MOVING BEYOND WIRING INSPECTIONS TO PREDICTIONS
effort will detail the incidence of chafing, nicks and cuts. However,
techniques also exist to "predict" the amount of life remaining
in aircraft wire [see ASW, July 27 and http://www.aviationtoday.com/reports/wiring.htm
]. The predictions - which project the probability
of the insulation becoming breached [cracked into the conductor] in the
future - can be used to target maintenance programs to replace wire in
key locations before problems occur. Such scheduled maintenance could
offer considerable cost savings over unscheduled maintenance that may
be required after problems are found.
The methodology has been developed by Dr. Armin Bruning, Ph.D., a recognized expert in aircraft wiring. Bruning heads a small company in Herndon, Va., that which specializes in aircraft wiring assessments.
The procedure involves the following steps:
Dividing an airplane into zones. Typically, " zones" will be identified, although the number could be as many as 20 zones, depending on aircraft type. These zones highlight where the most and the least wiring problems occur. Uniform conditions prevail in each zone. One of the zones is always identified with the least number of problems for comparative purposes. The trouble zones include: Electronic and Equipment [E&E] bays, Wheel wells [where wiring is subjected to flexing, temperature and moisture extremes]. The "cesspool", an area where the wings join the fuselage known to accumulate dirt and moisture, "swamp" at the bottom of the fuselage, Leading and trailing edges of the wings, Galleys, etc.
Extracting wire specimens. Each wire sample is about 1.5 ft. in length; about six specimens will be removed from each zone. Replacement wire will be spliced in. To be sure, improper splices can result in current leakage, but the number of splices required after the samples are removed is considered infinitesimal compared to the hundreds of splices on most jetliners. Extracting the wire samples, a process that can take a day or two, can be done during scheduled maintenance.
Proof testing. The wire samples are tested, in a manner analogous to checking a bicycle tube in a tub of water for air leaks. This testing reveals breaches in the insulation that might not be discovered visually. The exact point of the breach can be determined by slowly pulling the wire out of the water. When the point of breach hits the air, the change in current flow is very apparent on test instruments.
Dan Kasture, an engineer in Bruning's firm, says "Sometimes we don't see the breached insulation on site, but when we proof test it under current you can spot the leakage". Kasture and Bruning have concluded that visual inspections of wiring have limited utility detecting tiny but potentially significant breaks in insulation. Samples with breached insulation are considered to have zero percent of life remaining.
Accelerated testing. Wire samples that survive proof testing [i.e., no insulation breaches] are then subjected to accelerated testing, involving exposure to high humidity, temperature and strain. This testing continues for a period of a few days to a few weeks, until the insulation is breached. Every ten hours, the wire samples are subjected to a 2,500 volt current to assess if a breach in the insulation has occurred.
Determination of life remaining. The accelerated testing produces data which generate various reports of the airplane's wire, by zone. One such report will show the amount of insulation aging by zone. This type of information can be used to target wire replacement efforts to those areas where the insulation has been shown to be highly aged. Another report could show the probability, by zone, of insulation failure [breaching] over time. In the table illustrated, for example, the forward electrical load center is predicted to not have any insulation breakdowns for the next two years, but in a fleet of 100 aircraft, 24 failures are predicted between the end of the second year though the fifth year, with another 35 failures between the end of the fifth and the end of the tenth year. Note also the high probability of failures in the first year in the main wheel well. "We find a lot of problems with wire adjacent to hydraulic components", Bruning observed. This kind of information, Bruning said, can be used by top management for cost-effective targeting of the maintenance effort. "You would fix these areas in the first-year range first to avoid unscheduled maintenance", he suggested.
Another report could graphically portray the number of failures per 10-hour test interval over the life of the wire, providing a picture of least, most and average durability. This chart, Bruning said, " shows that the vast majority of the wire is better than the weakest link". But if you're hanging over a cliff on a chain, are you happy with the average link strength, or are you interested in the weakest link?", he asked.
Bruning offers these points: "I think I know what to do to reduce the probability of a wiring-related accident by a significant factor". "We can tell how bad the situation is, so the treatment can be managed, and inspections and repairs can be focused on the worst areas". "You can measure how delicate the insulating polymer has become, which has enormous implications for how you train your people. You can't pull bundles on wires that are 75% aged; the fracture point is much more delicate. So you find out the level of delicacy, and change the way your people handle the wiring".
In other words, efforts are focused on the pre-emptive replacement of wire in those areas where significant insulation breakdown is predicted. By this means, the prohibitive cost of replacing all the wiring in an older jetliner could be avoided, while assuring a continuing high level of safety.
Test method supports FAA goal. One concern might be the FAA's response to such testing. The discovery of significant problems could result in aircraft being grounded. However, these reports are of predicted insulation failures [other test methods can be employed to identify existing insulation failures]. On the other hand, FAA officials have publicly expressed their determination to find and fix problems before safety is compromised. As such, the test methodology described here could be employed to assess the life remaining on wiring in a sample of the fleet, as is already being suggested by the government-industry task force crafting the soon-to-be-announced wiring assessment program.
List of Annexes