Standard Practices

Standar AC 43.13-1B update provides valuable tips for maintenance of electronics By Jim Sparks April 2000 Over the years, various methods and techniques have been utilized to successfully maintain the worlds fleet of aircraft. Each...


Terminal strips
Terminal strips are a common means of coupling various segments of a circuit and may include barriers between adjacent studs. In all cases, the electrical current should be carried by the mated terminal surfaces — not the terminal stud, and no more than four terminals should be installed on any one stud. In the event more than four terminals need to be coupled, multiple studs should be utilized and connected by a small bus bar or a properly manufactured jumper wire. Terminal strips should be mounted so that loose objects cannot fall across and short the contacts. Another good practice is to leave several extra unused studs. This may serve for future circuit expansion or can even be useful in the event of a stud failure.

Terminal strips should be inspected for proper security for attaching hardware and the presence of corrosion. Like most other means of mechanical attachment, proper tightening torque is critical. Stacking of wires on a terminal stud should always find the wire with the greatest diameter on the bottom and the smallest diameter on the top. Generally, when locknuts are used for attachment of wire to terminal strips they are an all-metal construction. Sometimes a spring washer of an appropriate thickness is installed between the nut and a plain washer, which prevents damage to the terminal face. Grounding blocks often include a washer made of a sacrificial material, such as zinc, to take the effects of galvanic reactions. In such cases, this device will periodically require inspection and possibly replacement.

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Splicing
Splicing of electrical wire is permitted as long as it does not affect the reliability or strength of the wire and should be kept to a minimum. This method of joining wires should be avoided in areas subject to severe vibrations or in locations where periodic inspection is difficult. It should be considered that a splice is a mechanical connection and is therefore subject to various mechanical failures. It is for this reason that there should be no more than one splice in any wire between any two points of connection. These devices should also not be installed within 12 inches of a wire termination except in specific situations.

When numerous splices are contained in a wire bundle, their locations should be staggered as to not significantly increase the overall cross section of the bundle.

Many types of splice connectors are available and include the often preferred self-insulated type. When non-insulated connectors are used, they should be covered with a protective sleeve of an appropriate material, then secured at both ends. Environmentally sealed splices provide a reliable means of joining wires in hostile locations.

Clamping wire bundles
Proper clamping of wire bundles should not allow the bundle to move through the clamp but not be tight enough to crush the wire insulation. This will also apply to the installation of plastic tie wraps. Wire bundle mechanical loads will affect the spacing with 24 inches being the usual maximum distance between supports. Proper clamps are selected by static as well as dynamic loads on the bundle along with environment and compatibility of insulating materials. Wire routing should be accomplished so that fluids will drain away from the connectors and external areas such as wheel wells often require additional protection such as conduit or other protective jacket. These will generally include moisture drainage holes that will require periodic inspection. Anytime bundles are routed in areas where lines carrying oxygen, oil, fuel, hydraulic fluid, or alcohol, a minimum of 6 inches should be maintained between the bundle and the plumbing.

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Coaxial cables
All wiring needs to be protected from damage; however, coaxial cables are particularly vulnerable. For example, over-tightening of a tie wrap can cause a change in cable impedance. This can have an adverse effect on system operation. Most coax damage occurs as a result of improper maintenance.

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