Par 11-239 Page 11-580 (and 11-100) moisture in the connectors. A secondary benefit of potting is the reduced possibility of breakage between the contact and wire due to vibration.
Connectors 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 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.
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 heatshrinkable caps or crimped insulated end caps such as the MS 25274 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 placed in a drying oven at 100 °F for 3 to 4 hours. In all cases follow manufacturer’s instructions.
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.
Par 11-239 Page 11-581 (and 11-100) FIGURE 11-38. Spare wires for potting connector.
11-241.11-247. [RESERVED.]
Par 11-248 Page 11-582
SECTION 18. CONDUITS
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.
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 (I.D.) about 25 percent larger than the maximum diameter of the wire bundle.
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.
CONDUIT INSTALLATION.
Conduit problems can be avoided by following these guidelines:
Do not locate conduit where passengers or maintenance personnel might use it as a handhold or footstep.
Provide drainholes at the lowest point in a conduit run. Drilling burrs should be carefully removed.
Support conduit to prevent chafi ng against structure and to avoid stressing its end fittings.
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 11-23. 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.
TABLE 11-23. Bend radii for rigid conduit.
Nominal Tube O.D.
(inches) Minimum Bend Radii (inches) 1/8 3/16 1/4 3/8 1/2 5/8 3/4 1 1 1/4 1 1/2 1 3/4 2 3/8 7/16 9/16 15/16 1 1/4 1 1/2 1 3/4 3 3 3/4 5 7 8
Par 11-253 Page 11-583 FLEXIBLE CONDUIT. Flexible aluminum conduit conforming to Specification MIL-C-6136 is available in two types: Type I, Bare Flexible Conduit, and Type II, Rubber Covered Flexible Conduit. Flexible brass conduit conforming to Specification MIL-C-7931 is available and normally used instead of flexible aluminum where necessary to minimize radio interference. Also available is a plastic flexible tubing. (Reference 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 11-24.
TABLE 11-24. Minimum bending radii for flexible aluminum or brass conduit.
Nominal I.D. of conduit (inches) Minimum bending radius inside (inches) 3/16 1/4 3/8 1/2 5/8 3/4 1 1 1/4 1 1/2 1 3/4 2 2 1/2 2 1/4 2 3/4 3 3/4 3 3/4 3 3/4 4 1/4 5 3/4 8 8 1/4 9 9 3/4 10
11-254.11-259. [RESERVED.]
Par 11-260 Page 11-584
SECTION 19. PROTECTION OF UNUSED CONNECTORS
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.
QUICK REFERENCE CHART. A quick reference chart of unused connector contact cavity requirements is given in table 11-25. These requirements apply to harness manufacturing or connector replacement only.
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:
Firewall Connectors Installations. Firewall unused connector contact cavities should be filled with spare contacts and stub wires. (See figure 11-39.)
Construct stub wires using high temperature wire (260 ºC). Ensure that stub wires are of the same type of wires in the bundle.
Crimp the proper contact, for the connector and cavity being used, onto the wire. Install the crimped contact into the unused cavity.
Extend stub wires beyond the back of the connector clamp from 1.5 to 6 inches. Feather trim stub wires to taper wire bundle.
Secure wire ends with high temperature (greater than 250 ºC) 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
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 11-26 for dimensions.)
PRESSURIZED AREAS. Connectors installed in pressurized areas of the aircraft may be divided into two main installation categories, sealed and unsealed.
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.
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 additi on requirements. (See paragraph 11-234.)
Par 11-271 Page 11-585 TABLE 11-25. Contact cavity sealing-quick reference.
Connector Installation Types Unpressurized Area Sealing Means 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 11-26 for sealing rod dimensions.)
NOTE 2: Stub wires must be of the same type as the other wires of the bundle.
F IGURE 11-39. Stub wire installation.
TABLE 11-26. Sealing rod dimensions.
CONTACT SIZE DIAMETER ROD LENGTH (INCHES) (AWG) (INCHES) MIN MAX 20 1/16 5/8” 3/4” 16 3/32 7/8” 1” 12 1/8 7/8” 1”
FIGURE 11-40. Sealing unused contact cavities-unpressurized areas-(cut-away view).
11-264.11-270. [RESERVED.]
Par 11-271 Page 11-586 SECTION 20. ELECTRICAL AND ELECTRONIC SYMBOLS
GENERAL. The electrical and electronic symbols shown here are those that are likely to be encountered by the aviation maintenance technician. They are in accordance with ANSI-Y32.2-1975.
SYMBOLS. Only those symbols associated with aircraft electrical and electronic wiring have been listed in general. Refer to ANSI- Y32.2-1975 for more specific detail on each symbol.
TABLE 11-27. Electronic/Electrical Symbols.
Symbol Meaning
Adjustability Variability
Radiation Indicators
Physical State Recognition
Test-Point Recognition
Polarity Markings
Direction of Flow of Power, Signal, or Information
Kind of Current
Envelope Enclosure
Shield Shielding
Special Connector or Cable Indicator
Par 11-271 Page 11-587 TABLE 11-27. Electronic/Electrical Symbols (continued).
Symbol Meaning
Resistor
Capacitor
Antenna
Battery
Thermal Element Thermomechanical Transducer
Thermocouple
Spark Gap Ignitor Gap
Continuous Loop Fire Detector (Temperature Sensor)
Ignitor Plug
Par 11-272 Page 11-588 TABLE 11-27. Electronic/Electrical Symbols (continued).
Symbol Meaning
Transmission Patch Conductor Cable Wiring
Distribution Lines Transmission Lines
Alternative or Conditioned Wiring
Associated or Future
Par 11-272 Page 11-589 TABLE 11-27. Electronic/Electrical Symbols (continued).
Symbol Meaning
Intentional Isolation of Direct-Current Path in Coaxial or Waveguide Applications
Waveguide
Strip-Type Transmission Line
Termination
Circuit Return
Pressure-Tight Bulkhead Cable Gland Cable Sealing End
Switching Function
Electrical Contact
Par 11-272 Page 11-590 TABLE 11-27. Electronic/Electrical Symbols (continued).
Symbol Meaning
Basic Contact Assemblies
Magnetic Blowout Coil
Operating Coil Relay Coil
Switch
Pushbutton, Momentary, or Spring-Return
Par 11-272 Page 11-591 TABLE 11-27. Electronic/Electrical Symbols (continued).
Symbol Meaning
Two-Circuit, Maintained, or Not Spring-Return
Nonlocking Switching, Momentary, or Spring- Return
Locking Switch
Combination Locking and Nonlocking Switch
Key-Type Switch Lever Switch