Flight
into Danger
(Published
New Scientist 13 May, 2000)
Could
major air disasters have been caused by faulty wiring and how can
we avoid this in the future?
Gerry
Byrne investigates
A LOUD BANG
behind a panel above the pilot's head was the first sign of
trouble. Sparks showered into the pilot's lap as red "out of
order" flags popped up all over the instrument panel in
front of him. The aircraft was losing electrical power rapidly,
and more warnings erupted: the aircraft's batteries were
discharging, the navigation instruments went down, and the
auto-throttle failed. With only a few instruments operational,
the plane was in trouble.
It was just
after midnight on 22 June 1998, and the aircraft, a Boeing 757,
had just taken off from Larnaca in Cyprus, bound for Manchester
with 217 people on board. A major disaster appeared to be in the
making. The pilot alerted air traffic control, and without
pausing to dump fuel turned straight back to Larnaca.
Fortunately, he managed to land the heavily laden aircraft
safely.
When
technicians removed the panel where the explosion had occurred,
they discovered that the insulation on two wires was damaged
where they had rubbed against a support bracket. The fault had
caused a short circuit, which led to the sparking and the
near-catastrophic loss of power.
In an
industry as heavily regulated as aviation, you might imagine that
incidents like this are rare. But a recent study by the US Air
Line Pilots Association (ALPA) suggests that last year, on
average, one US airliner a day was forced to make an emergency
landing because of sparks, smoke or fire.
Faulty
wiring is the leading culprit. And while the 757 involved in the
Larnaca incident had been flying for only five years, it is older
planes that are most likely to give trouble. Usually this is
little more than a nuisance, no more serious than a dodgy power
lead on a coffee pot. But sometimes the consequences are
potentially catastrophic.
Some
organisations have taken drastic action to deal with the problem.
The US Navy in 1987 ordered the removal of the most vulnerable
wiring from its planes, and last year NASA grounded its entire
fleet of space shuttles when a wiring fault led to a launch being
aborted. Yet every day, millions of passengers are still carried
by commercial aircraft that are equipped with old wiring that
cannot be properly tested for faults.
In the US,
the Federal Aviation Administration (FAA) has been mounting a
probe into the problems that may afflict aircraft that have been
flying for more than 20 years . The Aging Aircraft Program has
been running since 1988, prompted by an accident in which part of
the roof peeled off an elderly Aloha Airlines Boeing 737 in the
sky over Hawaii. It has involved exhaustive investigations into
the structural integrity of things like wings, fuselages, control
surfaces, landing gear and engine mountings aboard older
aircraft. But nobody ever thought of looking at the electrical
wiring. Until recently.
In 1996,
TWA flight 800 came down off the coast of Long Island, killing
all 230 people on board. Faulty wires inside a fuel tank were
blamed as the most likely cause of the explosion. In the wake of
that crash, checks on other airliners around the world led to the
discovery of several other potential "flying bombs" in
which the insulation on ageing wiring leading to sensors in fuel
tanks had rubbed away through vibration, or had been damaged
during routine maintenance. Then, in 1998, 229 lives were lost
when Swissair flight 111 crashed off Nova Scotia minutes after
the crew reported smoke in the cockpit. The cause of that
accident has not yet been pinned down, but faulty wiring is one
of the leading suspects.
Long before
these disasters, warnings of the danger of old wiring--and of one
kind of insulating material in particular--had begun to emerge.
At the time of the TWA 800 disaster, one question haunted Richard
Healing: "I kept asking myself, what did we know in the Navy
that the commercial aviation sector didn't?" Healing, a
former US Coast Guard captain, now director of safety and
survivability for the US Navy, soon discovered that the answer
was a tremendous amount.
After a
series of baffling mid-air fires that killed several pilots in
the 1980s, the Navy traced the problem to wiring in exposed areas
of planes that had seen service aboard aircraft carriers: places
like wheel wells, flaps and the hinged sections of folding wings.
Attention focused on an insulation material made from a type of
polymer known as an aromatic polyimide, often referred to by its
common Du Pont proprietory brand name, Kapton. At first glance,
Kapton, and its relatives, seemed to be everything an electrical
insulator should be. It was tough, light, had high fire
resistance and gave off relatively little toxic smoke if it did
burn. As a result, aromatic polyimide coated wiring was used
throughout the aircraft industry. But the Navy soon discovered
that Kapton and other aromatic polyimides had a dark side.
When
exposed to a combination of salt air and the solvents used to
wash down aircraft aboard carriers, these wires experienced what
was, in effect, accelerated ageing. The insulation became liable
to crack if it was placed under strain. Worse, when faulty wires
short-circuited or arced, the material changed from being an
insulator to a partial conductor. Under the right circumstances,
a bundle of arcing aromatic polyimide-coated wires could explode
into a searing fire, generating temperatures of more than 1000 C--so hot that the wire's copper core would melt and spray
outwards.
On their
own, such fires will usually burn out harmlessly. But they can
get hot enough to rupture and set fire to hydraulic pipes, fuel
lines, insulation or any other flammable material nearby. So the
US Navy spent hundreds of millions of dollars ripping the
aromatic polyimides out of vulnerable areas aboard P-3 Orion
surveillance aircraft and F-14 Tomcat fighters. In Britain, the
Ministry of Defence heeded the warnings and started a programme
to remove them from RAF and Royal Navy aircraft wherever
possible.
Healing
says the military were quite open about the urgency with which
they removed aromatic polyimide or Kapton-type wiring. But
manufacturers continued to install the same wiring in civilian
airliners: Boeing used it until 1993, and Airbus Industrie
continues to use limited amounts of it even now. "Did we
have a problem communicating our findings to the other parts of
the aviation world?" Healing asks. "I had a bucketful
of information and I tossed it over the fence without checking
that anybody knew about it."
Healing has
helped to set up an industry organisation to combat both the
cause and effect of aircraft fires--the Aircraft Wiring and Inert
Gas Generator Group. He is concerned about the effect of ageing
on all types of insulation used for aircraft wiring, not just
Kapton. "People don't fully understand the seriousness of
that degradation," he says.
Ed Block
also has a strong interest in aircraft wiring: he is
vice-chairman of the International Aviation Safety Association
and a delegate on the Aging Transport Systems Rulemaking Advisory
Committee. ATSRAC is charged by the FAA with investigating the
condition of wiring in older aircraft. Block has been sounding
alarm bells about Kapton and other wiring problems for more than
a decade.
He points
out that many older aircraft are flying long past their original
design life. This is made possible by frequent inspections under
"accelerated maintenance" programmes, but aircraft
wiring poses special problems. Wiring, Block says, is designed
for a maximum flying life of 60 000 hours. "TWA 800 had 93
303 hours on the clock when it crashed," he says.
"That's 33 000 hours overtime."
ATSRAC is
tackling the issue of ageing wiring on two fronts. The first, a
series of visual inspections of wiring in a sample of older
aircraft, is already complete. This identified more than 3000
"anomalies" aboard 81 aircraft, representing 26 per
cent of the US fleet aged 20 years or more. These anomalies
occurred in wires with aromatic polyimide insulation and in wires
with other insulators, and only 140 of them were listed in
ATSRAC's final report as being significant. The report also
concluded that none of the anomalies posed a danger to flight.
However, in a minority report, Block registered strong
disagreement with this view.
Part of the
problem with surveys like this is that paper-thin aircraft wiring
insulation is so delicate that it can be damaged even by gently
probing a thick bundle. Many of the anomalies that have already
been found are held to be the result of careless handling by
maintenance crews over the years.
ATSRAC is
now moving on to the second phase of its work, a series of more
intrusive inspections on retired aircraft, where damage caused by
the inspection process itself doesn't matter. Block fears this
may turn up even more problems. "It is generally accepted
you can only visually and non-intrusively inspect 25 per cent of
the wiring aboard an aircraft."
His view is
partly borne out by a March 2000 report into the condition of
NASA's fleet of ageing space shuttles, which were grounded for a
time last year following the discovery of a series of faults,
including wiring problems. In one incident, an aromatic
polyimide-insulated wire had shorted out on a burred screw head,
knocking out a primary and a back-up engine controller and
leaving two of the shuttle's three engines without any controller
back-up.
The NASA
report lists 818 wiring problems on three shuttles, many of them
relating to Kapton wiring. It concludes that most of the damage
arose during maintenance and recommends that intrusive
inspections be limited in order to minimise damage to wiring,
which becomes increasingly delicate as it ages. It also warns
that 20 per cent of wiring cannot be inspected without
dismantling a large part of the shuttle. This means, the report
says, that the job is best done when heavy maintenance is taking
place.
The report
also carries an even more ominous warning. It focuses on the
shuttle's circuit breakers--switches designed to isolate the
craft's electrical components when they sense problems such as a
short circuit. The report says that the circuit breakers
installed in the shuttle fleet do not always protect against an
arcing fault, where sparks jump intermittently between damaged
wires, or to ground. Instead, they interpret intermittent arcing
as a varying load, so they may fail to trip even when current
spikes exceed 10 times the danger level.
Similar
circuit breakers are used in commercial aircraft, and American
pilots are angry that the FAA is not taking their shortcomings
seriously. Assuming that a circuit breaker will always trip out
if there is a serious problem, pilots often reset tripped circuit
breakers to see if a fault has rectified itself. If Kapton or
other aromatic polyimide wiring is arcing, says Block, that
action could be lethal. "Each time you re-energise the
circuit you may be setting it up for an even more dangerous
failure," he says.
In 1991,
the FAA issued an advisory circular on the resetting of circuit
breakers, warning that it could be dangerous. The FAA has left it
up to pilots' discretion whether to continue with the practice,
but in Britain the authorities have taken a stricter line. The
Civil Aviation Authority has firmly instructed pilots never to
reset circuit breakers, except in "exceptional
circumstances". Airbus says it has issued similar
recommendations for its aircraft. The ALPA is demanding that the
FAA should tighten up its ruling, too.
The problem
might be solved by installing different equipment. Arc fault
circuit interrupters, a type of circuit breaker designed to
detect and prevent arcing are now widely available for use in
homes and cars. So why not put these breakers in airliners too?
"There are some very unique things about aircraft
power," says Jim Shaw, manager of the In-Flight Fire Project
Team at ALPA. "You get what we call dirty power," he
says. The current and voltage can jump when the pilot switches
from one power generator to another. "That could cause all
your arc protection circuit breakers to pop."
For
Healing, that's not the end of the story, however. He says a
research contract is being finalised between the US Navy, the FAA
and two electronics manufacturers to miniaturise a smart circuit
breaker that should be able to handle dirty power. A design may
be completed within six months, though Block believes it could be
up to two years before the regulatory authorities approve it for
use on aircraft.
The main
problem remains that there is at present no system that can
report accurately on the condition of an aircraft's wiring. NASA
routinely puts 1500 volts through some 115 volt space shuttle
wiring systems in a bid to detect insulation faults. But it isn't
foolproof, as defects only register if they are within a
millimetre or two from a connection to ground.
A more
thorough test involves soaking the wiring in a conducting
solution and checking for stray voltages coming through the
electrolyte. However, this system is mostly used for wiring that
has already been removed from the plane, as operators are
reluctant to spray corrosive conducting solutions onto their
aircraft. And even if other insulation-checking systems become
available, they may not give reliable results unless the testers
also have baseline data for perfect, brand-new aircraft.
Much of the
emphasis in the Aging Aircraft Program has been on planes which
were designed and built in the late 1970s and early 1980s. Shaw
warns that over the next few years much more complicated
"fly-by-wire" aircraft, which have many more electrical
and electronic systems, will start to come into the
"ageing" bracket. With more than 300 kilometres of
wiring in the average large jetliner, the problem may call for
more radical solutions.
One answer,
says Healing, might be to replace low-voltage wires--the sort
suspected of causing the TWA explosion--with optical fibres or even to
use VHF radio signals to transmit commands around the aircraft,
doing away with most wires altogether. But refitting an old
aircraft from scratch would probably cost more than the plane is
worth.
Should all
wires like those insulated with Kapton and other aromatic
polyimides be removed from aircraft immediately? Safety advocates
like Block certainly think so. He says the issue is a matter of life and
death. "It's
sad," he says. "We have come this far yet we are still
tiptoeing around this subject because of its economic
ramifications."
But others
disagree. According to DuPont, there have been no accidents which
upon analysis can be linked to Kapton. Airbus says it uses Kapton
for cabin wiring because it is hard to ignite and, should a fire
start, it gives off far less toxic fumes than comparable
insulators. In the event of a fire it may improve survivability,
says Airbus spokesman David Vailypilai. Healing agrees.
"There are many places where it is the best solution
provided it is not used in situations where it is
vulnerable."
Whether the
aviation industry changes its mind on the issue could depend on
the results of ATSRAC’s wiring study, which will be
completed by September.
Gerry Byrne is based in
Ireland and is the winner of an IBM/STI Science and Technology
Journalism Award
Further Reading:
NASA's
Space Shuttle Report is available at
www.hq.nasa.gov/osf/shuttle_assess.html
The
progress of the TWA 800 disaster investigation can be followed at
www.ntsb.gov/events/twa800/default.htm
Details of
the Swissair 111 investigation are at www.tsb.gc.ca
Comments
from DuPont on Kapton insulation can be found at
www.kapton-dupont.com/
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