Editor’s Notebook

Wiring Maintenance

When it comes to electrical wiring maintenance, consider the basic rule in medicine: "First, do no harm." Perhaps the best guidance for avoiding inadvertent maintenance malpractice is a Federal Aviation Administration (FAA) document known as the Aircraft Wiring Practices Job Aid 1.0. It presents good, bad and outright dangerous wiring practices.

Job Aid 1.0 offers cautionary cases of what to avoid while illustrating "best practices" for prudent and proactive wiring maintenance and modification.

Job Aid 1.0 is in the form of a heavily illustrated PowerPoint presentation of some 164 slides. In the interests of brevity, we highlight here key elements that were summarized in one page by the Lectromechanical Design Company of Dulles, Virginia. The company specializes in wiring integrity and aging issues for the aerospace industry. Good husbandry of wiring complements the FAA's major initiative to inspect wiring (see story page 24). Lectromec's summary of Job Aid 1.0 takes the form of basic commandments:

1. Thou shalt not consider wire as immortal. At least half a dozen agents of aging are involved: degraded wire repairs or splices, heat-damaged or burnt wire, vibration damage or chafing, cracked insulation, arcing, and delaminated insulation.

2. Thou shalt hold Job Aid 1.0 as the first guidance before thee. Job Aid 1.0 warns that there are many texts: "For now, there is no one rule or AC [advisory circular] that ties everything together." But Job Aid 1.0 may come the closest to a single source document.

3. Thou shalt not take proven wiring routing practices in vain. The overarching guidance in Job Aid 1.0 is to eliminate the potential for chafing against structure or other components: wires should not ride on structure, wires should not ride on other wires, and wires should not ride on weight-reducing lightening holes cut into bulkheads, ribs and spars.

4. Thou shalt honor clamping criteria. Supporting the wiring with proper clamping can minimize the deleterious effects of vibration and mechanical strain. As a general rule, support clamps should be spaced at least every 24 inches, and closer in high vibration areas, or locations where the wiring must be routed around structural intrusions.

5. Thou shalt not crimp or crush wire. The operative rule-of-thumb is that when the wire is properly clamped, the fitting will be snug enough to prevent the wiring from sliding freely through the clamp, but not so tightly clamped that the wiring won't move when a light tug is applied.

6. Thou shalt not adulterate bend radii. Insulation is put under strain when a wire is bent at too sharp a radius, which can lead to accelerated topcoat flaking and breaches in the insulation.

7. Thou shalt not bear false witness to unsafe wiring. There are times when the wiring's condition cannot be overlooked or ignored, and replacement is in order: if it's chafed or frayed; the insulation is so brittle that slight flexing causes it to crack; discoloration shows that the insulation has been heat-damaged; sections of the wire have been spliced at less than 10-foot intervals.

8. Thou shalt not splice unless necessary. Splicing of wire should be kept to a minimum and avoided entirely in locations subject to high vibrations.

9. Thou shalt covet true terminals. Electrical system malfunctions frequently have been traced to poor terminal connections. For example, loose contacts can produce localized heating that may ignite nearby combustible materials or inflict damage on the insulation of adjacent wiring.

10. Thou shalt clean as thou goest. If "cleanliness is next to Godliness" for the human body, cleanliness of aircraft wiring is essential for safety. Job Aid 1.0 urges a basic standard: keep wiring clean though the life of the aircraft.

Job Aid 1.0 was produced, as it says, to "preclude accidents that may result from wire degradation." In this respect, Job Aid 1.0 exemplifies a basic precept--the devil is in the details. (Job Aid 1.0 may be found at www.academy.jccbi.gov/AIRDL/wiringcourse).