September, 2000

NTSB Electrical Short Scenario
 
Excerpt from Washington Post "Letter to the Editor":

"Despite this overwhelmingly consistent eyewitness testimony, the NTSB
clings to its story that an electrical short jumped from a high-voltage wire
to a low-voltage wire and ignited the center fuel tank of the 747--something
that Boeing safety engineers adamantly claim is impossible."
                                *******
Below, Concerned Aviation Professional, who participated in the building of
hundreds of 747s, comments on the above:

Since we are dealing with potentials, that is, voltages, it helps to use the
analog of the waterfall.  We should all know by now that water will seek the
lowest level, when given a course to follow. That being the case, if I
construct a waterway, and in the course of it I construct a double dam, such
that they both share the same lower reach, that is their head pressures are
referenced to the same point (where the water is taken to feed the turbines)
, water falling from 500 feet, will not run up-hill to a dam 500 feet taller
. Instead, the water from both will fall to the lowest point. If I suppose
the pressure gradient in the 500 foot dam is a pound per cubic foot (linear
height) of water, then the head pressure will be 500 pounds with 500 feet of
water at the spillway.

The same relationship will exist for the 1000 foot dam, except that the
potential difference between the two -- because they are connected -- is
only a value of 500. This is to say that the water falling from the 1000
foot dam into the 500 foot dam, will have only half the effect than if it
fell all the way to the lower reach reference point.

The lowest potential in an electrical circuit is that to which all other
potentials are referenced.  In an aircraft, the airframe has the lowest
potential in the electrical circuit. A spark will seek the point that has
the greatest differential (potential difference) between itself and another
object. Why, then, when one considers the physics of this event, is another
wire bundle more attractive than aircraft ground?

If I have two wires, and one is at 1000 volts, and another is a 500 volts,
and both of them run parallel to structure, then structure is more
attractive to either, because the 1000 volt wire sees something that is 1000
volts away from itself, whereas the 500 volt wire is only half as attractive
as the ground. If all the wires were at 1000 volts, then none of the wires
would appear attractive to the wire that is going to arc. If an arc-over is
to occur, then it is ground that will receive the arc, not the surrounding
wire bundles, as they are also powered to a degree that makes them less
attractive. 

If the aircraft structure, which happens to be closer to the 'hot' wire
bundle, isn't enough of an attraction, then neither is another wire bundle -
- which is at a potential less attractive than aircraft ground.

Now, further exacerbating the contention of an arc-over, is the fact that
wire bundles are clamped at about every 20 to 22 inches in the 747. The
clamps are rubber or plastic coated metal or are simply plastic for the
smaller wire bundles. The rubber is a good insulator, but not as good as air
. When one speaks of how good an insulator is, they use the scientific term
of Dielectric constant. Air has a constant of 1; most other things have a
constant of less than one. That means that air is better than almost all
other materials as an insulator. Therefore, before an arc can happen through
normal air pressure, it must first be impeded from finding any other path
through which to follow. Since the insulation in the rubber coated metal
clamps is not very thick, the break-over would likely happen through the
rubber to the metal, and direct to the metal of the aircraft structure.
Additionally, this arc path would happen at every place where the arcing
wire bundle was clamped to structure.

Now, ask yourself this: If a structure point is available at fixed distances
for the entire run of the errant wire bundle, and structure is more
attractive to a high potential than a lesser potential in another wire,
where -- in all likelihood -- is that arc going to go?

Additionally, since the wire bundles for the FQIS (Fuel Quantity Indicator
System) are all shielded to remove any external noise, and since those
shields are all at aircraft ground potential, then even if they were
subjected to an arc-over, the arc would be shunted to ground anyway, thereby
eliminating any high voltages from the tanks.