AIRCRAFT WIRING

Madam Chair and Members of the Subcommittee, I am Captain Paul McCarthy, Executive Air Safety Chairman of the Air Line Pilots Association (ALPA), which represents 55,000 professional pilots who fly for fifty-three commercial airlines in the United States and Canada. ALPA appreciates this opportunity for me to appear before you today to join with other members of the aviation community to discuss deficiencies associated with the wiring and circuit protection practices in today's transport aircraft.

Although the subject of aircraft wiring has been a central part of at least two recent airline accidents, I wish to make it clear that nothing I say here should be construed as being directed at any of these specific investigations. Rather, they should be taken as general comments on this aspect of aviation safety. I also want to make it clear that we at ALPA are not trying to issue any sort of general alarm about wiring concerns. Our members continue to operate these airliners, including the older models, with abundant confidence in their airworthiness.

Having said that, I do want to thank the Subcommittee for holding these hearings on what we perceive to be an important component of aviation safety. Various events and accidents have made it imperative that we examine all aspects of aircraft wiring, from the design characteristics, to the materials used, to how they are installed in the aircraft. We also must revise our previous notions of how well wiring holds up under aging and use. And wherever possible, we must pursue technological improvements that provide better alternatives to electrical wiring.

I will deal with all of these in more detail in the remainder of my statement.

Introduction

First generation airliners had a relatively minimal need for wiring. Their cockpits contained primarily mechanical instrumentation, and much of the wiring that was installed was used to provide electrical power for lighting and basic radio equipment. In most cases, the thickness of the wire insulation was the same as or greater than the wire thickness.

Times and technology have changed. In terms of their dependence on wiring, modern transports such as the MD-11 and the B-777 (one of the newest aircraft entering the fleet), are very different from early airliners. In today's aircraft, nearly all of the instrumentation and the majority of the aircraft systems are electrically powered or controlled. These wires are quite thin, and there insulation even thinner, often about the thickness of 3 human hairs. The reason for this decreased insulation thickness is very simple - weight. Modern wide-body aircraft can contain nearly 200 miles of wiring, so the appeal of reduced insulation thickness (& therefore, weight) is obvious. This vastly increased use of wiring, in conjunction with decreased insulation thickness, has increased the incidence of wire-related aircraft safety problems, including short circuits, electrical equipment malfunctions and failures, and fire.

This paper addresses some of the causes of and contributors to these safety problems, the current state of the associated industry practices and regulatory guidance, and some potential solutions.

Ticking Faults

Over 30 years ago, an electrical wiring phenomenon known as a 'ticking fault' was identified. This phenomenon was also cited as having caused or been a causal factor in aircraft accidents. Aircraft longevity has increased, and correspondingly, the average age of the fleet has also increased. The resultant aging of the wiring insulation in these aircraft has increased the potential for this ticking fault phenomenon to occur.

On some aircraft, wire bundles contain many different wires with several different types of insulation. We presently have wire bundles that are composed of AC power cables, DC power cables, signal (circuit controlling) wires, and circuit ground wires. We also have bundles that carry power from different power sources ('busses'). These conditions make it extremely difficult to protect any circuit in such a bundle, where an insulation failure could result in an electrical problem that has multiple power sources and current paths to feed it. It is not difficult to envision the complexity or severity of problems that could arise from shorting, arcing or some other type of damage to a bundle with this mix of wires.

Think of these bundles as I have described them and now imagine a 'ticking fault.' A ticking fault is electrically much different than a 'short to ground' type of fault. The 'ticking fault' is an arcing event that has duration of just a few milliseconds. In such an event, the voltage will typically drop to some lower level, while the amperage (current) will increase by a factor of 10 or more. This results in a localized discharge of a great amount of heat and energy. This phenomenon has been shown in laboratory test to explosively track along the wire. Visually, it resembles the burning of an old-fashioned black powder fuse.

Mr. Jerome Lederer, founder of the Flight Safety Foundation, encountered moisture-related wire faults over 30 years ago. He determined that these "wet wire" fires, as he called them, caused more than a few accidents. The phenomenon appears to have been a form of the ticking fault now known as 'wet arc tracking.' Wet arc tracking is enabled and supported by moisture-damaged insulation.

Wet arc tracking causes a carbonization of the insulation. The high temperature of the arcing event "dries out" the area, but leaves a carbon deposit behind. This carbon deposit is conductive. Over time a carbonaceous path forms and lengthens until it is close enough to a ground or open circuit. This is when the dry arc tracking occurs, and the explosive loss of an entire wire bundle can occur. Because of the need for free carbon, this type of failure only occurs in insulation types with a high carbon content, such as polyimide or Kapton.

Finally, microscopic cracking occurs in wire insulation as the insulation ages. Aging wiring and associated deterioration of the insulation increases the probability that ticking faults will occur in our aircraft.

Aging Wire

Laymen tend to think of aging only in terms of elapsed time. However, when it comes to aircraft structure and components, including wiring, the aging or degradation of these items is also a function of the operating environment and conditions that they are exposed to. The term 'aging' when used to refer to wiring primarily denotes the deterioration or failure of the insulation, as opposed to the conductor. Aircraft wiring aging rate is principally affected by the four factors discussed below.

Vibration is one of the factors affecting wire aging. Vibration is not at a single, constant level throughout the aircraft. It varies greatly as a function of location in the aircraft, and as such it affects the wiring running through those areas differently. Mixing wires of different types in the same bundle has been shown to be detrimental to wire life, because the harder coating on one wire can cut through the other when the wire bundle is subject to vibration. Wheel wells, engine compartments, and areas near the air-conditioning packs all have different vibration cycles, and yet the current regulatory and industry approaches to wiring do not take those differences into account.

As mentioned previously, moisture is another major contributor to wire aging. Most insulation material is a very complex long-chain polymer, and moisture accelerates changes to this complex polymer. These changes decrease the insulating qualities, and can occur over a very short period of time.

Temperature is another factor affecting the aging rate of wire insulation. Elevated temperatures increase the aging rate, as do large, repeated temperature variations ('thermal cycles'). Wiring insulation is subjected to heat generated by the wire itself, as well as the heat from the surrounding equipment.

The configuration of the installation of the wire can also affect wire aging rate. Each aircraft is slightly different, and it is common for the same wire runs on different aircraft to differ slightly from one another. Most insulation types are unable to withstand tight radius bends. During our examinations of aircraft wiring, we have seen wires with bend radii exceeding the industry bend radius standards. In addition, the wire clamping and bundling devices add to the stress and strain on the insulation. Some insulation types are tolerant of these installation irregularities while others are vulnerable to them. All these factors can and do affect the aging rate.

Location of the wiring in the aircraft determines the combination of vibration, moisture, heat and physical installation stresses that the wire will be exposed to. Different locations will have some or all of these factors, and at varying levels, and as such, the aging rates will vary. A company called Electromec has been working with the US Navy on predictive failure rates of insulation. They are quantifying the different aging rates as a function of the environment that the insulation is exposed to. It is expected that these data will be used to determine when to remove and replace wiring, before failure becomes likely, in order to prevent some of the problems we are experiencing today. In summary, aging wiring effects must be accounted for in the regulations and practices which govern the design and maintenance of the aircraft.

Wiring Guidance, Standards and Practices

The Federal Aviation Regulations (FARs) which regulate the installation of electrical equipment on today's aircraft go back to 1964; the latest revision to them was made over 20 years ago, in 1978. FAR Part 25.1353, paragraph (a) defines the requirement to prevent a single point failure in electrical equipment, controls and wiring, and states: "Electrical equipment, controls, and wiring must be installed so that operation of any one unit or system will not adversely affect the simultaneous operation of any other electrical unit essential to the safe operation." Paragraph (b) deals with the wire grouping, routing and spacing, and states: "Cables must be grouped, routed, and spaced so that damage to essential circuits will be minimized if there are faults in heavy current carrying cables." On numerous occasions, ALPA has found that what the manufacturer considers a significant loss of essential equipment differs greatly from what the average line pilot considers that to be.

As evidenced above, FAR Part 25 is vague in it's wiring guidance. Advisory Circular (AC) 43.13-1A (last updated 1977) does provide some additional guidance in this regard. However, much of the determination as to what constitutes good installation practice is left up to the discretion of the manufacturers and operators. The Air Transport Association has produced and made available to its members a standard wiring practices manual. This document is quite comprehensive, but there are indications that many maintenance personnel are not aware of most of these suggested practices.

In ALPA's view, polyimide-only (as opposed to composite polyimide) wire insulation is not satisfactory in any wire application which carries a significant load. Polyimide-only wire insulation was used in many transport aircraft for many years, and is still being widely used by Airbus today. This insulation has been proven by numerous studies to be susceptible to deterioration and cracking when subjected to moisture and heat, which in turn can lead to arc tracking & subsequent failure. Other wire types have not been found to have this negative quality. There have been many events where this has happened, but because of poor data collection the breadth of this can not be readily identified. As one effort to quantify this problem, the Aging Transport Systems Rulemaking Advisory Committee (ATSRAC) is looking at improved Service Difficulty Report (SDR) reporting of wire faults.

It appears that there is a general lack of awareness of many of the line maintenance personnel with regards to the proper handling and installation of wires, and the problems associated with intermixing different wire and insulation types in single bundles. In addition, electrical equipment and wiring have changed a great deal since the applicable FARs and Advisory Circular were last revised. The number and type of "black boxes", computers and entertainment systems have grown exponentially. For all these reasons, we need a more fully defined FAR Part 25.1353 and AC 43.13 in order to satisfactorily address these changes.

Ticking Fault Arc Protection

Existing design, fabrication and maintenance standards rely on circuit breakers to prevent damage and fires from electrical wiring problems. A transient arcing event is sometimes, but not always, a precursor to a hard fault, which normal circuit breakers will protect against. However, the short duration of a ticking fault arc prevents normal circuit breakers from protecting the wiring and aircraft against damage from such an arc. The temperature of an arcing event is well above the lower flammability limit of the insulation and virtually all other aircraft materials. Thus, an arcing event is usually hot enough to burn the insulation from the subject wire, and will likely also damage other wires in the same bundle. If there is sufficient flammable material in the area, a fire could result. Clearly, this is an unacceptable condition in an aircraft.

New technology, in the form of arc fault detection circuitry, can provide the necessary protection. Such systems would protect critical aircraft wiring, equipment and functions while preventing nuisance circuit breaker tripping. ALPA believes that these new arc fault detection technologies hold great promise for improving and ensuring the safety of modern, wire-intensive transport aircraft.

Conclusion

The recommendations I would like to present are really quite simple:

1. Enhance FAR 25.1353 and AC 43.13: Considering the expansion in the quantity and complexity of electrical devices in use on today's aircraft, and the resultant amount of wire, existing regulatory guidance does not adequately address the current technologies and practices.
2. Revamp design and installation practices: Consideration must be given to recent industry knowledge gained from incident and accident investigations. Specific areas to be addressed should include power sources, intermixing of insulation types in a single wire bundle, fastener design, and bundling methods.
3. Incorporate modular construction: Insulation does not last indefinitely, and therefore provisions for relatively easy replacement (such as modular construction) should be incorporated into new aircraft wiring designs. Modular wiring would allow for ease of replacement on a scheduled basis.
4. Evaluate/incorporate fiber optics: Not all the systems in our aircraft need wires for signal transmission; in fiber optics, light is the signal carrier. Aircraft interior lighting and entertainment systems seem to two good candidates for this type of technology
5. Evaluate/incorporate alternative signal transmission methods: Newer types of signal transmission methods such as infrared (IR) and frequency modulated (FM) radio technology are used in other industries; maybe there is a way for us to use these as well.

Madam Chair, thank you again for the opportunity to appear before you today on this important subject, and I would be happy to answer any questions you and the members of the subcommittee may have.