Re: B747 technical questions

Date:         01 Aug 97 04:04:18 
From:         "Michel Gammon" <jmgammon@sympatico.ca>
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shumaker@eisner.decus.org wrote in article
<airliners.1997.1560@ohare.Chicago.COM>...

> The most common reason for using nitrogen for tire inflation where the
> pressure must remain constant or at least predictable for wide
> temperature variations (e.g.: racing car tires which routinely exceed
> 180 F tread temperature) is that bottled nitrogen is _dry_.  It's the
> water and water vapor that cause wild pressure increases with
> temperature; any dry gas would work as well.  Nitrogen happens to
> be widely available and cheap.

As a chemist I dispute this.  Gas volume when uncontained or gas pressure
when contained is directly related to temperature.  That is why  you are
always admonished to check tire pressures on your car when they are cold,
for reproducible accuracy.

Water is a liquid and therefore will not be subjected to the same rules and
also is incompressible.  Water vapour is microscopic liquid water droplets,
and steam is water in the gaseous phase.

My guess for the reason for nitrogen instead of air is that nitrogen is
inert.  During an emergency stop before V1 when maximum braking is used, I
suspect the internal tire temperatures can get very high (the external
surface will at least be afforded some air cooling, while the gas in the
tire will be insulated from external air and thus will not be cooled).  I
suspect that if air is used to fill the tires, the internal temperature of
the tire can theoretically exceed the auto-ignition temperature of the
rubber, thus igniting the tire and causing a catastrophic failure (which
often occur anyway probably as the pressure limit on the tire is exceeded).
 Nitrogen does not support combustion.  Furthermore, there is a small
possibility that in a wheel well fire caused by overheated brakes or
failure of another tire at rotation, the other tires may burst from the
fire flooding the wheel well with nitrogen and helping to douse the fire.
At least in theory, but the crash of a Nationair DC-8 in Saudi Arabia a few
years back was attributed to a wheel well fire caused by a burst tire
during the takeoff run (before V1 too, they should have aborted).

The gas laws apply to all gases.

Mike Gammon