The hole in the bottom of your airplane window is actually a very
important safety feature. It's all-too-easy to let your mind wander when you're
confined to a tiny box of space while hurtling 40,000 feet in the air at
hundreds of miles per hour, but rest assured: every single window on the
airplane has the same hole. More officially, it's called a breather hole and
it's used to regulate the amount of pressure that passes between the window's
inner and outer panes. In short, the system ensures that the outer pane bears
the most pressure so that if there were a situation that caused added strain on
the window, it's the outside panel that gives out (meaning you can still
breathe). As shown in the Boeing 737 maintenance manual (the most widely
produced jet airliner in aviation history), the window structure consists of
three layers of acrylic – a tough, transparent and flexible resin – although
only two of them have an actual structural function.
As can be noted in
the diagram shown above, the breather hole is located in the middle layer of
the window. This little puncture acts as a bleed valve ensuring that the
pressure between the last two layers and the cabin always remains the same.
This is necessary as a way of preserving the middle layer (the extra safety
one) so it is only exposed to severe pressure differences in cases of emergency
– that is, if the last layer the window is fractured in some way.
Furthermore, any
possible cracks in the outermost layer of the window is enough to justify an
emergency landing – even if the middle layer is fully capable, in principle, to
maintain the appropriate cabin pressure conditions
The breather hole
also serves to prevent freezing and fogging between the outer layers of the
window.
