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AS4373 – Testing Aerospace Cables – Part 2

Testing & Assessment

When is a wire good enough to be placed onto an aircraft? What environmental tests should be considered to clear it for flight? Are there ways to determine how easy it will be to handle for maintainers? Over the last couple of decades, a single test standard has evolved to with test methods to address the assessment requirements for aircraft wire.

aircraft wire
Environmental testing may be conducted in high temperature ovens, low temperature freezers, or humidity chambers such as this one. Lectromec can help you with each of your wire testings needs.

In our previous article, we reviewed the various electrical and mechanical tests included in the AS4373 standard. We will conclude the two-part series by examining the remaining test groups (environment, conductor, and quality evaluation). The following is a brief discussion of each of these test methods.

Environmental Assessment

Perhaps more than anything else, the environment to which wires are exposed to will determine wire performance and longevity. These tests subject the wire to a wide range of environmental conditions and seek to determine performance.

Test Method

Test Name

Short Description

601

Fluid Immersion

The specimen is submerged in a variety of common aerospace fluids. No insulation breaches or swelling shall be permitted.

602

Forced Hydrolysis

The specimen is placed in a high-temperature salt water bath for thousands of hours. No insulation breaches shall be permitted. Good test for wires installed in external aircraft zones and for those installed on naval aircraft.

603

Humidity Resistance

The specimen is subjected to humidity cycling for a couple weeks. No performance degradation is permitted.

604

Weight Loss Under Temperature and Vacuum

The specimen is placed in a heated vacuum chamber for weeks then the specimen is weighed. This test is particularly important for space applications.

606

Weathering Resistance

The specimen is exposed to water and cycles of UV light. Important for outdoor applications.

607

Wicking

The specimen in placed vertically into a dye and the wicking is then measured.

608

Fluoride Off gassing

The insulation is removed from the specimen, placed in heated water for a week, and the extracted fluoride is measured. Important for silver plated conductors with XLETFE insulation and space applications.

801

Flammability

The specimen is set at an angle and a flame is placed on the mid-section. Any propagation of sustainment of the flame is considered a failure. Very close FAA flammability test.

802

High Pressure/High Temperature Air Impingement (Burst Duct)

A high pressure high temperature air flow is placed across a wire harness constructed of the test sample wire. Any breach of the wire insulation constitutes a failure.

803

Smoke Quantity

The specimen is exposed to a radiant heat panel and the quantity of smoke is assessed.

804

Relative Thermal Life and Temperature Index

The specimen is exposed to months of high temperatures. This test provides data to project long-term specimen temperature rating.

805

Thermal Shock Resistance

The specimen is subjected to temperature cycling.

806

Property Retention After Thermal Aging

The specimen is placed in a high temperature oven for several weeks then posted with several mechanical tests.

807

Multi-day Heat Aging Test (Life Cycle)

The specimen is weighted on both ends and placed over a mandrel in a high temperature oven. After oven exposure, the specimen is exposed to the bend and DVW tests.

808

Blocking

Specimen is wrapped on itself and exposed to high temperatures. Test seeks to determine if insulation will adhere to itself at high temperatures.

809

Lamination Sealing

For multilayer constructions, the specimen is cut flush on both ends and placed into an oven. Any separation of the insulation layers constitutes failure.

810

Topcoat Cure

This is a quick high temperature steam test to create cracks in insulation, particularly polyimide.

811

Cross-link Proof Test

Similar to Method 807, but intended for Crosslinked insulation types.

812

Flame Resistance

Wire sample is placed on vibrating jig above a aviation fuel fueled flame. Test of particular importance for fire zone wire.

813

Insulation State of Sinter

A Differential Scanning Calorimeter is used to determine the chemical state of wire insulation to verify proper processing.

Conductor Assessment

The performance of the conductor is also a critical part of wire performance. Many of the tests in the electrical performance evaluate the conductors often in combination with the insulation. These five tests specifically examine the conductor.

Test Method

Test Name

Short Description

401

Conductor Diameter

The conductor is removed for the insulation and the diameter is measured.

402

Conductor Elongation and Tensile Breaking Strength

The conductor is removed from the insulation and pulled longitudinally until break.

403

Conductor Resistance

The conductor resistance is measured.

404

Conductor Strand Blocking

The conductor strands are separated and examined to determine if a conductor strand to strand adhesion exists.

Workmanship/Manufacturing Quality Assessment

The last section that is covered in the AS4373 test standard is the final product quality and workability of the products. Depending on the particular application, these tests may or may not be important for a given project.

Test Method

Test Name

Short Description

101

Concentricity and Wall Thickness

The specimen is cut flush and examined under a microscope for wire concentricity and insulation wall thickness. Extruded insulation types tend to have more variability in wall thickness.

102

Insulation Bonding to Potting Compounds

Wire is placed in a jig and bonding computer is cured onto the specimen. Once cured, the wire is then subjected to a break force test to measure adhesion strength.

103

Insulation Pull-off Force

A short slug of insulation is cut from the wire and the required force to pull off the slug is measured. Too high a value can indicate an insulation construction that may be difficult to handle from an installation/maintenance perspective.

104

Insulation Shrinkage/Expansion

The specimen is briefly subjected to a high temperature and measurements are made to determine if insulation shrinkage (or expansion) has occurred.

105

Solderability

Common flux and solder are applied to the specimen conductor. Coverage and adhesion are assessed.

106

Thermal/Mechanical Resistance – Single Wire

A heated soldering iron with a fixed weight in placed on the specimen. Time to touching the conductor is recorded.

107

Thermal/Mechanical Resistance – Bundle

Similar to method 106 except performed on a 10-wire harness.

108

Solder Pot Test for Insulation Shrinkage

The specimen’s insulation shall be stripped back and the conductor submerged in solder. The insulation shrinkage is measured.

109

Percent Overlap of Insulating Tapes

(Tape wrap constructions only) The specimen’s insulation is examined to determine insulation overlap.

110

Outer Layer Insulation Smoothness

The specimen is cut flush, cast in a mold, and the smoothness is measured.

Summary

In this two-part article series discussing AS4373 test methods, we covered over 60 test methods each of which can provide some benefit in either assessing performance, design, or wire/cable manufactured quality. While some of these test methods are specifically focused toward aerospace wires, the bulk can be applied or adjusted to address the needs of many other industries. If you are interested in any of these tests and would like to find out more about how they can be applied to your application, contact Lectromec.

Michael Traskos

Michael Traskos

President, Lectromec

Michael has been involved in wire degradation and failure assessments for more than a decade. He has worked on dozens of projects assessing the reliability and qualification of EWIS components. In September 2014, Michael was appointed as an FAA DER with a delegated authority covering EWIS certification.