-T / T / +T | Comment(s)

Wednesday, March 1, 2006

Wiring Inspections

David Evans, Editor

Proposed to Assure Safety of Transport Aircraft

It may be the most lengthy regulatory proposal in recent memory, although its impact is far less certain than the number of pages suggests. Nevertheless, because it deals with aircraft wiring, the proposal has be to considered significant, and although it's focused on transport category aircraft, the issues it raises involve all aircraft and, as such, the proposal is worth the attention of manufacturers, operators and maintainers across the spectrum.

In effect, the Federal Aviation Administration (FAA) is saying that aircraft wiring systems can no longer be considered "out of sight, out of mind" until a failure commands repair. No, the stewardship of wiring must be an ongoing matter to assure reliability and safety.

The document in question is a notice of proposed rulemaking (NPRM) on electrical wiring issued October 6, 2005, for all Part 121 (regularly scheduled) airplanes of 30 or more passenger seats and 7,500 pounds payload capacity. Comments on the proposal were due February 3, and as of this writing, in late January, relatively few comments were received (although a flood of commentary from the industry is expected nearer to deadline).

The NPRM (Docket No. FAA-2004-18379) is the long-awaited requirement to assess and inspect wiring in commercial airliners that has been the attention of the Aging Transport Systems Rulemaking Advisory Committee (ATSRAC). This committee of government and industry experts was convened after the 1996 TWA Flight 800 disaster, in which electrical arcing was the prime culprit in the fuel tank explosion that destroyed the plane, and the committee's work was given impetus by the 1998 Swissair Flight 111 crash in Canada, in which an uncontained fire in the so-called "attic" space above the cockpit and forward cabin led to loss of control. The fire was believed started by electrical arcing in a bundle of wires, as evidenced by globs of molten conductor found on the wire remnants retrieved from the wreckage. Inspections of other aircraft revealed a host of problems with wiring, which lent impetus to these proposed inspections.

The ATSRAC recommendations have been translated into the proposed rulemaking. The intent of the action is to enhance the safety of commercial airliners by improving the design, installation and maintenance of wiring systems, with inspections of wiring to commence in 2007 (although it appears 2008 is a more realistic deadline). As such, all in the industry can profit from the proposal, as it represents at least a coherent approach to wiring safety, albeit one with limitations. The document says, "The FAA believes that the traditional ways of addressing wiring are no longer enough."

Improved awareness of wiring system design and maintenance will be accomplished through a variety of means. First, certification requirements for wiring are proposed to be consolidated in a single section of the Part 25 certification requirements, known as Appendix H. Second, the proposal introduces a new term, electrical wire interconnection system (EWIS), to acknowledge the fact that wiring is just one of many components. "The term EWIS means any wire, wiring device, or combination of these, including termination devices, installed in the airplane for transmitting electrical energy between two or more termination points," the FAA says, adding, "There are many examples of inadequate EWIS design that have later been determined to be unsafe." Adopting the proposed actions will go a long way toward redressing this situation, the FAA maintains.

Raising the bar on safety

Third, the rulemaking proposes new system safety assessments for existing, proposed and modified EWIS by "raising the quality of the safety assessment with respect to EWIS failures that would not be identified using the traditional methods," the FAA says. "Airplane manufacturers have delivered airplanes that have wiring problems when they leave the factory, or such problems have later developed in service, as evidenced by resulting mandatory corrective actions [e.g., airworthiness directives]," the FAA noted.

Fourth, and especially important for readers of this magazine, the rule mandates new maintenance requirements, and training of maintainers.

Fifth, the rule is intended to align efforts regarding fuel system wiring and components with the EWIS requirements proposed by this action for the rest of the airplane.

A comprehensive program

The rule is accompanied by a number of proposed advisory circulars (ACs) which, although not directive in nature, need to be considered along with the NPRM.

The rule is a significant development, as it applies to foreign operators flying to the U.S. and, in addition, must be harmonized with European regulations. As such, the rule runs to more than 110 pages. The associated ACs and other documents double, at the very least, the number of pages involved in this rulemaking. It requires type certificate (TC) holders (e.g., manufacturers) to "complete a comprehensive assessment of the EWIS of each `representative' airplane for which they hold a TC, develop inspection and maintenance instructions for them, and incorporate those instructions into the airplane's ICA [instructions for continued airworthiness]."

Under the proposal, manufacturers have until December 17, 2007, to complete the improved ICAs and, if approved, inspections of aircraft will follow. According to the FAA, the timeline is based on a number of considerations, including:

  • Aligning the proposal with other rules in the aging airplane program, to include the fuel tank safety initiatives now under way and the pending widespread fatigue damage proposal.
  • It allows adequate time for manufacturers to develop ICA and for operators and maintainers to implement them.
  • It establishes June 16, 2008, as the deadline for applying the ICA to supplemental type certificates (STCs), which means modifications and additions to the factory-installed EWIS. The proposed rule is not applicable to already issued STCs.

    As indicated, the rule is an outgrowth of the National Transportation Safety Board (NTSB) investigation of the TWA Flight 800 catastrophe and of the Transportation Safety Board (TSB) of Canada investigation into the loss of Swissair Flight 111. In their respective inquiries, both the NTSB and the TSB had much to say about the state of wiring:

  • NTSB: The center wing tank of Flight 800, a B747, exploded when the flammable vapors therein were ignited by a spark. The source could not be determined with certainty but was believed to have been from a wiring failure outside the center wing tank, which arced, causing the energy to jump to a fuel quantity indication system (FQIS) wire, which led to an explosion inside the tank. During the course of its investigation, which included examination of a number of B747s and other aircraft, the NTSB found several "potentially unsafe conditions" indicating that good wire husbandry was lacking in the industry.


  • TSB: Again, the source of the fire that downed the aircraft could not definitely be determined, but the TSB stated, "A segment of in-flight entertainment system network power supply unit cable exhibited a region of resolidified copper on one wire that was caused by an arcing event. This resolidified copper was determined to be located in the area where the fire most likely originated." The TSB noted that a system safety analysis is to be accomplished on every system installed in an aircraft (in this case, the in-flight entertainment system) but the requirements "are not sufficiently stringent to ensure that all systems, regardless of their intended use, are integrated into the aircraft in a manner compliant with the aircraft's type certificate." That is a nice way of saying the in-flight entertainment system was connected to a flight critical bus, rather than the cabin electrical bus.


    In its own assessments of wiring in aging transports, the FAA found unacceptable conditions, running the gamut from certification deficiencies to unacceptable maintenance practices. "Adoption of the proposed new and revised requirements and advisory material would help prevent future occurrences of the types of incidents and accidents described in the NPRM," the FAA said.

    The FAA estimates that the benefits of the total proposal outweigh the costs by a factor of 1.6 to 1. Specifically, the costs are placed at $474.3 million over 25 years, to include the cost of $35.40 per hour for maintenance personnel. The benefits, in terms of accidents avoided and unscheduled landings that do not occur as a result of greater EWIS reliability, are estimated at $755.3 million over 25 years. This figure appears low for at least two reasons. One, the value of the accidents avoided is placed at $507 million. It should be observed that this figure is based on $3 million for the cost of a life; note that TWA Flight 800 and Swissair Flight 111 together involved the loss of more than 450 lives, which implies a cost of at least $1 billion, to which the cost of the airframe must be added. Obviously, the FAA is not assuming that two aircraft are going to be lost from EWIS failures over the next two decades.

    The FAA is also assuming that unscheduled landings are avoided over 25 years, thereby saving about $152 million in costs. Given the gaps in service difficulty reports (SDR), the number of unscheduled landings the FAA is using may be low. The FAA says there were nearly 400 aircraft wiring failures between 1995-2002. Loose, chafed and broken wires accounted for nearly 84 percent of all wiring problems. Regarding the 400 figure, and given the long-standing and continuing parlous state of SDR reporting, it is suggested that figures used by the FAA (e.g., "wiring failures cause 22.1 flight delays per year ...") at best understate the incidence.

    Chafing and other problems

    A couple of citations in the NPRM merit comment. The executive summary says that in the past, "system safety assessments usually addressed only the effect of a wire failure on the system itself." Addressing wire failure in the singular avoids the issue of cascading system failures wrought by wiring bundle failure due to arc tracking, in which the damage from arcing affects not just the one wire but multiple wires.

    In the background section, the NPRM mentioned electrically conductive silver sulfide deposits on terminal blocks, a condition that is unrelated to aging, as the deposits can be found on near-new aircraft as well. The relative merits of silver-plating connectors, in comparison to nickel-plating, of electrical components in fuel tanks merits a mention here.

    In the general discussion section, the FAA discusses the role of vibration in eroding wire: "Causes of wire degradation must be addressed separately and collectively, and analyzed in relation to the entire airplane." Well and good, but by mentioning "vibration" in passing and not tying it to chafing directly, the impact of high frequency vibration normally inherent to flight may be lost. This vibration occurs in even smooth-air flying, and examples of the amplitude of movement experienced by wires under tension would have underscored the danger posed by chafing through very thin layers of insulation, and damage as well to cable ties and supports. It might be said that high frequency, low amplitude vibration eats through wiring insulation.

    Specific mention should be made of wiring required to regularly flex in position (such as that for doors and hatches), which probably should be annotated specifically for inspection on a regular basis. A classic example of this problem was the wire chafing caused by the ceiling-stowed doors on the MD-11, an anomaly detected only after the Swissair accident.

    Regarding circuit protection devices (e.g., breakers), the document talks about the need for breakers to be compatible with their associated wiring electrical loads, but the silent assumption that a fault wouldn't cause a breaker to trip, which in fact is often the case with the thermally activated breaker, is not in the FAA hazard syllabus.

    The document does say that if the wiring system hazard analysis uncovers the potential for a "catastrophic event," that basically it's back to the drawing board for an in-depth evaluation. For the problem of multiple systems damage caused by an arcing flashover in a bundle, it may be eminently logical to specify color-coding of non-cohabitable systems wiring, with resulting separation of these wires into separate bundles. This stratagem would be one way of coping with the seemingly intractable problem of multiple systems routed in the same bundle - and would be an improvement on simple labeling.

    Having said this, the discussion in the NPRM posed by wiring failures is still better and more comprehensive than anything to emerge from the FAA in years.

    Conduct of the inspections

    Details of how to conduct the wiring inspections are set forth in the proposed advisory circular, AC 120-XX, with a mouthful of a title: "Program to Enhance Transport Category Airplane Electrical Wiring Interconnection System Maintenance." Basically, according to this AC, the aircraft is divided into zones. If a particular zone (a) contains wiring and (b) there is the likelihood that the zone contains combustible materials, such as thermal/acoustic insulation blankets or lint, and (c) the wiring runs two inches or closer to both primary and back up flight controls, the zone must be inspected.

    There are a number of issues attendant to the approach the FAA has proposed. First, the two inch separation is a minimum and may not be enough. To quote from the National Transportation Board'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 specified that wiring for certain systems (including FQIS and other fuel system wiring, fire warning system wiring, primary generator feeder cables, and electro-explosive devices) must be separated by at least 3 inches from other electrical wiring. In contrast, 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 � inch in pressurized areas and � inch in unpressurized areas.

    "The potential for short circuits to damage nearby wiring (more than 1� inches away) has been documented in Safety Board investigations of numerous accidents and incidents."

    Some experts on wiring argue that three inches is a better standard, and some claim that six inches is better yet, given the potential of collateral damage from arcing.

    The limits of visual inspections

    The other problem with these inspections is that they are restricted to what is described as General Visual Inspections, or GVI. These inspections are done close enough to touch the wiring, or with a hand-held mirror. Yes, 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.

    So 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 less reputable operators, with indifferent attitudes about wiring husbandry and dirty airplanes, the program is probably well worth doing. The FAA has put forth the idea that wiring is a system, and it needs unique care, just like the black box or actuator it is connected to.

    The inspections are to be repeated at intervals for the life of the airplane. Obviously, they will get more onerous over time. And that raises a pertinent question: if the proper functioning and safety of wiring is so important, why not just replace it at, say, 15 years (the standard definition of an "aging" airplane)? This is what Federal Express did when converting the DC-10 to the two-pilot cockpit MD-10. The wholesale replacement of wiring was done by Air Canada on its aging fleet of DC-9s a few years ago. Under the so-called Energizer Program, Air Canada removed and replaced all the wiring, primarily to improve dispatch reliability, but also for safety's sake. The DC-9 featured "grandfathered" PVC wiring insulation, now acknowledged to be dangerous and not approved for aviation use.

    The case for rewiring

    Most recently, the U.S. Navy has embarked upon the complete rewiring of 30 C-2 aircraft, which amounts to some 23 miles of wiring per aircraft. These turbine-powered aircraft are used for carrier on board delivery (COD). "The old wires were susceptible to moisture and becoming brittle," said project engineer Don Sana.

    Complete rewiring has a number of advantages over mere visual inspections. For one thing, high power circuits can be better separated from low power signal wires. Right now, the two types of circuits are run in the same bundle on many aircraft, which just aggravates the damage potential of electrical arcing.

    Second, complete rewiring goes a long way towards eliminating the troublesome intermittent faults. And it does so by replacing the old wire with the newest and best types (thereby eliminating the aromatic polyimide wiring insulation in the Navy's C-2).

    Third, separation can be increased from the bare minimum two inches to three or even six inches, especially of high power wire and cable from other low power signal circuits.

    Fourth, wiring inside bundles will be replaced. And the bundles themselves can be, shall we say, disaggregated, from one 4-inch thick assemblage of wires to two 2-inch diameter groupings. By this means, improved segregation of vital wiring is accomplished, yet both bundles, with protection between the two, will fit within the same structural cutouts in the airplane.

    Fifth, the new wire starts the clock at zero, not 10,000 hours or more.

    The visual inspections proposed by the FAA are just the first step. For new aircraft, there is an opportunity to design the wiring system better. One suspects that wiring runs have been designed for ease of manufacture, but there is a growing imperative, as exemplified by the FAA's inspection program, to design them for ease of maintenance as well. As engineer Tadeuz Sitek of Poland said in his commentary on the NPRM, "Minimal entropy generation is a general evolution criterion for an aircraft." One way of assuring that is through periodic replacement, in this case the wire; another is by designing the wire installation such that it can be inspected.

    Incidents Involving Electrical Wiring

    From supplemental materials associated with the FAA's NPRM on wiring inspections

    1997: A Cessna 650 business jet experienced an in-flight fire during approach. While the airplane was descending through 4,000 feet, the crew smelled smoke. A navigation display went blank, and the airplane lost radio communications. After an emergency landing, ground personnel told the crew of flames burning a hole through the aft fuselage. An NTSB investigation traced the fire's cause to arcing between 115VAC electrical wiring and the hydraulic pump line above the baggage compartment. A fleetwide inspection of Cessna 650s found nine more airplanes with wire chafing against the same hydraulic line.

    2001: The FAA received a report from an aviation safety inspector that a scheduled maintenance check on a 757 freighter had uncovered auxiliary power unit (APU) generator power feeder cables that had chafed on their supporting clamp and bracket. Because of this chafing, the insulation of the power feeder cable became damaged and the exposed conductors arced to the cable clamp and supporting bracket, causing burning and melting of the supporting bracket assembly. The wire bundle, which was very heavy and stiff, changed its forward to aft direction about 45� after leaving the clamping point. As it changed direction, it exerted heavy pressure on the aft side of the cushioned clamp, thus squeezing out the cushion and contacting the bared metal clamp.

    When the operator of the airplane had seven more of its 757s inspected, four were found to have the feeder cables repaired or repositioned. The cables appeared to have been installed according to the installation drawings. The FAA issued an airworthiness directive to require that operators provide adequate clearance between the wire bundle and structural components to prevent wire chafing.

    August, 2002: About 17 minutes after departure from Sydney airport, a 747 experienced a cargo smoke warning and signs of smoke in the cabin and on the flight deck. The cargo fire extinguisher system activated and the airplane returned to Sydney and made an overweight landing. The airplane was evacuated on the runway with fire services present. After evacuation, the cargo hold was opened and smoke was found. Investigation revealed that a wire bundle had chafed against the chiller boost fan in the left hand cargo bay sidewall and the resulting arcing had burned the wire bundle and ignited the surrounding insulation blankets, causing smoldering and smoke. The 20-amp circuit breaker for the fan had not tripped. The fan had a known vibration problem and this particular fan showed signs of vibration damage.

    August 2003: Not included by the FAA, but a similar scenario to Swissair 111. A Lufthansa 747 inflight entertainment system was installed as a minor modification without approvals by the LBA (the German equivalent of the FAA). A plug carrying 115VAC was not sealed and because it was located in the lower dado area just above the cabin floor, took in air conditioning condensation water, shorted out and started a fire alongside the passenger in seat 24. The cabin crew ripped out the liner panels and expended two fire extinguishers to control the blaze. The flight station CB did not trip. See imagery at www.iasa.com.au/blaze.htm. Note scorched insulation blankets.

  • Live chat by BoldChat