John,

My intent is to keep an open mind on the root cause of the failure.
Prematurely stating this is fatigue issue is a mistake.

The investigation will have to look at many aspects. How was the glider
used or abused? It might include metallurgic analysis to ensure the
aluminum spare was made from the correct type of alloy, tempered properly,
and has no inclusions, etc.

As a person that has looked through a microscope at granular structure of
metal I can tell you this. This is not a lab experiment where all the
variables are controlled. There will be a lot of unknowns because of the
age of the glider, how it was flown, and how it was cared for. As such,
this is not a text book case where engineers can pull out a chart and look
up data to determine a resultant.

As an aerobatic pilot, I can tell you this. We make mistakes. If trained
properly a pilot will be able to recover from any situation without
exceeding the glider’s limits. However, inverted recoveries are not
intuitive. A simple mistake while inverted can quickly lead to the glider
going over Vne or exceeding G limits. It’s easy to do. Something happens;
the pilot gets caught off guard, takes a few seconds to figure things out,
mean while the glider is accelerating towards the ground. If inverted,
pulling on the stick will cause the glider to accelerate. The pilot may
never have known the ASI was above Vne. I’m not saying this happened to
the glider in question.

What I am saying is this “Don’t label the root cause as fatigue until the
investigators have done their due diligence and reached a conclusion based
on ALL the data!”. Labelling the issue as fatigue without data is not just
when there are thousands of these gliders with tens of thousand hours
without failures.

Andrew



At 14:44 31 August 2010, jcarlyle wrote:
Andrew, there would be no masking due to oxidation. Anyone who has
ever looked under a microscope at a fatigue crack scenario leading to
overload failure would agree with Vaughn above, who said "accident
investigators can fairly positively tell the difference between a
structure that has failed from overstressing and one that has failed
from fatigue." Believe me, if this failure had fatigue as its initial
cause, the evidence will be very clear - the fatigue "bench marks"
won't disappear. An overload due to high Gs that blunted the fatigue
crack temporarily will also be evident.

-John


On Aug 31, 10:12 am, Andrew Corrigan
wrote:
if the limits were exceeded 10-20 years ago, the oxidization build up
on
the aluminum might mask that damage. A fresh fracture would be shiny
because of the absence of oxidation.