Re: Airbus Gust-Load Alleviation systems

From:         cleyman@cix.compulink.co.uk (Clive Leyman)
Date:         05 Aug 96 04:21:18 
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>In a paper presented at the International Committee on Aircraft Fatigue
>(ICAF) 1987 Ottawa Canada, Ian Gray from Bae Filton presented "Fatigue
>Crack Propagation Test Programme for the A320 Wing".  Showed that the
>flight was more comfortable for the PAX, but the cracks grew faster with
>gust alleviation on over crack rates with gust alleviation off.  If you
>don't get the gust overloads, you don't get the fatigue life!

This sounds crazy but it isn't really. Cracks (and we are talking
about design for damage tolerance here), grow steadily under
normal flight loads, but if a big load comes along, either from a
gust or a big manoeuvre, then as I understand it the material at
the tip of the crack becomes work-hardened and, as after
cold-working, is left with a bit of residual compressive stress
that delays further crack growth.

So yes, if you don't get large gust loads you will get faster
crack growth (taken over reasonable time span) and will end up
with shorter inspection intervals.

I don't believe the bit about 'more comfortable for the PAX
though, as the A320 GLA was designed to alleviate the design loads
or something close to that, and that almost by definition is
something that happens only once in 50,000 hrs. So the impact on
day to day passenger comfort can't be all that much.

Clive