8 November 2007
FR Doc E7-21434 - Docket No. FAA-2004-18379
FAA

This new rule should boost wiring safety by imposing new wiring-related maintenance and inspection tasks. The rule applies to all transport category airplanes with 30 or more seats or a maximum payload of 7,700 pounds for which type certificates (TCs) have been issued and to all applicants for pending Supplemental Type Certificates (STC). The rule requires manufacturers to develop Instructions for Continued Airworthiness (ICA) that will treat wiring as a separate system and will ensure the integrity of the wiring installed on the airplane (TCs) or modified in any way (STCs).

The rule requires manufacturers to complete FAA-approved instructions for new wiring related maintenance within 24 months of the effective date, 10 December 2007. Scheduled air carriers have 39 months from the effective date to implement the manufacturer-developed wiring inspections. Obviously, the sooner the manufacturers complete their procedures for wiring inspection and maintenance, the more time operators will have to implement them (for a current example of what this rule is intended to prevent, see Figure A).

Figure A

The Hazard Intended to be Caught by This Rule

The FAA Aviation Maintenance Alerts contain information on incidents seen on aircraft. The above photo is the result of a failed protection device. Damage can be seen to a wide area including to the connectors and wires. Source: www.faa.gov/aircraft/safety/alerts/aviation_maintenance/media/2007/2007_04_Alert.pdf

The FAA explains:

“The FAA has concluded that current maintenance practices do not adequately address wiring components, wiring inspection criteria are too general, and maintenance instructions do not describe unacceptable conditions, such as improper repairs and installations, in enough detail.

“With this final rule, we are introducing new maintenance inspection and design criteria for airplane wiring to address conditions that put transport airplanes at risk of wire failures, smoke, and fire. We are adding requirements for type certificate holders and applicants for type certificate and supplemental type certificates to analyze the zones of their airplanes for the presence of wire and for the likely accumulation of contaminant materials [e.g., dirt, grease, lint]. This final rule also requires them to develop maintenance and inspection tasks to identify, correct, and prevent wiring conditions that introduce risk to continued safe flight.…

“The EWIS [Electrical Wiring Interconnection System] ICA must not conflict with the ICA for fuel tanks, and must avoid duplication and redundancy. Too frequent disturbance to electrical wiring by repeated moving, pulling, and flexing of the wire bundles will induce unnecessary stress on the wiring and its components, which in turn could lead to degradation, expedited aging, and failures. Thus, it is important that redundant tasks and unnecessary disturbances to the electrical wiring be minimized.…

“Since the [6 October 2005] Notice of Proposed Rulemaking (NPRM), the … NTSB has issued Safety Recommendations A-06-29 through -35 pertaining to fires on one particular model of regional jet [a Bombardier CRJ-200]. In the 6 months between October 2005 and March 2006, there were a total of 6 fires on regional jets. A seventh fire occurred prior to that 6-month period. The NTSB stated that, in addition to the danger posed by the fires, 2 of the incident airplanes temporarily lost all flight displays. The NTSB’s investigation revealed that all of the fires originated from the same electrical component – an electrical contactor located in the avionics compartment beneath the floor of the captain’s seat. The fires were caused by moisture-induced short circuits between the electrical terminals of the contactors. We have issued airworthiness directives (ADs) to correct this unsafe condition [see www.ntsb.gov/Recs/letters/2006/A06_29_35.pdf]. However, if the requirements in this final rule had been in effect, the type of failure that caused these 7 fires would not have occurred. This is because several of the new requirements directly address the design issues that led to the fire.

“This final rule is meant to proactively address wiring conditions existing in the transport airplane fleet that we now know affect safe flight and can be detected, corrected, or prevented.”

The ruling affects virtually the entire U.S.-registered fleet of airliners and has implications for better wiring-related maintenance on foreign registered aircraft as well.

According to the FAA, the rule will cost $416 million to implement over a 25-year period, over which time it will save an estimated $801 million. This positive cost-benefit comes primarily from avoided unscheduled landings due to electrical problems (smoke, fire, etc.), which are estimated to amount to some $274 million in avoided precautionary landings, and some $294 million in averted fatal and non-fatal accidents. Other benefits, making up the remainder of the $801 million in avoided costs, accrue from positive operational impacts of improved electrical system reliability.

The fact of the matter is that wiring was neglected by manufacturers and operators, and that improved precepts for installing and maintaining wire and related connectors and such were long overdue.

There are a number of aspects to this final rule that warrant comment:

a. To implement this 93-page rule, there are a dozen new and related advisory circulars (ACs) that must be published, read, and assimilated. These ACs collectively total at least another 90 pages of guidance. According to sources, only one of the dozen ACs has been published in final form. Assurances have been given that the remaining 11 ACs will be published by the effective date of the rule. These ACs in total provide one way, but not the only way to comply with this rule.

b. Compliance with ACs is not mandatory. This final rule represents the work of the Aging Transport Systems Rulemaking Advisory Committee (ATSRAC), which recommended some form of required EWIS training program. This recommendation was based on the evident widespread ignorance throughout the industry of good wiring practices. The notion of requiring such training was dispensed with in favor of a non-binding AC outlining recommended maintenance practices. Given the almost 2:1 favorable cost benefit ratio of the final rule, one wonders if the cost of a required training program would have turned the cost-benefit ratio negative.

c. This final rule comes some 9 years after the formation of the ATSRAC, which was announced about two years after the TWA 800 disaster. A whole lot of industry resistance was beaten down during this extended period. At early ATSRAC meetings, manufacturer’s representatives basically dismissed concerns about electrical wiring. “What problem? We don’t have a problem,” might sum up the prevailing attitude. A mountain of evidence to the contrary led to the belief that better husbandry of wiring was essential. Kent Hollinger of Mitre Corp., the chairman of the ATSRAC effort, reflected the view of the committee as a whole:

“The delivery of electrons throughout the aircraft is such an essential function that wiring can no longer be considered as ‘fit and forget.’ Past practices, such as using wire bundles as ladder rungs to reach higher areas, and hanging maintenance lights from wiring, can no longer be tolerated.”

One might say that the change in attitude represents a journey from “data-free declarations” to “data-driven safety.”

d. In the early ATSRAC deliberations, it was proposed that a one-time detailed visual inspection be conducted in three areas: (1) cockpit wiring, (2) electronic and equipment (E & E) bay wiring, and (3) power feeder cables. These inspections have been dropped. Indeed, they weren’t discussed in the NPRM either. The subject of such inspections was argued repeatedly and heatedly in ATSRAC meetings.

e. The visual inspections are to be conducted by zone in the aircraft. If the area (1) contains wiring and (2) there is the likelihood that the zone contains combustible materials, such as thermal/acoustic insulation blankets, or (3) the wiring runs 2 inches or closer to both primary and back up flight controls, the zone must be inspected. Exactly how much of the airplane’s wiring (25%, 50%, 75% or more) is not at all clear.

The 2 inch separation is a minimum and may not be enough. To quote from the NTSB’s final report of the TWA 800 investigation:

“Although airplane manufacturers generally provide protection for certain critical electrical circuits, there is no FAA regulation that specifies wire separation criteria or identifies which circuits must be protected.… Safety Board investigators reviewed the general wire separation standards and practices of several airplane manufacturers and found that these standards are not uniform. For example, Douglas Aircraft Company specifies that wiring for certain systems (including FQIS [fuel quantity indicator system] wiring, fire warning system wiring, generator feeder cables, and electro-explosive devices) must be separated by at least 3 inches from other electrical wiring. Boeing specifications do not require protection for some of the systems specified by Douglas (such as the FQIS) and, for those systems that are designated as protected, the required separation distance is only ¼ inch in pressurized areas and ½ inch in unpressurized areas. The potential for short circuits to damage nearby wiring (more than 1½ inches away) has been documented in Safety Board investigations of numerous accidents and incidents.”

Some experts on wiring argue that 3 inches is a better standard, and some claim 6 inches is better yet, given the potential of collateral damage from arcing (see Figure B).

Figure B

The Anatomy of Spew

The material ejected from an arcing event is a combination of the insulation, conductor, and the target of the arcing.  The material is usually hot enough to give off light. The damage caused by such spew is one reason some have called for inspection of wiring 3 inches from flight control cables or pushrods, rather than the rule's requirement for inspection of wiring up to only 2 inches from flight control runs.

Most of the collected spew from the tests looked like small ball bearings, though some had rough or jagged edges.

Using the 7 mil diameter wire strand for scale, the range of diameters can be estimated as between 2 and 35 mils.

A 35mil piece of spew may not sound like much, but it is hot enough to melt though a plastic sheet (melting temperature less than 140oC), and potentially the insulation of another wire.           Photo Source: Lectromec & FAA

f. The rule does not mention swarf. This term is applied to fine metallic shavings removed by a cutting tool. Swarf is considered by many to include metal dust and not just drill shavings. It should be noted that the elimination of metal shavings and dust, lint and swarf are prime housekeeping concerns for good wiring husbandry.

g. Perhaps insufficient attention is paid to sulfide deposits on wire connectors, a form of contamination that may have played a role in a major accident.

h. The homily about the three elements of fire (fuel, oxidizer, ignition source) may be simplistic and deficient, as it ignores two additional elements of propagation. One is the direction of airflow. The other is the concealment of wiring below or above the cabin. Hidden fire and the effect of an aircrew checklist in switching off busses and inadvertently reversing the airflow beneath the cabin lining are an intrinsic part of the problem with wiring fires and should be spelled out. The problem of reversed airflow was the fatally terminal clincher in the fire that downed Swissair flight 111 in 1998.

i. The rule discusses “vibration” only in passing. By not tying vibration directly to chafing, the impact of high frequency vibration, normally inherent to flight, is lost. The rule would have profited from a brief discussion of the frequencies of vibration present in smooth-air flying, with some examples of the amplitudes of movement experienced by wires under tension (and their effect upon adjacent wires and substructures, over time, in terms of chafing through very thin layers of wire insulation and thereby exposing the conductor.

j. The rule does not provide an adequate discussion of the ramifications of an entire wiring bundle failure due to an arcing event. It discusses localized “collateral damage” from a single wire failure, but this restrictive definition does not give due concern to multiple essential systems in a wiring bundle.

k. The other problem is that the wiring is subjected only to visual inspections. These inspections are done close enough to touch the wiring, or they’re done with a hand-held mirror. This approach will catch really egregious wiring faults, such as severe chafing, heat damage, and such. But visual inspections will not catch all breaches in insulation, which often manifest as intermittent faults (which can be maddening to track down and repair).

Nor will the visual inspections involve a breakdown or disassembly of wire bundles to uncover and examine all the wiring buried deep inside (see Figure C).

Figure C

In this example from NASA, only about 20% of the wiring in a bundle is amenable to visual inspection, and that assumes 360-degree coverage of each wire.

Given these limitations to the protocol, one has to ask if these inspections are going to be worth the effort. For the operator running a tight maintenance operation, the payoff may be marginal, but positive. For the less reputable operators, with indifferent attitudes about wiring and tolerant of dirty airplanes, the program seems well worth doing. The FAA has put forth the idea that wiring is a system, and that it needs unique care, just like the black box or actuator it is connected to.

It has taken 11 years after TWA 800 blew up from a wiring fault to issue this final rule. The FAA’s stately and deliberate rulemaking process pushes mandatory, fleet wide wiring inspections at least two years into the future, meaning it may take 13 years to achieve actual implementation. The thoroughness and effectiveness of the visual inspections of just some of the wire may well reflect what some critics regard as a minimalist approach, given the long history of wiring faults uncovered in numerous NTSB investigations and as reflected in the FAA’s own maintenance database. Supporters will argue that these wiring inspections represent a good beginning, and there is nothing to prevent operators from doing more to safeguard the integrity of their aircraft wiring.

Figure D

United Airlines distributed this poster of wiring maintenance practices to get a leg up on the wiring rule published by the FAA. The poster is an excellent summary of what the new rule requires of operators. The last bullet point above (“Reference pertinent manual procedures …”) would be clearer if it had read, “Refer to proper procedures in the pertinent manual.” Regarding the caption to the lower right photo above, the word “egressing” is awkward. It would have been better to describe moisture as infiltrating, permeating, penetrating or percolating.