AS4373 - Testing Aerospace Cables - Part 2

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.

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

President, Lectromec

michael.traskos@lectromec.com

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.

AS4373 – Testing Aerospace Cables – Part 2