Each of the last four articles has focused on a subpart of the FAA 25.1707 regulation.
- Jan 20, 2014 Part I: Damage assessment for 25.1707
- Jan 28, 2014 Part II : EMI compliance in 25.1707
- Feb 7, 2014 Part III: Heavy Current Cables in 25.1707
- Feb 15, 2014 Part IV: Grounding and Power Separation
Because subparts e, f, g, and h are very similar, they will all be addressed here. Rather than present each of these regulations, we have combined them into one:
Combined revision of FAA 25.1707 subparts e, f, g, and h: Except to the extent necessary to provide electrical connection to the fuel/hydraulic/oxygen/water/waste systems components, the EWIS must be designed and installed with adequate physical separation from fuel/hydraulic/oxygen/water/waste lines and other fuel/hydraulic/oxygen/water/waste system components, so that: (1) An EWIS component failure will not create a hazardous condition. (2) Any fluid leakage onto EWIS components will not create a hazardous condition.
To understand the consequences of EWIS component failure, we only have to look back at the loss of an US Air Force F-22 in November 2012.
So, what does this mean from a certification perspective?
1) You must have safe separation distances defined for all of your fluid carrying system components. Not all tubes are going to fail under the same conditions. Below are a couple of articles on this topic:
- Regulatory Compliance with 25.1707 – Part I
- Presentation on failure of pressurized hydraulic lines
- Arc Damage Modeling Tool (ADMT) failure modeling
2) EWIS components should be installed, so that they are above the fluid/oxygen carrying components; this will help to minimize the likelihood of fluid contamination of the EWIS.
In the figure below, there are three wire harnesses routed near a fuel line:
- Harness #1 is routed beneath the fuel line, but is protected by a shroud. While this can be used to protect a harness from fluid exposure, it does add to weight and potentially inspection procedures for maintaining airworthiness.
- Harness #2 is routed above and perpendicular to the fuel line, but the supporting clamps are too far apart to prevent sagging and chaffing on the tube. Additional clamping could be installed to resolve this issue.
- Harness #3 is routed horizontally, parallel to the line, which is of no issue, but in the presented system, the separation distance is less than the allowed distance as defined by a damage assessment. Redesign, separation, or protective sleeving must be considered for harness #3.
This part of the 25.1707 regulation should also be considered in coordination with the recommended practices of AS50881 section 3.11.11 “Gas and Fluid Carrying Lines and Tubes.” Whereas the standard suggests that wiring should be routed with, “the maximum practicable separation from all fluid carrying lines,” this regulation allows for wiring to be installed in close proximity, as long as the failure consequences have been considered.
The support clamps for separation between the EWIS and fluid/oxygen lines, should be given additional consideration. Recommendation from AS50881 suggests that supporting clamps for EWIS should not be connected to fluid/oxygen components unless the separation is less than two inches.
If you are familiar with the recommendations of AS50881, then you are aware that the recommended standard practice is that wiring should, “… be installed to maintain positive separation [from gas and fluid carrying lines and tubes] of at least 0.500 inch.” First, while this separation distance may be fine for some system configurations, it is not an acceptable blanket statement for separation. Lectromec has done testing showing that tube failure can occur at an even greater distance from the arc plume generated during a wire failure event.
Second, conformity to the regulation takes precedent over industry guidance. Certification relies on data to verify the safety of design, and the recommendations of AS50881 do not provide sufficient data to support certification.
The next article in this series will cover Section ‘i’ of the 25.1707 regulations – separation with mechanical control systems.
For the past three decades, Lectromec has worked with federal agencies to make sure aircraft wiring is safe and reliable. Lectromec is responsible for testing, inspecting, and analyzing a wide range of electrical wire interconnect systems. For a white paper on “Addressing Fleet Management Needs” complete the form to the right.