Airplane travelers addicted to worry sometimes fret about an engine failing or a wing falling off, but they rarely stress about wiring. But they might — if they knew there are miles of aging wiring, intertwined like spaghetti, behind the side panels of a commercial airliner’s fuselage. An intermittent electrical short due to frayed insulation can make lights blink or air conditioning falter, or even cause fatal crashes, as with flights SwissAir 111 or TWA 800.
The challenge to engineers is how to locate a wiring fault before — not after — it becomes a problem. That’s quite a trick. Sandia researchers believe they have helped achieve a solution.
The newly patented method — called PASD, for Pulse Arrested Spark Discharge — relies on a Sandia specialty called pulsed power, developed over decades of research. Usually the public thinks of this research — if it thinks of it at all — in terms of Sandia’s massive Z machine, which sends great bursts of electrical current down conduits as big around as a horse’s girth.
But the PASD device in its experimental state was only the size of a small refrigerator.
Now, licensed in late April to Astronics Advanced Electronic Systems of Redmond, Wash., and combined with that company’s other patented test methods, it’s the size of a small suitcase. It can be plugged into aircraft-installed wire harnesses, 40 wires at a time, to check them for the very small insulation breaks associated with intermittent faults.
These sporadic short circuits occur where two exposed conductors, or a conductor and aircraft frame, make temporary contact during flight. Vibrations caused by turbulence may cause wires to touch, interrupting power to sensitive electronics and possibly damaging wires. These conditions are tricky to diagnose when the aircraft is on the ground because the shorting wires often will have shifted back to a non-shorted state. Sometimes these breaks can barely be seen by the naked eye because missing insulation may be the size of a pinhole, or nearly invisible like a fine cut from a razor blade. Traditional wire-test systems have great difficulty finding these faults.
Location method simple
PASD locates faults by a method simple to understand at its basis.
What Sandia has done, under project lead Larry Schneider (1650), is send a high-voltage pulse down the wire. The pulse is very brief (nanoseconds — a nanosecond is a billionth of a second) so the energy is very low.
The situation is analogous to a waterfall — very high, higher even than Niagara Falls — with only a trickle of water going over it, and even the trickle lasting for only a fraction of a second. Water will certainly fall far and fast, but the tiny amount arriving at the bottom will cause only a tiny splash — not enough to do damage.
But because the voltage is so high, the little bit of harmless energy will jump like a rabbit from the smallest insulation break to the bulkhead or to another nearby damaged wire. That jump — like static electricity leaping from hand to doorknob — in effect lights up the damaged spot like a tracer bullet at night lights up on its trajectory to a target. The amount of time it takes for the current to return to its source is analyzed by the automated test-set to tell within inches how far the break is from the test entry point.
The simple method should make it financially feasible for airlines to quickly diagnose and locate intermittent faults that have plagued the industry and cost millions of dollars in lost revenue due to aircraft downtime.
Is the technique in reality more complicated than that? You bet. That’s why the National Nuclear Security Administration and the US Navy supported the research, followed by the Federal Aviation Administration, to the tune of about $2 million. It’s why it took two years for Astronics to adapt it to its suite of tools, called (fittingly) ArcSafe, a method developed over four years of research to locate wire breaches with the potential for electrical shorting. ArcSafe is expected, by the way, to be on the market by September.
“Rather than ripping apart the fuselage for access to a faulty harness that may run the length of the plane, airline mechanics will be able to use this new tool to efficiently locate and repair the fault,” says Larry.
Says Astronics team leader Mike Ballas, "We really value PASD technology. We licensed it, turned it into a practical portable test unit targeted for the aviation industry to find intermittent faults, and we believe it’s the best way now to do the job. It’s a nice complement to our patented technology.”
Says Robert Pappas, the Federal Aviation Administration’s project manager for aging aircraft research and the first to recognize the value of Sandia’s original research proposal in 1998, “It would have been unfortunate if PASD had been developed and then remained stuck in a lab. Integration of the technique [with those of Astronics’ ArcSafe] is a real success story.”
Faults detected early
“Rather than reacting to a problem, these systems can find a fault before it manifests into a catastrophic event,” says Larry, who predicts PASD will one day become a final test for the wiring harnesses of passenger cars and new homes, as well as for military tanks and the hard-to-reach wiring behind the steel bulkheads of submarines.
There’ll be problems, just the same, in getting the method accepted, says Mike Walz, current FAA overseer of the project. For one thing, he says, “What PASD looks like is an electrostatic discharge [ESD] — something aircraft manufacturers work hard to keep out of their wiring system.”
One researcher responds with humor, “PASD is a little like homeopathy: Uncontrolled ESD can kill you, but a little bit can help cure you.” (Homeopathy holds that dangerous material, extremely diluted in solution, can be helpful in healing.)
Other problems involve the varying resistance of wires over long distances, called electrical impedance, particularly in the branched wiring systems prevalent in aircraft. This was a problem for earlier versions of ArcSafe, which used a DC current to detect breaks. Varying impedance meant it was difficult to accurately locate an intermittent fault, since electrical return signals were inconsistent, especially on complex wire geometries. Still, the DC method is most effective for identifying ordinary faults and has been retained for quick fault screening. To enhance its fault-locating ability, a new Astronics method allows the PASD pulse to ride upon the DC current like a rider on a horse. The DC current provides support for the high-voltage pulse, which then can be effective even a hundred feet from its starting point in accurately locating critical breaches in wire insulators, even those occurring on branched wire harnesses. The distance to a fault is computable, regardless of changes in impedance produced by the wiring as it reacts to the PASD pulse at various voltage levels.
Insulation defects hard to find
“Wiring insulation grown defective over time can cause malfunctions or even fires, but is devilishly hard to spot and even harder [once spotted,] to [exactly] locate,” says Larry. “Other methods have faltered when confronted with the varying impedances of bundles of wires, or the difficulty of providing the exact location of the defect as wiring bundles branch into other bundles. This nondestructive, inexpensive method not only detects cracking or pinholes but also is able to pinpoint the defect’s precise location to facilitate wire replacement.”
The actual location of the defect may require examining several possible branches since the same distance-to-short may exist along several paths, but the problem is minor compared with the alternative.
The hybridized system is an improvement be-cause of its greater robustness on complex systems.
The technique probably would be best used to check those wiring subsystems that are known problem areas, says Larry. To check all the wiring in a plane might take several days.
Involved in the project over the years for Sandia have been Mike Dinallo, Steve Glover, and Gary Pena (all 1654), Kevin Howard (1653), Tom Lockner (5445), and John Barnum (6452).
“We’re advertising the system now and we’d love to take orders,” Mike Ballas says.