Product Focus: Wire and Cable

Aircraft wiring, fundamental to the safety of airplane operations, is the subject of perennial debate. Incidents such as the TWA crash in 1996 and the Swissair crash in 1998 focused attention on wiring issues. A new government wiring policy document, one of the latest in a line of Federal Aviation Administration (FAA) activities in this area, has set the stage for future regulatory changes.

"Aircraft wiring damage is not unusual and, in most cases, does not cause problems. However, any given airplane could have several hundred insulation breaks extending to the conductor," says Vince Press, director of marketing for Lectromec, a wiring test company in Dulles, Va. (There are more than 150 miles [241 km] of wiring in a widebody airplane.) Data from a two-year-old Air Line Pilots Association (ALPA) study, reported by our sister publication, Air Safety Week, suggests that there were 1.5 service difficulty reports (SDRs), specifically citing wiring, wire harnesses and connectors, per week in the first 10 months of 1999.

One of the results of FAA’s renewed wiring emphasis was the publication in February 2002 of a "notice of final policy" on wiring for Federal Aviation Regulation (FAR) Part 25 airplanes. This document, ANM-01-04, addresses a small segment of wiring concerns–improper installation or repair of aircraft equipment. The policy addresses applicants for type certificates (TCs) and supplemental type certificates (STCs)–anyone developing design data for the installation of wire bundles. But it bears upon maintainers and operators, as well. (STC applicants frequently include designated alteration stations and repair stations.) The policy statement applies to any project started after July 2, 2001.

The policy statement doesn’t change already existing requirements, FAA says, but clarifies and reemphasizes various aspects. The agency seems to be trying to change substandard practices, short of new regulations. Regulatory changes are expected eventually, perhaps by 2004.

The Enhanced Airworthiness Program for Airplane Systems (EAPAS) is the means by which FAA seeks to improve wiring system safety. The program’s approximately 40 activities include airworthiness directives, maintenance procedures, FAA inspector/engineer training, and longer-term regulatory changes in the areas of inspection, maintenance and certification. EAPAS is to implement the recommendations of the three-year-old industry/government Aging Transport Systems Rulemaking Advisory Committee (ATSRAC). Recommendations for rulemaking are expected by August 2002. The effort also seeks to harmonize U.S. FAR and European requirements.

The FAA policy supplemental information identifies areas, such as the following, where wiring practices in some cases have been deficient:

Design data packages,
System safety assessments,
Installation drawings,
Manufacturing process specs,
Compatibility with original type design standards, and
Instructions for continued airworthiness (ICA).

But the document states: "[The FAA policy] is not intended to establish a binding norm. It does not constitute a new regulation, and the FAA would not apply or rely upon it as a regulation." The disposition of comments, furthermore, removes phrases such as "complete," "completely define," "strictly," and "definitive."

Nevertheless EAPAS is developing Part 25 requirements that, among other things, "will give [the policy] more teeth," says Chuck Huber, ATSRAC executive director, with FAA’s Aircraft Certification Service. "The proposed Part 25 changes will directly require most of the information in the policy memo," he adds.

Nothing New?

"There’s nothing new in the policy I see as onerous–they’re talking about things people should have been doing all along," says Jack Raia, director of modification engineering for TIMCO Aviation. Greensboro, N.C.-based TIMCO holds about 140 STCs on almost every Boeing airplane.

"But we’ve seen STC packages in the past that have exhibited some of the problems being discussed by the policy document," Raia says. For example, a windshear weather radar installation has to interface with the airplane’s power system, inertial system and other systems. But the design data package "might just give you weather radar wiring," he says, which would make later troubleshooting more difficult.

The policy "is an attempt to clarify the existing regulations and how [FAA] expects us to comply," says Aaron Duncan, engineering manager with Garrett Aviation’s facility in Springfield, Ill. "It’s all about trying to get more detail in the design data." But at this point "it’s still subject to interpretation and is being applied differently in different regions." Furthermore, he adds, FAA designated engineering representatives (DERs)–who review and approve the [installation] drawings–haven’t been formally trained yet in the design theory or methodology behind the document.

Garrett specializes in mid-sized to large corporate aircraft, but is covered by the Part 25 policy statement. The Springfield unit averages 65 to 75 STCs a year.

Nevertheless, the ideas reflected in the wiring policy represent a step forward in the awareness of problems and the intent to do something about them. The guideline could be said to provide a set of reasons–in the design data area–for what ATSRAC has been asked to do, Huber says.

Little of what’s generally thought to be "aging" can be attributed to the passage of time, he contends. Aging, defined as degradation, is typically caused by mechanical damage, contamination, improper installation or repair, and inadequate maintenance."

"Three years ago there was little regard for wiring," Press says. "But FAA has commissioned several important projects in this area." There are studies on how to change general visual inspection of wiring–the arms-length examination of its condition–to more detailed visual inspection. That’s a step forward, but there are also five or six technologies that offer more thorough, instrumented testing, although the airlines have not yet invested in that approach. Among wire testing equipment/services vendors are: DITMCO, Lectromec, Phoenix Aviation, Honeywell, Northrop Grumman, CM Technologies, Cirris and Eclypse (see list).

FAA is developing technology for an instrument that a mechanic can tap non-intrusively onto a wire bundle to detect cracks, Huber says. He estimates it will be "a couple years" before a tool is available that can reliably do that. "Systems that are out there now have limited use and will not detect everything we’re looking for." It is desirable, for example, to have prognostic capability, he says, in order to anticipate a wiring problem.

Installation Drawings

The FAA policy emphasizes installation drawings, which depict the exact path a wire takes from starting point to termination–around spars, through bulkheads, etc. In some cases, in corporate aviation, "people have been getting away with a silhouette view of the airplane, says Gary Shroyer, Garrett senior systems engineer. "They’ll simply have a black line that runs from the nose, branches off and goes to the instrument panel, branches off and goes to the right rack. It really doesn’t tell you if it’s routed along with an oxygen line or along with the high-current-carrying cables." (Corporate aviation, with its wide range of aircraft and large number of repair stations, probably would show greater variance in design practices than would air transport.)

"Depending on how old the aircraft is and how often it has been modified–some aircraft in the corporate fleet are over 30 years old–some wire harness installations might be found to have been modified in a substandard manner, resulting in system failures." Shroyer says. Wires, for example, might be found twisted together, using electrical tape as insulation.

"Garrett, however, must ensure that any systems it has modified, including existing wiring associated with that system, comply with appropriate regulations before returning the aircraft to service," says Shroyer. In some cases this means the company must replace existing wiring harnesses.

Standard Practices Manual

The FAA policy document likewise stresses the need for conformity in wiring practices between a new installation and the original installation. The original equipment manufacturer (OEM) wiring practices manual describes the general philosophy of how an airplane was wired–the type of clamps or sleeving, Huber explains. "The policy says, to comply with our existing regulations, you need to do a thorough job of considering and documenting wire separation, types of clamping, sleeving, drip shields, and so on."

Although the information in these manuals is general, it’s a necessary item, so FAA (through EAPAS) plans to propose as a requirement a standard wiring practices manual for each installation, whether a TC or an STC. ATSRAC also is recommending revisions to standard wiring practices manuals, to provide common format and content across all transport-category OEM documents.

Today’s manuals also contain little or no routing instructions. A manual, typically, is "a book of wiring diagrams without much in the way of mechanical layout drawings which could be used to locate the connectors or determine harness routing," Shroyer explains.

To be perfectly compatible with the OEM’s intent, one needs to have the company’s process specifications. "But making sure that a modification meets the certification requirements of the [OEM’s] original data package is very difficult for anyone in the aftermarket," for competitive and liability reasons, Garrett’s Duncan says. In the absence of that information, one can develop one’s own process specs, based on knowledge of the regulations and special conditions applicable to an aircraft and physical inspection of the installation to be modified.

The policy likewise says that when the OEM information is unavailable, the applicant should physically inspect the airplane to be modified, to ensure compatibility between the original installation and the modification, and/or develop processes and procedures to address compatibility.

System Safety Assessment

The policy information concedes: "Insufficient emphasis has been placed on an examination of failures of wiring external to the actual line replaceable units being installed." It adds: "Failure of wiring in bundles due to chafing, contamination or other causes may affect the continued safe operation of the airplane."

FAA emphasizes "zonal analysis," or consideration of the area of the aircraft in which the wiring would be installed. In the past, analysis "has not focused very directly on the installation itself," Huber says. He expects a Special FAR requiring manufacturers to develop an enhanced zonal analysis procedure (EZAP). This could include looking for contaminants and flammable material on wires, as well as wire separation issues. According to the policy, the analysis should consider the possible effects of wire system failures on safety-critical systems. Furthermore, "any possible interaction between systems [should] be examined."

Continued Airworthiness

Currently, few regulations address how or how frequently to inspect wiring. Even in a D-check, only a general visual inspection is usually performed. The policy background information states: "The maintenance aspects of system wire external to the installed equipment [are] not being adequately addressed."

The policy adds: "Where wire cannot be inspected visually, the ICA [instructions for continued airworthiness] should address wire removal for inspection, when necessary, and the use of inspection techniques that do not rely on visual inspection alone… wire in metal conduits may require repeated inspections for wear." Installers–and by extension, STC applicants–the policy document says, are responsible "to address any special maintenance requirements."

COMPANIES

A.E. Petsche Co. www.aepetsche.com

Aeroflite Enterprises www.aeroflite.com

Aircraft Engineering & Installation Services Inc. www.aeisinc.com

AMETEK Aerospace www.ametek.com

Astro Industries www.astro-ind.com

Chippewa Aerospace 843-828-1160

CM Technologies www.ecadusa.com

Cirris www.cirris.com

Delphi Connection Systems www.delphiconnectionsystems.com

DITMCO www.ditmco.com

Eclypse International Corp. 909-371-8008