Re: What is the small hole in the window?

From:         phipps@ionet.net (Stephanie Phipps)
Organization: IONet
Date:         29 Jul 96 02:29:32 
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In article <airliners.1996.1416@ohare.Chicago.COM>,
drranu@holly.ACNS.ColoState.EDU says...
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>
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>        Every commercial plane I have been in, the windows have a small,
>        what looks like a hole (or metal insert), at the bottom of it.
>        It is the window pane between the inner plexiglass one and the
>        outer pane.  What is this thing, and why is it there?

It is a hole, not a metal insert.  That small hole (.060" typically) is an air
bleed hole or "weep hole".  In order to understand why it is there, you need
to know how a common airliner cabin windows (B747/737/727,MD-80,L1011,DC9,etc)
are constructed.

There are two main window panes: the outer window (approx 0.3" thick) and the
inner window (approx 0.2" thick-the one with the weep hole).  These two
windows sit in a thick rubber gasket.  The outer window serves as the
aircraft's skin, and provides protection from the outside elements
(weather,foreign particles, wind-blast, etc.).  The inner window is sort of a
fail-safe pane.  It would serve as the outer window if the outer window were
to fail.  The weep hole in the inner window allows air pressure between the
two windows to equalize during cabin pressurization cycles (normal for
ascent/decent).  Since the windows sit in the rubber gasket, the hole is
important to keep the windows from unnecessary flexure, and allows any excess
moisture to escape.

The third window (approx 0.1" thick), the one you can touch, is the "scratch
pane".  This serves only to keep the inner window from becomming damaged,
smudged-up or tampered with.

In addition, the outer and inner windows are made of acrylic, while the
scratch pane is polycarbonate.

I hope this info is useful to you.


James Phipps - phipps@ionet.com
Manufacturing Engineer
NORDAM Transparency Div., Tulsa, OK