EWIS Separation from Fluid Lines under FAA 25.1707(e)–(h)
Subparts (e)–(h) of FAA 25.1707 require EWIS to be adequately separated from fuel, hydraulic, oxygen, water, and waste lines and components, so neither an EWIS failure nor fluid leakage onto EWIS can create a hazardous condition. AS50881 §3.11.11 recommends ≥0.500″ separation, but Lectromec testing has shown tube failure at greater distances.
“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.”
Two failure directions matter. EWIS-to-line: an arc or component failure damages a nearby line and creates fire or fluid-loss hazard. Line-to-EWIS: a leak contaminates harness or connector hardware and creates either an electrical or corrosion-driven hazard. Both must be assessed.
The consequence side of this regulation is not hypothetical. The USAF investigation of a November 2012 Lockheed Martin F-22 crash near a Florida runway attributed the loss to a chafed power feeder cable, with total cost approaching $150 million. The same failure mode is the design driver for 25.1707(e)–(h).
Two specific design rules follow. (1) Safe separation distances must be defined for every fluid-carrying system component; tubes of different materials, pressures, and wall thicknesses do not fail under the same conditions. (2) EWIS should be routed above fluid- and oxygen-carrying components wherever practicable, to minimize fluid contamination of the harness under leak conditions.
Industry guidance must be reconciled with the regulation. AS50881 §3.11.11 recommends maintaining positive separation of at least 0.500″ from gas and fluid lines and recommends that EWIS support clamps not be attached to fluid or oxygen components unless separation is less than two inches. Lectromec arc-plume testing has shown tube failure can occur at distances exceeding the AS50881 default, meaning the 0.500″ figure is not an adequate blanket statement and a damage assessment can supersede it. Conformity to the regulation takes precedence over industry guidance, and certification depends on supporting data.
Citing AS50881's 0.500″ as the basis for fluid-line separation is one of the most common ways to write a finding into a cert package. The regulation does not adopt that number, and Lectromec testing shows it is sometimes insufficient. A damage assessment for the actual tube material, pressure, and proximity is the defensible record.
Frequently asked
Is the AS50881 0.5-inch separation enough for FAA 25.1707 compliance?
Not always. The 0.500″ recommendation in AS50881 §3.11.11 is industry guidance, not the regulatory text. Lectromec arc-plume testing has shown tube failures at greater separations. Conformity to the regulation requires a damage assessment for the actual tube material and configuration; certification depends on that supporting data.
Should aircraft wiring be routed above or below fluid lines?
Aerospace standard practice — and the design preference under 25.1707(e)–(h) — is to route wire harnesses above fluid lines whenever practical, so that a fluid leak below cannot contaminate the EWIS.
Can a protective shroud substitute for separation distance?
Yes, with a caveat. A shroud or sleeve can protect a harness routed beneath a fluid line, but the resulting weight and the inspection burden during continued airworthiness must be included in the certification package and the Instructions for Continued Airworthiness.