Re: Causes and solutions for window crazing.

From:         phipps@ionet.net (James Phipps)
Organization: IONet
Date:         05 Aug 96 23:32:58 
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In article <airliners.1996.1552@ohare.Chicago.COM>, slacker@arlut.utexas.edu
says...

>Are they not a safety item? I really wonder about this. Isn't it likely that
>the scratches could eventually lead to stress fractures as the windows flex
>during pressure cycles? Or is this simply not a problem in the case of the
>window material?


Yes, scratches do lead to crazing of airliner cabin windows.  The windows can
also craze as a result of several thousand pressure cycles.  Chemical
reactions between water, sulfuric acid and the acrylic material can rapidly
promote crazing.  Cabin windows routinely flex as much as 0.25" during
pressure cycles.  I have intentionally induced craze in acrylic cabin windows
on a test stand.  I was able to cause a severally crazed window to fail (blow
apart) after it had flexed 0.75" at a pressure 10 times that of normal
pressure differential at altitude.

Cabin windows are not considered by the FAA to be a critical components of the
airframe.  (As a passenger, I would strongly argue that they are critical!)
Remember, there is a fail-safe window pane (the inner one with the vent hole
at the bottom) to take the place of the outer pane if it happens to fail.  And
this fail-safe pane is rarely crazed, cracked or scratched.  As long as the
maintenance inspectors can see through the window, they normally won't remove
the window until a D check.  It's a matter of economics for the airlines.

Cabin windows, which are made of acrylic, can be repaired by removing the
pits, scratches and crazing.  This can be done by removing material from the
window.  Most repair stations will polish the windows to do this.  My company
machines the material off the window.  When we are done, it looks like, and
basically is, a new window.  Cabin windows are good for 2 to 3 repairs.  If
you saw good windows manufactured in 1987, chances are they have been
repaired.  Normally, most cabin windows start to have noticeable crazing after
2 years.  The crazing gets so bad after 4 or 5 years, that the window will
need replaced.  This usually corresponds to the aircraft going through a D
check.

In summary, crazing is more of an aesthetic issue than a safety issue.  If
airlines care that their passengers can see through a window, they will spend
the extra effort and money to keep them in good shape.  If your flight has bad
windows, let the airlines know.  I do!

James Phipps   phipps@ionet.net
Manufacturing Engineer
NORDAM Transparency Division - Tulsa
"I only do windows."