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
green. The intensity of the green color is proportional to the concentration of carbon monoxide in the air sample at the time and location of the tests.
Another type indicator may be worn as a badge or installed on the instrument panel or cockpit A functional check of the detector should be made after installation, and at frequent intervals thereafter.
wall. It is a button using a tablet which changes from a normal tan color to progressively darker shades of gray to black. The transition time required is relative to the concentration of CO. At 28 v.d.c. Test switch a concentration of 50 ppm CO (0.005%), the indication will be apparent within 15 to 30 minutes.
A concentration of 100 ppm CO (0.01%) will change color of the tablet from tan to gray in 2-5 minutes, from tan to dark gray in 15 to 20 minutes.
Detector 7 Photoelectric Smoke Detectors Beacon This type of detector consists of a photoelectric lamp cell, a beacon lamp, a test lamp, and a light trap, all mounted on a labyrinth. An accumulation of 10% smoke in the air causes the photoelectric cell to | To amplifier and conduct electric current. Figure 10-29 shows the fire warning circuit details of the smoke detector, and indicates how the smoke particles refract the light to the photoelectric cell. When activated by smoke, the detector supplies Test relay a signal to the smoke detector amplifier. The ampli- Test fier signal activates a warning light and bell. lamp FIGURE 10-30. Smoke detector test circuit.
Photoelectric cell Beacon lamp
• Visual Smoke Detectors On a few aircraft visual smoke detectors provide Light retracted to photoelectric cell particles Smoke the only means of smoke detection. Indication is by smoke particles provided by drawing smoke through a line into the Labrynth indicator, using either a suitable suction device or cabin pressurization.
• 00 When smoke is present a lamp within the indicator is illuminated automatically by the smoke detec- Light -Test lamp rendered visible in the appropriate window of the tor. The light is scattered so that the smoke is FIGURE 10-29. Photoelectric smoke detector. indicator. If no smoke is present the lamp will not be illuminated. A switch is provided to illuminate the lamp for test purposes. A device is also pro- A test switch (figure 10-30) permits checking the vided in the indicator to show that the necessary operation of the smoke detector. Closing the switch airflow is passing through the indicator.
connects 28 v. d.c. to the test relay. When the test relay energizes, 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, the photoelectric cell, the smoke detector amplifier, and associated circuits are operable.
The efficiency of any detection system depends on the positioning and serviceability of all the components of the system. The foregoing information is intended to provide familiarization with the various systems. For details of a particular installation, refer to the relevant manuals for the aircraft concerned.
The maximum allowable concentration, under (parts per million) which is equal to 0.005% of Federal Law, for continuing exposure is 50 ppm carbon monoxide. (See figure 10-31.)
The maximum allowable concentration under Federal Law for continuing exposure is 50 ppm (parts per million) which is equal to 0.005% of carbon monoxide.
Parts Per Million Percentage Reaction 50 0.005% Maximum allowable concentration under Federal Law.
100 0.01 % Tiredness, mild dizziness.
200 0.02 % Headaches, tiredness, dizziness, nausea after 2 or 3 hours.
800 0.08 % Unconsciousness in 1 hour and death in 2 to 3 hours.
2,000 0.20 % Death after 1 hour.
3,000 0,30 % Death within 30 minutes.
10,000 1.00 % Instantaneous death.
FIGURE 10-31. Human reactions to carbon monoxide poisoning.
GENERAL The satisfactory performance of any modern aircraft depends to a very great degree on the continuing reliability of electrical systems and subsystems.
Improperly or carelessly installed wiring or improperly or carelessly maintained wiring can be a source of both immediate and potential danger. The continued proper performance of electrical systems depends on the knowledge and techniques of the mechanic who installs, inspects, and maintains the electrical system wires and cables.
Procedures and practices outlined in this section are general recommendations and are not intended to replace the manufacturer's instructions and approved practices.
For the purpose of this discussion, a wire is described as a single, solid conductor, or as a stranded conductor covered with an insulating material. figure 11-1 illustrates these two definitions of a wire.
Conductors Wire single solid conductor Solid conductor. Stranded condactor 002 1 8 19 1 FICURE 11-1. Two types of aircraft wire.
The term cable, as used in aircraft electrical installations, includes:
(1) Two or more separately insulated conductors in the same jacket (multi-conductor cable).
CHAPTER 11
AIRCRAFT ELECTRICAL SYSTEMS
(2) Two or more separately insulated conductors twisted together (twisted pair).
(3) One or more insulated conductors, covered with a metallic braided shield (shielded cable).
(4) A single insulated center conductor with a metallic braided outer conductor (radio frequency cable). The concentricity of the center conductor and the outer conductor is carefully controlled during manufacture to ensure that they are coaxial.
Wire Size Wire is manufactured in sizes according to a standard known as the AWG (American wire gage). As shown in figure 11-2, the wire diameters become smaller as the gage numbers become larger.
The largest wire size shown in figure 11-2 is number 0000, and the smallest is number 40. Larger and smaller sizes are manufactured but are not commonly used.
A wire gage is shown in figure 11-3. This type of gage will measure wires ranging in size from number zero to number 36. The wire to be measured is inserted in the smallest slot that will just accommodate the bare wire. The gage number corresponding to that slot indicates the wire size. The slot has parallel sides and should not be confused with the semicircular opening at the end of the slot. The opening simply permits the free movement of the wire all the way through the slot.
Gage numbers are useful in comparing the diameter of wires, but not all types of wire or cable can be accurately measured with a gage. Large wires are usually stranded to increase their flexibility. In such cases, the total area can be determined by multiplying the area of one strand (usually computed in circular mils when diameter or gage number is known) by the number of strands in the wire or cable.
Factors Affecting the Selection of Wire Size Several factors must be considered in selecting the size of wire for transmitting and distributing electric power.