In aircraft wire systems, how much slack should I give a wire?

Clearly wires shouldn’t be pulled so tight that they are under constant regular strain, but is there some guidance as to how much slack should be provided in aircraft wire systems? One source for guidance comes from the FAA in AC 25.1701. In this advisory circular, the guidance suggests that “EWIS components should have sufficient mechanical strength for their service conditions” and provides design recommendations to maintain the in-service system reliability.

The first recommendation is for wires connected to terminal lugs. The recommendation suggests that wires should “have sufficient slack to allow for two reterminations without replacement of wires.”

Next, there should be sufficient slack in the wiring to allow for a drip loop. A ‘drip loop’ is a short section of the harness immediately before the connection point. The drip loop is designed to prevent fluid ingress into the connector. An example of a drip loop is shown in the following figure.

This prevents any fluid contamination on the harness from interfering with the connections and cause shorting between circuits. Also, the use of drip loops provides additional slack on the harness limiting the stress on connectors.

An article related to this topic is Lectromec’s The 7 Tenets of Wire Systems Installation.

If the harness is in a zone or application where there are movable parts (e.g. doors, landing gear, etc.), the harness should “have sufficient length to allow for full travel without tension.” This may also require that additional clamp points be created to prevent the additional slack from becoming pinched when the equipment is retracted or closed.

In summary, there should be sufficient slack on the wire harness to serve the aircraft systems, not be placed under continuous mechanical stress, and provide additional length to allow for drip loops and retermination.