Finding airplane short circuits before they cause trouble — continued
Gaining acceptance
There are still problems to surmount
in gaining acceptance of the method, 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.”
Kevin Howard prepares to
acquire PASD data on electrical
wiring in the wheel well of a
retired Boeing 727.
(Photo by Randy Montoya)
Astronics welcomed the addition of
PASD because of other problems involving
the varying resistance of wires over
long distances, particularly in the branched
wiring systems prevalent in aircraft. This
was a problem for earlier versions of the
company’s ArcSafe, which used a DC current
to detect breaks. Varying resistance
meant it was difficult to accurately locate
an intermittent fault, since electrical return
signals were inconsistent, especially on
complex wire geometries. (The DC method
remains the most effective for identifying
ordinary faults, and Astronics has retained
it for quick fault screening.)
To enhance its fault-locating ability,
Astronics developed a method that allows a
PASD pulse to ride on the DC current like
a rider on a horse. The DC current provides
support for the high-voltage pulse, which
then can accurately locate critical breaches
in wire insulators, even those occurring on
branched wire harnesses, even a hundred
feet from its starting point in.
Devil in the details
“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 Sandia’s Schneider. “Other methods
have faltered when confronted with the
varying resistances 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.”
Technical Contact: Larry Schneider (505) 845-7135, lxschne@sandia.gov
Media Contact: Neal Singer (505) 845-7078, nsinger@sandia.gov
A quarterly research and development magazine.
