From: Michael Lorrey (retroman@together.net)
Date: Thu Feb 18 1999 - 20:08:22 MST
Spike Jones wrote:
> the fuel pump on my truck is inside the tank, submerged in gasoline,
> unless the tank is nearly empty. i would think the vapor pressure
> of octane on a cool day would be lower than that of kerosene on
> a warm day, yet there is never enough oxygen in a car's gas tank
> to cause an explosion, even assuming a faulty fuel pump. that
> kapton has problems is acknowledged, but why would a spark
> inside a fuel tank cause an explosion?
>
> what is your experience? i fly on these boeings about 40 times
> a year. i would hate to hafta worry that a tank could explode
> at any time. {8-[ spike
There are several ways:
a) fuel leaks. aircraft fuel tanks can be notoriously leaky, especially planes on
international flights. the fuels used in different countries tend to have some
important variances to them, and ground crews in many other countries are not english
literate and can easily mix up a request for JP-4 for JP-5, 4 for 3, etc. if they are
not properly trained. I do know that JP-5, for example has no trouble at all leaking
through the standard seals used on tanks made to hold JP-4. When I worked on F-15's in
the USAF we deployed to the Navy's Miramar NAS to fly as agressors against the Navy
Top Gun students. The F-14's use JP-5, while the F-15 uses JP-4, but there was no JP-4
on hand. What did we do? We used JP-5. Within the first 24 hours all of our fuels
specialists were working non-stop to plug the leaks that were developing in the seals.
The Navy boys helped out immensely. Apparently one specific brand of chewing gum works
as an excellent temporary sealant for JP-5 tanks....I don't recall the brand other
than it was a bubblegum.
Given this potential for fuel leaks (also likely to increase when under increased
structural loads from climbing, I can imagine that a sufficient leak could have
developed through the seals, around screws, etc. and the leaked fuel could have
ignited on a neighboring wire harness, especially if it leaked on a cannon-plug at a
bulkhead interface connection.
b) arc-throughs. The power supply cables for the forward distribution busses run by
the fuel tanks as I recall, but any wire with sufficient current that had chafed
against the fuel tank wall could possibly have chafed through, arced into the fuel
tank wall, and arc-welded a hole in the tank, providing a point of ignition.
Keep in mind that this particular tank was nearly empty. Nearly its entire volume was
filled with fuel vapor, and and air that had filled the space when it had been vented
as it emptied. Such a situation is far more potentially explosive than a full tank of
gas.
I don't think your statement that there is never any oxygen in a tank is true. The
reason why tanks don't blow up so often is that the pumps and sensors are built so
well that they almost never expose bare conductors to the inside of the tank. One
interesting feature that some planes have on their fuel systems is that they have
current sensors on both the hot and ground legs of the circuit. This allows the system
to measure if the insulation is breaking down and permitting too much current to go to
ground outside of the ground leg of the circuit. I do not know if that model of 747
have such a monitoring system or not.
The reason why I think we will never have a conclusion to this case is because it
happened over water. If it had happened over land, we would have known long ago what
happened to the plane, but there is so much of the plane (especially in the area of
that fuel tank) that is still missing, that I am afraid we will never know.
--
TANSTAAFL!!!
Michael Lorrey, President
Lorrey Systems
Webmaster, Mallnetworks.com
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