Let's face it, the average age of the nation's aircraft fleet is getting older. As we struggle to maintain these older aircraft, aging wiring issues come to the forefront of critical issues. Swiss Air Flight 111 and TWA Flight 800 have given industry a heightened awareness of aging wiring issues. Damaged wiring can result in loss of function of critical controls or instruments. In extreme cases, damaged wiring can start a chain reaction that results in an on-board fire. In the past, visual inspection was about the only tool mechanics had to inspect wiring during routine maintenance. But visual inspection doesn't always provide the clues necessary to detect flaws, especially within wire bundles. So what can we do to prevent incidents or accidents from happening? What kinds of tools are available to help us detect faults in old wiring? We will discuss those topics in this article.
One of the biggest dangers associated with aging wiring is arc faults. As wiring gets older, its protective insulation gets brittle. Normal flexing of the wires can cause breaks in the insulation. It is at these break points that an arc fault can occur. An arc fault is a brief arcing event between two wires or between a wire and ground. The bad thing about arc faults is that they typically aren't of a severity to trip a traditional circuit breaker.
Traditional circuit breakers are meant to protect the circuit's wires from overheating due to a short within the circuit. When there is a short, bi-metallic elements within the circuit breaker heat up and cause it to trip.
Arc faults, however, are seldom of a duration to cause the wiring or circuit breaker to heat up to the point that the circuit breaker will trip. Despite that, an enormous amount of heat is generated at the spot of the arc fault. If there are any combustibles nearby, an arc fault can easily initiate a combustion situation.
In the case of arc faults, one of the ways that they can be detected is through the use of smart circuit breakers. The FAA teamed up with the Naval Air Systems Command (NAVAIR), the Office of Naval Research, and industry to develop "smart" circuit breakers that can detect arc faults. Several companies such as Eaton Aerospace, Texas Instruments, and AMETEK are developing "smart" circuit breaker products. These circuit breakers use advanced circuitry to detect faults in the wiring down to the arc fault level and trip before excessive damage is caused by the arc fault event. In many cases, these units are even able to tell the difference between an arc fault and a normal spike in electrical loads (such as a starter coming on-line) and prevent itself from tripping during normal operating conditions, which is known as nuisance tripping. If a "smart" circuit breaker is not able to differentiate between arc faults and normal conditions, then nuisance tripping can result. On the other hand, if the circuit breaker is designed to ignore the smaller anomalies in order to reduce nuisance tripping, it could miss a legitimate arc fault discrepancy. The "smart" breakers being developed need to take these factors into consideration in order to offer protection from arc faults without the problems of nuisance tripping.
What to do now?
Well, whether through circuit breaker tripping or system malfunction, if you know you have a problem, where do you look? Sometimes, wire runs can be very long and inaccessible, behind sidewalls or other paneling. Even if you know which wire is causing the trouble, you probably don't know where along that wire to look.
Well, there are some products available now that can narrow down the location of the fault. Several handheld testers are now available that can not only tell you whether or not a fault exists in a wire run, but where the location of that fault is.
Advanced system tester
One of the products available for this type of wire testing is 3M's new Advanced System Tester 900 AST. The tester was unveiled at the NBAA show last October.
The 900 AST is the size of a typical digital multimeter. In addition to offering all of the functions of a digital multimeter, the 900 AST offers a time domain reflectometer (TDR) to identify the distance to an open circuit as well as a function called resistive fault locate (RFL) that calculates the distance to a short or ground.
The TDR on the 900 AST can examine single conductor wires with a reference wire, twisted pair cables, shielded wires, and coax cables.
While the TDR function of the unit works well with open circuits, it does not detect all faults or grounds. This is where the RFL function comes in. This function will display the distance to a fault or ground accurately. A color-coded ground on the screen shows the mechanic how to set up this test.
The 900 AST has the ability to store up to 200 TDR traces in its internal memory. Past traces can be recalled in order to compare it to a live trace. In addition, wire traces can be uploaded to a computer using a built-in infrared port.
Eclypse International Corporation has launched its own fault location meter. Dubbed the ESP Meter, it can also locate faults in wiring. It uses Standing Wave Reflectometry to locate faults in wiring up to 1,000 feet. It can also store data and link to a PC for upload via a RS-232 data port. It is being deployed for operational evaluation with units at Marine Corps Air Station, Cherry Point, NC and Tinker AFB in Oklahoma.
The technology available to help us do our jobs is increasing each day. With these additional tools available, mechanics may be able to meet the demands that aging wiring poses to the safe operation of the aircraft we work on. AMT