This is a question that Lectromec regularly receives. The reason for this question is that a parts-supplier or system-integrator is looking to find wires in compliance with EWIS requirements. Since there are so many ways to evaluate any component, the important question to consider is if a defined requirements list for aerospace wire exists? Here, we run through several different means of assessing wires and identify what requirements should be considered.
FAA Requirements
As a starting point for any commercial or private vehicles, reviewing FAA guidance is usually a good place to start. Unfortunately, the FAA does not have and published Technical Standard Orders (TSOs) for wires or cables (but they do have one for clamps). FAA regulation 25.1703 states that all components should be selected such that they can operated within their limitations. Further, the environmental conditions within those locations must also be considered when making part selection. Obviously, a low temperature wire (max temperature <85C) should not be installed in the engine compartment. The material properties must be evaluated to ensure compatibility with the local environment.
To go beyond this, the implications of the FAA requirements mean that it is not just the straightforward environmental considerations but also the electrical requirements of the component as well. Wires and cables should not be stressed to such a level that they are overheating when the electrical load is turned on. As an example, wires with single core conductors should not be part of applications that have regular flexing because of conductor fatigue and the increased likelihood of breaking.
Each wire should be viewed from the system level and with consideration for its application.
Similar to FAA requirements on other systems, FAA does not specifically state what properties a wire must have; it simply states that it must be able to perform adequately and without degradation during normal operations.
Industry Standards
What constitutes the acceptable performance of an aerospace wire will vary from organization to organization and standard to standard. SAE wire installation standard AS50881 identifies several classifications of wires/cables that are explicitly recommended for aerospace use. The wires/cables explicitly recommended are those that fall into the AS22759, AS6070, and NEMA27500 categories.
Those familiar with the requirements outlined in each standard know the large number of tests to necessary show compliance and conformity to the standards. Many of the tests that have been developed and now levied on these wire and cable types exist because of lessons learned from wire/cable manufacturing processes to in-service problems.
While some might argue that the tests in assessment requirements on these wires is in excess, many of these wires and cables provide the basis for many of the general-purpose wire used on aircraft. Because there such a variety of thermal, electrical, and mechanical stresses in aerospace application, these wires must be designed to handle as wide a range as possible. From a maintainers perspective, having common wire types throughout much of the aircraft makes easier to maintain aircraft.
Commercial standards
There are applications where fully robust general-purpose aerospace wire is not necessary. Often, these wires are selected when system integrators are looking for means to reduce cost. Some OEMs will allow for this, but in doing so, they also identify a set of criteria a wire must pass. Often times, this testing will include evaluation of the smoke generation, toxicity, and flammability of the wire/cable. The performance assessments of the other properties are then reduced and/or ignored.
From an EWIS certification standpoint, as long as the failure can be mitigated and does not create a hazardous or catastrophic event, regulators have no means of limiting their use. As such, the limited purpose wire/cable are permissible when installed in a limited number areas of the aircraft.
Achieving the EWIS qualified wire status
“EWIS qualified wire” is not a term that should be easily assigned to any wire. What may be qualified for one application may not be applicable for other EWIS applications. Products that have been evaluated as part of some quality management system, already have a leg up on other wire/cables. In order to achieve the same recognition and acceptance as part of the EWIS design, it becomes necessary to evaluate these wires and cables to a set of parameters that match the intended application.
As an ISO 17025 accredited lab, Lectromec has the capabilities and experience to test the wires and cables designated for aircraft installation. If you are looking to determine if a wire or other EWIS component can be placed onto a vehicle, Lectromec can help you get the data you need to prove you have an EWIS qualified part.
Michael Traskos
President, Lectromec
michael.traskos@lectromec.comMichael 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. Michael is an FAA DER with a delegated authority covering EWIS certification and the chairman of the SAE AE-8A EWIS installation committee.