From: Mike Lorrey (mlorrey@datamann.com)
Date: Tue May 28 2002 - 10:47:35 MDT
Chuck Kuecker wrote:
>
> At 11:40 AM 5/27/02 -0400, you wrote:
>
> >It came down in four separate pieces (no details on what pieces, but I'm
> >guessting front fuselage, back fuselage, and two wings, indicating
> >failure in the wingroot area of the fuselage). The wing root area
> >contains two major things: fuel tanks and landing gear, as well as lots
> >of wiring that heads out through the wings. The claim that it is
> >counterintuitive of a fuel tank explosion is wrongly based. It doesn't
> >matter what the oxygen content outside the plane is, what is important
> >is what the oxygen content inside the fuel tank was. Fuel tanks are
> >pressurized, and thus retain the O2 concentrations of the airport they
> >were last fueled at. Any explosion occuring within the fuel tank would
> >occur irrespective of altitude.
>
> I seem to remember reading somewhere that fuel tanks on jetliners are
> pressurized with combustion bleed from the engines - which is much lower in
> O2 than the atmosphere. Sort of a natural fire preventer...do I misremember?
>
It's not combustion bleed, it is compressor bleed. This is air bled out
of the compressor core of a turbine engine just before the combustion
chambers (so it has whatever O2 levels are ambient at the time). It
generally comes out at about 180 psi and 600 degrees F. It requires both
mixing with cold ram air and passage through one or more heat exchangers
and turbo-expanders to cool it down enough to be useful in a safe manner
for fuel tank pressurization, and failure of this Environmental Control
System could result in hot air being fed into the fuel tanks (if it's a
poor design). For example, a bird strike to the ECS ram air intake would
cause the heat exchanger to fail. The problem with this scenario is that
it would light off warning sensor lights in the cockpit, something which
did not occur here, and a bird strike to the ram air intake would likely
result in a rather interesting and not unnoticable indicator of bloody
snow in the cockpit....
While some planes may use bleed air to maintain fuel tank
pressurization, not all do so, nor would this evacuate the pre-existing
sea-level air already in the fuel tanks. I'm still betting on kapton
detonation as the root cause, assuming the failure was in the wing root
area.
I worked on ECS systems while in the Air Farce, so I'm pretty familiar
with these sorts of systems.
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