When it comes to aircraft wire maintenance, consider the basic rule in medicine: “First, do no harm.” But where to find the best guidance to avoid inadvertent maintenance malpractice? Fortunately, for those in need of a quick reference, the Federal Aviation Administration (FAA) has produced a single, well-illustrated document known as the Aircraft Wiring Practices Job Aid 1.0. It presents essential examples of good, bad and outright dangerous wiring practices.
Job Aid 1.0 is an outgrowth of the need for better training in wiring maintenance identified by the Aging Transport Systems Rulemaking Advisory Committee (ATSRAC). The inspections done by the members of this government-industry task force showed that wire ages in service, it can be abused during maintenance, and that the problems fall into clear categories of improper installation, aging or traumatic damage. Job Aid 1.0 offers cautionary cases of what to avoid while illustrating “best practices” for prudent and proactive wiring maintenance, modification or modernization.
1. Thou shalt not consider wire as immortal
At least a half-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 of any of the materials produced to date. Moreover, it points out conflicting information that can mislead the layman. For example, one AC says the circuit breaker must protect against any downstream component failure. But this same AC also says that breakers are designed such that they DO NOT protect components. Job Aid 1.0 clarifies the matter: “Breakers are sized to protect the aircraft wiring as the main design constraint.” Job Aid 1.0 also notes that most circuit breaker (CB) failures are latent in nature, “So you won’t know they have failed until you need them.”
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. Above all, the doctrine of three applies: 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, spars, etc.
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.
In high vibration areas, or locations where the wiring must be routed around structural intrusions, the clamping intervals should be reduced. To minimize strain at the terminal, some slack should be allowed between the last clamp and the termination of a wire run.
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 wire won’t move when a light tug is applied.
6. Thou shalt not adulterate bend radii
Insulation is put under strain when a wire bent at too sharp a radius, which can lead to accelerated topcoat flaking and breaches in the insulation. For a bundle of wires, the minimum bend radius must not be less than 10 times the outside diameter of the largest wire in the bundle.
7. Thou shalt not bear false witness to unsafe wiring
In other words, there are times when the wiring’s condition cannot be overlooked or ignored and replacement is in order. Here are some of the conditions in which wire replacement is in order: it’s chafed or frayed; the insulation has been penetrated to conductor; 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. In these areas, even well supported, clamped and semi-tautened wiring may experience accelerated wear (e.g., chafing).
If you are interested in wire maintenance you may want to read Lectromec’s 7 Tenets of Wire Systems Installation article.
9. Thou shalt covet true terminals
The beginnings and endings of wiring are as important as the routing and support in between. Connectors and terminals in aircraft require special attention to ensure a safe installation. 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 heat 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 throughout the life of the aircraft.
Job Aid 1.0 was produced 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.