Electrical Wiring Interconnect Systems

Planning is the Key to Meeting the Mandate

The electrical wiring interconnect systems (EWIS) requirement is mandated in U.S. Federal Aviation Regulations (FAR) Part 25, Subpart H as a precursor to type certification for all transport category aircraft. The mandate is far reaching and open to some interpretation. From the perspective of the company attempting to comply with the mandate, a re-evaluation of the current program should be the starting point. Then, an EWIS strategy must be defined and instituted. The key to meeting the mandate in a cost-effective fashion is planning so that appropriate processes and tools can be in place.

History

Until recently, wiring was not thought to be a significant safety risk on passenger jets. Then, in the span of just over two years, two major crashes claimed the lives of all on board the two aircraft. Both crashes were, in part, blamed on arcing of electrical wiring which subsequently caused an onboard fire.

The first fatal crash occurred on July 17, 1996. The plane departed JFK airport in New York. Just 12 minutes later, the Boeing 747 exploded and crashed, killing 230 passengers and crew. The NTSB listed the cause of the accident as:

The National Transportation Safety Board determines that the probable cause of the TWA flight 800 accident was an explosion of the center wing fuel tank (CWT), resulting from ignition of the flammable fuel/air mixture in the tank. The source of ignition energy for the explosion could not be determined with certainty, but, of the sources evaluated by the investigation, the most likely was a short circuit outside of the CWT that allowed excessive voltage to enter it through electrical wiring associated with the fuel quantity indication system.¹

Just over two years later on September 2, 1998 a tragically similar accident took the lives of another 229 people. Swissair flight 111 also left New York’s JFK airport, bound for Geneva, Switzerland. Instead of an explosion, the MD-11 experienced a fire that quickly spread, disabling avionics and eventually causing the crash. The Transportation Safety Board of Canada determined the crash was a result of “in-flight fire leading to electrical failure, spatial disorientation and crew distraction.²

The findings that part of the wiring system was the ignition source for both accidents led to an extensive investigation of wiring systems and, in late 2007, the release of FAR 25. Per § 25.1701(a), an EWIS is:

Any wire, wiring device, or combination of these, including termination devices, installed in any area of the airplane for the purpose of transmitting electrical energy between two or more intended termination points.³

EWIS Compliance

While the FARs define EWIS in a single sentence, the actual breadth and ramifications of EWIS are more substantial. In particular, EWIS applies to:

• Wires, cables, and bus bars
• Termination points on electrical devices
• Connectors, and connector accessories
• Electrical grounding and bonding devices and their connections
• Electrical splices
• Wire protection materials, including insulation, sleeving, and conduits
• Shields or braids
• Clamps and other devices used to route and support the wire bundle
• Cable tie devices
• Labels or other means of identification
• Pressure seals
• EWIS components inside shelves, panels, racks, junction boxes, etc.

Because of the reach of the mandate up the supply chain, the mandate has forced companies to reevaluate and improve their internal business processes as well as their supplier relationships. Without planning, the cost impacts to the company could be substantial. There is also substantial risk of a program delay. Finally, the EWIS mandate can indirectly present a challenge with knowledge retention.

Prior to EWIS, engineers and designers used due diligence to ensure that the components used in the aircraft design fully met all specifications. Now, the EWIS mandate requires a more structured and rigid decision process, and must now include considerations such as component design limitations, functionality, and susceptibility to arc tracking and moisture. Within the context of the mandate, consistency of decision making is of utmost importance — a part that one designer selects as being compliant should be the same part that another designer would select given the same set of design inputs.

In addition to the challenge of selection of appropriate parts, the factors and reasons for the decision must be documented, traceable, and detailed enough to pass the EWIS certification process. But the requirement goes even further than the selection of parts and the parts themselves. FAR §25.1711 requires consistent methods of distinguishing EWIS components, allowing easy identification of the component, its function and its design limitations. Identification of the EWIS objects is specified in detail. Most important, the identification must provide uniqueness in the context of the aircraft configuration and also convey important information about its role, EMC segregation, and other information.

Given the number of designers involved with a commercial aircraft design as well as the volume of EWIS components, it’s quite obvious that managing just the identification requirement can be quite a challenge. Manual methods that have worked well in the past are not only inadequate to meet the mandate, but can require a significant amount of time, which could delay the entire program.

Automation Lowers Costs and Increases Quality

A significant part of the EWIS compliance plan should be automation to make the myriad of tasks more manageable, more thorough, and most importantly to ensure complete compliance for a safer aircraft. Modern automation tools are clear and easy-to-use, making the EWIS compliance task easier to manage. These tools also present a very structured design environment that creates procedures that ensure requirement satisfaction.

Figure 1 illustrates how automation tools can be used to validate part selection early in the process to eliminate errors before any hardware is constructed. (Click here for large version in a new window)

From the standpoint of failure analysis, automation can streamline the process and improve overall aircraft safety — which is of paramount importance in commercial aircraft design, as illustrated in Figure 2 (Click here for large version in a new window). FAR §25.1709 leverages FAR §25.1309 to provide a thorough and structured analysis of aircraft wiring and its associated components.

For example, even for systems covered by EWIS, the safety analysis requirements have not always been applied to the associated components. The integrated nature of EWIS must also consider internal factors like a single wire arcing or chafing event. So the failure modes of the EWIS components must be examined at the airplane or platform level to determine whether the failures could have an impact on the safe operation of the aircraft. The impact of §25.1709 is that the time necessary to conduct the safety assessment has the potential to increase. Couple that with the trend in newer aircraft to become “more electric” and it can be seen that the potential program time impact to ensure that a sufficient safety analysis is performed can only increase.

With those “more electric” airplanes come other issues that can drive up costs if the company is not prepared to deal with them. With EWIS, the challenges of system separation become more acute. FAR §25.1707 looks at system separation and details what must be maintained to ensure that a hazardous event impacting one part of the EWIS doesn’t cause catastrophic failure of nearby EWIS components. In addition, EMI and high intensity radiated field (HIRF) effects need to be considered.

Platform-level Design Methodology

With the EWIS mandates, engineers must take a more holistic, or platform-based approach to the wiring systems development process. Many tools provide a design-centric view of data but a platform-based approach means the designer can and should evaluate design decisions in the context of the entire platform.

Until recently data was evaluated separate from the design tools and took a great deal of time to organize,

evaluate and report. This made evaluating a design rapidly enough to make corrections early on and thus saving costs very difficult.

Platform-level assessment tools can evaluate all electrical wiring systems from a platform context. An example is shown in Figure 3: on the right is a topological representation together with an EWIS wire routing map. The primary wires, shown on the left, are selected and wire routes highlighted in red dashed line font, showing the paths in which the wires travel. (Click here for large version in a new window)

The EWIS routing rules were captured in the application and used to automatically drive the signal and wire routing. The same solution would have resulted regardless of which engineer instructed the system to route the wires. Also shown is an EMC category graphic that depicts the various types of signal separation running in each bundle, so that the engineer can get a holistic, platform-centric view of the EWIS. These types of tools can quickly help engineers understand whether their EWIS separation objectives have been met and support many types of virtual verification.

Conclusion

The business challenges from the EWIS requirements mean that companies must investigate new approaches and processes while keeping costs and program risks at bay. Without a doubt, the only way to effectively meet the requirements and keep costs under control is to leverage automation in the context of a structured design process as much as possible. The platform-level approach, which considers the entire aircraft platform, not the isolated wiring systems, provides the best analysis in the least time, and also provides analysis quickly enough to act early on discovered issues. These innovative tools and approaches provide the technology for companies to meet business goals while complying with federal aviation requirements.

References

1. National Transportation Safety Board Aircraft Accident Report, In-flight Breakup Over the Atlantic Ocean Trans World Airlines Flight 800 Boeing 747-131, N93119 Near East Moriches, New York July 17, 1996, August 23, 2000.
2. Transportation Safety Board of Canada, Aviation Investigation Report: In-Flight Fire Leading to Collision with Water Swissair Transport Limited McDonnell Douglas MD-11 HB-IWF Peggy’s Cove, Nova Scotia 5 nm SW, 2 September 1998, Report Number A98H0003.
3. Federal Aviation Regulations, FAR § 25.1701(a).

John Low is worldwide aerospace programs manager for Mentor Graphics’ Integrated Electrical Systems Division. John has been actively involved in the aerospace industry for over 30 years. He started with design and manufacturing engineering responsibilities on rocket propulsion systems followed by work with The Boeing Company in a variety of design engineering roles on commercial and military applications. Following that, John was a consultant for IBM’s Aerospace Solutions team focusing on aircraft systems for the business aviation sector. John has published multiple articles ranging from the need for COTS software tools to focus on the specific challenges of EWIS development through to technical articles that focus on the electrical systems development process. He can be reached at john_low@mentor.com.