Stuart & Kathryn Fields wrote:

aluminum control tube failed causing helicopter crash. The helo had an
anodized aluminum control tube that failed in fatigue with less than 400hrs.
The loading was low producing stresses in the area of 460psi.

It seems unlikely to me that anodizing was the
real culprit here, since the anodized layer is
normally only .001" thick or so.

Discussions
with anodizing folks say "Never anodize flight critical components" Some
research has provided data indicating that some cleaners can also shorten
the fatigue life of aluminum significantly.

I'm not an expert on anodizing, but I am an
engineer, and I've studied anodizing because I own
two sailboats that have aluminum components
exposed to a salt-water environment, and anodizing
is a great way to passivate aluminum to prevent
corrosion. The aluminum hydroxide layer formed by
anodizing is around an 8 on the hardness
scale--roughly the same hardness as rubys.

Do you have any links to the research you are
citing? If this is really a problem, I'd like to
learn more about it.

I've seen gyrocopters with long
anodized aluminum control tubes shaking like your English teacher's No-No
finger. The May 07 issue of Experimental Helo has a more indepth article on
this with pictures of the helo before and after..

If there was an existing flaw in the aluminum tube
_before_ it was anodized, the anodizing might very
well make the flaw worse, as it would eat away at
the edges.

Just my thoughts... I could be wrong. Have been
before, and will be again.

Don W.