure gauge is provided for quick reference of the container pressure. The bonnet assembly contains an electrically ignited discharge cartridge, or squib, which fires a projectile into the frangible disc. Once the disc breaks, the pressurized nitrogen forces the extinguishing agent out of the sphere. A strainer is installed in the bonnet assembly to prevent the broken disc fragments from getting into the distribution lines. As a safety feature, each extinguishing container is equipped with a thermal fuse that melts and releases the extinguishing agent if the bottle is subjected to high temperatures. If a bottle is emptied in this way, the extinguishing agent will blow out a red indicator disc as it vents to the atmosphere. On the other hand, if the bottle is discharged normally, a yellow indicator disc blows out. The indicator discs are visible from the outside of the fuselage for easy reference. The diagram shows "DISCH REL SYST. I&II" and "SYST. I&II". #### Fire extinguisher discharge indicators When installed on a multi-engine aircraft, the fire extinguishing agent containers are typically equipped with two firing bonnets. The two discharge ports allow one container to serve both engines. On 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. The diagram shows a "CONTAINER" and "FIRING BONNETS". #### A typical extinguishing agent container ## Fire Extinguishing System Identification Extinguishing agent pipelines are identified by brown-coloured tape. This tape also has diamond-shaped symbols and the words "fire protection.” The fire extinguishing agent is distributed in the engine/APU sections by spray nozzles and perforated tubing. This is also marked with the brown fire tape. The diagram shows how "FIRE PROTECTION" is repeated four times all with the same diamond-shaped logo. #### Fire protection system identification tape ## Extinguishing Agent The most common agent is halon, which is a CFC based chemical. - Halon is a colourless, odourless, nontoxic, gas. - It is fast acting; it cools the surface and chemically interferes with the combustion process, i.e. removes heat and oxygen from Fire Triangle - The gas is attracted to heat, moves around barriers and tracks to a fire. It blankets the fire area and reduces the risk of flashback. - Halon is clean and evaporates completely; it is non-staining, non-corrosive and non-conductive. - Halon will not damage any surface, including electronics, fabrics and painted surfaces. - Halon does cause damage to the atmosphere, and its use its tightly regulated. The diagram shows two "FIXED DUAL BONNET FIRE EXTINGUISHER CONTAINERS". #### Fixed dual bonnet fire extinguisher containers ## Configurations of Engine Fixed Fire Extinguishing Systems ### Single Bottle (One-shot) Twin-Engine Fixed Fire Extinguishing System The diagram shows how "ESS 28 VDC BUS' is connected to "R/FUEL S/O VALVE", "CROSSFEED FUEL VALVE" and " L/FUEL S/O VALVE." It shows how "RIGHT ENGINE FIRE WARNING LIGHT" is connected to "FIRE" which is connected to "RIGHT ENGINE FUEL SHUTOFF VALVE", "OPEN", and "CLOSE". It also shows "FIRE EXTINGUISHER BOTTLE". It also shows how "ESS LT PWR 28 VDC" and "ESS 28 VDC BUS" are connected to "FIRE EXTINGUISHER DISCHARGE SWITCH" which is connected to "READY", "DISCHG" and "FUEL CROSSFEED VALVE" which is connected to "OPEN" and "CLOSE". It also shows how "LEFT ENGINE FUEL SHUTOFF VALVE" is connected to "OPEN", "CLOSE", and "LEFT ENGINE FIRE WARNING LIGHT" which is connected to "FIRE". It also shows how "RIGHT ENGINE CARTRIDGE" and "LEFT ENGINE CARTRIDGE" are connected to "FIRE EXTINGUISHER BOTTLE". It also shows how a "TO RIGHT ENGINE" and "TO LEFT ENGINE" are connected to the "FIRE EXTINGUISHER BOTTLE". #### 'One-shot' twin-engine fixed fire extinguishing system ### Two Bottle (Two-Shot) Twin-Engine Fixed Fire Extinguishing System The diagram illustrates a twin-engine aircraft system with one bottle in each engine nacelle, operated by a pull fire handle which also runs the motorised fuel shutoff valve closed. Each bottle has two outlets. Assume the left engine is on fire; the bottle selector switch should be in the first shot-position. The pilot shuts down the engine and pulls the fire handle. The fire handle switch completes a circuit to the close side of left fuel