Jim, well that's a bummer! Someone writes an order to bring clarity to
a murky area, then that order is made obsolete by a superceding
directive which leaves us in the dark again. Perhaps this is why my
ASTM buddy hasn't been able to find anything to answer George's
question. It may be that use of pressure systems is not driven by
regulation so much as common sense, fear of litigation, and the
influence of insurance companies.

To answer JW's questions, aluminum and steel are pretty equal with
regard to safety. Of course, aluminum is generally more susceptible to
stress corrosion cracking than steel, and it will have a lower fatigue
life, so I would chose steel if you can stand the weight. The metal
alloys chosen for this application are ductile, so in neither case
would shrapnel be likely to be an issue. (The exception might be if the
bottle were cold enough at altitude to be below its ductile to brittle
transition temperature.) But, exploding in flight would be a pretty
unlikely event; refilling is the dangerous process as it puts rising
tension on any crack tip that may exist.

That said, you really don't want to be anywhere near a failing high
pressure cylinder! There will be an extremely high speed, high pressure
jet going in one direction, and a metal bottle going in the opposite
direction. Failing bottles have been known to go through solid concrete
walls, while high pressure jets have been known to cut people apart.

No one has remarked on the engineering behind these bottles. There are
three main concerns, flaws, fatigue life and corrosion. Flaws are
mostly found through hydrotesting; if a bottle passes it means any flaw
is below the critical crack size for the material. The crack growth
rate per fatigue cycle figure is determined for the material, and the
retest interval is chosen such that a sub-critical crack cannot grow
through refill cycling to critical crack size during the interval. But
fatigue cracking isn't the only mechanism of failure, stress corrosion
crack growth can occur simply by the passage of time, without any need
for stress cycles (it can happen to a full cylinder sitting on the
shelf).

A careful and prudent individual will get his high pressure cylinder
hydrotested every five years (or whatever the manufacturer recommends).
Remember, it's not just the number of refills, it's time passing itself
that can get you. If you think hyrotesting is expensive or
inconvenient, then I recommend you keep in mind Dirty Harry's question
"do you feel lucky?"

-John

jphoenix wrote:
John,

That Order was cancelled. It is replaced by HBAW 02-01B.

http://www.airweb.faa.gov/Regulatory...light=8000.40d

Jim