Re: Inert Gas in Fuel Tanks

From: (Scott Odle)
Organization: Earthlink Network, Inc.
Date:         Mon, 28 Oct 1996 21:56:44 GMT
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In article <>, says...

>   Is there any reason that it would be impractical to put a nitrogen
>fuel tank pressurization system similar to that on the C-17 on an
>airliner?  Could it be retrofitted, or would it have to be
>incorporated in new models only? Aside from the weight and space, I
>can't see any drawbacks, in that the failure of such a system would
>not in itself endanger the aircraft.  Ambient air would still be
>available for venting, so it would be no more dangerous than present
>practice, as long as maintenance on the electrical system was kept to
>the current standard.

Weight and Space are not the only consideration.  While I recognize
that under much older certification standards there have been
accidents related to this, but have there been any attributed to this
for aircraft certificated under more recent FAR requirements (We can't
use TWA 800 as an answer since a cause has not yet been deterined.

Under todays laws, concerning rulemaking (the process of changing the
FARs) the FAA must show that the increase in the level of safety
provided by requiring such a change justifies its cost.  While many
people disagree with this premiss you can't blame the FAA for it when
these types of requirements are set by congress.  The FAA can't just
ignore the regulations any more that any operator can.

As for the other considerations:

1. Can it be justified (see above) or are the current standards
2. Added maintence costs (this would appear to be a BIG cost)
3. retrofiiting many older aircraft may be impossible (or maybe the
word is impratical) due to the design of there fuel systems.

There are probably others also.

Moderator's note: The little (compared to airliners) SR-71 uses LN2 to
pressurize the fuel tanks.  The plane has three LN2 bottles for this
purpose.  These bottles, which have to contain a lot of pressure, are
not light.  In addition, there's quite a bit of plumbing required.
Maintainence is not straightforward.  Furthermore, servicing is a real
pain.  Such a system would be very impractical, both to retrofit and
to turn around at the destination.  

Obviously, this old airplane doesn't use the system to collect N2 from
the ambient air as does the C-17, however, I have seen that C-17
system and it's quite large and looks very heavy.  It too has a lot of
plumbing.  When you consider that airlines switched from glass to
plastic for the little liquor bottles purely for the saving of fuel,
you can understand that every ounce of added weight is very expensive.

Unlike the laity, who frequently say that "you can't put a price on a
human life", you can do exactly that.  When I was in college in the
late '60s, the price the California Department of Transport (now
Caltrans) put on one life was $100,000 per year.  That is, if an
improvement was projected to save one life per year, the Department
would only proceed with the improvement if the cost was less than
$100,000 per year.  For improvements that didn't require continuing
maintainence, I believe the criterion was one life in five years.

Since airliners aren't falling out of the sky with great regularity
because the ullage isn't filled with inert gas, I'd say that it's not
likely to happen.  Maybe someone would like to ask the FAA, NTSB, and
major airlines what their criterion (price per life) is?  Also ask the
airlines how much money they're saving on those plastic bottles, of
course.  MFS]