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recommendations as applicable to the mixing of wire types in aircraft applications.
1.2 BACKGROUND.
1.2.1 Wire Installation.
Original aircraft manufacturers (OAM) have designed and built aircraft with various wire types.
This occurred because the technology, materials, and processing of wire has continually
improved. Aircraft wiring is not subject to a service life and mandatory replacement. The
airworthiness of aircraft wiring is managed through inspections and maintenance processes.
Wiring sections may be modified or rewired with the same wire that was originally installed in
the aircraft, or with a newer, more advanced wire, thus mixing wire types in bundles. As aircraft
systems are modified or updated, the same airframe wire may not be available or desirable to
use. During maintenance and repair, newer or different wire may be used to replace older wire.
In these cases, different wire types may be placed together within a harness, or may be located in
adjacent harnesses.
1.2.2 Design Options.
It is not uncommon to have multiple wire types used for a new aircraft design because of
extreme environmental differences between aircraft zones. Some zones require more rugged
wire types than general-purpose wire zones. The designer has the option to protect the generalpurpose
wire in the extreme zones or to use an alternate wire type. In many cases, the alternate
wire type is a specialty wire used for enhanced performance or weight savings. The end result is
mixed wire bundles for the life of the aircraft.
1.2.3 Investigation Reason.
Very little data has been generated or published indicating whether mixing wire types creates
short- or long-term problems. Since recent Aging Transport Systems Rulemaking Advisory
Committee aircraft inspections indicated mixed wire might be a problem [1], it was determined
that an investigation was warranted for aged aircraft.
2. APPROACH.
The current requirements from various regulatory and military documents that have been used as
guidance regarding the mixing of different wire types in electrical wiring interconnection system
(EWIS) bundles were reviewed. Aircraft wiring practices referencing mixed wire types were
also examined. Past test results, formal or informal, were analyzed. Based on the background
information gathered, a test program was developed and executed to assess the actual
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interactions between different wire types within the same bundles or in adjacent harnesses in the
EWIS.
3. REVIEW OF DOCUMENTATION.
A collection of information related to mixing of dissimilar wire types that was compiled through
extensive research of military specifications, Federal Aviation Administration (FAA) regulations
and advisory material, technical manuals, and other documentation are found in appendix A. It
also summarizes the results of the search and provides a description of how, and to what extent,
the subject is currently addressed in the aerospace industry.
3.1 REVIEW OF REGULATIONS, PROPOSED REGULATIONS, AND DESIGN
SPECIFICATIONS.
FAA regulations, Title 14 Code of Federal Regulations (CFR), Airworthiness Directives, ACs,
military and commercial design documents such as SAE AS50881 (formerly MIL-W-5088) and
SAE ARP4404, and other documents were researched extensively for information related to the
mixing of dissimilar wire types in bundles (as shown in appendix A).
Two major points are evident from the research. First, there are no specific statements in any of
the documents that specifically address how mixing of dissimilar wire types in bundles should or
should not be performed. Second, the documents imply that mixing different wire types is not
restricted. However, regulations do stipulate that the wire selected for an application must meet
the environmental and application requirements demanded of the wire, wiring components
interface requirements, and design recommendations such as minimizing the number of
components used in the EWIS.
3.2 AIRCRAFT WIRING PRACTICES.
3.2.1 Original Aircraft Manufacturers Wiring Practices.
OAM standard wiring practices were reviewed to identify guidelines related to mixing wire types
in installation, maintenance, repair, and inspection of the EWIS. The wiring practices of the
following OAMs, as described through the standard wiring practices manuals, were reviewed for
material related to mixing of dissimilar wire types.
• Airbus Industrie Electrical Standard Practices Manual
• Boeing D6-54446 Standard Wiring Practices Manual (chapter 20)
Lockheed Martin also provided informal information but no documentation.
Only limited relevant processes and practices of the operators and the OAMs were provided,
making it difficult to provide a clear picture of what the current commercial practices are in
　
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