Evolution of Complexity

The amount of wiring in transport category aircraft has grown steadily over time, with no plateau visible yet. The more of it, the greater the potential exposure to wiring failures, as evidenced by numerous airworthiness directives issued recently (see ASW, May 3, for examples of electrical and wiring system hazards).

The increase in total length of wiring is a function of aircraft size, complexity, evolution to fly-by-wire, and added features such as in-flight entertainment systems. Indeed, in its investigative report of a severe in-flight arcing event involving a United Airlines [UALAQ] B767, which was forced to make an emergency landing at London's Heathrow airport, the UK's Air Accidents Investigation Branch (AAIB) touched on the safety challenge presented by the increasing density of electrical components in modern jets:

"The increasing reliance on electrical power on modern and future public transport aircraft for flying control, engine and flight management systems with the associated increase in the use of computers, in addition to passenger services and entertainment systems, makes such aircraft more vulnerable to electrical fires and their potential affects, particularly if the flight crew do not receive timely warnings of electrical fire initiation." (The link to the full AAIB report is at end of this article.) A few general observations (questions, more like) may be in order. First, the big ETOPS twins now have a fourth generator, emblematic of the general increase in electrical power- generating capability, with more power feeder cables to main load centers and from there to bus-ties and busses. At the same time, much of the additional wiring is low voltage sensor and signal wiring. The fact that high and low power wires are often routed in the same bundle or adjacent bundle(s) raises a question about required separation and redundancy of flight critical systems. The end-use components may be separated but the associated wiring is often run in bundles (see ASW, April 19).

Second, circuit protection device (CPD) technology may not have kept pace with the increase in wiring. Mechanical push-pull thermal circuit breakers still dominate, with efforts under way to field arc fault circuit breakers (AFCB) and possible universal fault interrupters (UFI). These advanced CPDs are on the cusp of widespread deployment (see ASW, Sept. 16, 2002 and March 22). Some of the new AFCBs actually are now undergoing trials in non-critical circuits on some airliners.

How often are circuit breakers checked (i.e., calibrated) for their trip-threshold levels? A 2002 study of aging circuit breakers found they hold up well in years of service, but that there has been a tendency to use them inappropriately as "on-off" switches (see ASW, Feb. 10, 2003).

Third, wiring represents the arteries and veins of the airplane's electrical system. More wiring is associated with a commensurate growth in electrical componentry and a quantum jump in circuit complexity. At the solid-state level, there has been a marked increase in circuit and power density. Every new system adds wiring (traffic alert collision avoidance systems, TCAS; Mode S; enhanced ground proximity warning systems, EGPWS; in-flight entertainment systems, IFE; online Internet access; reduced vertical separation minimums, RVSM; cargo hold fire detection, etc., to name a few). One is reminded of triffids, creatures from the British science fiction thriller "Day of the Triffids." Triffids were long, thin-stalked plants that thrived in colonies, quietly multiplying, eventually attacking with a sudden, violent sting. Every additional electrical component adds a tentacle to the electrical triffid.

Fourth, is fault-finding and troubleshooting technology keeping pace? Is BITE (built-in test equipment) reliably pointing to faults after power-up? Are computer-monitored power-up routines now sophisticated enough to pick up an individual LRU (line replaceable unit) that passes a BITE check but may be underperforming and on its way to failure? The day may be coming when aircraft systems-management computers monitor for incipient catastrophic electrical faults and activate a software-driven CPD. To what extent will quick access recorders (QARs) be able to contribute in the future to policing the integrity of electrical systems?

Fifth, are electrical systems adequately protected against power failures, spikes and surges? Based on the number of recent total cockpit instrument blackouts, it is fair to suggest that not all sensitive electronics are adequately protected against arcing or its external electrical equivalent, lightning (see ASW, Jan. 5, "Blankety, Blank, Blank').

Sixth, there is the matter of inherent reliability of all components and the inclusion of the MTBF (mean time between failure) into any safety calculation. Note the industry's resistance to any consideration of wiring reliability as a system co-equal with other airplane systems, an objection that could unravel much of the wiring safety-related work of the Aging Transport Systems Rulemaking Advisory Committee, or ATSRAC (see ASW, May 5, 2003).

To be sure, the sheer length of wiring is a paramount parameter, in addition to installation, routing, maintenance, exposure to accidental trampling, and so forth. Not all failures will be catastrophic, of course, but the potential points of failure clearly are increasing along with the growing complexity of interconnected electrical systems. Thus, the quality, type and positioning of CPDs becomes more critical.

Seventh, when a wiring fault occurs (e.g., arcing), the first warning will be late (at least with today's detection and CPD technology). Wiring is concealed, so smell and smoke are vague indicators - not pointers. Closed circuit television cameras behind the linings may not be the best answer to pinpointing the source in time. However, IR (infrared) mapping may be a nascent technology that could be used to pinpoint fire (see ASW, April 26). Pinpointing an electrical fire remains an (as yet) unacknowledged factor in ETOPS flying. As in the January 1998 fire in the electronics and equipment (E&E) bay of a B767, the captain radioed, "We would like to get her on the ground just as soon as we possibly could." The UK's Air Accidents Investigation Board (AAIB) report is a spine-chilling read (see http://www.dft.gov.uk/stellent/groups/dft_avsafety/documents/
pdf/dft_avsafe ty_pdf_503084.pdf). Extracts of the report reveal some of the hazards involved in a case typical of so many in-flight arcing events (see 'Spatter' below).

Eighth, Federal Aviation Regulations (FARs) have not kept pace with the march of technology. The Oct. 16, 2002, final report of the ATSRAC's Working Group No. 6, "Wire Systems Certification Requirements," is instructive:

"Current regulations do not adequately address requirements for wires in system separation, safety assessments, protection of wires in fire zones, protection of wires in cargo and baggage compartments, and accessibility of wires for inspection, maintenance, repair, etc.

"Current requirements also do not clearly define the necessity and method for wire identification ... necessary for safe flight.

"The proposed regulations and associated advisory materials are considered necessary to address these specific concerns and more."

Yet the regulations proposed by ATSRAC, addressing these shortcomings, are generating objections from the industry. At best, the proposed regulations will be issued formally for industry comment in early 2005, according to recent ATSRAC documents. In other words, it would be mid-2006, at the earliest, before the new regulations take effect. That means the new wiring-related regulations may not take force in time to affect the design of the Airbus A380, which is expected to enter service in 2006 and will feature an industry- leading 300 miles of wiring, or the electrical system design of Boeing's new 7E7, which is expected to enter service in 2008.


--------------------------------------------------------------------------------

Arc Induced Copper 'Spatter'

Electrical arcing forced a United Airlines B767-300 on a planned Jan. 9, 1998, ETOPS [extended operations] flight from Zurich to Washington, D.C., to make an emergency landing at London's Heathrow Airport. Investigated by the UK's Air Accidents Investigation Branch [AAIB], the August 2000 final report captures many of the typical elements associated with in-flight electrical arcing and fires. This particular emergency stemmed from a food chiller installed in the electronic and equipment [E&E] bay the day before by two mechanics at Washington's Dulles Airport. They had never performed this task before and did not strictly follow the maint-enance manual. A jagged edge on the chiller scraped insulation off some of the wiring as the chiller was pushed into position.

Pertinent extracts from the AAIB report (numbers equate to sections in the report):

1.11 Despite the number of recorded parameters, the DFDR [digital flight data recorder] contained no information that was of assistance in determining the cause, or the time of initiation, of the electrical failure.
1.12.3.6 Examination throughout the E&E bay revealed the presence of small curled aluminum swarf, typical of that produced from the drilling of holes. [Swarf is the industry term for drill shavings.]
2.3.3 Localized extreme temperatures within the [wiring] loom probably existed in flight for at least 30 minutes, supporting the concern that further wiring damage could have occurred to previously undamaged wires should the flight have continued for a longer period of time. The ETOPS clearance for the Boeing 767-300 is that it should always remain within a distance equivalent to 180 minutes single engine flying time from a suitable diversion airfield.
2.3.4 The concentration of smoke in the E&E bay throughout the event had not been sufficient to activate the sole smoke detector for the bay.
Finding #12. Arc-induced copper 'spatter' could rapidly spread fire effects to adjacent areas.