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On a commercial airliner, there is a hole in one of the layer of the passenger windows.

See here:

Airplane window

and here:

Another airplane window

Or in more detail:

Zoom on hole in airplane window

Can you explain the physical reasons for this design choice?

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Not exactly a dupe, but the question is adressed here: aviation.stackexchange.com/q/673/95 –  yankeekilo Mar 19 at 10:14
3  
@yankeekilo Well spotted. The title and slightly different subject of the other question made me not find it when searching before asking. Since there is not a lot of details in the answer that address it aviation.stackexchange.com/a/681/1599 I suggest we keep this one open. –  Guillaume Mar 19 at 10:17
    
I agree with @Guillaume –  Pato Sáinz Mar 19 at 22:03

1 Answer 1

Aircraft windows are in multiple layers thick. The side facing the passengers is just a plastic sheet, since they wouldn't want the passengers scratching up the actual glass window.

However, since pressure changes in the cabin during the flight, it's also necessary to equalize the pressure between the window panes, and hence the little hole.

787window

787 Window

This picture includes the electric shade system of the 787, but you still get an idea of the different layers of the window.

I'd guess they might have been able to put the hole facing sideways and hidden away internally, but this might perhaps cause problems with condensation, or perhaps risk sucking in dust, but this is just a guess.

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So on the 787, where the hole would be? On the interior window pane? –  Guillaume Mar 19 at 10:20
    
I'm not sure how the 787 is, but here's a picture: airlinereporter.com/wp-content/uploads/2012/11/787-wing.jpg and it looks like it might have two holes, but i'm not sure where these are located. i'm not sure if you can just drill through these electrochromic panels, or if there might be a bypass on the side to fulfill the same purpose... –  MikeFoxtrot Mar 19 at 10:23
    
I'm still confused by the need of the 2 panes if an hole in the interior pane make the pressure adjust to the cabin. –  Guillaume Mar 19 at 10:25
    
There's a seal on every window and therefore every pressure until the final 'real' window has to be equalized since otherwise these would blow or crack. does this answer your question? :) –  MikeFoxtrot Mar 19 at 10:30
1  
I'm pretty sure this is for safety/redundancy purposes. Both are designed to be strong enough for the pressure difference, but in case there's a structural defect in one, it will not lead to the complete blowout and loss of cabin pressure. I'm very sure the cockpit windows are double paned as well for the same reason. Looking on a few forums, it seems to be that the outer panel is designed to carry the load, since there's a very small bleed between the inner window and cabin air to equalize the space inbetween. –  MikeFoxtrot Mar 19 at 10:44

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