Usually taken for granted, wiring is emerging as a major safety concern in aircraft and the FAA is taking the threat seriously
Graham Warwick/WASHINGTON DC
Certain aircraft accidents have become milestones in aviation safety. Just as the Aloha Airlines Boeing 737 fuselage failure in 1988 alerted the world to the problem of ageing structures, the ongoing investigation of the Trans World Airlines Boeing 747 inflight explosion in 1996 has turned a spotlight on ageing systems.
While all aircraft systems are coming under scrutiny, the possibility that arcing in the centre wing fuel tank caused the explosion which downed TWA Flight 800 has focused attention on electrical system safety, and particularly problems associated with wiring.
The Federal Aviation Authority's ageing systems plan is modelled on the agency's successful ageing structures programme. It has begun with a review of aircraft service histories and fleet sampling "in an effort to get information on how wiring and other systems age over time", says Beth Erickson, director of the FAA's aircraft certification service. These data are being used to define an action plan which may include airworthiness directives, maintenance programmes and research into new technology.
Fleet sampling has involved the visual inspection of several aircraft while they were undergoing heavy maintenance. The FAA's on-site inspections of three McDonnell Douglas DC-10s more than 24 years old, a 29-year-old DC-9 and a 19-year-old Boeing 727 have revealed some wiring deterioration and found that maintenance practices needed improvement.
As aircraft wiring ages, it becomes stiff and easily cracked if improperly handled or inadequately restrained. Wire bundles are difficult to inspect and contamination with dust, fluids and even metal shavings during maintenance is a common occurrence, the FAA has found. At the same time, inspection criteria and maintenance guidelines for wiring are inadequate.
A series of "intrusive inspections" of retired aircraft is under way to determine the exact condition of wiring in older types. Inspections have been completed on four aircraft - an Airbus A300, a Boeing 747 and two DC-9s - covering the four main wiring systems used. Two more will be inspected this month: a Boeing 737 and a Lockheed L-1011.
Before starting, an "in-depth protocol" was developed to ensure the inspections were conducted in a consistent manner, says Erickson. The inspections involve aircraft around 20 years old that have been retired from service only recently, to ensure that any wiring degradation "is not an artifact of storage". After completion, the aircraft will be used as testbeds for new technologies and maintenance procedures.
Erickson describes the effort as a "comprehensive programme to increase our scientific knowledge of how wiring ages over time, which will result in a multi-faceted approach to the use and maintenance of wiring in aircraft. As with ageing structures, we will get well based with data before figuring out the right action."
The result of the ageing systems effort is expected to be the definition of improved inspection and maintenance guidelines; enhanced design practices and training for certification engineers evaluating the designs and FAA inspectors overseeing the airlines. "We will also try to get better data reporting on ageing systems," says Erickson.
"The priority is on wiring, but we will go back and do the other systems," she says. Wiring is a priority as "we have seen it is an area where we need to pay attention", she says. "No immediate safety concerns have been identified to date, but there are issues we want to follow up."
Wiring maintenance is of particular concern. "You have to have very good maintenance practices and procedures, and we've seen room for improvement, "Erickson says. "You have to take care with repairs and modifications."
Sparking research
Research into new technologies to improve wiring safety is an integral part of the programme. To that end, the FAA has joined the US Navy in funding development of arc-fault circuit interruption technology, which promises to overcome some of the problems caused by wiring degradation.
Arc faults have been described as "ticking timebombs", says Steve Eisenberg, vice-president of Eaton Aerospace - recipient of one of two FAA/USN research contracts. Although arcing can occur in new aircraft, he says, the problem is exacerbated by the "ageing and abuse" of wiring in older aircraft.
The problem with arcing is that the intermittent faults caused by two wires with damaged insulation touching within a bundle cannot be detected and prevented by conventional circuit breakers, says Joe Engel, manager of electronic research and development at Eaton's Cutler-Hammer division.
"Circuit breakers in aircraft are simple, reliable devices that respond to the heating caused by an overcurrent," he says, "With an arcing fault, the wires touch, blow apart, the current ceases, then they touch again. The heating caused by the [intermittent] current is below the normal 3-5A rating of the circuit breaker."
When two wires touch, the current can reach 150A, but only momentarily. What is needed is a device that can detect arcing and interrupt the electrical supply to the affected wires before the whole bundle is involved, says Engel. "We want to prevent damaged wires propagating faults to undamaged wires."
The Eaton approach is based on arc-fault interruption technology that will be installed as standard in US homes from 2002. In essence, the device keeps a count of each momentary fault and breaks the circuit when the number of faults exceeds a threshold.
"The device continuously looks at the current. When a fault occurs it goes into a register. If it occurs again, it gets added to the previous value and begins accumulating. If it accumulates to the threshold, and the frequency indicates a danger of arcing, the device trips the circuit," Engel explains.
Adapting the technology for use in aircraft will be a challenge, he says. One reason is that power transients which are normal in aircraft electrical systems have characteristics that are similar to arcing faults. This requires detailed analysis of the signature, or fingerprint, of arc faults to enable the device to reject false alarms. The signature will have to be valid for all wires in an aircraft. "We cannot program one circuit breaker for each individual wire,"Engel says. "The device cannot be circuit-specific."
Eaton has been working for two years on adapting the technology for aircraft use, initially to characterise the electric system and understand the known problems. "We have collected a lot of waveforms," says Engel, "and we have ideas we need to work on."
The biggest challenge, he says, will be making the arc-fault interrupter small enough and cheap enough. The device will have to be miniaturised to fit inside today's circuit breakers and will have to be "very inexpensive and extremely reliable", Engel says.
Under its FAA/USN contract, Eaton is to build 20 flight-ready arc-fault breakers for certification testing. "We have joined with the navy to spur development of this technology for aircraft," says the FAA's Erickson. "The current technology is 10 times the size of a normal circuit breaker, but it is developing rapidly."
Whether arc-fault interrupters re-place conventional circuit breakers will be up to the airlines, Erickson believes. One benefit cited by Eaton's Eisenberg is that the technology "is predictive in nature. It eliminates the possibility of catastrophic failure later."
Other lower-technology approaches are also receiving attention. Device Technologies reports increased interest in its Spring-Fast composite grommet edging, which provides increased protection against chafing of wire bundles passing through openings in the aircraft's internal structure.
Developed originally as a labour-saving device, the snap-on spring-steel grommet is used by a number of aircraft manufacturers, says Nick Petri, president of the company, based in Marlborough, Massachusetts. Now Device Technologies is hoping to persuade airlines to replace conventional nylon grommet edging during maintenance.
Bare metal edges
Nylon grommets require adhesive and deteriorate with age, often falling off and exposing wires to bare metal edges, Petri says. A study to determine whether the company's composite grommet edging improves wiring safety will be completed in November, he says.
Whatever the result, it is clear that the focus on wiring safety is likely to intensify in the coming months as the FAA begins to roll out the results of its data-gathering efforts. And attention will not be restricted to older aircraft.
Wiring design issues uncovered during investigation of last year's Swissair Boeing MD-11 crash, coupled with the increasing electronic complexity of today's airliners, will ensure that electrical system safety stays at the top of the agenda.
Source: Flight International
