快猫短视频

A LOUD BANG behind a panel above the pilot鈥檚 head was the first sign of
trouble. Sparks showered into his lap as red 鈥渙ut of order鈥� 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鈥檚 batteries were
discharging, the navigation instruments had gone down, and the auto-throttle had
failed. With only a few instruments operational, the plane was in trouble.

It was a few minutes 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 think 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 made an
emergency landing because of sparks, smoke or fire.

Faulty wiring is the leading culprit. And while the Boeing 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鈥攏o more serious than a dodgy power lead on a coffee pot, for
instance. But sometimes the consequences are potentially catastrophic.

Some organisations have taken drastic action to deal with the problem. In
1987, the US Navy 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 almost no one
thought to look at 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
鈥渇lying bombs鈥� 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鈥攁nd
of one kind of insulating material in particular鈥攈ad begun to emerge. At
the time of the TWA 800 disaster, one question haunted Richard Healing: 鈥淚 kept
asking myself, what did we know in the Navy that the commercial aviation sector
didn鈥檛?鈥� 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 and flaps.
Attention focused on an insulation material made from a type of polymer known as
an aromatic polyimide, often referred to by its proprietory name, Kapton. At
first glance, Kapton and its relatives seem to be everything an electrical
insulator should be. They are tough, light, have high fire resistance and give
off relatively little toxic smoke if they do burn. As a result, aromatic
polyimide-coated wiring was used throughout the aircraft industry. But the Navy
soon discovered that Kapton and other polyimides have a dark side.

When exposed to a combination of salt air and the solvents used to wash down
aircraft aboard carriers, Kapton 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, Kapton-coated wires could explode into a searing fire,
generating temperatures of more than 1000 掳C鈥攕o hot that the wire鈥檚
copper core could melt and spray outwards.

Fire in the sky

On their own, such fires usually burn out harmlessly. But they can get hot
enough to rupture and set fire to hydraulic pipes, fuel lines or any other
flammable material nearby. So the US Navy spent hundreds of millions of dollars
ripping the Kapton out of vulnerable areas aboard P-3 Orion surveillance
aircraft and F-14 Tomcat fighters. In Britain, the Ministry of Defence heeded
the warnings about Kapton and started a programme to remove it from RAF and
Royal Navy aircraft wherever possible.

Healing says the military were quite open about the urgency with which they
removed polyimide-insulated wiring. But manufacturers continued to install it in
airliners: Boeing used it until 1993, and Airbus Industrie continues to use
limited amounts of it. 鈥淒id we have a problem communicating our findings to the
other parts of the aviation world?鈥� Healing asks. 鈥淚 had a bucketful of
information and I tossed it over the fence without checking that anybody knew
about it.鈥�

Healing has helped to set up an industry organisation to combat both the
cause and effect of aircraft fires鈥攖he 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. 鈥淧eople don鈥檛 fully
understand the seriousness of that degradation,鈥� he says.

Ed Block also has a strong interest in aircraft wiring: he is vice-chairman
of a pressure group called the International Aviation Safety Group 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 the 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鈥斺�漚ccelerated
maintenance鈥� programmes鈥攂ut aircraft wiring poses special problems.
Wiring, Block claims, is designed for a maximum flying life of about 60 000
hours. 鈥淭WA 800 had 93 303 hours on the clock when it crashed,鈥� he says. 鈥淭hat鈥檚
33 000 hours鈥� overtime.鈥�

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 鈥渁nomalies鈥� aboard 81 aircraft,
representing 2.6 per cent of the US fleet aged 20 years or more. These anomalies
occurred both in wires with polyimide insulation and in wires with other
insulation, and only 140 of them were listed in ATSRAC鈥檚 final report as being
significant. The report also concluded that none of the anomalies posed a danger
to flight. However, a minority of the committee, including Block, registered
strong disagreement with this view.

Part of the problem with surveys like this is that insulation on aircraft
wiring is paper-thin and 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鈥檛 matter. Block fears this may turn up even more problems.
鈥淚t is generally accepted you can only visually and non-intrusively inspect 25
per cent of the wiring aboard an aircraft.鈥�

His view is partly borne out by a report published in March, into the
condition of NASA鈥檚 fleet of ageing space shuttles, which were grounded last
year following the discovery of a series of faults, including wiring problems.
In one incident, a 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鈥檚 three engines without any controller back-up.

The NASA report lists 818 wiring problems on three shuttles, most 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鈥檚 circuit breakers鈥攕witches designed to isolate the craft鈥檚
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. 鈥淓ach time you re-energise the circuit you may be setting it up
for an even more dangerous failure.鈥�

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鈥�
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 鈥渆xceptional circumstances鈥�.
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? 鈥淭here are some very unique things about aircraft
power,鈥� says Jim Shaw, manager of the In-Flight Fire Project Team at ALPA. 鈥淵ou
get what we call dirty power,鈥� he says. The current and voltage can jump when
the pilot switches from one power generator to another. 鈥淭hat could cause all
your arc protection circuit breakers to pop.鈥�

For Healing, that鈥檚 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鈥檚 wiring. NASA routinely puts 1500
volts through some 115 volt space shuttle wiring systems in a bid to detect
insulation faults. But this isn鈥檛 foolproof, as defects only register if they
are within a millimetre or two of 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 test
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 tests become available, they
will not give reliable results unless the testers also have baseline data for
perfect, 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 鈥渇ly-by-wire鈥� aircraft, which have many
more electrical and electronic systems, will start to come into the 鈥渁geing鈥�
bracket. With almost 300 kilometres of wiring in a large jetliner, the problem
may call for more radical solutions.

The solution, says Healing, might be to replace low-voltage wires with
optical fibres or to use VHF radio signals to transmit commands around the
aircraft, doing away with most wires altogether. But refitting an old aircraft
from scratch could cost more than the plane is worth.

Should all wires like those insulated with Kapton be removed from aircraft
immediately? Safety advocates like Block certainly think so. He says the issue
is a matter of life and death. 鈥淚t鈥檚 sad,鈥� he says. 鈥淲e have come this far yet
we are still tiptoeing around this subject because of its economic
谤补尘颈蹿颈肠补迟颈辞苍蝉.鈥�

But others disagree. According to DuPont, there have been no accidents which
upon analysis can be linked to Kapton. And 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:
鈥淭here are many places where it is the best solution, provided it is not used in
situations where it is vulnerable.鈥�

Whether the aviation industry changes its mind on this issue could depend on
the results of ATSRAC鈥檚 wiring study, which will be completed by September.