AC 43.13-1A CHG 1 Page 165 1975 Chapter 10. HYDRAULIC AND PNEUMATIC SYSTEMS Section 1. HYDRAULIC SYSTEMS
392. GENERAL. Maintain, service, and adjust connection fittings: AN-818 nut and AN-819 airplane hydraulic systems in uccordance with sleeve.
manufacturers' maintenance manuals and pertinent component maintenance manuals. Certain Minimum bend radii * general principles of maintenance and repair Wrench torque range measured at inside for tightening tube nuts of bend.
which apply are outhned below. Tube O.D. (inch pounds) Dimension in inches. * (Inches)
393. HYDRAULIC LINES AND FITTINGS. Carefully Alum. alley 1100-H14, Alum. alloy 1100-114, inspect all lines and fittings at regular intervals 5052-0 Steel 5052-0 Steel to insure airworthiness. Investigate any evidence of fluid loss or leaks. Check metal lines for 30-70 leaks, loose anchorages, scratches, kinks, or other 60-80 40-65 50-90 70-120 ½o 21⅛2 damage. Inspect fittings and connections for 75-125 00-150 1% leakage, looseness, cracks, burrs, or other damage. 150-250 155-250 1010 1¾e Replace or repair defective elements. 200-350 300-400 1¾ 300-500 430-575 13½ 2%6
a. Replacement of Metal Lines. When inspec- 500-700 550-750 1¾ 2% tion shows a line to be damaged or defective, 1 600-900 3½ replace the entire line or if the damaged section 1¾ 600-900 3¾ 4% is localized, a repair section may be inserted. In replacing lines, always use tubing of the same 1¾ 2 5 8 5¼ 8% size and material as the original line. Use the old tubing as a template in bending the new line, FIGURE 10.1.--Tube data.
unless it is too greatly damaged, in which case a template can be made from soft iron wire. In forming flares, cut the tube ends square, file Soft aluminum tubing (1100, 3003, or 5052) smooth, remove all burrs and sharp edges, and under ¼-inch outside diameter may be bent by thoroughly clean. The tubing is then flared hand. For all other tubing use an acceptable using the correct 37° aviation flare forming tool hand or power tube bending tool. Bend tubing for the size of tubing and type of fitting.
carefully to avoid excessive flattening, kinking, double flare is used on soft aluminum tubing or wrinkling. Minimum bend radii values are ⅛-inch outside diameter and under, and a single shown in figure 10.1. A small amount of flatten- flare on all other tubing. In making the connec ing in bends is ncceptable but do not exceed an tions, use hydraulic fluid as a lubricant and then amount such that the small diameter of the flat- tighten. Overtightening will damage the tube tened portion is less than 75 percent of the origi or fitting, which may cause a failure; undernal outside diameter. When installing the tightening may cause leakage which could result replacement tubing, line it up correctly with the in a system failure, mating part so that it is not forced into line by means of the coupling nuts. CAUTION Mistaken use of 45° automotive fare forming tools will
b. Tube Connections. Many tubing connections result in improper tubing fare shape and angle causing are made using flured tube ends, and standard misfit, stress and straln, and probable system failure.
Chap 10 Par 392
Page 166 AC 43.13-1A CHG 1 1975
c. Repair of Metal Tube Lines. Minor dents and scratches in tubing may be repaired. Scratches or nicks no deeper than 10 percent of the wall thickness in aluminum alloy tubing, that are not in the heel of a bend, may be repaired by burnishing with hand tools. Replace lines with severe die marks, seams, or splits in the tube.
any crack or deformity in a flare is also unac.
ceptable and cause for rejection. A dent less than 20 percent of the tube diameter is not objectionable unless it is in the heel of a bend.
Dents may be removed by drawing a bullet of proper size through the tube by means of a length of cable. A severely damaged line should be replaced; however, it may be repaired by cutting out the damaged section and inserting a tube seotion of the same size and material. Flare both ends of the undamaged and replacement tube sections and make the connection by using standard unions, sleeves, and tube nuts. If the damaged portion is short enough, omit the insert tube and repair by using one union and two sets of connection fittings.
d. Replacement of Flexible Lines. When replacement of a flexible line is necessary, use the same type, size, and length of hose as the line to be replaced. If the replacement of a hose with swaged-end-type fittings is necessary, obtain a new hose assembly of the correct size and composition. Certain synthetic oils require a specially compounded synthetic rubber hose which is compatible. Refer to the aircraft manufacturer's service information for correct part number for replacement hose. If the fittings on each end are of the collet type or sleeve type, a replacement may be fabricated as shown in figure 10.2. Typical aircraft hose specifications and their uses are shown in figure 10.3. Install hose assemblies without twisting. (See fgure
10.4.) Never stretch a hose tight between two fittings as this will result in overstressing and eventual failure. The length of hose should be sufficient to provide about 5 percent to 8 percent alack. Avoid tight bends in flex lines as they may result in failure. Never exceed the minimum bend radii as indicated in figure 10.5.
Teflon hose is used in many aircraft systems because of its superior qualities for certain applications. Teflon is compounded from tetrafluorothylene resin which is unaffected by fluids normally used in aircraft. It has an operating range of -65° F. to 450° F. For these reasons, Teflon is used in hydraulic and engine lubricating systems where temperatures and pressures preclude the use of rubber hose. Although Teflon hose has excellent performance qualities, it also has peculiar characteristics that require extra care in handling. It tends to assume a permanent set when exposed to high pressure or temperature. Do not attempt to straighten a hose that has been in service. Any excessive bending or twisting will cause kinking or weakening of the tubing wall. Replace any hose that shows signs of leakage, abrasion, or kinking.
Any hose suspected of kinking may be checked with a steel ball of proper size. Figure 10.6 shows hose and ball sizes. The ball will not pass through if the hose is distorted beyond limits.
If the hose fittings are of the reusable type, a replacement hose may be fabricated as described in figure 10.2. When a hose assembly is removed the ends should be tied as shown in figure 10.7, so that its preformed shape will be maintained. Refer to figure 10.9 for minimum bend radii of Teflon hose.
All flexible hose installations should be supported at least every 24 inches. Closer supports are preferred. They should be carefully routed and securely clamped to avoid abrasion, kinking, or excessive flexing. Excessive flexing may cause weakening of the hose or loosening at the fittings.
e. O-Ring Seals. A thorough understanding of O-ring seal applications is necessary to determine when replacement must be made. The simplest application is where the O-ring merely serves as a gasket when it is compressed within a recessed area by applying pressure with a packing nut or screw cap.
Leakage is not normally acceptable in this type of installation. In other installations the O- ring seals depend primarily upon their resiliency to accomplish their sealing action. When moving parts are involved, minor seepage may be normal and acceptable. A moist surface found on moving parts of hydraulic units is an indication the seal is being properly lubricated.
In pneumatic systems, seal lubrication is provided by the installation of a grease-impreg- Par 393 * U.S. GOVERNMENT PRINTING OFFICE: 1975-631-513/928 3-1 Chap 10
AC 43.13-1A Page 167 1972
LENE ST
COYER WIN ENE
TO WHE DAG
TWIST OFF
COVER WITH PLIERS
1. Place hose in vise and cut to 2. Locate length of hose to be cut desired length using fine tooth off and slit cover with knife to wire hacksaw or cut off wheel. brald. After slitting cover, twist off with pair of pliers. (see nole below)
3. Place hose in vise and screw 4. "Lubricate inside of hose and socket on hose counterclockwise. nipple threads liberally.
NOTE:
Hose assemblies fabricated per MIL-H-8790 must have exposed wire braid coated with a special sealant.
NOTE:
Step 2 applies to high pressure hose only.
Screw nipple into socket using wrench on hex of nipple and leave .005 inches to .031 inches clearance between nipple hex and socket.
*CAUTION:
Do not use any petroleum product with hose designed for synthetic fluids "SKYDROL".
For a lubricant during assembly use a vegetable soap liquid.
DISASSEMBLE IN REVERSE ORDER FIGURE 10.2.-Hose assembly instructions.
Chap 10 Par 393
Page 168 AC 43.13-1A 1972
SINGLE WIRE BRAID FABRIC COVERED NUSE HOSE HOSE RECOMM. MIN. MAX. MIN.
MIL SIZE SIZE SIZE OPER. BURST PROOF BEND PART NO. O.D. I.D. O.D. PRESS. PRESS. PRESS. RADIUS MIL-H-B794- 3-1 45 3,000 12,000 6,000 3.00 MIL-H-8794- 4-1 -52 3,000 12,000 6,000 3.00 MIL-H-8794. 5-1 -56 3,000 10,000 5.000 3.38 MAIL-H-8794 6-1 -07 2,000 9,000 4,500 4.00 MIL-H-B794. 8-1 2,000 8,000 4,000 4.63 MIL-H-8794-10-6 -92 1,750 7,000 3,500 5.50 MIL-H-8794-12-L 1.08 1,500 6,000 3,000 6.50 MIL-H-8794-16-L 1.23 800 3,200 1,600 7.38 MIL-H-8794-20-1 1¼ 1.50 600 2,500 1,250 9.00 MIL-H-8794-242 1% 175 500 2,000 1,000 11.00 MIL-H-8794-32-L 1'Хo 2.22 350 1,400 700 13.25 MIL-M-0794-40-1 2% 2.88 200 1,000 300 24.00 MIL-M-8794-48-L 3.56 200 800 300 33.00 Construction: Seamless syn- Uses:
thetic rubber inner tube rein- Hose is approved for use in forced with one fiber braid, one braid of high tensile aircraft hydraulic, pneumatic, coolant, fuel and oil systems.
steel wire and covered with an oil resistant rubber im- Operating pregnated fiber braid. Temperatures:
Identification: Hose is iden- tified by specification num- Sizes -3 thru - 12: Minus 65°F. to plus 250°F.
ber, size number, quarter Sizes - 16 thru - 48: Minus year and year, hose manu- 40°F. 1o plus 275°F.
facturer's Identification. NOTE, Maximum temperatures and pres.
sures should not be used simullaneously.
MULTIPLE WIRE BRAID RUBBER COVERED TUBE HOSE HOSE ВЕСОМИ. NIM. MIM. Лік.
SIZE SIZE SIZE OFER BURST PROOF SEND PART NO. O.D. I.D. O.D. PRESS. PRESS. PRESS BADIUS MIL-H-9788. 4-1 ,63 3,000 16,000 8,000 3.00 MIL-H-B788-51 70 3,000 14,000 7,000 3.36 MIL-H-B788- 6-L 3,000 14,000 7,000 3.00 MIL-H-8788- S-L ,86 3,000 14,000 7,000 5.75 MIL-H-8788-10-1 1.03 3,000 12,000 6,000 6.50 MIL-H-8788-12-L ¼ 1.22 3,000 12,000 6,000 775 MIL-H-8788-16-1 1.50 3,000 10,000 5,000 9.63 Hose Constructions Seamless synthetic rubber inner tube reinforced with one fabric braid, two or more steel wire braids, and covered with a synthetic rubber cover (for gas applications request perforated cover).
Identification: Hose is identified by specification number, size number, quarter year and year, hose manufacturer's Identification.
Usos: High pressure hydraulic, pneumatic, coolant, fuel and oil.
Operating Temperatures:
Minus 65°F. to plus 200°F.
FIGURE 10.3.-Aircraft hose specificationa.
Par 393 Chap 10
AC 43.13-1A Page 169 1972
RIGHT WAY WRONG WAY Do not bend or twist the hose as 1001 319 illustrated.
Allow enough slack in the hose line to provide for changes in length when pressure is applied.
The hose will change in length from +296 to -19%.
Metal and fittings cannot be considered as part of the flexible portion of the assembly.
The use of elbows and adapters will assure easier installation and in many installations will remove the strain from the hose line and greatly increase service life.
At all times keep the minimum bend radil of the hose as large (pee as possible to avoid tube collapsing.
FIGURE 10.4.-Proper hose installations.
Chap 10 Par 393
Page 170 AC 43.13-1A 1972 INSIDE BEND RADII V.S. OPERATING PRESSURE MIL-H-8794 HOSE WITH NO FLEXING 10 = • 200 400 600 800 1000 1500 2000 2500 3000 OPERATING PRESSURE -POUNDS/SO. IN. (-3 TO -24 INCLUSIVE) MINIMUM BEND RADII FOR - 32, - 40, AND - 48 AT ALL PRESSURES ARE AS FOLLOWS: -32 13.25" MIL-M-8788 HOSE WITH -40 24ª NO FLEXING -48 33" DASK NO. SEND RADHI
1.3 3.000
3.375
1.2 5.000
5.750 10 8.500 12 7.750 20 40 60 80 100 120 140 160 180 16 9.625
TOTAL FLEXING RANGE OF INSTALLED HOSE (DEGREES) MINIMUM BEND RADIUS OF HOSE UNDER FLEXING CONDITIONS = IN" × NO FLEXING BEND RADIUS OF EITHER MIL-H-8794 OR MIL-H-8788 HOSE.
EXAMPLE: FOR MIL-H-8794 HOSE SIZESIZE AT T5ОD PSI AND HAVING A FLEXING RANGE OF 60 MINIMUM BEND RADIUS = 1.16 × 6.5 = 7½ INCHES (MEASURED AT INSIDE OF BEND).
MINIMUM BEND RADI MEASURED AT INSIDE OF BEND DIMENSIONS IN INCHES.
FIGURE 10.5.-Minimum bend radil.
Par 393 Chap 10
AC 43.13-1A Page 171 1972 nated felt wiper ring. When systems are static, seepage past the seals is not normally acceptable.
(1) Inspection. During inspection, consider the following to determine whether seal replacement is necessary:
(a) How much fluid or air is permitted to seep past the seals? In some installations minor seepage is normal. Refer to manufacturers' maintenance information.
(b) What effect does the leak have on the operation of the system? Know the system.
(c) Does the leak of fluid or air create a hazard or affect surrounding installations? A check of the system fluid and a knowledge of previous fuid replenishment is helpful.
(d) Will the system function safely until the next inspection?
(2) Do's and Don'ts That Apply to O-ring Seal Maintenance.
(a) Correct all leaka from static seal installations.
(b) Don't retighten packing gland nuts;
retightening will, in most cases, increase HOSE SIZE BALL SIZE rather than decrease the leak.
-4 5/64 9/64 (e) Never reuse O-ring seals because they -6 13/64 tend to swell from exposure to fluids, and be- -8 9/32 come set from being under pressure. They may -10 3/8 have minor cuts or abrasions that are not read- -12 1/2 ily discernible by visual inspection.
- 16 -20 61/64 47/84 (d) Avoid using tools that might damage the seal or the sealing surfaces.
(e) Do not depend upon color-coding.
FIGURE 10.6.-Ball diameters for testing hose restric- tions or kinking. Coding may vary with manufacturer.
(f) Be sure that part number is correct.
(g) Retain replacement seals in their package until ready to use. This provides proper identification and protects the seal from damage and contamination.
(h) Assure that the sealing surfaces are clean and free of nicks or scratches before installing seal.
(¡) Protect the seal from any sharp sur- a core th SUPPORT WIRE faces that it must pass over during installation. Use an installation bullet or cover the sharp surfaces with tape.
(¡) Lubricate the seal so it will slide into place smoothly.
(k) Be sure the seal has not twisted during installation.
(I) Allow sufficient time for the seal to cold-flow to its original size before continuing with the installation.
(3) Storage of Replacement Seals,
(a) Store O-ring seals where temperatures do not exceed 120° F.
(b) Keep seals packaged to avoid exposure to ambient air and light, particularly sun- FIGURE 10.7.-Suggested handling of preformed hose. light.
Chap 10 Par 393
AC 43.1:
•606-20° MINIMUM BEND RADIUS (INCHES ) -667-20° -666:10° 667-10 -006.12" 666-10 600-8 666-5 -006-4- 000-3 10 12 15 OPERATING PRESSURE, PSI ( 00'S OMITTED I FIGURE 10.8.-Minimum bend radil-Teflon hose.
(c) Avoid storing O-ring seals on pegs or hooks, as in hardware stores. This practice leads to loss of identification, loss of cure dates, impairs cleanliness, and tends to cause the seals to become distorted and lose their resiljency from exposure to light and ambient air.
394. HYDRAULIC COMPONENTS. Hydraulic components such as pumps, actuating cylinders, selector valves, relief valves, etc., should be repaired or adjusted following the airplane and component manufacturer's instructions. Inspect hydraulic filter elements at frequent intervals and replace as necessary.
395.-405 RESERVED.
Par 393 Chap 10
AC 43.13-1A Page 173 1972 Chapter 11. ELECTRICAL SYSTEMS Section 1. CARE OF ELECTRICAL SYSTEMS
406. GENERAL. The satisfactory performance of an aircraft is becoming more dependent upon the continued reliability of the electrical system. Reliability of the system is usually proportional to the amount of maintenance received and the knowledge of men who perform such maintenance. It is, therefore, important that maintenance be accomplished using the best techniques and practices to minimize the possibility of failure. This handbook is not intended to supersede or replace any government specification or specific manufacturer's instruction regarding electrical system maintenance.
The term "electrical system" as used in this circular means those parts of the aircraft which generate, distribute, and utilize electrical energy, including their support and attachments.
407. INSPECTION AND OPERATION CHECKS.
Frequently, inspect equipment, electrical assemblies, and wiring installations for damage, general condition, and proper functioning to assure the continued satisfactory operation of the electrical system. Adjust, repair, overhaul, and test electrical equipment and systems in accordance with the recommendations and procedures set forth in aircraft and/or component maintenance instructions. Replace components of the electrical system that are damaged or defective with identical items or with equipment equivalent to the original in operating characteristics, mechanical strength, and the ability to withstand the environmental conditions encountered in the operation of the aircraft. A suggested list of items to look for and checks to be performed are:
a. Damaged or overheated equipment, connections, wiring, and installation.
b. Excessive resistance at high current carrying connections as determined by millivolt drop test.
c. Misalinement of electrically driven equipment.
d. Poor electrical bonding.
e. Dirty equipment and connections.
f. Improper support of wiring and conduit.
g. Loose connections, terminals, and ferrules.
h. Continuity of fuses.
i. Condition of electric lamps.
I. Insufficient clearance or poor insulation of exposed terminals.
k. Broken or missing safety wire, cotter pins, etc.
I. Operational check of electrically operated equipment such as motors, inverters, generators, batteries, lights, ete.
m. Voltage check of electrical system with portable precision voltmeter.
n. Miscellaneous irregularities such as poorly soldered or loose swaged terminals, loose quick disconnects, broken wire bundle lacing, broken or inadequate clamps, and insufficient clearance between exposed current-carrying parts and ground.
408. CLEANING AND PRESERVATION. Frequent cleaning of electrical equipment to remove dust, dirt, and grime is recommended. Fine emery cloth may be used to clean terminals and mating surfaces if they are corroded or dirty.
Crocus cloth or very fine sandpaper may be used to polish commutators or slip rings. Do not use emery cloth on commutators since particles from the cloth may cause shorting and burning.
Chap 11 Par 406
409. ADJUSTMENT. Accomplish adjustmenta to items of equipment such as regulators, generators, contactors, control devices, inverters, and relays outside the airplane on a test stand or test bench where all necessary instruments and test equipment are at hand. Follow the adjustment procedures outlined by the equipment manufacturer.
410. INSULATION OF ELECTRICAL EQUIPMENT.
In some cases, a unit of electrical equipment is connected into a heavy current circuit, perhaps as a control device or relay. Such equipment is normally insulated from the mounting structure, since grounding the frame of the equipment may result in a serious ground fault in the event of equipment internal failure. If a ground connection for a control coil must be provided, use a separate small gauge wire.
411. BUS BAR MAINTENANCE, Periodically check bus bars used in aircraft electrical systems for general condition and cleanliness. Grease, oxide, or dirt on any electrical junction may cause the connectors to overheat and eventually fail. Clean bus bars by wiping with a clean soft cloth saturated with stoddard solvent and drying with a clean soft cloth.
412. JUNCTION BOXES.
a. Junction Box Construction. Fabricate replacement junction boxes using the same material as the original or from a fire-resistant, nonabsorbent material, such as aluminum alloy or an acceptable plastic material. Where fireproofing is necessary, a stainless steel junction box is recommended. Rigid construction will Par 409 AC 43.13-1A 1972 prevent "oll-canning" of the box sides, which could result in internal short circuits. In all cases, provide drainholes in the lowest portion of the box.
b. Internal Arrangement. The junction box arrangement should permit easy access to all installed items of equipment, terminals, and wires. Where marginal clearances are unavoidable, insert an insulating material between current-carrying parts and any grounded surface. It is not good practice to mount equipment on the covers or doors of junction boxes, since inspection for internal clearance is impossible when the door or cover is in the closed position.
c. Junction Box Installation. Securely mount junction boxes to the aircraft structure in such a manner that the contents are readily accessible for inspection. When possible, face the open side downward or at an angle so that loose metallic objects, such as washers or nuts, will tend to fall out of the junction box rather than wedge between terminals.
d. Junction Box Wiring. Junction box layouts must take into consideration the necessity for adequate wiring space and possible future additions. Lace or clamp electric wire bundles inside the box in a manner that terminals are not hidden, relay armatures are not fouled, and motion relative to any equipment is prevented.
Protect cable at entrance openings against using grommets or other suitable
413.-423. RESERVED.
Chap 11
AC 43.13-1A Page 175 1972 Section 2. EQUIPMENT INSTALLATION
424. ELECTRICAL LOAD LIMITS. When installing additional equipment which consumes electrical power in an aircraft, determine that the total electrical load can be safely controlled or managed within the rated limits of the affected components of the aircraft's electrical powersupply system. Also determine that storage batteries forming a part of the affected system will be properly charged during flight.
a. Wires, Wire Bundles, and Circuit Protective Devices. Before any aircraft electrical load is increased, check the associated wires, wire bundles, and circuit protective devices (fuses or circuit breakers) to determine that the new total electrical load (previous maximum load plus added load) does not exceed the rated limits of the existing wires, wire bundles, or pro- . tective devices. Where necessary, add or replace with units having the correct rating.
425. GENERATORS. Compare the generator or alternator output ratings and limits prescribed by the manufacturer with the electrical loads which can be imposed on the affected generator or alternator by installed equipment. When the comparison shows that the probable total connected electrical load can exceed the output load limits of the generator (s) or alternator (s), reduce the load so that an overload cannot occur or provide means whereby the pilot or flight crew can readily and safely manage the electrical load so that it can be held within the prescribed limits. When a storage battery is part of the electrical power system, assure that the battery is continuously charged in flight, except when short intermittent loads are connected such as operation of radio transmitter, a landing gear motor, or other similar devices which may place short-time demand loads on the battery.
426. ACCEPTABLE MEANS OF CONTROLLING OR
MONITORING THE ELECTRICAL LOAD,
o. The use of placards is recommended to inform the pilot and/or crewmembers the combination (s) of loads that may be connected to the power source.
b. Installations where the ammeter is in the battery lead, and the regulator system limits the maximum current that the generator or alternator can deliver, a voltmeter can be installed on the system bus. As long as the ammeter never reads "discharge" (except for short intermittent loads such as operating the gear and flaps) and the voltmeter remains at "system voltage," the generator or alternator will not be overloaded.
c. In installations where the ammeter is in the generator or alternator lead, and the regulator system does not limit the maximum current that the generator or alternator can deliver, the ammeter can be redlined at 100 percent of the generator or alternator rating. If the ammeter reading is never allowed to exceed the red line, except for short intermittent loads, the generator or alternator will not be overloaded.
d. Where the use of placards or monitoring devices is not practical or desired, and where assurance is needed that the battery in a typical small aircraft generator/battery power source will be charged in flight, the total continuous connected electrical load may be held to approximately 80 percent of the total rated generator output capacity. (When more than one generator is used in parallel, the total rated output is the combined output of the installed generators.)
e. When two or more generators are operated in parallel and the total connected system load can exceed the rated output of one generator, provide means for quickly coping with the sud- Chap 11 Par 424
den overloads which can be caused by generator or engine failure. Employ a quick load reduction system or procedure whereby the total load can be reduced to a quantity which is within the rated capacity of the remaining operable generator or generators.
f. Consider the total electrical load of devices that a pilot or flight crewmember would normally be expected to use as the probable continuous load. The use of placards or other load-monitoring devices or methods notwithstanding, the probable continuous load must not exceed the output capacity of the generator (s).
427. INVERTERS, ALTERNATORS, AND SIMILAR AIRCRAFT ELECTRICAL POWER SOURCES.
a. Connect the electrical load to inverters, alternators, and similar aircraft electrical power sources so that it cannot exceed the rated limits of the power source, unless effective monitoring means are provided whereby the pilot or flight crewmember can keep the load within the prescribed limits. Assure load circuit protective devices are time coordinated to trip prior to the inverter protective device or prior to electrical collapse of the inverter due to overload or fault in the load circuit.
With the exception of the battery-charging provisions in the preceding paragraph, the same basic consideration with respect to load limits, inverters, alternators, and similar devices is applicable.
428. DETERMINATION OF ELECTRICAL LOAD.
o. The connected load of an aircraft electrical system may be determined by any one or a combination of several acceptable methods, techniques, or practices. However, regardless of the methods, techniques, or practice involved, any person who has need to know the status of a particular aircraft's electrical system should have available, current, and accurate data concerning the capacity of the installed electrical power source(s) and the load (s) imposed by installed electrical power consuming devices. Such data should provide a true picture of the status of the electrical sys- Par 426 AC 43,13-1A 1972 tem. Do not install new or additional electrical devices in an aircraft, nor change the capacity of any power source until the status of the electrical system in the aircraft has been determined accurately and found not to adversely affect the integrity of the electrical system.
429. CIRCUIT PROTECTION DEVICES.
a. General. Protect the wire with circuit breakers or fuses located as close as possible to the electrical power-source bus. Normally, the manufacturer of the electrical equipment specifies the fuse or breaker to be used when installing the respective equipment.
b. Matching Protector to Wire. The circuit breaker or fuse should open the circuit before the wire emits smoke. To accomplish this, the time-current characteristic of the protective device must fall below that of the associated wire. In order to obtain maximum utilization of the connected equipment, match the circuit protector characteristics.
c. Circuit Protector Chart. Figure 11.1 may be used as a guide for the selection of circuit breaker and fuse rating to protect copper conductor wire. This chart was prepared for the conditions specified by the notes which accompany the chart. If actual conditions deviate materially from those stated, ratings above or below the values recommended may be justified. For example, a wire run individually in the open air may possibly be protected by the circuit breaker of the next higher rating to that shown on the chart. In general, the chart is conservative for all ordinary aircraft electri-
(1) All resettable type circuit breakers should open the circuit irrespective of the position of the operating control when an overload or circuit fault exists. Such circuit breakers are referred to as "trip free."
(2) Do not use automatic reset circuit breakers (which automatically reset themselves periodically) as circuit protective devices.
430. SWITCHES.
a. General. In all circuits where a switch Chop 11