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As
part of the Swissair Flight 111 debris examination, every inch of
wire had to be examined visually for signs of burning or pre-crash
damage. This was no small task considering there were more than 250
km of wire onboard. Photo #1 shows part of the MD-11 overhead
area with its abundance of electrical wires. Photo #2 shows
one of the Transportation Safety Board members who meticulously
searched through every inch of retrieved wire for indications of
damage.
Wires
ranged in size from extremely fine gauge to large cables about 2 cm
or more
in diameter. The debris pieces varied from hundreds of feet long
down to ½ inch in length. The vast bulk was the relatively fine
wire shown in photo #2, about one quarter of the diameter of the
individual wires that make up normal
household wires and known publicly as the Kapton coated wires.
Wires became the focus of the investigation when
numerous short-circuited wires were located. Typical of this is the
molten wire shown in Photo #3. However, as in any structural
fire involving live wires, there are bound to be numerous electrical
short circuits that occur during the progression of the fire. In
an attempt to determine if there was an initiating wire, very
advanced testing techniques including
Auger Electron Spectroscopy
were undertaken for the first time in this country.
Founded
on basic theories of
Physics is SIEVERT’s
rule. In general terms it states that the solubility of a gas in a
liquid is proportional to the square root of the gas pressure and
increases with increasing temperature. The concentration of
dissolved gases in the arc bead is proportional to the square root
of their mole fraction in the atmosphere. In other words, any
liquid will have an attraction for the gas that surrounds it which
is proportional to the air pressure and the temperature of that
liquid. Therefore an initiating
short-circuited wire at the instant that it heats sufficiently to
melt will absorb the atmospheric gases that surround it. Because
it is a bare wire, there is no actual material fire at that instant,
so the absorbed gases will be
normal air with levels of oxygen and carbon dioxide that reflect
usual levels. The molecules of these gases, following Dalton’s
Theory of Gases and Liquids, are absorbed and trapped in the outer
levels of the copper wire that has now frozen to form a solid bead. |
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Due to the
extremely high temperatures reached at the moment of the electrical
short circuit (think of an electrical welder), the heat created
starts a fire that burns the insulation of other wires to create
further electrical short-circuits. However, the atmosphere has
now become contaminated with higher levels of carbon oxides and
lower levels of oxygen, along with other chemicals from plastics and
other flammables that are not present in the normal atmosphere.
At the instant of the follow-on electrical short circuits, the
molten bead of copper will absorb this contaminated atmosphere in
the same manner as the first wire, but the elements are now
different and in different levels. This difference in the
elements that are trapped and their amounts allows one to identify
the initiating wire and subsequent follow-on events.
To examine the short-circuited wires in the Swissair 111
investigation,
Auger Electron Spectroscopy
was used as one of several procedures. It allows for the analysis
of the bead’s sub-surface contents by using a method that has been
described for nearly a century but that could never be undertaken
until the space age provided the ability to create a nearly perfect
vacuum. The main vacuum chamber and measuring scopes are shown
in Photo #4.
In a vacuum chamber that is about a billion
times better than that used for vacuum CA fingerprinting, a focused
beam of argon is used to etch layers of the bead’s surface only
several microns square to a depth ranging from several hundred to
two thousand angstrom units. An angstrom unit is
1/10-billionth of a meter or about the size of an atom.
The test results show the amounts of each element present in the
examined portion of the bead. Those elements are
individualized and are not shown as compounds. They then have
to be interpreted by using comparison data gained from previous
exemplar tests already conducted.
As one can
imagine, the process is time consuming and requires highly
specialized equipment and expertise. The procedure is
basically non-destructive as such a small amount of material is
actually etched from the bead’s surface as to be invisible to the
naked eye. As in most advanced and highly technical testing
procedures, there is a specific method required for exhibit
collection to eliminate the potential for post fire contamination.
However, if faced with an insurance claim of several millions of
dollars, this process is certainly worth consideration.
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