Par 11-232 Page 11-111 FIGURE 11-37. COAX CABLE CONNECTORS (CONTINUED) 11 -233. VOLTAGE AND CURRENT RATING. Selected connectors must be rated for continuous operation under the maximum combination of ambient temperature and circuit current load. Hermetic connectors and connectors used in circuit applications involving high-inrush currents should be derated. It is good engineering practice to conduct preliminary testing in any situation where the connector is to operate with most or all of its contacts at maximum rated current load. When wiring is operating with a high conductor temperature near its rated temperature, connector contact sizes should be suitably rated for the circuit load. This may require an increase in wire size also. Voltage derating is required when connectors are used at high altitude in nonpressurized areas. Derating of the connectors should be covered in the specifications.
11-234. SPARE CONTACTS (Future Wiring). To accommodate future wiring additions, spare contacts are normally provided. Locating the unwired contacts along the outer part of the connector facilitates future access. A good practice is to provide: 2 spares on connectors with 25 or less contacts; 4 spares on connectors with 26 to 100 contacts; and 6 spares on connectors with more than 100 contacts. Spare contacts are not normally provided on receptacles of components that are unlikely to have added wiring. Connectors must have all available contact cavities filled with wired or unwired contacts. Unwired contacts should be provided with a plastic grommet sealing plug.
Par 11-235 Page 11-112
11-235. INSTALLATION.
a. Redundancy. Wires that perform the same function in redundant systems must be routed through separate connectors. On systems critical to flight safety, system operation wiring should be routed through separate connectors from the wiring used for system failure warning. It is also good practice to route a system’s indication wiring in separate connectors from its failure warning circuits to the extent practicable. These steps can reduce an aircraft’s susceptibility to incidents that might result from connector failures.
b. Adjacent Locations. Mating of adjacent connectors should not be possible. In order to ensure this, adjacent connector pairs must be different in shell size, coupling means, insert arrangement, or keying arrangement. When such means are impractical, wires should be routed and clamped so that incorrectly mated pairs cannot reach each other. Reliance on markings or color stripes is not recommended as they are likely to deteriorate with age.
c. Sealing. Connectors must be of a type that exclude moisture entry through the use of peripheral and interfacial seal that are compressed when the connector is mated. Moisture entry through the rear of the connector must be avoided by correctly matching the wire’s outside diameter with the connector’s rear grommet sealing range. It is recommended that no more than one wire be terminated in any crimp-style contact. The use of heat-shrinkable tubing to build up the wire diameter, or the application of potting to the wire entry area as additional means of providing a rear compatibility with the rear grommet is recommended. These extra means have inherent penalties and should be considered only where other means cannot be used. Unwired spare contacts should have a correctly sized plastic plug installed. (See Section 19.)
d. Drainage. Connectors must be installed in a manner which ensures that moisture and fluids will drain out of and not into the connector when unmated. Wiring must be routed so that moisture accumulated on the bundle will drain away from connectors. When connectors must be mounted in a vertical position, as through a shelf or floor, the connectors must be potted or environmentally sealed. In this situation it is better to have the receptacle faced downward so that it will be less susceptible to collecting moisture when unmated.
e. Wire Support. A rear accessory back shell must be used on connectors that are not enclosed. Connectors having very small size wiring, or are subject to frequent maintenance activity, or located in high-vibration areas must be provided with a strain-relief-type back shell.
The wire bundle should be protected from mechanical damage with suitable cushion material where it is secured by the clamp. Connectors that are potted or have molded rear adapters do not normally use a separate strain relief accessory. Strain relief clamps should not impart tension on wires between the clamp and contact.
f. Slack. Sufficient wire length must be provided at connectors to ensure a proper drip loop and that there is no strain on termination after a complete replacement of the connector and its contacts.
g. Identification. Each connector should have a reference identification that is legible throughout the expected life of the aircraft.
Par 11-236 Page 11-113
11-236. FEED-THROUGH BULKHEAD WIRE PROTECTION. Feed-through bushing protection should be given to wire bundles which pass through bulkheads, frames, and other similar structure. Feed-through bushings of hard dielectric material are satisfactory. The use of split plastic grommets (nylon) is recommended in lieu of rubber grommets in areas subject to fluids, since they eliminate the unsatisfactory features of rubber grommets and are resistant to fluids usually encountered in aircraft.
11-237. SPECIAL PURPOSE CONNECTOR. Many special-purpose connectors have been designed for use in aircraft applications, such as: subminiature connector, rectangular shell connector, connectors with short body shells, or connector of split-shell construction used in applications where potting is required. Make every attempt to identify the connector part number from the maintenance manual or actual part, and the manufacturer’s instruction used for servicing.
11-238. POTTING COMPOUNDS. Many types of potting compounds, both commercial and per military specifications, are available and offer various characteristics for different applications. Carefully consider the characteristics desired to ensure the use of the proper compound. Preparation and storage of potting materials should receive special attention. Careful inspection and handling during all stages of the connector fabrication until the potting compound has fully cured is recommended. Potting compounds selected must not revert to liquid or become gummy or sticky due to high humidity or contact with chemical fluids.
a. Po tting compounds meeting MIL -S-8516 are prepared in ready-to-use tube-type dispensers and in the unmixed state, consisting of the base compound and an accelerator packed in paired containers. To obtain the proper results, it is important that the manufacturer’s instructions be closely followed.
b. Potting compounds normally cure at temperatures of 70 °F to 76 °F. If the mixed compound is not used at once, the working pot life (normally 90 minutes) can be prolonged by storing in a deep freeze at -20 °F for a maximum of 36 hours. The time factor starts from the instant the accelerator is added to the base compound and includes the time expended during the mixing and application processes.
c. Mixed compounds that are not to be used immediately should be cooled and thawed quickly to avoid wasting the short working life. Chilled compounds should be thawed by blowing compressed air over the outside of the container. Normally the compound will be ready for use in 5 to 10 minutes.
CAUTION: Do not use heat or blow compressed air into the container when restoring the compound to the working temperature.
11-239. POTTING CONNECTORS. Connectors that have been potted primarily offer protection against concentration of moisture in the connectors. A secondary benefit of potting is the reduced possibility of breakage between the contact and wire due to vibration.
a. C onnectors specifically designed for potting compounds should be potted to provide environment resistance. An O-ring or sealed gasket should be included to seal the interface area
Par 11-239 Page 11-114 of the mated connector. A plastic potting mold that remains on the connector after the potting compounds have cured should also be considered. To facilitate circuit changes, spare wires may be installed to all unused contacts prior to filling the connector with potting compound.
b. Connect wires to all contacts of the connector prior to the application of the potting compound. Wires that are not to be used should be long enough to permit splicing at a later date.
Unused wires should be as shown in Figure 11-38 and the cut ends capped with heat-shrinkable caps or crimped insulated end caps such as the AS25274D prior to securing to the wire bundle.
Clean the areas to be potted with dry solvent and complete the potting operation within 2 hours after this cleaning. Allow the potting compound to cure for 24 hours at a room temperature of 70 °F to 75 °F or carefully place in a drying oven at 100 °F for 3 to 4 hours. In all cases, follow manufacturer’s instructions.
FIGURE 11-38. SPARE WIRES FOR POTTING CONNECTOR 11-
240. THROUGH BOLTS. Through bolts are sometimes used to make feeder connections through bulkheads, fuselage skin, or firewalls. Mounting plates for through bolts must be a material that provides the necessary fire barrier, insulation, and thermal properties for the application. Sufficient cross-section should be provided to ensure adequate conductivity against overheating. Secure through bolts mechanically and independently of the terminal mounting nuts, taking particular care to avoid dissimilar metals among the terminal hardware. During inspection, pay particular attention to the condition of the insulator plate or spacer and the insulating boot that covers the completed terminal assembly.
11-241.–11-2 47. [RESERVED.]
Par 11-248 Page 11-115 SECTION 18. CONDUITS
11-248. GENERAL. Conduit is manufactured in metallic and nonmetallic materials and in both rigid and flexible forms. Primarily, its purpose is for mechanical protection of cables or wires.
Conduit should be inspected for: proper end fittings; absence of abrasion at the end fittings;
proper clamping; distortion; adequate drain points which are free of dirt, grease, or other obstructions; and freedom from abrasion or damage due to moving objects, such as aircraft control cables or shifting cargo.
11-249. SIZE OF CONDUIT. Conduit size should be selected for a specific wire bundle application to allow for ease in maintenance, and possible future circuit expansion, by specifying the conduit inner diameter (ID) about 25 percent larger than the maximum diameter of the wire bundle.
11-250. CONDUIT FITTINGS. Wire is vulnerable to abrasion at conduit ends. Suitable fittings should be affixed to conduit ends in such a manner that a smooth surface comes in contact with the wire. When fittings are not used, the end of the conduit should be flared to prevent wire insulation damage. Conduit should be supported by use of clamps along the conduit run.
11-251. CONDUIT INSTALLATION. Conduit problems can be avoided by following these guidelines:
a. Do not locate conduit where passengers or maintenance personnel might use it as a handhold or footstep.
b. Provide drain holes at the lowest point in a conduit run. Drilling burrs should be carefully removed.
c. Support conduit to prevent chafing against structure and to avoid stressing its end fittings.
11-252. RIGID CONDUIT. Conduit sections that have been damaged should be repaired to preclude injury to the wires or wire bundle which may consume as much as 80 percent of the tube area. Minimum acceptable tube bend radii for rigid conduit are shown in Table 11-25.
Kinked or wrinkled bends in rigid conduits are not recommended and should be replaced. Tubing bends that have been flattened into an ellipse and the minor diameter is less than 75 percent of the nominal tubing diameter should be replaced because the tube area will have been reduced by at least 10 percent. Tubing that has been formed and cut to final length should be deburred to prevent wire insulation damage. When installing replacement tube sections with fittings at both ends, care should be taken to eliminate mechanical strain.
Par 11-252 Page 11-116 TABLE 11-25. BEND RADII FOR RIGID CONDUIT Nominal Tube OD (inches) Minimum Bend Radii (inches) 1/8 3/8 3/16 7/16 1/4 9/16 3/8 15/16 1/2 1 1/4 5/8 1 1/2 ¾ 1 3/4 1 3 1 1/4 3 3/4 1 1/2 5 1 3/4 7 2 8
11-253. FLEXIBLE CONDUIT. Flexible aluminum conduit conforming to AS6136A is available in two types: Type I, Bare Flexible Conduit, and Type II, Rubber Covered Flexible Conduit. Flexible brass conduit conforming to specification AS3136 is available and normally used instead of flexible aluminum where necessary to minimize radio interference. Also available is a plastic flexible tubing. (Refer to MIL-T-8191A.) Flexible conduit may be used where it is impractical to use rigid conduit, such as areas that have motion between conduit ends or where complex bends are necessary. The use of transparent adhesive tape is recommended when cutting flexible tubing with a hacksaw to minimize fraying of the braid. The tape should be centered over the cutting reference mark with the saw cutting through the tape. After cutting the flexible conduit, the transparent tape should be removed, the frayed braid ends trimmed, burrs removed from inside the conduit, and coupling nut and ferrule installed. Minimum acceptable bending radii for flexible conduit are shown in Table 11-26.
Par 11-253 Page 11-117 TABLE 11-26. MINIMUM BENDING RADII FOR FLEXIBLE ALUMINUM OR BRASS
CONDUIT Nominal ID of Conduit (inches) Minimum Bending Radius Inside (inches) 3/16 2 1/4 1/4 2 3/4 3/8 3 3/4 1/2 3 3/4 5/8 3 3/4 3/4 4 1/4 1 5 3/4 1 1/4 8 1 1/2 8 1/4 1 3/4 9 2 9 3/4 2 1/2 10
11-254.–11-259. [RESERVED.]
Par 11-260 Page 11-118 SECTION 19. UNUSED CONNECTORS AND UNUSED WIRES
11-260. GENERAL. Connectors may have one or more contact cavities that are not used.
Depending on the connector installation, unused connector contact cavities may need to be properly sealed to avoid damage to the connector, or have string wire installed. Unused wires can be secured by tying into a bundle or secured to a permanent structure; individually cut with strands even with insulation; or pre-insulated closed end connector or 1-inch piece of insulating tubing folded and tied back.
11-261. QUICK REFERENCE CHART. A quick reference chart of unused connector contact cavity requirements is given in Table 11-27. These requirements apply to harness manufacturing or connector replacement only.
TABLE 11-27. CONTACT CAVITY SEALING-QUICK REFERENCE Sealing Means Connector Installation Types Unpressurized Area Firewall Non-Firewall Sealing Plugs or Teflon Sealing Rods No Yes Stub Wires (Note 2) Yes No Spare Contacts Yes Yes NOTE 1: Sealing plugs may be included with the spare connector and may be used for sealing unused contacts. Sealing rods are procured from stock by the foot. (See Table 11-28 for sealing rod dimensions.)
NOTE 2: Stub wires must be of the same type as the other wires of the bundle.
11-262. UNPRESSURIZED AREA CONNECTORS. Connectors may be installed in unpressurized areas of the aircraft. Unused connector contact cavities installed in unpressurized areas should be properly sealed as follows:
a. Firewall Connectors Installations. Firewall unused connector contact cavities should be filled with spare contacts and stub wires. (See Figure 11-39.)
Par 11-262 Page 11-119 FIGURE 11-39. STUB WIRE INSTALLATION (
1) Construct stub wires using high-temperature wire (260 °C). Ensure that stub wires are of the same type of wires in the bundle.
(2) Crimp the proper contact, for the connector and cavity being used, onto the wire.
Install the crimped contact into the unused cavity.
(3) Extend stub wires beyond the back of the connector clamp from 1.5 to 6 inches.
Feather trim stub wires to taper wire bundle.
(4) Secure wire ends with high-temperature (greater than 250 °C) lacing cord. Nylon cable ties are not allowed for this installation.
NOTE: Both connectors mating through the engine fire-seal are considered firewall connectors. Connectors mounted on or near, but not through, the engine fire-seal are not considered firewall connectors.
b. Non-firewall Connector Installations. In this type of installation, all unused connector cavities must also be filled with spare contacts. It is not required, however, to crimp stub wires on filling contacts. Fill unused contact cavities with spare contacts and Teflon sealing plugs or rods. (See Figure 11-40.) Rods shall be cut so that they extend 1/8 to 1/4 inch beyond the surface of the grommet when bottomed against the end of the spare contact. (See Table 11-28 for dimensions.)
Par 11-262 Page 11-120 FIGURE 11-40. SEALING UNUSED CONTACT CAVITIES—UNPRESSURIZED AREAS (CUT-AWAY VIEW) TAB LE 11-28. SEALING ROD DIMENSIONS
CONTACT SIZE (AWG)
DIAMETER (INCHES)
ROD LENGTH (INCHES)
MIN MAX 20 1/16 5/8” 3/4” 16 3/32 7/8” 1” 12 1/8 7/8” 1”
11-263. PRESSURIZED AREAS. Connectors installed in pressurized areas of the aircraft may be divided into two main installation categories, sealed and unsealed.
a. Sealed Connector Installations. Sealed connectors installed in pressurized areas must have their unused contact cavities filled with Teflon sealing plugs or rods. (See Figure 11-40.)
Installation of spare contacts is optional, except for future wiring addition requirements. (See paragraph 11-234). No stub wires are required.
b. Unsealed Connector Installations. It is not required to fill unused contact cavities of unsealed connectors installed in pressurized areas with Teflon sealing plugs or rods. Installation of spare contacts is optional, except for future wiring addition requirements.
(See paragraph 11-234.)
11-264.–11-270. [RESERVED.]
Par 11-271 Page 11-121 SECTION 20. ELECTRICAL AND ELECTRONIC SYMBOLS
11-271. GENERAL. The electrical and electronic symbols shown here are those that are likely to be encountered by the Aviation Maintenance Technician (AMT). They are in accordance with IEEE/ANSI 315-1975.
11-272. SYMBOLS. Only those symbols associated with aircraft electrical and electronic wiring have been listed in general. Refer to IEEE/ANSI 315-1975 for more specific detail on each symbol.
TABLE 11-29. ELECTRONIC/ELECTRICAL SYMBOLS
TABLE 11-29. ELECTRONIC/ELECTRICAL SYMBOLS (CONTINUED) Symbol Meaning Resistor Capacitor YVDE Anterna ЛГ Battery Thermal Element Thermomechanical Transducer Thermocouple Spark Gap Ignitor Cap Continuous Loop Fire Detector (Temperature Sensor) Ignitor Flug Par 11-272 Page 11-122