Fire Extinguishing Systems

Engine fire switch (P8) Switch positions LEFT Disch 1 2 L E F T 1 2 DISCH Switch locked Switch released electrically Switch pulled Switch released manually Electrical connector Push-pull contacts (internal) Engine fire override switch Solenoid energized Engine fire warning light Spring return (switch open) Rotary contacts (internal) Discharge 2 (switch closed) Spring return (switch open) Discharge 1 (switch closed) Center Engine fire override switch pushed 1. There is a separate approved smoke detector or fire detector system to give warning at the pilot or flight engineer station. 2. There is an approved built-in fire extinguishing or suppression system controllable from the flight deck.

3. There are means to exclude hazardous quantities of smoke, flames, or extinguishing agent from any compartment occupied by the crew or passengers. 4. There are means to control ventilation and drafts within the compartment so that the extinguishing agent used can control any fire that may start within the compartment. Class E Class E cargo compartment is one on airplanes used only for the carriage of cargo and in which: 1. There is a separate approved smoke or fire detector system to give warning at the pilot or flight engineer station. 2. The controls for shutting off the ventilating airflow to, or within, the compartment are accessible to the flight crew in the crew compartment.

3. There are means to exclude hazardous quantities of smoke, flames, or noxious gases from the flight crew compartment. 4. The required crew emergency exits are accessible under any cargo loading condition. Cargo & Baggage Compartment Fire Detection & Extinguisher System The cargo compartment smoke detection system gives warnings in the flight deck if there is smoke in a cargo compartment. [Figure 17-18] Each compartment is equipped with a smoke detector. The smoke detectors monitor air in the cargo compartments for smoke. The fans bring air from the cargo compartment into the smoke detector. Before the air goes in the smoke detector, in-line water separators remove condensation and heaters increase the air temperature. [Figure 17-19] Smoke Detector System The optical smoke detector consists of source light emitting diodes (LEDs), intensity monitor photodiodes, and scatter detector photodiodes. Inside the smoke detection chamber, air flows between a source LED and a scatter detector photodiode. Usually, only a small amount of light from the LED gets to the scatter detector. If the air has smoke in it, the smoke particles reflect more light on the scatter detector.

This causes an alarm signal. The intensity monitor photodiode makes sure that the source LED is on and keeps the output of the source LED constant. This configuration also finds 17-14 Cargo fire/engine control panel • Fire/overheat test switch • FWD cargo fire warning light • AFT cargo fire warning light Speaker (2) Glareshield panel • 2 Master warning lights Air sampling port (typical) Air inlet tube (3) Exhaust ducts Lower cargo smoke detector Smoke detector fans FWD Smoke detector Zone 1 sampling ports Zone 2 sampling ports Zone 3 sampling ports Water separator (3) Heater (3) Manifold Suction lines contamination of the LED and photodiodes. A defective diode, or contamination, causes the detector to change to the other set of diodes. The detector sends a fault message.

The smoke detector has multiple sampling ports. The fans draw air from the sampling ports through a water separator and a heater unit to the smoke detector. [Figure 17-20] Cargo Compartment Extinguishing System The cargo compartment extinguishing system is activated by the flight crew if the smoke detectors detect smoke in the cargo compartment. Some aircraft are outfitted with two types of fire extinguisher containers. The first system is the dump system that releases the extinguishing agent directly when the cargo fire discharge switch is activated. This action extinguishes the fire. The second system is the metered system. After a time delay, the metered bottles discharge slowly and at a controlled rate through the filter regulator. Halon from the metered bottles replaces the extinguishing agent leakage. This keeps the correct concentration of extinguishing agent in the cargo compartment to keep the fire extinguished for 180 minutes.

The fire extinguishing bottles contain Halon 1301 or equivalent fire extinguishing agent pressurized with nitrogen. Tubing connects the bottles to discharge nozzles in the cargo compartment ceilings. The extinguishing bottles are outfitted with squibs. The squib is an electrically operated explosive device. It is adjacent to 17-15 a bottle diaphragm that can break. The diaphragm normally seals the pressurized bottle. When the cargo discharge switch is activated, the squib fires and the explosion breaks the diaphragm. Nitrogen pressure inside the bottle pushes the Halon through the discharge port into the cargo compartment.

When the bottle discharges, a pressure switch is activated that sends an indication to the flight deck that a bottle has been discharged. Flow control valves are incorporated if the bottles can be discharged in multiple compartments. The flow control valves direct the extinguishing agent to the selected cargo compartment. [Figure 17-21] The following indications occur in the flight deck if there is smoke in a cargo compartment: • Master warning lights come on. • Fire warning aural operates. • A cargo fire warning message shows. • Cargo fire warning light comes on. CARGO FIRE FWD ARM AFT ARMED ARMED The master warning lights and fire warning aural are prevented from operating during part of the takeoff operation.

Lavatory Smoke Detectors Airplanes that have a passenger capacity of 20 or more are equipped with a smoke detector system that monitors the lavatories for smoke. Smoke indications provide a warning light in the flight deck or provide a warning light or audible warning at the lavatory and at flight attendant stations that would be readily detected by a flight attendant. Each lavatory must have a built-in fire extinguisher that discharges automatically. The smoke detector is located in the ceiling of the lavatory. [Figure 17-22] Lavatory Smoke Detector System Refer to Figure 17-23. The lavatory smoke detector is powered by the 28-volt DC left/right main DC bus. If there is smoke in the sensing chamber of the smoke detector, the alarm LED (red) comes on. The timing circuit makes an intermittent ground. The warning horn and lavatory call FWD AFT AFT in-line pressure switch DISCH Flow valves DISCH FWD in-line pressure switch Metered bottle (3) Lower forward cargo compartment Discharge nozzle (8) AFT and bulk cargo compartments (ref) Dump bottle (2) Filter/regulator FWD cargo compartment (ref) Identification plate Warning plate Squib Discharge port Safety relief and fill port Handle (2) Mounting bracket (3) Pressure switch Dump bottle Discharge assembly Metered bottle 17-16 Lavatory smoke detector Power indicator Alarm indicator Interrupt switch Self-test switch Sensor Alarm horn light operate intermittently. The smoke detection circuit makes a ground for the relay. The energized relay makes a ground signal for the overhead electronics unit (OEU) in the central monitoring systems (CMS). This interface gives these indications: lavatory master call light flashes, cabin system control panel (CSCP) and cabin area control panel (CACP) pop-up window shows, and the lavatory call chime operates.

Push the lavatory call reset switch or the smoke detector interrupt switch to cancel the smoke indications. If there is still smoke in the lavatory, the alarm LED (red) stays on. All smoke indications go away automatically when the smoke is gone. Lavatory Fire Extinguisher System The lavatory compartment is outfitted with a fire extinguisher bottle to extinguish fires in the waste compartment. The fire extinguisher is a bottle with two nozzles. The bottle contains pressurized Halon 1301 or equivalent fire extinguishing agent. When the temperature in the waste compartment reaches approximately 170 °F, the solder that seals the nozzles melt and the Halon is discharged. Weighing the bottle is often the only way to determine if the bottle is empty or full.

[Figure 17-24] Fire Detection System Maintenance Fire detector sensing elements are located in many high- activity areas around aircraft engines. Their location, together with their small size, increases the chance of damage to the sensing elements during maintenance. General maintenance of a fire detection system typically includes the inspection and servicing of damaged sections, containment of loose material that could short detector terminals, correcting connection joints and shielding, and replacement of damaged sensing elements. An inspection and maintenance program for all types of continuous-loop systems should include the following visual checks.

Note: These procedures are examples and should not be used to replace the applicable manufacturer’s instructions. 28V DC Left/right main DC LAV smoke detect L/R P110/210 PWR MGMT panel Smoke Sensing chamber Smoke detect circuit Lavatory smoke detector Lavatory Timing circuit Horn Call light Reset Power on (green) Alarm (red) Smoke Test Interrupt alarm CMS Master call light (amber) Test Smoke Indications • Chime • CACP/CSCP • Pop-up window Pulsed ground if smoke detected A Test Interrupt LAV call light/ reset switch 28V AC 17-17 180° 200° 230° Temperature Indicator Strip 250° 180° 200° 230° Temperature Indi cator Strip 250° Fire extinguisher bottle Sensing elements of a continuous-loop system should be inspected for the following: 1. Cracked or broken sections caused by crushing or squeezing between inspection plates, cowl panels, or engine components.

2. Abrasion caused by rubbing of the element on cowling, accessories, or structural members. 3. Pieces of safety wire, or other metal particles, that may short the spot-detector terminals. 4. Condition of rubber grommets in mounting clamps that may be softened from exposure to oils or hardened from excessive heat. 5. Dents and kinks in sensing element sections. Limits on the element diameter, acceptable dents and kinks, and degree of smoothness of tubing contour are specified by manufacturers. No attempt should be made to straighten any acceptable dent or kink, since stresses may be set up that could cause tubing failure. [Figure 17-25] 6. Nuts at the end of the sensing elements should be inspected for tightness and safety wire. [Figure 17-26] Loose nuts should be retorqued to the value specified by the manufacturer’s instructions. Some types of sensing element connection joints require the use of copper crush gaskets. These should be replaced any time a connection is separated.

7. If shielded flexible leads are used, they should be inspected for fraying of the outer braid. The braided sheath is made up of many fine metal strands woven Sharp bendCrushed section Long unsupported loop Kink into a protective covering surrounding the inner insulated wire. Continuous bending of the cable or rough treatment can break these fine wires, especially those near the connectors. 8. Sensing element routing and clamping should be inspected carefully. [Figure 17-27] Long, unsupported sections may permit excessive vibration that can cause breakage. The distance between clamps on straight runs, usually about 8 to 10 inches, is specified by each manufacturer. At end connectors, the first support clamp usually is located about 4 to 6 inches from the end connector fittings. In most cases, a straight run of one inch is maintained from all connectors before a bend is started, and an optimum bend radius of 3 inches is normally adhered to.

9. Interference between a cowl brace and a sensing element can cause rubbing. This interference may cause wear and short the sensing element. 10. Grommets should be installed on the sensing element so that both ends are centered on its clamp. The split end of the grommet should face the outside of the nearest bend. Clamps and grommets should fit the element snugly. [Figure 17-28] Fire Detection System Troubleshooting The following troubleshooting procedures represent the most common difficulties encountered in engine fire detection systems: 1. Intermittent alarms are most often caused by an intermittent short in the detector system wiring. Such shorts may be caused by a loose wire that occasionally touches a nearby terminal, a frayed wire brushing against a structure, or a sensing element rubbing against a structural member long enough to wear through the insulation. Intermittent faults often can be located by moving wires to recreate the short.

17-18 Heat-sensing element Rub point Loose clamp Heat-sensing element Bracket Clamp hinge Grommet Clamp screw 2. Fire alarms and warning lights can occur when no engine fire or overheat condition exists. Such false alarms can be most easily located by disconnecting the engine sensing loop connections from the control unit. If the false alarm ceases when the engine sensing loop is disconnected, the fault is in the disconnected sensing loop, which should be examined for areas that have been bent into contact with hot parts of the engine. If no bent element can be found, the shorted section can be located by isolating the connecting elements consecutively around the entire loop.

3. Kinks and sharp bends in the sensing element can cause an internal wire to short intermittently to the outer tubing. The fault can be located by checking the sensing element with an ohm meter while tapping the element in the suspected areas to produce the short. 4. Moisture in the detection system seldom causes a false fire alarm. If, however, moisture does cause an alarm, the warning persists until the contamination is removed, or boils away, and the resistance of the loop returns to its normal value. 5. Failure to obtain an alarm signal when the test switch is actuated may be caused by a defective test switch or control unit, the lack of electrical power, inoperative indicator light, or an opening in the sensing element or connecting wiring. When the test switch fails to provide an alarm, the continuity of a two-wire sensing loop can be determined by opening the loop and measuring the resistance. In a single-wire, continuous- loop system, the center conductor should be grounded.

Fire Extinguisher System Maintenance Regular maintenance of fire extinguisher systems typically includes such items as the inspection and servicing of fire extinguisher bottles (containers), removal and reinstallation of cartridge and discharge valves, testing of discharge tubing for leakage, and electrical wiring continuity tests. The following paragraphs contain details of some of the most typical maintenance procedures. Container Pressure Check Fire extinguisher containers are checked periodically to determine that the pressure is between the prescribed minimum and maximum limits. Changes of pressure with ambient temperatures must also fall within prescribed limits. The graph shown in Figure 17-29 is typical of the pressure-temperature curve graphs that provide maximum and minimum gauge readings. If the pressure does not fall within the graph limits, the extinguisher container is replaced.

Discharge Cartridges The service life of fire extinguisher discharge cartridges is determined from the manufacturer’s date stamp, which is usually placed on the face of the cartridge. The cartridge service life recommended by the manufacturer is usually in terms of years. Cartridges are available with a service life of 5 years or more. To determine the unexpired service life of a discharge cartridge, it is usually necessary to remove the electrical leads and discharge line from the plug body, which can then be removed from the extinguisher container. 17-19 Temperature (°F) Pressure (psig) −40 −30 −20 −10 0 10 20 30 40 50 60 70 80 900 800 700 600 500 400 300 200 100 0 Min. gauge reading Max. gauge reading chart.

Agent Containers Care must be taken in the replacement of cartridge and discharge valves. Most new extinguisher containers are supplied with their cartridge and discharge valve disassembled. Before installation on the aircraft, the cartridge must be assembled properly in the discharge valve and the valve connected to the container, usually by means of a swivel nut that tightens against a packing ring gasket. [Figure 17-30] If a cartridge is removed from a discharge valve for any reason, it should not be used in another discharge valve assembly, since the distance the contact point protrudes may vary with each unit. Thus, continuity might not exist if a used plug that had been indented with a long contact point were installed in a discharge valve with a shorter contact point.

Note: The preceding material in this chapter has been largely of a general nature dealing with the principles involved and general procedures to be followed. When actually performing maintenance, always refer to the applicable maintenance manuals and other related publications pertaining to a particular aircraft. Fire Prevention Leaking fuel, hydraulic, deicing, or lubricating fluids can be sources of fire in an aircraft. This condition should be noted, and corrective action taken when inspecting aircraft systems. Minute pressure leaks of these fluids are particularly dangerous for they quickly produce an explosive atmospheric condition. Carefully inspect fuel tank installations for signs of external leaks. With integral fuel tanks, the external evidence may occur at some distance from where the fuel is actually escaping. Many hydraulic fluids are flammable and should not be permitted to accumulate in the structure.

Sound-proofing and lagging materials may become highly flammable if soaked with oil of any kind. Any leakage or spillage of flammable fluid in the vicinity of combustion heaters is a serious fire risk, particularly if any vapor is drawn into the heater and passes over the hot combustion chamber. Oxygen system equipment must be kept absolutely free from traces of oil or grease, since these substances spontaneously ignite when in contact with oxygen under pressure. Oxygen servicing cylinders should be clearly marked so they cannot be mistaken for cylinders containing air or nitrogen, as explosions have resulted from this error during maintenance operations.

Mounting lugs (4) Nut Pressure gauge Bottle Packing ring Cartridge Strainer Contact assembly Gasket Discharge plug (or valve) body Swivel nut Bottle outlet 17-20