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regard to mixing different wire types within the same wire bundles during installation, repair,
modification, and routine maintenance.
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It is generally accepted that the use of fewer parts on an aircraft will help to keep the overall
costs down. The logistical costs of maintaining aircraft with multiple wire types and tooling are
often cost prohibitive. OAMs prefer to use one wire type for all major wiring on the aircraft;
however, special applications or environments may require that a different type of wire be used
that can meet these special requirements. These various wire types will usually lay together in
the same wiring runs and harnesses, or cross other wires. Since one wire length may pass
through multiple areas of an aircraft with vastly different environmental conditions, the wire
must be able to meet the most stringent of those application requirements.
3.2.2 Commercial Maintenance and Operator Wiring Practices.
Maintenance facilities and operators usually follow the instructions and recommendations of the
OAM for a specific airframe. Some facilities attempt to decrease parts inventories through
standard process initiatives and may substitute parts through that process. In areas where
modifications are made to the EWIS, mixing of wire types will occur if substitute wire types are
used. Many of the older aircraft have airframe wire that is no longer allowed in the new 14 CFR
Part 25 aircraft and is discouraged from use in aerospace applications. When repairs are made to
the EWIS, these wires or portions of these wires could be replaced with alternative wire types.
3.2.3 Modification Facilities.
Facilities that hold a Supplemental Type Certificate may perform modifications to the aircraft
with different wire types than were originally used on the aircraft when approved by FAA
authorities.
3.2.4 Military Maintenance and Operation Wiring Practices.
As with many military maintenance documents, NAVAIR 01-1A-505 and U.S. Air Force T.O.
1-1A-14 “Installation Practice Aircraft Electric and Electronic Wiring” manuals and other
documents (listed in appendix A) do not specify that mixing of dissimilar wire types can or
cannot take place in the aircraft. However, there is wire substitution information contained in
NAVAIR 01-1A-505.2 concerning wire replacement for repair and maintenance. For example,
work package 006 contains very specific instructions for replacing M81381 aromatic polyimide
insulated wire types with M22759 cross-linked ethylene-tetrafluoroethylene (XLETFE) insulated
wire types for single conductor repair. For aircraft that were originally manufactured with
polyimide insulated wires, the instructions in the work package tacitly call for an eventual
mixing of wire types over time as old polyimide is replaced with fluorocarbon-based wire during
maintenance and repair actions.
The U.S. Air Force does not appear to have published policy related to specific replacement of
wire across the board. However, specific platforms can and do make decisions, such that older
wire types can be replaced with available alternative wire types. Over time, this will lead to the
presence of mixed wire types in U.S. Air Force aircraft.
The Naval Aerospace Vehicle Wiring Action Group guideline, D5-GI-1188, assists maintenance
personnel with replacement of polyvinylchloride (PVC) and aromatic and aliphatic-type
polyimide insulated wire types. The guidance specifies the replacement of PVC insulated wire
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and aromatic and aliphatic polyimide insulated wire with XLETFE insulated wire. The guideline
also provides wire substitutions for canceled wire specifications and for certain common wire
types stored in wiring shops.
These directives and guidelines indicate that mixing of wire types in U.S. Navy and U.S. Air
Force aircraft is a recognized practice that is necessary and acceptable.
3.3 REVIEW OF SERVICE DATA.
Review of field service data did not indicate that failures occur as a result of mixed wires. There
were significant insulation abrasion failures reported that may or may not be attributed to mixed
wire, but there was no indication by the repair actions that mixed wires were the issue (see
appendix A).
3.4 REVIEW OF PRIOR RESEARCH AND TEST DATA (INDUSTRY, GOVERNMENT,
MILITARY, ETC.).
Some parts of the military are presently looking at the issue of mixed wire, but the only formal
report found is the Naval Avionic Center (NAC) report TR-2333 [2]. The results indicate that
the dielectric characteristics of harder insulations are reduced significantly more than those of
softer insulation types. In some cases, the withstand voltage capability was reduced nearly 50%
from the initial values, but it should be understood that even at these reduced levels, the wire
　
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