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How to identify an aircraft wire to understand wire system degradation

Management

If you try to identify an aircraft wire, particularly in service conditions, is not an easy task. However, we know it can assist with understanding details of wire system degradation. With the a variety of codes combined with the difficulty of finding the code on smaller gauge wires can try the patience of a skilled technician. The following is a set of guidelines developed by Lectromec to help with wire identification. Editors Note: This article was updated August 31, 2014 for content clarification.

  1. Read the stamped identification markings, usually about every 12 inches (30 cm) along a wire
    • If there is no identification marking go to the next main heading
    • If there is an identification marking the following will help in the recognition
      • Top or left most alphanumerics give the main wire identification, e.g., MIL-W-22759
      • Slash number, if used, follows for further identification, e.g., MIL-W-22759/35
      • Further alphanumerics beyond the slash number give information about gauge, wire color, etc.
      • Identification codes area different between the military and civilian communities both in the United States and Europe
      • Go to the wire identification page to find the material or materials used
  2. In some instances there will be a number that is a functional identifier (a wire for a light, etc.)
    • This identification most likely will not follow any of the wire identification type codes
    • If available from the airplane’s service or wiring manuals, this number should give an identification of the wire type
    Wire System Degradation
    Looking to identify a wire? This quick guide can help
  3. Sometimes only a manufacturer’s identification is the only information available
    • Maybe of limited use even if the manufacturer’s name is on the wire
    • Contacting the manufacturer might give the material identification
    • An example with Boeing BMS 13-42A is in the figure above, courtesy of the NTSB TWA 800 Accident report
  4. Cables (e.g., twisted pair, etc.) have a different coding scheme; see the Information about Cable (twisted pair, etc.) wire identification page.

If you are interested in wire system degradation, you may want to read Wire Degradation and Looking in the Arc.

Why is an identification marking not available and what to do

  1. Why isn’t the identification available
    • The wire is too short
    • If the wire of interest is in a bundle, the identification could be below another wire
    • The marking has been worn or dissolved
    • The wire is covered with dirt, e.g., grease, dust, etc.
    • If the wire is dirty, careful, quick cleaning may uncover the identification marking
    • There was no identification marking at the time of manufacture or installation
  2. How to overcome this problem
    • Check the type of aircraft in order to help identify a wire
      • See if the aircraft is in the database
        • Caution – the manufacture date should be checked as changes of wire types during production may have happened and the dates are only approximate
        • The age of the airplane may offer some further clues (see below)
        • Caution – the wire listings usually do not include some special wires that are used sparingly
      • If available, specifications from the aircraft manuals can be used
        • Caution – the wire may have been replaced by another type and the change was not recorded
        • Caution – unrecorded changes may have happened during the aircraft’s manufacture
      • Where the airplane was manufactured may give hints
        • Polyimide insulation with European constructed wire is somewhat different than most of that from the United States
        • Other insulation types usually are more similar in construction
    • Check the construction of the wire
      • Cross-section the wire and perhaps make a small slice at an angle with the conductor
        • If there is a tape layer or tape layers of insulation, then some of the material probably is a polyimide
          • Polyimides usually are tan to brown in color
          • Some insulation is wrapped tape, e.g., MIL-W-22759/1
          • The “top coat” sometimes can aid in identification
          • Check the material information part of the database for more details
          • See the figure below for an example of tape construction (MIL-W-22759/88)
          Wire Faiure
          Example of multi-layer wire insulation
        • If the material has a layer or layers of extruded material (i.e., completely continuous and uniform)
          • PVC is layered with Nylon and sometimes glass fiber
          • ETFE or XL-ETFE usually are single insulation types
          • XL-PVDF is the outer layer with a cross-linked poly alkene inner layer
          • Poly-X insulation was extruded with at least two layers and a poly imide”top coat”
          • Color variations between the layers often can be seen
          • Check the material information part of the database for more details
          • See the figure below for an example of extruded construction (MIL-W-5086/7B)
      • Color identification of the insulation materials may or may not be useful
        • Color additives are often added to extruded materials
        • Sometimes aged Nylon will be discolored
        • Poly imides are tan or brown
      • Measure the conductor diameter
        • This can help determine the wire gauge size
        • Most of the wire is stranded
        • Go to the stranded wire table at the bottom
    • Simple chemical and/or physical tests for identification and confirmation
      • The following procedures should be done only by qualified individuals
      • Nylon will dissolve in 10% hydrochloric acid and more quickly in concentrated hydrochloric acid
        • Hydrochloric acid is also known as muratic acid
        • Care is required when handling this acid
        • Would be useful if glass fiber layer is not present and as a confirmation
      • PMDA-ODA dissolves in very caustic or concentrated sulfuric acid solutions
        • For safety reasons a caustic solution (sodium hydroxide) is preferable
        • With either solution caution must be used
        • Dissolution may take some time with either sodium hydroxide or sulfuric acid with stirring
        • Not know if “T” film is soluble in sodium hydroxide
        • “T” film has been reported to dissolve in sulfuric acid
      • Poly-X dissolves in a caustic or concentrated sulfuric acid solutions
        • Prediction from the structural similarity to PMDA-ODA
        • Also based on reported weaknesses
      • PVC will dissolve in solvents used for PVC pipes
      • Other materials, e.g., PTFE, PVDF, PE, and ETFE, can not be identified as easily as the above insulation with simple tests
      • Note that XL PE/XL PVDF insulation is thicker than XL ETFE or ETFE
    • Various instrumental analysis methods
      • Fourier Transform Infrared Spectroscopy (FTIR)
      • Thermogravimetric analysis (TGA) and/or Differential Scanning Calorimetry (DSC) may be useful
      • Relatively small amounts are needed for either method
      • Much higher level of certainty for identification
      • Much higher level of certainty for identification
      • Any instrumental analysis will be somewhat costly

Age of the aircraft and the wire insulation

  1. An aircraft’s age may provide a clue to the insulation type
  2. Some insulation materials usage has been discontinued in newer airplane
  3. Replacement with “older” materials often is done to conform with OEM original specifications
  4. In some cases “older” materials are replaced with other insulation and probably newer types
  5. Some materials have been introduced or discontinued over the years
  6. Some newer materials were introduced while older ones were still being employed
  7. The following list can be used as arough guide especially with many large transport and some military airplanes for the common wire types
    • PVC/Nylon/Glass was installed up to about 1980 for some aircraft types
    • PTFE was approved for some uses in the 1960’s but appears to not have been used widey
    • XL-PVDF/XL-poly alkene was installed up to about 1970 in most airplanes and still may be employed in some cases
    • “Poly – X” started and stopped being installed in the 1970’s
    • PEK started and stopped being installed in the 1970’s
    • Poly imide started to be installed in the early 1970’s and is still being fitted in some new aircraft
    • XL-ETFE started to be installed in the late 1970’s and is still being used in new aircraft
    • “TKT” started to be installed in the middle 1990’s and is still being utilized in new aircraft

Overall diameter for various stranded wire sizes-does not include insulation

AWG Size UK Size code # of strands Diameter (min) in Diameter (max) in Diameter (min) mm Diameter (max) mm
26 001 19 0.018 0.019 0.46 0.48
24 002 19 0.023 0.024 0.58 0.61
22 004 19 0.029 0.031 0.74 0.79
20 006 19 0.037 0.038 0.94 0.97
18 010 19 0.046 0.048 1.17 1.22
16 012 19 0.052 0.054 1.32 1.37
14 020 19 0.065 0.068 1.65 1.73
12 030 37 0.084 0.087 2.13 2.21
10 37 0.106 0.111 2.69 2.82
8 133 0.158 0.017 4.01 0.43
6 133 0.198 0.212 5.03 5.38
4 133 0.250 0.268 6.35 6.81
2 665 0.320 0.340 8.13 8.64
0 1045 0.474 0.506 12.04 12.85
00 1330 0.546 0.578 13.87 14.68
000 1665 0.614 0.654 15.60 16.61
0000 2109 0.674 0.718 17.12 18.24
Michael Traskos

This article was written by the Lectromec technical team. Aircraft wiring is our passion and we strive to make a contribution to the field by sharing our expertise through blogs, podcasts, and videos. We hope you find this information helpful. We also encourage you to submit comments and spur discussions.