Proper aircraft wire installation begins with selecting the right components. There will never be a perfect component, because there will always be tradeoffs between performance, cost, weight, and reliability. Engineering is what allows us to weigh all these factors and determine which one is the most important. The decision matrix includes requirements of the electrical system, the environmental stresses, size and weight requirements, and regulatory requirements.
Regulatory considerations are often the last item considered when making the decision of what should be selected, but compliance can create significant problems if not addressed early in the design process.
For the US, the requirements for function and installation of Electrical Wiring Interconnect Systems (EWIS) components is defined by FAA regulation 25.1703 Function and Installation.
This regulation has the following four clauses:
(a) Each EWIS component installed in any area of the aircraft must:
(1) Be of a kind and design appropriate to its intended function.
(2) Be installed according to limitations specified for the EWIS components.
(3) Perform the function for which it was intended without degrading the airworthiness of the airplane.
(4) Be designed and installed in a way that will minimize mechanical strain.
(b) Selection of wires must take into account known characteristics of the wire in relation to each installation and application to minimize the risk of wire damage, including any arc tracking phenomena.
(c) The design and installation of the main power cables (including generator cables) in the fuselage must allow for a reasonable degree of deformation and stretching without failure.
(d) EWIS components located in areas of known moisture accumulation must be protected to minimize any hazardous effects due to moisture.
To help designers and maintainers understand the intent behind these regulations, the FAA released guidance document AC25.1701-1. We will go through each of these items and discuss the implications of each.
A.1 – use for intended function.
Selecting EWIS components for installation on aircraft should focus on those components that have already been qualified for aircraft use – use of consumer grade components (e.g. PVC wire) is unacceptable. That is because aircraft components have been tested and evaluated for aerospace environments (e.g. outgassing in the case of fire).
Further, EWIS components should be appropriately rated for their particular application. Wires that are rated for 150oC should not be used in environments with temperatures higher that this rating. Component selection should consider both the environmental factors as well as nearby equipment heating and resistive heating of the wire harness (for more information, you can read Lectromec’s Derating Tool for EWIS article).
Beyond using aircraft grade components, an engineer should also consider the performance of the components in the right application as well as the service life. The Instructions for Continued Airworthiness (IAC) provides a basis for understanding the ideas of expected service life:
“Expected service life means the expected service lifetime of the EWIS. This is not normally less than the expected service life of the aircraft structure. If the expected service life requires that all or some of the EWIS components be replaced at certain intervals, then these intervals must be specified in the ICA as required by § 25.1529.”
During design, the aircraft’s expected service life is defined. Those components able to function safely for the entire duration are periodically inspected through the EZAP process. For operators looking to go beyond the existing certified service life of the aircraft, additional analysis is necessary.
In future articles, Lectromec will discuss the other parts of the 25.1703 requirements and considerations necessary for certification (Part II).