# Why the holes in aeroplane windows?

(I was sure this would be the sort of question that has already been addressed on the SDMB, but I can’t find anything using the search. Apologies if I missed it.)

Why do the window panes on airliners have those little holes near the bottom? I can only assume it’s something to do with allowing pressure to equalise, but surely if this is the case then it makes one pane redundant? If you have two panes of glazing, but one has a hole in it, then presumably all the pressure is being taken by the other one. I would have thought that having two independent panes, both able to withstand the cabin pressure, would be a safer situation.

Similarly, it seems to go against the anti-condensation principle of double glazing, which avoids having warm interior air directly contacting the cold outer pain of glass, by having an insulating layer in-between. Surely having a hole in one pane defeats the purpose?

I presume it does let pressure equalise between the inner pane and the main window, but I always thought the inner pane was more to protect the main window from being hit, scratched, damaged, and weakend by the hyper-active toddler strapped into the seat next to it, rather than for thermal protction. Isn’t the inner pane made of plastic?

Sunspace is correct. The inner window is not structural. The holes are there to prevent condensation between the protective inner window and the structural outer panes.

I don’t think you can do that very easily. If there is an air gap between the panes, the force (due to air pressure) won’t be shared among the two panes. The outer pane would have to deform quite a bit before it exerts much force on the inner pane.

And what pressure do you set for the air gap anyway? If you set it for cabin pressure at cruising altitude, it’d be at partial vacuum when the plane is on the ground. You’d need a vacuum pump to achieve that pressure after you do any maintenance work on the window. If there’s a slow leak, the gap might equalize to ambient pressure on the ground, and then not depressurize fast enough as the plane goes up.