Scope of Wiring Safety Program Debated by Task Force

There is a limit to the cost of safety. As a prime example, consider the deliberations earlier this month of a government-industry task force on wiring system safety.

At the July 7 meeting of the Aging Transport Systems Rulemaking Advisory Committee (ATSRAC), the challenge of how to proceed while keeping costs under control was vigorously discussed.

Having developed the concept of wiring as a system - the electrical wiring interconnection system (EWIS) - the ATSRAC developed a comprehensive program for training and inspection. The heart of the safety-assurance was an enhanced zonal analysis program (EZAP) for the EWIS. And if the inspection and maintenance protocols envisioned by EZAP were to be implemented (an added cost to industry), people would have to be trained to implement them (another cost). However, the full program was rejected by the Federal Aviation Administration (FAA) as too expensive to pass the requisite cost-benefit calculation. Asked to reduce the requirements, the ATSRAC has examined two areas in particular: who should receive voluntary training in EWIS inspection and maintenance, and which supplemental type certificates (STCs) should be subjected to detailed analysis for possible inclusion in EZAP?

The schoolhouse conundrum
For purposes of mandatory and voluntary EWIS-specific training, eight target groups have been identified. Federal Aviation Regulations (FAR US dollars 121.375 to be specific) require that maintenance personnel must be trained to perform their functions. Accordingly, the ATSRAC has proposed a full set of required instruction for the maintainers, Target Group 1. For Target Group 2, inspectors, the ATSRAC has recommended that they receive about three-quarters of the specific subject matter training for Target Group 1. Unlike the maintainers, the inspectors would receive specific training in zonal area inspections and human factors in inspections. In other words, mandatory training has been proposed for those maintaining EWIS and for those inspecting their work.

However, to minimize the training burden, it appears that EWIS awareness training will be limited to a voluntary program. For example, it was suggested that Target Group 4 (those performing general maintenance) should at least receive mandatory training in EWIS housekeeping. This suggestion was shot down. In other words, metalworkers will not be required to receive training on the damage they could cause, such as by allowing drill shavings (swarf) to fall onto an unprotected wire bundle. The metal shavings, lodged between the packed wires, could over time "saw" their way through the insulation, creating the potential for dangerous electrical arcing. Practices vary widely from one repair station to another. One station may cover the wiring with plastic before conducting such work, while at another the debris may be vacuumed up afterward (which does not get all the swarf).

To encourage operators and repair stations to provide training on a voluntary basis, a number of considerations were proposed, among them that a voluntary training effort:


Improves safety.
Saves face.
Responds to public perception and political pressure.
May prevent mandatory action in the future.
Raises awareness of EWIS maintenance issues.
Avoids unintended damage to EWIS.

These items are direct quotes from the briefing slide. The "saves face" rationale was criticized for its bald-faced tokenism. One ATSRAC member noted that the business case for the voluntary training provides the best argument for the voluntary training (i.e., increased dispatch reliability, reduced unscheduled maintenance - the most costly kind). But, at this point in time, Target Groups 1 and 2 are the only candidates for required EWIS training.

The supplemental type certificate conundrum
The larger challenge, which could involve far more of a burden on the industry in terms of money and time, concerns EWIS-related modifications to aircraft. These are the supplemental type certificates (STC).

To put the problem in context, consider the fuel system safety evaluation effort, required as part of the SFAR 88 process (see ASW, May 14, 2001). In this project, post-manufacturing modifications to fuel systems had to be considered as part of the overall safety analysis. Charles Huber, the FAA Executive Director of the ATSRAC, observed "the STC review for SFAR 88 was hugely expensive."

For the SFAR 88 program, ASW identified nearly 100 STCs that had to be evaluated. For the EWIS effort, the number of potential STCs could run as high as 40,000, sources say. Because of the horrific cost implications, the FAA has determined that "there will not be a parallel requirement" for existing STCs, according to an ATSRAC document. To be sure, not all of the extant STCs involve wiring, but of this total population, the number subjected to the full EZAP analysis has to be constrained in order to meet the cost-benefit test.

How to identify the STCs that might require revised inspections or inspection/maintenance intervals?

An ATSRAC working group presented a flow chart in which every STC would be assessed against four criteria - if any one or more applied, an EZAP analysis would be required. The idea behind this logic chart is to target only those STCs affecting or modifying wiring, or the environment in which the wiring exists. It was estimated that some 85 percent of existing STCs would not require an EZAP analysis. Thus, of 40,000 STCs, about 34,000 would pass through this initial "filter," as it were, and no further action would be required. However, the remaining 15 percent of STCs, or roughly 6,000, might have to undergo an EZAP analysis.

That is still an enormous number. One way to reduce the burden, suggested at the ATSRAC meeting, would group the STCs into major categories. For example, STCs involving avionics upgrades, installation of in-flight entertainment (IFE) systems, major cabin reconfigurations, or passenger-to-freighter conversions would all automatically undergo EZAP evaluation.

The rest would be run through the four-criteria filter. Some would fall into the EZAP pot, and for good reason. As one ATSRAC participant noted, "The less notable STCs may be the problem." The single wire installed per an STC routed next to a hydraulic line may present a greater threat to safety than a full- blown IFE and warrant EZAP analysis. This much is clear: if existing STCs simply were excused from analysis, then existing industry practices would prevail. And it is those practices, which the ATSRAC - through its aircraft inspections and laboratory analyses of wiring samples - has demonstrated are inadequate.

The status quo will not hold. Yet the specific amount of training and STC analysis to be mandated is, for the moment, held in thrall by the test of cost- benefit.


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Target Groups for EWIS-Specific Training

1. Personnel performing EWIS maintenance.

2. Personnel performing maintenance inspections on EWIS.

3. Personnel performing electrical/avionics engineering on in- service aircraft (i.e., people who may be involved in system modifications and maintenance processes).

4. Persons performing general maintenance and inspections not involving EWIS (Line replaceable unit, LRU, change is not considered wiring maintenance).

5. Personnel performing other engineering or planning work on in-service aircraft.

6. Other service staff with duties in proximity to EWIS.

7. Flight deck crew.

8. Cabin crew.

Source: ATSRAC, Working Group 11 Interim Report, July 1, 2004


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Outline of EWIS-Related Instruction

A. Introduction. Demonstrate the safe handling of airplane electrical systems, LRUs, tooling, troubleshooting procedures, and electrical measurement (includes safety practices such as not using bundles as handholds).

B. Wiring Practices Documentation. Know the construction of the applicable airplane wiring system.

C. Inspection. Know the different types of inspection, human factors in inspection, zonal areas and typical damage.

D. Housekeeping. Know the contamination sources, materials, cleaning and protection procedures.

E. Wire. Demonstrate the correct identification of different wire types, their inspection criteria, and damage tolerance, repair and preventive maintenance procedures (includes insulation damage limits, unused wire termination, electrical bonding and grounds).

F. Connective Devices. Know the procedure to identify, inspect and find the correct repair for typical types of connectors.

G. Connective Device Repair. Demonstrate replacement of all parts for typical connectors found on the technician's airplane.

Source: ATSRAC, Working Group 11 Interim Report, July 1, 2004


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Maintenance Guide Shows How to Avoid Wiring Malpractice

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 90º 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

http://www.academy.jccb i.gov/AIRDL/wiringcourse.

My remark: This is a great article but in the case of sr111 it would almost suffice to say- keep the thugs out of the airline business and the plane wouldn't have crashed. Nothing very complicated about that.