Par 11-186 Page 11-81 loosening under vibration. If the structure is fabricated of a material such as carbon fiber composite (CFC), which has a higher resistivity than aluminum or copper, it will be necessary to provide an alternative ground path(s) for power return current. Special attention should be considered for composite aircraft.
d. Current Return Paths for Internally Grounded Equipment. Power return or fault current ground connections within flammable vapor areas must be avoided. If they must be made, make sure these connections will not arc, spark, or overheat under all possible current flow or mechanical failure conditions, including induced lightning currents. Criteria for inspection and maintenance to ensure continued airworthiness throughout the expected life of the aircraft should be established. Power return fault currents are normally the highest currents flowing in a structure. These can be the full generator current capacity. If full generator fault current flows through a localized region of the carbon fiber structure, major heating and failure can occur. CFC and other similar low-resistive materials must not be used in power return paths.
Additional voltage drops in the return path can cause voltage regulation problems. Likewise, repeated localized material heating by current surges can cause material degradation. Both problems may occur without warning and cause nonrepeatable failures or anomalies.
e. C ommon Ground Connections. The use of common ground connections for more than one circuit or function should be avoided except where it can be shown that related malfunctions that could affect more than one circuit will not result in a hazardous condition. Even when the loss of multiple systems does not, in itself, create a hazard, the effect of such failure can be quite distracting to the crew.
(1) Redundant systems are normally provided with the objective of assuring continued safe operation in the event of failure of a single channel and must therefore be grounded at well separated points. To avoid construction or maintenance errors that result in connecting such ground at a single point, wires that ground one channel of a redundant system should be incapable of reaching the ground attachment of the other channel.
(2) The use of loop type grounding systems (several ground leads connected in series with a ground to structure at each end) must be avoided on redundant systems, because the loss of either ground path will remain undetected, leaving both systems with a potential single-point failure.
(3) Electrical power sources must be grounded at separate locations on the aircraft structure. The loss of multiple sources of electrical power, as the result of corrosion of a ground connection or failure of the related fasteners, may result in the loss of multiple systems and should be avoided by making the ground attachments at separate locations.
(4) B onds to thermally or vibration isolated structure require special consideration to avoid single ground return to primary structure.
(5) The effect of the interconnection of the circuits when ungrounded should be considered whenever a common ground connection is used. This is particularly important when employing terminal junction grounding modules or other types of gang grounds that have a single attachment point.
Par 11-186 Page 11-82
f. Grounds for Sensitive Circuits. Special consideration should be given to grounds for sensitive circuits. For example:
(1) Grounding of a signal circuit through a power current lead introduces power current return voltage drop into the signal circuit.
(2) Running power wires too close will cause signal interference.
(3) Separately grounding two components of a transducer system may introduce ground plane voltage variations into the system.
(4) Single point grounds for signal circuits, with such grounds being at the signal source, are often a good way to minimize the effects of EMI, lightning, and other sources of interference.
11-187. BONDING. The following bonding requirements must be considered:
a. Equipment Bonding. Low-impedance paths to aircraft structure are normally required for electronic equipment to provide radio frequency return circuits and for most electrical equipment to facilitate reduction in EMI. The cases of components which produce electromagnetic energy should be grounded to structure. To ensure proper operation of electronic equipment, it is particularly important to conform the system’s installation specification when interconnections, bonding, and grounding are being accomplished.
b. Metallic Surface Bonding. All conducting objects on the exterior of the airframe must be electrically connected to the airframe through mechanical joints, conductive hinges, or bond straps capable of conducting static charges and lightning strikes. Exceptions may be necessary for some objects such as antenna elements, whose function requires them to be electrically isolated from the airframe. Such items should be provided with an alternative means to conduct static charges and/or lightning currents, as appropriate.
c. Static Bonds. All isolated conducting parts inside and outside the aircraft, having an area greater than 3 in2 and a linear dimension over 3 inches, that are subjected to appreciable electrostatic charging due to precipitation, fluid, or air in motion, should have a mechanically secure electrical connection to the aircraft structure of sufficient conductivity to dissipate possible static charges. A resistance of less than 1 ohm when clean and dry will generally ensure such dissipation on larger objects. Higher resistances are permissible in connecting smaller objects to airframe structure.
11-188. BONDING INSPECTION. Inspect for the following:
a. If there is evidence of electrical arcing, check for intermittent electrical contact between conducting surfaces that may become a part of a ground plane or a current path. Arcing can be prevented either by bonding, or by insulation if bonding is not necessary.
b. The metallic conduit should be bonded to the aircraft structure at each terminating and break point. The conduit bonding strap should be located ahead of the piece of equipment that is connected to the cable wire inside the conduit.
Par 11-188 Page 11-83
c. Bond connections should be secure and free from corrosion.
d. Bonding jumpers should be installed in such a manner as not to interfere in any way with the operation of movable components of the aircraft.
e. Self-tapping screws should not be used for bonding purposes. Only standard threaded screws or bolts of appropriate size should be used.
f. Exposed conducting frames or parts of electrical or electronic equipment should have a low-resistance bond of less than 2.5 milliohms to structure. If the equipment design includes a ground terminal or pin, which is internally connected to such exposed parts, a ground wire connection to such terminal will satisfy this requirement. Refer to manufacturer’s instructions.
g. Bonds should be attached directly to the basic aircraft structure rather than through other bonded parts.
h. Bonds must be installed to ensure that the structure and equipment are electrically stable and free from the hazards of lightning, static discharge, electrical shock, etc. To ensure proper operation and suppression of radio interference from hazards, electrical bonding of equipment must conform to the manufacturer’s specifications.
i. Use of bonding testers is strongly recommended.
j. Measurements should be performed after the grounding and bonding mechanical connections are complete to determine if the measured resistance values meet the basic requirements. A high-quality test instrument (AN/USM-21A or equivalent) is required to accurately measure the very low-resistance values specified in this document. Another method of measurement is the millivolt drop test as shown in Figure 11-19.
k. U se appropriate washers when bonding aluminum or copper to dissimilar metallic structures so that any corrosion that may occur will be on the washer.
FIGURE 11-19. MILLIVOLT DROP TEST
Par 11-189 Page 11-84
11-189. BONDING JUMPER INSTALLATIONS. Bonding jumpers should be made as short as practicable, and installed in such a manner that the resistance of each connection does not exceed .003 ohm. The jumper should not interfere with the operation of movable aircraft elements, such as surface controls, nor should normal movement of these elements result in damage to the bonding jumper.
a. Bonding Connections. To ensure a low-resistance connection, non-conducting finishes, such as paint and anodizing films, should be removed from the attachment surface to be contacted by the bonding terminal. On aluminum surfaces, a suitable conductive chemical surface treatment, known as chromate conversion coating, should be applied to the surfaces within 24 hours of the removal of the original finish. Refer to SAE ARP1870 for detailed instructions. Electric wiring should not be grounded directly to magnesium parts.
b. Corrosion Protection. One of the more frequent causes of failures in electrical system bonding and grounding is corrosion. Aircraft operating near salt water are particularly vulnerable to this failure mode. Because bonding and grounding connections may involve a variety of materials and finishes, it is important to protect completely against dissimilar metal corrosion.
The areas around completed connections should be post-finished in accordance with the original finish requirements or with some other suitable protective finish within 24 hours of the cleaning process. In applications exposed to salt spray environment, a suitable noncorrosive sealant, such as one conforming to AMSS8802F, should be used to seal dissimilar metals for protection from exposure to the atmosphere.
c. Corrosion Prevention. Electrolytic action may rapidly corrode a bonding connection if suitable precautions are not taken. Aluminum alloy jumpers are recommended for most cases;
however, copper jumpers should be used to bond together parts made of stainless steel, cadmium plated steel, copper, brass, or bronze. Where contact between dissimilar metals cannot be avoided, the choice of jumper and hardware should be such that corrosion is minimized, and the part likely to corrode would be the jumper or associated hardware. Tables 11-16 through 11-18 and Figures 11-20 through 11-22 show the proper hardware combinations for making a bond connection. At locations where finishes are removed, a protective finish should be applied to the completed connection to prevent subsequent corrosion.
TABLE 11-16. STUD BONDING OR GROUNDING TO FLAT SURFACE
SCREW OR BOLT - WASHER A LOCKWASHER E • STRUCTURE
WASHER B WASHER C
PLAIN NUT -
WASHER D (ERITTO T LOCKWASHER F LOCKNUT Aluminum Terminal and Jumper Screw or Lock Lock Bolt and Plain Washer Structure Washer A Washer B Washer Washer Lock Nut C&D E Nut Aluminum Cadmium Cadmium Aluminum Aluminum Cadmium Cadmium Cadmium Alloys Plated Plated Alloy Alloy Plated Plated Plated steel Steel Steel or Steel Steel Aluminum Magnesium Cadmium Cadmium Magnesium Magnesium Cadmium Cadmium Cadmium Alloys Plated Plated Alloy Alloy Plated Plated Plated Steel or Steel Steel Aluminum Steel, Cadmium Cadmium None None Cadmium Cadmium Cadmium Cadmium Plated Plated Plated Plated Plated Plated Steel Steel Steel or Steel Steel Aluminum Steel, Corrosion Cadmium None None Cadmium Corrosion Cadmium Corrosion Resisting Plated Plated Resist Plated Resisting Steel Steel Steel or Steel Steel Aluminum Tinned Copper Terminal and Jumper Aluminum Cadmium Cadmium Aluminum Aluminum Cadmium Cadmium Cadmium Alloys Plated Plated Alloy Alloy Plated Plated Plated Steel Steel Steel Steel Steel Magnesium Alloys' Steel, Cadmium Cadmium None None Cadmium Cadmium Cadmium Cadmium Plated Plated Plated Plated Plated Plated Steel Steel Steel Steel Steel Steel, Corrosion Corrosion None None Cadmium Corrosion Corrosion Corrosion Resisting Resisting Plated Resisting Resisting Resisting Steel Steel Steel Steel Steel ' Avoid connecting copper to magnesium.
Par 11-189 Page 11-85
TABLE 11-17. PLATE NUT BONDING OR GROUNDING TO FLAT SURFACE
LOCKWASHER
SCREW OR BOLT LIMITED TO 4
WASHER A
WASHER B
RIVET NUT PLATE Aluminum Terminal and Jumper Structure Screw or Bolt Rivet Lock Washer Washer A Washer B and Nut Plate Aluminum Cadmium Aluminum Cadmium Cadmium None Alloys Plated Steel Alloy Plated Steel Plated Steel or Aluminum Magnesium Cadmium Aluminum Cadmium Cadmium None or Alloys Plated Steel Alloy Plated Steel Plated Steel or Magnesium Aluminum Alloy Steel, Cadmium Cadmium Corrosion Cadmium Cadmium None Plated Plated Steel Resisting Steel Plated Steel Plated Steel or Aluminum Steel, Corrosion Corrosion Corrosion Cadmium Cadmium Cadmium Resisting Resisting Steel Resisting Steel Plated Steel Plated Steel or Plated Steel or Cadmium Aluminum Plated Steel Tinned Copper Terminal and Jumper Aluminum Cadmium Aluminum Cadmium Cadmium Aluminum?
Alloys Plated Steel Alloy Plated Steel Plated Steel Alloy Magnesium Alloys!
Steel, Cadmium Cadmium Corrosion Cadmium Cadmium None Plated Plated Steel Resisting Steel Plated Steel Plated Steel Steel, Corrosion Corrosion Corrosion Cadmium Cadmium None Resisting Resisting Steel Resisting Steel Plated Steel Plated Steel I Avoid connecting copper to magnesium.
2 Use washers having a conductive finished treated to prevent corrosion, suggest AN960JD10L.
Par 11-189 Page 11-86
TABLE 11-18. BOLT AND NUT BONDING OR GROUNDING TO FLAT SURFACE
- SCREW OR BOLT
WASHER A
WASHER B
LOCKWASHER
TERMINAL LIMITED TO 4
LOCKNUT WASHER C Aluminum Terminal and Jumper Structure Screw or Bolt and Nut Plate Lock-Nut Washer A Washer B Washer C Aluminum Cadmium Cadmium Cadmium None Cadmium Alloys Plated Steel Plated Steel Plated Steel or Plated Steel or Aluminum Aluminum Magnesium Cadmium Cadmium Magnesium None or Cadmium Alloys Plated Steel Plated Steel Alloy Magnesium Plated Steel or Alloy Aluminum Steel, Cadmium Cadmium Cadmium Cadmium Cadmium Cadmium Plated Plated Steel Plated Steel Plated Steel Plated Steel Plated Steel or Aluminum Steel, Corrosion Corrosion Cadmium Corrosion Cadmium Cadmium Resisting Resisting Steel Plated Steel Resisting Steel Plated Steel Plated Steel or or Cadmium Aluminum Plated Steel Tinned Copper Terminal and Jumper Aluminum Cadmium Cadmium Cadmium Aluminum? Cadmium Alloy Plated Steel Plated Steel Plated Steel Alloy Plated Steel Magnesium Alloy!
Steel, Cadmium Cadmium Cadmium Cadmium None Cadmium Plated Plated Steel Plated Steel Plated Steel Plated Steel Steel, Corrosion Corrosion Cadmium Corrosion None Cadmium Resisting Resisting Steel Plated Steel Resisting Steel Plated Steel or Cadmium Plated Steel I Avoid connecting copper to magnesium.
2 Use washers having a conductive finished treated to prevent corrosion, suggest AN960JD10L.
Par 11-189 Page 11-87
FIGURE 11-20. COPPER JUMPER CONNECTOR TO TUBULAR STRUCTURE CAD. PL. STEEL AN 735 CLAMP SCREW COPPER
TERMINAL
ALUMINUM
CYLINDRICAL WASHERS
SURFACE CAD. PL. STEEL
LOCKWASHER CAD. PL. STEEL
LOCK NUT FIGURE 11-21. BONDING CONDUIT TO STRUCTURE CAD. PL. STEEL SCREW AN 735 CLAMP WASHE ALUMINU
STRUCTURE
- ALUMINUM
ALUMINUM ALLOY WASHER
OR CORROSION- RESISTING STEEL CAD. PL. STEEL
CONDUIT LOCKWASHER CAD. PL. STEEL
LOCKNUT FIGURE 11-22. ALUMINUM JUMPER CONNECTION TO TUBULAR STRUCTURE CAD. PL. STEEL SCREW AN 735 CLAMP
ALUMINUM WASHER
BARE ALUMINU
TERMINA
TLINDRICA URFACI ALUMINUM WASHER LOCKNUT CAD. PL. STEEL LOCKWASHER CAD. PL. STEEL
d. Bonding Jumper Attachment. The use of solder to attach bonding jumpers should be avoided. Tubular members should be bonded by means of clamps to which the jumper is attached. Proper choice of clamp material should minimize the probability of corrosion.
e. Ground Return Connection. When bonding jumpers carry substantial ground return current, the current rating of the jumper should be determined to be adequate and that a negligible voltage drop is produced.
Par 11-189 Page 11-88
Par 11-190 Page 11-89
11-190. CREEPAGE DISTANCE. Care should be used in the selection of electrical components to ensure that electrical clearance and creepage distance along surfaces between adjacent terminals, at different potentials, and between these terminals and adjacent ground surfaces are adequate for the voltages involved.
11-191. FUEL SYSTEMS. Small metallic objects within an aircraft fuel tank that are not part of the tank structure, should be electrically bonded to the structure so as to dissipate static charges that may otherwise accumulate on these objects. A practical bonding design would use a flexible braided jumper wire or riveted bracket. In such situations, a DC resistance of 1 ohm or less should indicate an adequate connection. Care should be taken, in designing such connections, to avoid creating continuous current paths that could allow lightning or power fault currents to pass through connections not designed to tolerate these higher amplitude currents without arcing. Simulated static charge, lightning, or fault current tests may be necessary to establish or verify specific designs. All other fuel system components, such as fuel line (line to line) access doors, fuel line supports, structural parts, fuel outlets, or brackets should have an electromechanical (bonding strap) secure connector that ensures 1 ohm or less resistance to the structure. Advisory Circular (AC) 20-53, Protection of Aircraft Fuel Systems Against Vapor Ignition Caused by Lightning, and associate manual DOT/FAA/CT-83/3, provide detailed information on necessary precautions.
11-192. ELECTRIC SHOCK PREVENTION BONDING. Electric shock to personnel should be prevented by providing a low-resistance path of 1/100 ohm or less between structure and metallic conduits or equipment. The allowable ground resistance should be such that the electric potential of the conduit or equipment housing does not reach a dangerous value under probable fault conditions. The current carrying capacity of all elements of the ground circuit should be such that, under the fault condition, no sparking, fusion, or dangerous heating will occur. Metallic supports usually provide adequate bonding if metal-to-metal contact is maintained.
11-193. LIGHTNING PROTECTION BONDING. Electrical bonding is frequently required for lightning protection of aircraft and systems, especially to facilitate safe conduction of lightning currents through the airframe. Most of this bonding is achieved through normal airframe riveted or bolted joints but some externally mounted parts, such as control surfaces, engine nacelles, and antennas, may require additional bonding provisions. Generally, the adequacy of lightning current bonds depends on materials, cross-sections, physical configurations, tightness, and surface finishes. Care should be taken to minimize structural resistance, so as to control structural voltage rises to levels compatible with system protection design. This may require that metal surfaces be added to composite structures, or that tinned copper overbraid, conduits, or cable trays be provided for interconnecting wire harnesses within composite airframes. Also, care must be taken to prevent hazardous lightning currents from entering the airframe via flight control cables, push rods, or other conducting objects that extend to airframe extremities. This may require that these conductors be electrically bonded to the airframe, or that electrical insulators be used to interrupt lightning currents. For additional information on lightning protection measures, refer to DOT/FAA/TC-22/11. Report DOT/FAA/CT 86/8, April 1987, Determination of Electrical Properties of Grounding, Bonding and Fastening Techniques for Composite Materials, may provide additional information for composite materials.
Par 11-193 Page 11-90
a. Control Surface Lightning Protection Bonding. Control surface bonding is intended to prevent the burning of hinges on a surface that receives a lightning strike; thus causing possible loss of control. To accomplish this bonding, control surfaces and flaps should have at least one 6,500 circular mil area copper (e.g., 7 by 37 AWG size 36 strands) jumper across each hinge. In any case, not less than two 6,500 circular mil jumpers should be used on each control surface. The installation location of these jumpers should be carefully chosen to provide a low-impedance shunt for lightning current across the hinge to the structure. When jumpers may be subjected to arcing, substantially larger wire sizes of 40,000 circular mils or a larger cross-section are required to provide protection against multiple strikes. Sharp bends and loops in such jumpers can create susceptibility to breakage when subjected to the inductive forces created by lightning current, and should be avoided.
b. Control Cable Lightning Protection Bonding. To prevent damage to the control system or injury to flight personnel due to lightning strike, cables and levers coming from each control surface should be protected by one or more bonding jumpers located as close to the control surface as possible. Metal pulleys are considered a satisfactory ground for control cables.
11-194. LIGHTNING PROTECTION FOR ANTENNAS AND AIR DATA PROBES.
Antenna and air data probes that are mounted on exterior surfaces within lightning strike zones should be provided with a means to safely transfer lightning currents to the airframe, and to prevent hazardous surges from being conducted into the airframe via antenna cables or wire harnesses. Usually, the antenna mounting bolts provide adequate lightning current paths. Surge protectors built into antennas or installed in coaxial antenna cables or probe wire harnesses will fulfill these requirements. Candidate designs should be verified by simulated lightning tests in accordance with RTCA DO-160G.
11-195. STATIC-DISCHARGE DEVICE. Means should be provided to bleed accumulated static charges from aircraft prior to ground personnel coming in contact with an aircraft after landing. Normally, there is adequate conductivity in the tires for this, but if not, a static ground should be applied before personnel come into contact with the aircraft. Fuel nozzle grounding receptacles should be installed in accordance with the manufacturer’s specifications. Grounding receptacles should provide a means to eliminate the static-induced voltage that might otherwise cause a spark between a fuel nozzle and fuel tank access covers and inlets. In addition, static discharging wicks are installed on wings and tail surfaces to discharge static changes while in flight.
11-196. CLEANING. In order to ensure proper ground connection conductivity, all paint, primer, anodize coating, grease, and other foreign material must be carefully removed from areas that conduct electricity. On aluminum surfaces, apply chemical surface treatment to the cleaned bare metal surface in accordance with the manufacturer’s instructions within 4–8 hours, depending on ambient moisture/contaminate content.
11-197. HARDWARE ASSEMBLY. Details of bonding connections must be described in maintenance manuals and adhered to carefully when connections are removed or replaced during maintenance operations. In order to avoid corrosion problems and ensure long-term integrity of the electrical connection, hardware used for this purpose must be as defined in these documents or at least be equivalent in material and surface. Installation of fasteners used in bonded or
Par 11-197 Page 11-91 grounded connections should be made in accordance with SAE ARP1870. Threaded fasteners must be torqued to the level required by SAE ARP1928.
11-198.–11-204. [RESERVED.]
Par 11-205 Page 11-92 SECTION 16. WIRE MARKING
11-205. GENERAL. The proper identification of electrical wires and cables with their circuits and voltages is necessary to provide safety of operation, safety to maintenance personnel, and ease of maintenance.
a. Each wire and cable should be marked with a part number. It is common practice for wire manufacturers to follow the wire material part number with the five-digit/letter CAGE code identifying the wire manufacturer. Existing installed wire that needs replacement can thereby be identified as to its performance capabilities, and the inadvertent use of a lower performance and unsuitable replacement wire avoided.
b. The method of identification should not impair the characteristics of the wiring.
CAUTION: Do not use metallic bands in place of insulating sleeves. Exercise care when marking coaxial or data bus cable, as deforming the cable may change its electrical characteristics.
11-206. WIRE IDENTIFICATION. To facilitate installation and maintenance, original wire-marking identification is to be retained. The wire identification marks should consist of a combination of letters and numbers that identify the wire, the circuit it belongs to, its gauge size, and any other information to relate the wire to a wiring diagram. All markings should be legible in size, type, and color.
11-207. IDENTIFICATION AND INFORMATION RELATED TO THE WIRE AND WIRING DIAGRAMS. The wire identification marking should consist of similar information to relate the wire to a wiring diagram.
11-208. PLACEMENT OF IDENTIFICATION MARKINGS. Identification markings should be placed at each end of the wire and at 15-inch maximum intervals along the length of the wire. Wires less than 3 inches long need not be identified. Wires 3 to 7 inches in length should be identified approximately at the center. Added identification marker sleeves should be so located that ties, clamps, or supporting devices need not be removed in order to read the identification. The wire identification code must be printed to read horizontally (from left to right) or vertically (from top to bottom). The two methods of marking wire or cable are as follows:
a. Direct marking is accomplished by printing the cable’s outer covering. (See Figure 11-23.)