For several decades now, the AS50881 (formerly the MIL-STD-5088) has been a fundamental knowledge base used for the installation of aircraft wiring systems. The requirements laid out in this standard cover the electrical wiring interconnection system (EWIS) design and recommendations on the selection of qualified parts. Additionally, the design must be verified to show compliance. For commercial aircraft, this may be done as part of the overall certification package. For the military, this may be part of design acceptance.
To get to EWIS certification, there are some areas of the AS50881 that require data supporting part selection and engineering decisions. Some of this data must be gathered through performance verification and/or lab testing. This article covers some of the tests that are necessary for showing compliance with the standard’s data gathering requirement.
Qualification of Parts
All parts that go on an aircraft must be qualified. To circumvent the process or to accept parts from untrusted third parties can have significant long-term impacts on aircraft reliability. The AS50881 Section 3.3 specified that any nonstandard part used for the EWIS, “must be equivalent to or better than qualified parts.” What does that mean to the implementer? The parts that are used in the EWIS (see what is included in the EWIS), must meet or exceed the existing standards for comparable equipment with the goal to not use non-compliance parts. See this list for an example of tests on wire/cables.
Each aircraft has a specified service life and the installed equipment must either be identified as a life limited part or last the entire service life. To determine the long-term reliability, accelerated aging may be necessary to show reliable performance data through the expected life of each component. This accelerated aging should be structured to be a more severe representation of the in-service conditions.
AS50881 specifies that epoxy adhesives are acceptable for use if they can be proven to be fully functional at maximum rated temperatures. So what does this mean for testing? It depends on the location within the aircraft. Consideration should be given for which zones the adhesive will be placed; it would not be wise to place an adhesive bond in moisture or fuel exposed zones if no verification had been performed. Such an assessment might include prolonged fuel/environmental condition exposure after which pull tests are performed. These tests may be done as focused tests or as part of DO-160 testing.
For EWIS routed in or near fuel tanks, special precautions are necessary and are thoroughly discussed in SFAR88 and FAA regulation 25.981 for fuel tank ignition prevention. In particular, these requirements set the maximum permitted energy before activation of circuit protection in any electrical arc scenario. The FAA AC 25.981-1C states the following:
“Laboratory testing has shown that the minimum ignition energy in an electrical spark required to ignite hydrocarbon fuel vapor is 200 microjoules. Therefore, for electrical or electronic systems that introduce electrical energy into fuel tanks, such as fuel quantity indicating systems, any electrical arcs or sparks that are created into any fuel tank should be less than 200 microjoules during either normal operation or operation with failures.”
Circuits and circuit protections must be evaluated that electrical sparks and electrical arcs are below this energy.
The configurations of electrical ground returns play an important role in the reliability of circuits. Often the electrical power source grounds are connected to the primary structure, but this can be difficult in composite structures or alternative structure designs. Whatever the structure may be, the electrical ground connections should meet the EMI requirements of MIL-STD-464 (Electromagnetic Environmental Effects Requirements for Systems). This may include contact resistance or bonding measurements.
Understanding and mitigating the physical impact of EWIS failure has been added to recent versions of the AS50881. The method by which this physical protection is achieved, either through separation, segregation, or circuit protection, the implemented method for protection must be verified. The specific requirements for this will vary depending on the certifying authority, but the basis may be simplified as an EWIS failure, regardless or probability should not impact the aircraft airworthiness. Lectromec has written on this topic:
- Seven part series on FAA regulation 25.1707
- Arc damage analysis
- Perspectives on arc fault protection
In particular, it is necessary for the user to show performance data of EWIS components if used at voltages above 115VAC and 28VDC. This is necessary because the long-term exposure of higher voltages to EWIS components has not been thoroughly examined and must be assessed. Within many of the standards groups, efforts are being made to develop test methods and quantify the long-term impact.
The AS50881 standard includes many of the hard lessons learned from aircraft EWIS design. Some of the lessons learned apply to the design level and some place a need for performance verification that requires testing and assessment. There are more requirements that exist on the designer and maintainer than are defined here, but this should provide a starting point for understanding the standard. For those in need of laboratory testing to generate data for AS50881 compliance, contact Lectromec.