EWIS regulatory compliance with 25.1707, Part VI

This is the sixth article in the series focused on FAA 25.1707 that discusses electrical wire interconnection system (EWIS). The previous articles in this series are listed below:

• 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
• Feb 21, 2014 Part V: Component Separation Guidance

This article considers the eighth section of the system separation requirement that focuses on EWIS separation from control cable. Section ‘i’ of 25.1707 states:

(i) EWIS must be designed and installed with adequate physical separation between the EWIS and flight or other mechanical control systems cables and associated system components, so that: (1) Chafing, jamming, or other interference are prevented. (2) An EWIS component failure will not create a hazardous condition. (3) Failure of any flight or other mechanical control systems cables or systems components will not damage the EWIS and create a hazardous condition.

The reason that this section was not included in the EWIS Regulatory Compliance with 25.1707–Part V article, is that it poses a separate set of challenges. Whereas sections e, f, g, and h focus on EWIS proximity and protection regarding fluid/oxygen lines, this section considers the proximity to mechanical cables.

Clearly moving cables can create a hazardous situation with EWIS as they can quickly abrade through the protective insulation. The obvious failure conditions here include:

• Electrical arcing damaging or destroying all wires in the cable
• Damage and loss of functionality of a flight control cable
• Damage to nearby systems due to arc plume or ejected molten materials

One of the terms popular within the EWIS communities is a critical clamp marker (see the figure below). These markers on harnesses indicate where a clamp should be placed on the harness. They provide a quick visual reference to a maintenance technician if there is a misalignment of a harness or a harness has slid from its designed location.

While not necessary, it is recommended that critical clamp markers be added to EWIS harnesses in areas with mechanical cables. This will ease inspections and ensure EWIS components moved during maintenance are reinstalled with the proper clearance.

An additional issue to keep in mind regarding mechanical cables is the jamming of mechanical components, such as a harness jamming a pulley. As such, areas routed near cables should have sufficient clamping to prevent this from occurring. In particular, the clamping and EWIS harness support must be designed to prevent a hazardous condition from occurring with the loss of a single clamp. Inspection of these items should also be included in ICA/EZAP.

The failure of a control cable would adversely affect any EWIS component and create a hazardous condition. Mechanical cable routing and a protection mechanism should be considered in these areas.

CCA must be done to ensure that the loss of a mechanical cable will not damage an EWIS component that supports a secondary/redundant system.

The next and final article in the series will cover the remaining sections of the 25.1707 regulation.