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
selector valve Outboard engine Fire extinguisher bottle Inboard engine selector valve Double check tee - Bottle pressure gage
- Manifold Safety discharge
- ports (red discs) Discharge indicator port (yellow disc) FIGURF 10-16. Dual container installation and fittings.
common alarm bell. After the alarm bell sounds, it can be silenced by activating the bell cutout switch (figure 10-19). The bell can still respond to a fire signal from any of the other circuits.
Most fire protection systems for turbine engine aircraft also include a test switch and circuitry which permit the entire detection system to be tested at one time. The test switch is located in the center of the panel in figure 10-19.
TURBINE ENGINE GROUND FIRE PROTECTION The problem of ground fires has increased in seriousness with the increased size of turbine engine aircraft. For this reason, means are usually -Discharges lase Pylon disturge extinguisher discharge tube outlets FIGURE 10-17. Fire extinguisher discharge tubes. FIcURE 10-18. Fire extinguisher discharge nozzle location.
Left system transfer test switch -Right system transfer switch Fire detection systems test switch - Engine fire switches No. 1 No. 2 No. 3 No. 4 Bell cutout Test fire Plastic cover Bottle discharge switches FIGURE 10-19. Fire detection system and fire switches.
provided for rapid access to the compressor, tailpipe, and/or burner compartmenta. Thus, many aircraft systems are equipped with spring-loaded access doors in the skin of the various compartments. Such doors are usually located in accessible areas, but not in a region where opening a door might spill burning liquids on the fire fighter.
Internal engine tailpipe fires that take place during engine shutdown or false starts can be blown out by motoring the engine with the starter. If the engine is running, it can be accelerated to a higher
r.p.m. to achieve the same result. If such a fire persists, a fire extinguishing agent can be directed into the tailpipe. It should be remembered that excessive use of CO, or other agents which have a cooling effect can shrink the turbine housing onto the turbine and may damage the engine.
FIRE DETECTION SYSTEM MAINTENANCE PRAC- TICES Fire detector sensing elements are located in many high-activity areas around aircraft engines.
Their location, together with their small size, increases the chances of damage to the sensing elements during maintenance. The installation of the sensing elements inside the aircraft cowl panels provides some measure of protection not afforded elements attached directly to the engine. On the other hand, the removal and re-installation of cowl panels can easily cause abrasion or structural defects to the elemeuts. A well-rounded inspection and maintenance program for all types of continuous-loop sys.
tems should include the following visual checks.
These procedures are provided as examples and should not be used to replace approved local maintenance directives or the applicable manufacturer's instructions.
Sensing elements should be inspected for:
(1) Cracked or broken sections caused by crushing or squeezing between inspection plates, cowl panels, or engine components.
(2) Abrasion caused by rubbing of element on cowling, accessories, or structural members.
(3). Pieces of safety wire or other metal particles which may short the spot detector terminals.
(4) Condition of rubber grommets in mounting clamps, which 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 or 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. (See illustration of kinked tubing in figure 10-20.)
(6) Loose nuts or broken safety wire at the end of the sensing elements (figure 10-21). Loose nuts should be re-torqued to the value specified in the manufacturer's instructions. Some types of sensing element connections require the use of 423
Ling unsupported loop
•Kisk FICURE 10-20. Sensing element defects.
copper crush gaskets. These gaskets should be replaced any time a connection is separated.
(7) Broken or frayed flexible leads, if used.
The flexible lead is made up of many fine metal strands woven 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. Broken strands can also protrude into the insulated gasket and short the center electrode.
(8) Proper sensing element routing and clamping (figure 10-22). Long unsupported sections may permit excessive vibration which can cause breakage, The distance between clamps on straight runs is usually about 8 to 10 in., and is specified by each manufacturer. At end connectors, the first support clamp is usually located about 4 to 6 in. from the end Heat sensing element FIGURE 10-21. Connector joint fitting attached to structure.
424 Loose clamp
- Rub point Loose clamp FIGURE 10-22. Rubbing interference.
connector fittings. In most cases, a straight run of 1 in. is maintained from all connectors before a bend is started, and an optimum bend radius of 3 in. is normally adhered to.
(9) Rubbing between a cowl brace and a sensing element (figure 10-22). This interference, in combination with loose rivets holding the clamps to skin, may cause wear and short the sensing element.
(10) Correct grommet installation. The grommets are installed on the sensing element to prevent the element from chafing on the clamp. The slit end of the grommet should face the outside of the nearest bend.
Clamps and grommets (figure 10-23) should fit the element snugly.
(11) Thermocouple detector mounting brackets should be repaired or replaced when cracked, corroded, or damaged. When replacing a thermocouple detector, note which wire is connected to the identified plus terminal of the defective unit and connect the replacement in the same way.
(12) Test the fire detection system for proper operation by turning on the power supply and placing the fire detection test switch in the "TEST" position. The red warning light should flash on within the time period established for the system.
some aircraft an audible alarm will also sound.
In addition, the fire detection circuits are checked for specified resistance and for an open or grounded condition. Tests required after repair of replacement of units in a fire detection system or when the system is inoperative include: (1) Checking the polarity, ground, resistance and continuity of systems that use thermocouple detector units, and (2) resistance and continuity tests performed on systems with sensing elements or cable detector units. In all situations follow the recommended practices and procedures of the manufacturer of the type system with which you are working.
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 which occasionally touches a nearby terminal, a frayed wire brushing against a structure, or a sensing element rubbing long enough against a structural member to wear through the insulation. Intermittent faults can often best be located by moving wires to recreate the short.
Grommet Clamp screw Clamp hinge Heat sensing element Bracket FIGURE 10-23. Typical fire detector loop clamp.
(2) Fire alarms and warning lights can occur when no engine fire or overheat condition exists. Such false alarms can most easily be located by disconnecting the engine sensing loop from the aircraft wiring. If the false alarm continues, a short must exist between the loop connections and the control unit. If, however, 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 which 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 and disconnecting 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 a megger 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 will persist 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 singlewire, continuous-loop system, the center conduct or should be grounded.
FIRE EXTINGUISHER SYSTEM MAINTENANCE
PRACTICES Regular maintenance of fire extinguisher systems typically includes such items as the inspection and servicing of fire extinguisher bottles (containers), removal and re-installation 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, and are in- 425
Maximum gage reading Pressure (p.s.i.g.)
200 Minimum gage reading 100 -40 -30 -20 -10 10 20 30 40 50 60 70 80 90 100 Temperature (°F) FicuRE 10-24. Fire extinguisher container pressure/temperature curve.
cluded to provide an understanding of the operations involved.
Fire extinguisher system maintenance procedures vary widely according to the design and construction of the particular unit being serviced. The detailed procedures outlined by the airframe or sys.
tem manufacturer should always be followed when performing maintenance.
Container Pressure Check A pressure check of fire extinguisher containers is made periodically to determine that the pressure is between the minimum and maximum limits prescribed by the manufacturer. Changes of pressure with ambient pressure must also fall within prescribed limits. The graph shown in figure 10-24 is typical of the pressure/temperature curve graphs that provide maximum and minimum gage readings.
If the pressure does not fall within the graph limits, the extinguisher container should be replaced.
Freon Discharge Cartridges The service life of fire extinguisher discharge cartridges is calculated from the manufacturer's date stamp, which is usually placed on the face of the cartridge. The manufacturer's service life is usually recommended in terms of hours below a predetermined temperature limit. Many cartridges are available with a service life of approximately 5,000 hours. To determine the unexpired service life of a discharge cartridge, it is necessary to remove the electrical leads and discharge hose from the plug body, which can then be removed from the extinguisher container.
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 properly assembled into the discharge valve and the valve connected to the container, usually by means of a swivel nut that tightens against a packing ring gasket.
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 which had been indented with a long contact point were installed in a discharge valve with a shorter contact point.
When actually performing maintenance, always refer to the applicable maintenance manuals and other related publications pertaining to a particular aircraft.
Freon Containers Bromochloromethane and freon extinguishing agents are stored in steel spherical containers.
There are four sizes in common use today ranging from 224 cu. in. (small) to 945 cu. in. (large).
The large containers weigh about 33 Ibs. The Container small spheres have two openings, one for the bonnet assembly (sometimes called an operating head).
and the other for the fusible safety plug (figure 10-25). The larger containers are usually equipped with two firing bonnets and a two-way check valve as shown in figure 10-26. • Firing bonnets Container Two way check valve FIGURE 10-26. Typical double bonnet extinguisher assembly.
Gage Disk Fusible safety plug Cartridge Strainer Electrical contact FIGURE 10-25. Single bonnet sphere assembly.
The containers are charged with dry nitrogen in addition to a specihed weight of the extinguishing agent. The nitrogen charge provides sufficient pressure for complete discharge of the agent. The bonnet assembly contains an electrically ignited power cartridge which breaks the disk, allowing the extinguishing agent to be forced out of the sphere by the nitrogen charge.
A single bonnet sphere assembly is illustrated in figure 10-25. The function of the parts shown, other than those described in the preceding paragraph, are as follows: (1) The strainer prevents pieces of the broken disk from entering the system,
(2) the fusible safety plug melts and releases the liquid when the temperature is between 208° and 220° F., and (3) the gage shows the pressure in the container. In this type of design, there is no need for siphon tubes.
In some installations the safety plug is connected to a discharge indicator mounted in the fuselage skin, while others simply discharge the fluid into the fire extinguisher container storage compartment.
The gage on the container should be checked for an indication of the specified pressure as given in the applicable aircraft maintenance manual. In addition make certain that the indicator glass is unbroken and that the bottle is securely mounted.
Some types of extinguishing agents rapidly corrode aluminum alloy and other metals, especially under humid conditions. When a system that uses a corrosive agent has been discharged, the system must be purged thoroughly with clean, dry, compressed air as soon as possible.
Almost all types of fire extinguisher containers require re-weighing at frequent intervals to determine the state of charge. In addition to the weight 427
check, the containers must be hydrostatically tested, usually at 5-year intervals.
The circuit wiring of all electrically discharged containers should be inspected visually for condition. The continuity of the entire circuit should be checked following the procedures in the applicable maintenance manual. In general this consists of checking the wiring and the cartridge, by using a resistor in the test circuit that limits the circuit current to less than 35 milliamperes to prevent detonating the cartridge.
Carbon Dioxide Cylinders These cylinders come in various sizes, are made of stainless steel, and are wrapped with steel wire to make them shatterproof. The normal storage pressure of the gas ranges from 700 to 1,000 p.s.i.
However, the state of the cylinder charge is determined by the weight of the CO.. In the container, about two-thirds to three-fourths of the CO? is liquefied. When the CO, is released, it expands about 500 times as it converts to gas.
The cylinder does not have to be protected against cold weather, for the freezing point of carbon dioxide is minus 110° F. However, it can discharge prematurely in hot climates. To prevent this, manufacturers put in a charge of dry nitrogen, at about 200 p.s.i., before they fill the cylinder Straight siphon tube for all vertical installations with carbon dioxide. When treated in this manner, most COz cylinders are protected against premature discharge up to 160° F. With a temperature increase, the pressure of the nitrogen does not rise as much as that of the CO, because of its stability with regard to temperature changes. The nitrogen also provides additional pressure during normal release of the CO, at low temperature during cold weather.
Carbon dioxide cylinders are equipped internally with one of three types of siphon tubes, as shown in figures 10-27 and 10-28. Aircraft fre extinguishers have either the straight rigid or the short flexible siphon tube installed. The tube is used to make certain that the CO, is transmitted to the discharge nozzle in the liquid state.
Cylinders containing either the straight rigid or short flexible types of siphoning tubes should be mounted as shown in figure 10-28. Notice that the straight rigid siphon tube is allowed a 60° tolerance, while the tolerance for the short flexible tube is only 30°.
CO, cylinders are equipped with metal safety disks designed to rupture at 2,200 to 2,800 p.s.l.
This disk is attached to the cylinder release valve body by a threaded plug. A line leads from the fitting to a discharge indicator installed in the fuselage skin. Rupture of the red disk means that Type of tube in cylinder is indicated by one of the following methods:
(1) By stamping on body bushing *SF" denotes short flexible tube "S" denotes straight tube
(2) By stamping or stenciling on body of cylinder "Straight rigid siphon" "Short flexible siphon" "Curved siphon" AH thread - - LH thread
- Siphon tube integral with body bushing Curved siphon tube Short flexible siphon tube for raft installations for horizontal installations FIcURE 10-27. Typical CO: cylinder construction.
Straight rigid siphon for vertical installation Short flexible siphon (for horizontal installations) FIcURF 10-28. Mounting positions of CO: cylinders.
the container safety plug has ruptured because of an overheat condition. A yellow disk is also installed in the fuselage skin. Rupture of this disk indicates that the system has been discharged normally.
FIRE PREVENTION AND PROTECTION Leaking fuel and hydraulic, de-icing, 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 will spontaneously ignite in contact with oxygen under pressure. Oxygen servicing cylinders should be clearly marked so that they cannot be mistaken for cylinders containing air or nitrogen, as explosions have resulted from this error during maintenance operations.
Fire prevention is much more rewarding than fire extinguishing.
COCKPIT AND CABIN INTERIORS All wool, cotton, and synthetic fabrics used in interior trim are treated to render them flame resistant. Tests conducted have shown foam and sponge rubber to be highly flammable. However, if they are covered with a flame-resistant fabric which will not support combustion, there is little danger from fire as a result of ignition produced by accidental contact with a lighted cigarette or burning paper.
Fire protection for the aircraft interior is usually provided by hand-held extinguishers. Four types of fire extinguishers are available for extinguishing interior fires: (1) water, (2) carbon dioxide, and
(3) dry chemical, and (4) halogenated hydrocarbons.
Extinguisher Types
(1) Water extinguishers are for use primarily on nonelectrical fires such as smoldering fabric, cigarettes, or trash containers. Water extinguishers should not be used on electrical fires because of the danger of electrocution. Turning the handle of a water extinguisher clockwise punctures the seal of a CO, cartridge which pressurizes the container. The water spray from the nozzle is controlled by a trigger on top of the handle.
(2) Carbon dioxide fire extinguishers are provided to extinguish electrical fires. A long, hinged tube with a non-metallic megaphone-shaped nozzle permits discharge of the CO, gas close to the fire source to smother the fire. A trigger type release is normally lockwired and the lockwire can be broken by a pull on the trigger.
(3) A dry chemical fire extinguisher can be used to extinguish any type of fire. However, the 429
dry chemical fire extinguisher should not be used in the cockpit due to possible interference with visibility and the collection of nonconductive powder on electrical contacts of surrounding equipment.
The extinguisher is equipped with a fixed nozzle which is directed toward the fire source to smother the fire. The trigger is also lockwired but can be broken by a sharp squeeze of the trigger.
(4) The development of halogenated hydrocarbons (freons) as fire extinguishing agents with low toxicity for airborne fire extinguishing protection systems logically directed attention to its use in hand type fire extinguishers.
Bromotrifluoromethane (Halon 1301) having a rating of 6 on the toxicity scale is the logical successor to CO2 as a hand type fire extinguisher agent. It is effective on fires in lower concentrations. Halon 1301 can extinguish a fire with a concentration of 2% by volume. This compares with about 40% by volume concentration required for CO2 to extinguish the same fire.
This quality allows Halon 1301 to be used in occupied personnel compartments without depriving people of the oxygen they require. Another advantage is that no residue or deposit remains after use. Halogen 1301 is the ideal agent to use in airborne hand held fire extinguishers because:
(1) its low concentration is very effective, (2) it may be used in occupied personnel compartments, (3) it is effective on all 3 type fires, and
(4) no residue remaining after its use.
Extinguishers Unsuitable as Cabin or Cockpit Equipment The common aerosol can type extinguishers are definitely not acceptable as airborne hand type extinguishers. In one instance, an aerosol type foam extinguisher located in the pilot's seat back pocket exploded and tore the upholstery from the seat. The interior of the aircraft was damaged by the foam. This occurred when the aircraft was on the ground and the outside air temperature was 90° F. In addition to the danger from explosion, the size is inadequate to combat even the smallest fire.
A dry chemical extinguisher was mounted near a heater vent on the floor. For an unknown reason, the position of the unit was reversed. This placed the extinguisher directly in front of the heater vent.
During flight, with the heater in operation, the extinguisher became overheated and exploded filling the compartment with dry chemical powder. The proximity of heater vents should be considered • hand fire coco.
when selecting a location for a hand fire extinguisher.
Additional information relative to airborne hand fire extinguishers may be obtained from the local FAA District Office and from the National Fire Protection Association, 470 Atlantic Ave., Boston, MA 02210.
SMOKE DETECTION SYSTEMS A smoke detection system monitors the cargo and baggage compartments for the presance of smoke, which is indicative of a fire condition. Smoke detection instruments, which collect air for sampling, are mounted in the compartments in strategic locations.
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 a heat detection system.
Smoke detection instruments are classified by method of detection as follows: Type 1 - Measure.
ment of carbon monoxide gas (CO detectors), Type II - Measurement of light transmissibility in air (photoelectric devices), and Type IIII - Visual detection of the presence of smoke by directly viewing air samples (visual devices).
To be reliable, smoke detectors must be maintained so that smoke in a compartment will be indicated as soon as it begins to accumulate. Smoke detector louvers, vents, and ducts must not be obstructed.
Carbon Monoxide Detectors The CO detectors, which detect concentrations of carbon monoxide gas, are rarely used to monitor cargo and baggage compartments. However, they have gained widespread use in conducting tests for the presence of carbon monoxide gas in aircraft cabins and cockpits.
Carbon monoxide is a colorless, odorless, tasteless, non-irritating gas. It is the byproduct of incomplete combustion, and is found in varying degrees in all smoke and fumes from burning carbonaceous substances. Exceedingly small amounts of the gas are dangerous. A concentration of .02% (2 parts in 10,000) may produce headache, mental dullness, and physical loginess within a few hours.
There are several types of portable testers (sniffers) in use. One type has a replaceable indicator tube which contains a yellow silica gel, impregnated with a complex silico-molybdate compound and is catalyzed using palladium sulfate.
• In use, a sample of air is drawn through the detector tube. When the air sample contains carbon monoxide, the yellow silica gel turns to a shade of 430