Hazardous wiring practices are vividly illustrated in a guide for safe wiring practices. The document, known as Job Aid 1.0, is the outgrowth of industry wiring practices under examination by the Aging Transport Systems Rulemaking Advisory Committee (ATSRAC).

ATSRAC is a government-industry task force formed in the wake of the 1996 TWA Flight 800 and the 1998 Swissair Flight 111 disasters. Poor wiring practices were uncovered by investigators in the wake of these accidents, and subsequent inspections of retired airliners showed that wiring problems were more pervasive than previously thought.

As a consequence, ATSRAC has recommended a comprehensive enhanced zonal analysis program (EZAP) to assure the safety of aircraft wiring on a continuing basis.

In the meantime, Job Aid 1.0 is perhaps the most notable output of the ATSRAC activity thus far. Produced by two Federal Aviation Administration (FAA) wiring experts, it shows in clear, visual form wiring practices to be avoided, and those to be emulated. This document is of direct utility to operators and repair stations, as it distills into one document guidance on wiring practices that is spread among numerous other documents, including federal aviation regulations (FARs), advisory circulars (ACs), airworthiness directives (ADs), wiring-related recommendations from the National Transportation Safety Board (NTSB), wiring-related research papers produced by the FAA's Technical Center, the 1996 "Gore Commission" on aviation safety and security, and other materials produced by manufacturers. As the Job Aid said, "For now, there is no one rule or AC that ties everything together."

For those in need of a single source of information on how best to ensure the safety of aircraft wiring systems, Job Aid 1.0 provides about 90 percent of the "immediate need" type information in a single PowerPoint presentation that can be reviewed in about an hour's time.

In fact, some of the extant guidance is contradictory. The Job Aid refers to advisory AC 43.13-1b, which deals with acceptable methods and practices for aircraft inspection and repair. Chapter 11 of this document deals with wiring practices yet contains internal inconsistencies. For example, paragraph 11-48 says, "A circuit breaker must always open before any component downstream can overheat and generate smoke."

But in a later paragraph (11-51) the AC cautions that breakers are designed such that they do not protect components or line replaceable units (LRUs): "Circuit breakers are designed as circuit protection for the wire, not for protection of black boxes or components."

The Job Aid explains, "In reality, breakers are sized to protect the aircraft wiring as the main design constraint. Any further protection of components or LRUs is desirable but not mandatory." (Emphasis in original)

Moreover, the Job Aid adds, "Thermal circuit breakers do not always detect against arcing events."

Furthermore, the Job Aid says, "Circuit breaker [CB] failures are, for the most part, latent in nature. So you won't know they have failed until you need them."

Consider the implications of this statement. Extrapolate the Job Aid statement by the large number of CBs in an airplane (not all visible on CB panels, some hidden away in nooks and crannies or in modular components themselves) and one can see that by design the airplane will contain what might be called some "fused bombs," in the form of CBs that just will not trip when and if required. By definition, the consequences of such functional failure will be circuit overload, and smoke/fire/system failure.

As a corollary to this point (and because of the reality), perhaps modular components should always be designed with an internal current limiting device that always will trip in the event of a CB failure, and well before the system's power-supply wiring becomes heat-stressed.

The Job Aid outlines the numerous causes of wiring degradation, to include: vibration, moisture, improper maintenance (drill shavings contaminating wire bundles), chemical contamination (e.g., solvents), exposure to heat (which can accelerate degradation, insulation dryness and cracking), and installation (e.g., wire installed under tension). Perhaps not sufficiently stressed is the hygroscopic nature of polyimide insulations, and their affinity for moisture which, over time, can make the insulation more conductive (the very opposite of the insulation's purpose).

Wherever vibration is mention in the Job Aid, it is in reference to "high vibration." However, the most damaging form of vibration may be the "high frequency low amplitude" buzz that is characteristic even of ultra-smooth flight. It is this sort of vibration that can set up the strumming in even well supported, clamped wiring, that causes chafing.

As one important means of preventing such chafing, the Job Aid warns against installations where wires are riding on other wires. This problem is not unique to aviation, as shown by the power wiring observed in a neighborhood near our Access Intelligence offices.

The Job Aid covers many of the basic precepts of good wiring husbandry (routing, clamping, splicing, replacing, etc.). About 15 major topics are covered. We address a few here, distilled into handy "rules."

• Rule #1: Prevent chafing. Wire should not ride on structure, lines or cables. More specifically, the wire should be routed in a manner intended to eliminate the potential for chafing against structure or other components.

• Rule #2: Secure unused wires. Unused wires, either left open- ended due to removal of a component, or included in anticipation of future modifications, should be secured the same way as the wiring.

• Rule 3: Clamping is critical. Analogous to holding a bird in one's hand, too loosely and the bird flies away or too tightly and the bird is crushed, clamping must be neither too loose nor too tight. If too loose, the wire can slide back and forth, leading to chafing, or droop to contact structure. If too tight, the wire can be crushed. Clamps should spaced no further than 24 inches apart, and closer in areas of high vibration, according to the Job Aid. The clamp should be oriented 90� to the wires it is supporting, and the clamp cushions should not cut into the wires.

• Rule #4: Wire should be installed with appropriate strain relief to minimize premature degradation from mechanical stress. In addition, wire lengths should be sufficient to allow for at least two reterminations (i.e., reconnections) during the life of the airplane.

• Rule #5: Terminals must be tight. After the terminal is completely assembled, a minimum of two or three threads should be showing on the stud when the nut is properly torqued. According to the Job Aid, "Service history has shown that hardware stack up at the terminals is prone to human error. Omission of lock washers, incorrect washers, improper sizing of washers, etc., has been a definite problem."

• Rule #6: Lock to prevent loosening. In locations where vibration is endemic, it may be necessary to provide a locking device to keep connectors from loosening.

Overall, the Job Aid provides a readily accessible answer to the question about wire husbandry: "How do you know what you're supposed to do?" It is a Cliffs Notes guide, readily accessible and useful enough to avoid committing harm through ignorance.

Since the Job Aid is intended to foster good maintenance practices, it does not dwell at great length on wiring system design criteria. That having been said, the document does not mention one important aspect of wire routing: given a choice, it is preferable to run wires and bundles horizontally rather than vertically. Under arcing conditions, the damage can be spread further in a vertically oriented bundle. Since the heat of electrical arcing rises, the arc tends to follow the hot gas upwards in what is described as a "chimney effect" (see ASW, April 10, 2000).

The full Job Aid, a 164-slide PowerPoint presentation, may be viewed at