Key Takeaways
- Location and design of EWIS in an aircraft directly affects the maintainability of surrounding system
- Frequency of aircraft maintenance and modification creates a potential to cause long-term EWIS problems in systems with little maintainability consideration.
- Frequency of aircraft maintenance and modification creates a potential to cause long-term EWIS problems in systems with little maintainability consideration.
Overview
The maintainability of the electrical wiring interconnection system (EWIS) is integral to the longevity of an aircraft. EWIS maintainability affects not only the EWIS itself, but any nearby system whose maintenance access is restricted by the location or function of the EWIS. Thus, high maintainability of the EWIS allows for faster, more efficient, and more precise maintenance of surrounding equipment and systems.
Original aircraft design must comply with EWIS regulations in development to ensure long-term maintainability. It is important to know which regulations apply and which standards to follow when designing an aircraft to determine the appropriate maintenance requirements.
Lectromec has published several articles on the dangers of poorly maintained EWIS which will not be discussed in this article, here we will explore the requirements of some of the more common standards for EWIS maintainability in aircraft design.
E-CFR
We will first take a look at the FAA regulations for EWIS in transport category airplanes. Note that the E-CFR (Code of Federal Regulations) is a regulation rather than a standard and thus defines federal requirements for aircraft design.
The E-CFR provides detailed requirements for component selection, EWIS certification, system separation, safety, component identification, fire & smoke protection, electrical bonding, circuit protection, accessibility, and maintenance instructions (to be provided by OEM or the modifier). Each of these requirements contributes to the design and implementation of EWIS in an aircraft and directly impacts the ease of maintenance on the EWIS and surrounding systems.
This regulation does not explicitly define maintainability requirements, but compliance with the regulations inherently increases maintainability of an aircraft.
ASTM F2696
The ASTM F2696 is an inspection standard rather than a design standard but can serve as a supplementary guideline for OEMs.
Some of the main topics relevant to EWIS and maintainability addressed in this standard include ease of EWIS displacement during maintenance and adequate identification of EWIS components.
The consideration of EWIS maintainability in aircraft design is imperative due to the limited amount of space available for EWIS routing.
The inspection standard calls for adequate amounts of slack in cable and wire to allow for ease of maintenance. With no specific quantitative definition, it is up to the designers and implementors to determine the appropriate slack required for high maintainability. From a design perspective, this should include sufficient slack for easily connecting Line Replaceable Units (LRUs) and the movement of bundles if needed to access other aircraft systems. Cable slack is also an important factor when attached to moving components (such as doors and panels).
Adequate identification should always be considered when discussing maintainability. Clear, visible identification allows for faster and more efficient maintenance in any system without a strict requirement of familiarity with the system. This aligns with the requirements of 25.1711.
Though the ASTM F2696 is intended for inspection applications, it is useful to incorporate in the design process to prepare for future inspections.
AS50881 and EN3197
Frequent maintenance around an EWIS creates potential for damage from repeated moving/ removing of the EWIS to access adjacent systems. Simply moving a wire harness out of the way to perform other maintenance may not seem particularly straining but can wear down the integrity of the harness over time if the system requires frequent repairs or modification.
The AS50881 and EN3197 are perhaps the most used standards for aircraft EWIS design. Both standards list explicit requirements for the maintainability of EWIS. These requirements are similar in many ways, particularly in emphasizing the importance of component choice when designing EWIS.
AS50881 (AIR 6808) EWIS wire selection
The AS50881 emphasizes the selection of appropriate wire based on the expected maintenance to be performed on/ around the EWIS. The specific section on maintainability in the AS50881 leaves much to be desired in terms of specificity, only stating:
Paragraph 3.8.9: “Wire selection shall be affected by considerations of the types and frequency of maintenance action.”
It is useful to reference the AIR 6808 document which provides the rationale for AS50881 for further understanding.
The AIR 6808 document goes on to suggest wire choices based on maintenance requirements. Specifically, in areas requiring frequent maintenance on an aircraft, the AIR 6808 recommends the use of wire composed of many conductors and abrasion resistant secondary harness protection.
The flexibility of multi-strand conductor wires allows for easier physical displacement of a wire harness during maintenance without causing undue stress or strain. Wire harnesses surrounding areas of frequent maintenance also benefit from abrasion resistant harness protection as these harnesses are subject to potential abrasion each time a nearby system is maintenanced.
AIR 6808 also urges the consideration of cost-benefit in EWIS component selection. Highly specialized components can create increased opportunities for poorly executed maintenance and ultimately become a detriment to the EWIS and surrounding systems.
EN3197 Commonality
The EN3197 emphasizes commonality among EWIS components, discouraging an excess in variety and complexity. A greater variety of components requires a greater variety of specialized maintenance tools and expertise; this can very quickly reduce the maintainability of a system. Commonality among components reduces the need for specialized training and diverse complex maintenance.
The EN3197 also emphasizes the consideration of EWIS maintenance and maintainability in component selection. To quote directly from the standard:
“All cabling should be accessible, repairable and/or replaceable at the maintenance level specified by the design authority.”
Ease of disassembly, reassembly, and modification greatly increase the maintainability of aircraft systems, including EWIS.
Conclusion
Regulations and standards provide useful and imperative guidelines for designing aircraft EWIS to extend service life by increasing maintainability. Even with the best design, progressive wear on the system will result in the need for repair/replacement; an EWIS that is well marked, easy to access, and uses common components will make the task easier and reduce the cost of the maintenance action. If you are considering an evaluation of the EWIS in your aircraft, Lectromec is here to help. Contact us today to explore EWIS maintenance planning and data review options.
Laura Wishart
Engineer, Lectromec
laura.wishart@lectromec.comLaura has been with Lectromec since 2019 and has been a key contributor on projects involving testing of EWIS/fuel system failure modes, the impact of poor installation practices on EWIS longevity, and wire/cable certification testing. Her knowledge and attention to detail ensure consistent delivery of accurate test results from Lectromec’s lab.