Fuel Tank Safety

From:         "P. Wezeman" <pwezeman@blue.weeg.uiowa.edu>
Organization: The University of Iowa
Date:         Mon, 3 Mar 1997 04:40:46 GMT
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   The current Aviation Week has an article on various proposals to
prevent the possible explosion of airliner fuel tanks. They
authorities are at the point of talking to hardware makers.  To
summarize:

   Foam filling has the advantage of no moving parts or sensors. It is
effective at suppressing explosions if a fuel tank is hit repeatedly
by cannon fire. Weight for a 747 is said to be between 1,200-1,400 lb.
with about 1-2% reduction in fuel capacity. Some say the foam tends to
trap debris from the fuel, but the makers say that's not necessarily a
bad thing as it's better to have the debris trapped in the foam than
going through the engine. The U.S. Air Force has had extensive and
satisfactory experience with foam over the last twenty years.

[Moderator's note:  The foam deteriorates and has to be completely
replaced fairly often, particularly compared to the lifespan of the
average airline.  As it deteriorates, it tends to clog the fuel pumps.
Also, the military uses it in part for its ability to damp out fuel
sloshing during maneuvering that is well outside the envelope of most
airliners, which is why they're willing to put up with its short
lifetime.  MFS]

   The other approach is nitrogen blanketing. The only state of the
art system of this sort in use is on the C-17. It weighs 2,000 lb.,
occupies 80 cubic feet, and costs 2 million. It was developed by
Mcdonnell-Douglas for an Air Force requirement, and filters the oxygen
out of bleed air from the engines. It is new, but an earlier system
using liquid nitrogen has worked well on the C-5. The total buy of
C-17s is pretty small compared to the number of airliners. Would the
cost come down much for these systems if more were built?

[Moderator's note: Even if the price did come down, a ton is a lot of
weight to carry in an industry that switched from glass to plastic for
the liquor bottles to save a few bucks per flight.  Also, the
Blackbird has been using N2 pressurization since the plane's
inception, as the fuel gets sufficiently warm that even JP-8
vaporizes.  The SR-71 carries LN2 in pressurized tanks and can't
launch for a hot flight without it.  Thus, this would add yet another
cluster of items to the MEL, as well as more weight.  Fuel explosions
are sufficient more rare than failed LN2 sensors that the cost of
acquiring, maintaining, and carrying such systems may not be worth the
reduced risk.  The value of a saved life is not infinite.  MFS]

   The article implies that some in the NTSB are skeptical about being
able to eliminate all possibility of an ignition source inside a fuel
tank.

                        Peter Wezeman, anti-social Darwinist

                             "Carpe Cyprinidae"