Dagger Dirk
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posted 7th February 2001 15:48
The Prof Furse
Article on Wiring Dangers (in full)
Aging, brittle
wiring within aircraft poses a hidden hazard that emerging
technologies aim to address
Down to the Wire
By Cynthia Furse & Randy Haupt, Utah State University
As today's military and commercial aircraft age past
their teen years, the many kilometers of wiring buried
deep within their structures begin to crack and fray.
Once thought to be rare and benign, such faults are
found by the hundreds in a typical aircraft. Unlike
obvious cracks in a wing or an engine, though, damaged
wire is extremely difficult to detect. But the resulting
arcing and electromagnetic emissions can be just as
deadly: faulty wiring has been blamed for the downing
of Swissair 111 near Nova Scotia in 1998 and of TWA
800 off New York's Long Island in 1996 [see http://www.spectrum.ieee.org/WEBONLY/publicfeature/feb01/wiref1.html
]. Indeed, any densely wired system is vulnerable--the
space shuttle, nuclear power plants, subways and railroads,
even the family car.
Public scrutiny has prompted strongly worded recommendations
from the likes of NASA, the U.S. Federal Aviation Administration,
and the National Transportation Safety Board (NTSB)
[see "Government and Industry Take Action" at http://www.spectrum.ieee.org/WEBONLY/publicfeature/feb01/wiresb1.html
]. "The safety of the nation's wire systems is an issue
of major importance to us all," noted a White House
report issued last fall. Several months earlier, the
NTSB concluded its lengthy investigation of TWA 800
with the verdict that a short circuit
sparked an explosion in the center wing fuel tank. The
condition of the wiring, it noted, was "not atypical
for an airplane of its age." Among the NTSB's recommendations
was to incorporate into aircraft "new technology, such
as arc-fault circuit breakers
and automated wire test equipment."
Solutions are not straightforward. Among the most promising
technologies are advanced reflectometry methods, for
routine maintenance; so-called smart wire systems, for
continual, on-the-spot wire testing; and arc-fault circuit
breakers and advanced fire suppression techniques,
for minimizing damage and injury should a fault occur.
Remaining challenges include detecting the minuscule
insulation breaks that encourage arcing; optimizing
the benefits and mitigating the risks of the various
wire testing techniques; and getting a better handle
on the labyrinthine complexity of aircraft wiring systems.
Failing the test of time
A healthy wire is perhaps the simplest, yet most important,
element in an electrical system. Typically, a copper
conductor (from 1 to 10 mm in diameter) is covered by
a thin outer insulation (from 0.5 to 2 mm thick). Damaged
insulation can expose the copper, giving rise to arcs,
shorts, and electromagnetic emission and interference.
Arcing occurs when current flows from the wire through
ionized air to another conducting object, such as a
second wire or the aircraft structure. A short circuit channels the current to an undesired
conductor. If an external shield or braid protecting
a wire is broken, the resulting antenna radiates the
signal on the wire.
As the wire ages, the insulation may become brittle
and crack. Vibration can also chafe the insulation as
wires vibrate against each other, a tie-down, or any
other hard surface. Maintenance can also be hard on
wires, as they may be nicked by workers' pliers, or
bent beyond their tolerable radius, or sprinkled with
metal drill shavings, chemicals or water, or even used
as stepladders in hard-to-reach places. [see Photos
at http://www.spectrum.ieee.org/WEBONLY/publicfeature/feb01/wiref1.html
] that show cracked and singed wiring taken from U.S.
Navy planes.]
But perhaps the greatest concern is the breakdown of
the wire's insulation when exposed to moisture. Insulation
made from polyimide film, often referred to by the brand-name
Kapton, was once thought to be the ideal wiring insulation
and was widely used in both military and commercial
aircraft during the 1970s and early '80s. A long-chain
polymer that is both tough and durable, with a very
high resistivity, Kapton provides excellent electrical
insulation even at a thickness of less than a millimeter.
What was not known initially was how Kapton held up
to the moisture that tends to condense in or near aircraft
wiring harnesses. This moisture is so prevalent that
most wires are outfitted with a drip loop, which prevents
water droplets from running down the cables and into
critical electronics. Exposed to this moisture, Kapton's
long polymer chains break down, and the insulation becomes
brittle, developing small cracks that in turn let in
even more moisture. So-called wet arcs begin to flow
along these cracks, creating intermittent arcs too small
to trip normal circuit breakers and often too small even to interfere
with the signal transfer along the wire. Nonetheless,
the tiny arcs do begin to carbonize the insulation,
and carbon is an excellent conductor. Once enough carbon
has built up ("enough" depends on the type and thickness
of the insulation, the power handling of the wire, and
other factors), there can be a large explosive flashover,
with exposed wires spewing molten metal.
One would hope that Kapton cracks are relatively rare.
Not so, according to a recent report by Lectromechanical
Design Co., an electrical research firm based in Sterling,
Va. Using a proprietary tool called the DelTest, Letromec
engineers tested the wiring in a Boeing 747, an Airbus
A300, a Lockheed L-1011, and two DC-9s that were each
over 20 years old and had been retired by commercial
airlines within the previous six months. The results:
13 cracks per 1000 meters of wire in the L-1011, down
to 1.6 cracks per 1000 meters in one of the DC-9s. With
approximately 240 km of wire in the L-1011, this amounted
to over 3000 cracks, each a potential cause of catastrophic
arcing.
Some time after Kapton's problems came to light, in
the late '70s, its use was cut back, and aircraft manufacturers
began replacing it in some of the most critical wiring
systems in planes in service. Alternatives to Kapton
include polyvinylchloride, glass, nylon, polyester,
and teflon. But polyimide can still be found on thousands
of aircraft in service, including the McDonnell Douglas
MD-11 and older Boeing 737s and 767s.
How old is too old?
Updating rather than replacing old planes has become
a standard way to save money. Some aircraft being designed
today, such as the Joint Strike Fighter, may fly 100
years. Similarly, the B-52s flown by the U.S. Air Force
were built in 1961–62 and are expected to remain operational
until 2045. Its designers would have never dreamed that
this plane would fly for over 80 years. Indeed, not
much thought was given to replacing or inspecting the
wiring, because the planes were to have been retired
long before any problems developed.
So when is it time to scrap an airplane because its
wires are too old? The answer depends on a complex array
of factors--among them calendar age, manufacturing variations,
exposure to water, ultraviolet light, temperature, vibration
and g-forces, and stress during normal use and maintenance.
Planes over 20 years old are virtually guaranteed to
have wiring problems, many of which turn up during routine
maintenance. The average age of civilian aircraft in
use today is 18 years, and the average age of military
planes is 16 years. [See table at http://www.spectrum.ieee.org/WEBONLY/publicfeature/feb01/wiret1.html
] Of course, most fleets are composed of a mix of aircraft
types and ages. Trying to relate this information to
wiring failure probability rates, such as those in the
table at http://www.spectrum.ieee.org/WEBONLY/publicfeature/feb01/wiret2.html
, gives some idea why wiring problems are endemic today.
Short of replacing an entire aircraft, how about replacing
just the wiring system? That also turns out to be hugely
expensive--anywhere from US $1 million to $5 million
for a typical aircraft. Determining what, when, or whether
to replace then means weighing cost against risk--a
decision complicated by the fact that neither the cost
nor the risk has yet been fully characterized. What
is more, military planes get exposed to more hostile
environments than the average commercial plane, so extrapolation
to other types of planes is not necessarily accurate.
The maintenance nightmare
Snaking through an aircraft are many kilometers of wire--some
17.5 km in a Navy F-18C/D fighter, 240 km in a typical
wide-body jet. The wire is literally built into the
aircraft, running through fuel tanks, and twisted around
hydraulic lines. Just reaching the wiring harness often
entails dismantling an aircraft's external structure.
And merely touching a wire, let alone disconnecting,
handling, and reconnecting it, heightens the risk that
the wire will be damaged.
But maintenance workers do not always show due respect.
They have been known to stand on wires instead of step
stools, to cut and splice them poorly to get them out
of the way of difficult-to-reach places, and to smack
connectors with hammers to loosen them. Tiny razor-sharp
metal shavings from maintenance or upgrades, coupled
with ordinary aircraft vibration, create the perfect
conditions for insulation damage.
Other parts of the aircraft never get touched, but are
no less problematic. The dust bunnies and chaff that
collect in these out-of-sight areas are excellent tinder
to turn sparks into smoke and flames. Then there's the
sticky "syrup" that collects in and around wire bundles.
This well-aged potion of condensation, toilet and galley
leaks, dust, hydraulic fluid, and various unnamable
ingredients is intensely caustic to most kinds of insulation.
One of the Navy and FAA directives for making aging
wiring safer has been simply to improve cleanliness
within aircraft!
Compounding the maintenance nightmare is its high cost.
By one estimate, the Navy spends 1.8 million person-hours
each year to troubleshoot and repair its aircraft wiring
systems.
Why state of the art isn't enough
Wire troubleshooting is still very much a hands-on art
that has changed little over the last 40 years. Among
the techniques in current use are visual inspection,
several versions of reflectometry, and impedance testing.
Each technique has its advantages and, more importantly,
disadvantages.
Visual inspection is still the most common way to check
for wiring failures. It entails accessing the cables
and then carefully checking the insulation for holes
and cracks, often no larger than the head of a pin.
Whole sections of wiring never get inspected: chafed
insulation can be hidden under clamps or around corners,
or within multiwire bundles, each consisting of 75 or
more wires. And many wire bundles are built right into
the walls of the aircraft.
Another approach involves measuring the cable's resistance
from end to end. A low resistance means the cable is
"good," and a high resistance means that it is broken.
When a very high voltage (500 V or more) is placed between
adjacent, supposedly unconnected wires, current leakage
from one wire to another can indicate degraded insulation.
There is some concern, though, that high voltage may
in itself damage the insulation. So nondestructive resistance
tests, such as those developed by Eclypse International
Corp., Corona, Calif., use voltages of 28 V or less.
A floating comparator analyzes the currents on the cable
as the input current is stepped through several levels.
In a healthy cable, Ohm's Law predicts that the resistance
will stay the same for all current levels. If it does
not, then something is wrong with the cable. The method
has been used to locate cold solder joints, bad crimps,
carbonization of the cable or connectors, and foreign
matter on or near the cables. And unlike the high-voltage
tests, it can be used on a fueled airplane. It does,
though, still require disconnecting and reconnecting
the cables.
Several techniques now used or under development involve
reflectometry. Common to all these methods is the sending
of a signal (a pulse, sine wave, or the like) down the
wire and sensing the reflection that returns from the
wire's end. They are most useful for detecting so-called
hard errors, such as short circuits, but have not proven
useful for less obvious wire problems.
Time domain reflectometry (TDR) is customarily used
when a wiring problem is already suspected. A short,
typically rectangular pulse is sent down the cable,
and the cable impedance, termination, and length give
a unique temporal signature to the reflected signal.
A trained technician then interprets the signature to
determine the health of the cable. Such signal interpretation
is particularly necessary for aircraft systems, where
wires branch into complicated network structures and
connect to active avionics. The running joke about TDR
is that it requires a Ph.D. to use.
Standing-wave reflectometry (SWR) involves sending a
sinusoidal waveform down the wire. A reflected sinusoid
is returned from the wire's end, and the two signals
add to a standing wave on the line. The peaks and nulls
of this standing wave give information on the length
and terminating load of the cable; a healthy line's
wave pattern will be distinct from that of a line with
an open or short circuit. The edge this method has over TDR
is that the electronics are simpler and therefore less
expensive.
Like SWR, frequency domain reflectometry (FDR) uses
sine waves. FDR, though, directly measures the phase
difference between the incident and reflected waves;
any faults in the line will generate resonances between
the two signals. This method is being developed for
in situ wire testing by researchers at Utah State University
with support from Management Sciences Inc., Albuquerque,
N.M., and the Naval Air Systems Command. The goal is
to allow preflight testing of cables with the touch
of a button, and without the risk of damaging the cables
by disconnecting them.
On the horizon
Because of the shortcomings in the above techniques,
researchers are now looking at several new technologies.
These include automated reflectometry testing; smart
wire systems for real-time, on-the-spot testing; and,
in the event of an in-flight failure, advanced fire
suppression methods and arc-fault circuit breakers.
Automating the reflectometry methods now in use may
one day mean that maintenance workers will be able to
gauge a cable's health with minimal physical intervention.
A hand-held unit would clamp around the wire, rather
than directly connecting to it. Recently, a fully automated
TDR unit was developed by Phoenix Aviation and Technology.
It provides a wider range of fault diagnostics and prognostics,
with precise location and interpretation of the fault.
The same software can be easily embedded into application-specific
IC format or similar small computing platforms, thus
paving the way for real-time embedded conductor monitoring.
All the same, reflectometry is pushing the state of
the art when it comes to finding small insulation cracks,
detecting chafed insulation before arcs occur, and locating
an arc's source. Better detection of these tiny anomalies
may be achievable by wetting the cable with water or
saline solution, or filling the plane with inert gas.
Perhaps the maintenance worker's greatest nightmare
is finding faults that come and go. These so-called
ticking faults arise from vibration, temperature change,
moisture, g-forces, electromagnetic interference, and
so on. Diagnosing the problem requires systems that
can function in flight, where ticking faults usually
occur.
Smart wire systems are thus being designed for testing
cables continuously, both before takeoff and during
a flight. Systems now under development include a frequency
domain reflectometer, on-board processor, environmental
sensors, and wireless communication system integrated
into a single miniaturized unit, hundreds of which can
be embedded in the wiring system. They will monitor
the health of the cable and guide cable maintenance,
and even detect any faults that occur and correct them
in real time.
For the aircraft being designed today, a novel kind
of wiring with a complete array of embedded sensors
is being proposed. This is particularly critical for
long-lived planes such as the Joint Strike Fighter.
Weight and space constraints are likely to drive this
technology to nanoscale sensors, emerging material science
technologies, and microelectromechanical system devices.
Of course, wire failures will still occur. New technologies
that can help limit the damage in such an event include
arc-fault circuit breakers and fire suppression methods.
--------------------------------------------------------------------------------
Smart wire systems will continuously monitor the
cable's health and correct faults as they occur
--------------------------------------------------------------------------------
Ordinary circuit breakers
are heat-sensitive bimetal elements that trip only when
a large current passes through the circuit long enough to heat the element. This
power may be on the order of 1000 percent of the rated
current for 0.35 to 0.8 seconds. By comparison, a single
arc fault may last only 1.25 ms, and a series of events
may last 20–30 ms. Too fleeting to trip the circuit
breaker, these arc faults can nonetheless cause catastrophic
local damage to the wire. Fires have been known to break
out with the breaker still intact.
Arc-fault circuit breakers contain
sophisticated electronics to sample the current on the
wire at submillisecond intervals. Both time and frequency
domain filtering are used to extract the arc-fault signature
from the current waveform. This signature may be integrated
over time to discriminate, by means of pattern-matching
algorithms, between a normal current and a sputtering
arc-fault current. And so ordinary transients, due to,
say, a motor being turned on and off, can be distinguished
from the random current surges that occur with arcing.
Arc-fault breakers are already
required in new home wiring in the United States and
are now being miniaturized for use on aircraft. Normally
these breakers either are used in tandem with a traditional
heat-sensitive breaker or else include a heat-sensitive
element in addition to the pattern-matching electronics.
Ideally, circuitry will also be added to locate the
fault after the breaker has tripped.
Once a fire starts on an aircraft, it spreads rapidly,
aided by Mylar-backed insulation in the cabin walls,
limited access to fire extinguishers, and so on. New
extinguisher designs that rely on super-fine, high-pressure
mists of water, inert gases, and other techniques are
now being developed to put out all types of aircraft
fires, including those due to faulty wiring.
Amazingly little is known about how and why wires age,
but polymer scientists are making up for lost time.
Among other things, they are studying the chemical and
physical changes and resultant effects on electrical
insulation properties that occur as wires age. One goal
is to find new materials to replace copper wiring in
signal-transfer and electromagnetic interference shielding
on aircraft, as well as new types of wire insulation
that resist chafing and have extended life and built-in
diagnostics.
Not to panic
If you happen to read this article while flying, do
not panic. Few wiring problems end in disaster. There
is cause for concern, though, as the air fleet continues
to age, and our reliance on air transport grows. While
an aircraft's other major systems undergo preflight
testing and regular inspection and maintenance, its
central nervous system--wiring--has been long neglected.
Sorely needed are new maintenance methods that account
for the aging of wires, as is done for aging structural
and computer systems.
Diagnosis is good. Prognosis is better. And prevention
is better still. This last may require a new way of
thinking for electrical engineers, who tend to be more
at home with obsolescence than geriatrics. For aging
aircraft wiring, diagnostics and prevention are improving,
and prognostics are on the horizon. What remains to
be seen is how all of these methods will be implemented
in practical systems, so that disasters like TWA 800
and Swissair 111 can be prevented.
Original Article (Down to the Wire)
Read the original Full article (with links and images)
here:
Cynthia Furse Article
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