EMI Compliance under FAA 25.1707(b)
FAA 25.1707(b) requires EWIS to be installed so that any electrical interference likely to be present in the airplane cannot produce hazardous effects. Per AC 25.1701-1, designers must consider bus-bar noise, inter-cable coupling, parasitic currents, lightning, static discharge, and dissimilar generator frequencies.
“Each EWIS must be designed and installed so that any electrical interference likely to be present in the airplane will not result in hazardous effects upon the airplane or its systems.”
The advisory material in AC 25.1701-1 identifies eight EMI sources for assessment: bus-bar noise, cable-to-cable coupling, cable-to-aerial coupling, equipment operating out of spec, parasitic eddy currents and voltages in the EWIS and grounding systems, lightning currents, static discharge, and dissimilar frequencies between the generation system and other systems.
Mitigation options include EMI/EMP filter-protection connectors, separating power and signal wires through different connectors, shielded constructions sized to the signal's frequency content, harness-level shielding schemes with appropriately grounded shields, and physical separation of power harnesses from signal harnesses where space permits.
Systems necessary for continued safe flight, landing, and egress should be given the highest priority when assessing EMI impact. A representative compliant example places a signal harness within two inches of a power harness with shielded construction shown — through measurement — to be sufficient to keep interference below the threshold of effect on the protected circuits.
EMI compliance is failed quietly. The harness functions on the ground, the airplane flies, and the issue only surfaces as intermittent flight-control behavior under specific generator loadings. Treating 25.1707(b) as a measured, signed assessment — not a checklist of mitigations applied — is what separates a defensible cert package from one that gets challenged in service.
Frequently asked
What is the minimum separation distance between power and signal harnesses under 25.1707(b)?
There is no minimum distance fixed by the regulation. Lectromec's reference example uses a 2.0″ minimum, validated by interference measurement against the protected circuit. Closer routing is permitted when mitigations such as shielded cable or filter connectors are shown to keep interference below the threshold of effect.
Which EMI sources must be considered under FAA 25.1707(b)?
Per AC 25.1701-1: bus-bar noise from connected equipment, cable cross-talk, cable-to-aerial coupling, equipment operating out of spec, parasitic eddy currents and voltages in EWIS and grounding, lightning effects, static discharge, and dissimilar frequencies between the generation system and other airplane systems.