AC 43.13-1B update provides valuable tips for maintenance of electronics
By Jim Sparks
Over the years, various methods and techniques have been utilized to successfully maintain the worlds fleet of aircraft. Each manufacturer has the responsibility to evaluate any special requirements of their products and then make those of us involved in the upkeep process aware of unique details. These may include inspections, removals, repairs, and even functional tests. The Air Transport Association (ATA) has developed a method for organizing technical documentation in standardized chapters. As the aviation industry's dependency on electronics has exploded in the past 20 years, today's technician needs to have adequate information available to ensure airworthiness.
ATA Chapter 20 is the designated area for manufacturers to publish information pertinent to Standard Practice. This may include, but is not limited to, aircraft cleaning, torque values, plumbing installation and bonding tests. Some manufacturers even have gone as far as to create dedicated manuals of standardized maintenance practices. It is impractical to believe that each manufacturer can document each and every situation.
The Federal Aviation Administration has solicited the creation of Advisory Circular 43.13-1B. This document was issued in September 1998 and supersedes AC 43.13-1A. Contained within are methods, techniques, and practices considered acceptable to the Administrator for repairs and inspection of non-pressurized civil aircraft where no specific manufacturer instructions will apply. To those active in aircraft maintenance who have not as yet reviewed this Advisory Circular, they will find it contains data for inspecting and maintaining newer technology systems.
Following are some useful pointers for properly maintaining electronics systems in aircraft:
Proper bonding and grounding
One of the important factors in the installation and operation of aircraft electrical systems is proper bonding and grounding. A poor ground can lead to improper system operation as well as cause damage due to Electro Static Discharge (ESD). Low impedance paths to aircraft structure are normally required for electronic equipment to provide radio frequency return circuits and for most equipment to reduce the possibility of Electro Magnetic Induction (EMI).
All external aircraft surfaces that are capable of conducting electricity should be electrically connected to other surfaces through mechanical joints. Exceptions such as some antenna elements require electrical isolation from the rest of the airframe. Coupling the return paths from multiple sources should never occur as electrical noise maybe transmitted from one source to the other and can be a significant problem in digital systems.
Bonding inspections should include checking for the presence of electrical arcing. In all cases, arcing should be suppressed either by enhancing bonding or increasing resistance. Any metal conduit should be bonded to the aircraft structure at each terminating point and break. All connections should be free of corrosion and tightly secured as well as installed in such a way as not to interfere with operation of other movable components. Standard threaded screws are typically used in bonding applications, and for the most part, self-tapping screws should be avoided unless specifically called out by the aircraft manufacturer.
Proper wire termination is an area that requires several considerations. First of all, the tensile strength of the wire to terminal joint should be at least equivalent to the strength of the wire.
Proper selection should be based on current rating, wire size and insulation thickness, conductor material, size of the attaching device (stud size or hardware diameter), compatibility with the insulating material, operating environment, and method of attachment.
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.
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.
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.
The complexity of wiring systems has resulted in an increase in the use of electrical connectors and proper application is essential. Both electrical and environmental requirements are considered for any installation. Connectors are designed for installation in fluids, high vibration areas, and thermal extremes or may be needed to prevent penetration of fire or the loss of pressurization; in other types, Electro Magnetic Interference (EMI) or Radio Frequency (RF) penetration is the primary consideration. Connectors must be rated for continuous operation under the combination of maximum ambient temperature and circuit load. In the event exposure to moisture is not avoidable, a means of draining or sealing is required. The integrity of potting compound is one of the important considerations when inspecting a connector.
Receptacles installed in pressurized areas of an aircraft are divided in two areas: sealed and unsealed. A sealed connector installed will have all unused contact holes filled with sealing plugs, which is typical in firewall connectors. In areas where air leaks are not a factor, it is not as important to have all holes in a connector plugged.
Once a connector has been removed, a cap should be placed on both the plug and receptacle to prevent foreign material from entering. It may also be advantageous to clean the contacts prior to reconnection. Connectors of the same type and having like numbers of pins are often color-coded or otherwise identified to prevent improper attachment to an inappropriate component. Once the connection has been made, it is important to ensure that locking has occurred. In the case of certain Deutsch™ connectors (common brand), an orange collar will appear when properly tightened. Frequently, electrical plugs will have to be tightened to a specific torque value and then safetied. Periodically, it may be necessary to install or remove contacts from a connector. This should only be attempted if proper tooling is available. For example, when using a size 20 contact in a Deutsch™ connector, only a RED and WHITE insertion/extraction tool should be utilized. The purpose of the tool is to disengage the locking device within the connector to allow a contact to either be removed or installed.
The insulation on wires is another factor that can lead to adverse conditions. In some cases, routing of wires with dissimilar types of insulation in a bundle can have adverse effects particularly if one type of insulation is softer than the other and relative motion is present. The subsequent abrasion could lead to an electrical short. Insulating materials are chosen for abrasion resistance, corrosion resistance, strength, dielectric abilities, flame resistance, heat distortion, impact and mechanical strength, resistance to fluids, and smoke emission.
Kapton wires and cables are widely used throughout the aerospace industry and newer technology enable them to be smaller, lighter and have improved temperature resistance over earlier generations. Certain types of this wire will include a Polyimide-based insulation surrounded by several layers of Polyimide varnish. If the thin outer film wears away, this varnish may be mistaken for the inner copper conductor. Kapton insulation is sensitive to cuts and scratches — the slightest damage may result in the loss of insulation. Therefore, piercing type test probes should not be used. Care should be taken to make sure these cables do not chafe against any part of the aircraft structure. Cable bending is another concern and normally the radius is 10 times the diameter of the strand.
Any and all repairs on Kapton wiring should be carried out in accordance with wiring repair procedures of the aircraft in question. Stripping is a common operation and can be successfully carried out if done correctly using the appropriate tools. Pliers designed for stripping external insulating materials are not appropriate for Kapton. There is a danger of tearing this Polyimide insulation, cutting the strands or scraping the alloy and damaging the varnish. A Kapton sheath should only be stripped using a tool that cuts to a specific depth.
Thorough inspection of wires and electrical connections are a requirement to allow satisfactory operation of the high technology equipment used in today's aircraft. As a rule, the specific manufacturers guides should be used for evaluation, but don't forget about our old friend from A&P school, AC 43.13-1B. The new data included in this addition gives a baseline for areas not clearly addressed by manufacturers.