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?

  • 1
    $\begingroup$ Not exactly a dupe, but the question is adressed here: aviation.stackexchange.com/q/673/95 $\endgroup$
    – yankeekilo
    Commented Mar 19, 2014 at 10:14
  • 5
    $\begingroup$ @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. $\endgroup$
    – Guillaume
    Commented Mar 19, 2014 at 10:17
  • $\begingroup$ I agree with @Guillaume $\endgroup$
    – ppp
    Commented Mar 19, 2014 at 22:03

1 Answer 1


Aircraft windows are multiple layers thick. The side facing the passengers is just a plastic sheet, since they wouldn't want the passengers scratching up the actual expensive glass window. Beyond the inner pane there are two actual structural window panes. The outer one is normally the one bearing the load, while the inner one serves as a backup.

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. A hole of that size would not jeopardize pressurization even if the outer pane blew out. Note the label "Breather Hole" in the diagram below.

enter image description here

There is a rubber seal around the outside of the two panes so it would be a bit of a pain to open a hole to cavity from the side. Apart from that, the windows are pretty tight together and by keeping it where it is there is nothing in the vicinity such as dust that could gradually be pulled in.

Since the air in between the panes is colder, it's not unusual to see ice crystals forming around the hole as the more humid cabin air enters the cavity.

(source: gentex.com)

787 Window

This diagram includes the electric shade system of the 787 which is fairly similar. Since the two outermost panes carry no structural purpose you can just have them equalize air as you please.

  • 1
    $\begingroup$ So on the 787, where the hole would be? On the interior window pane? $\endgroup$
    – Guillaume
    Commented Mar 19, 2014 at 10:20
  • $\begingroup$ 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... $\endgroup$ Commented Mar 19, 2014 at 10:23
  • $\begingroup$ 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. $\endgroup$
    – Guillaume
    Commented Mar 19, 2014 at 10:25
  • 1
    $\begingroup$ 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? :) $\endgroup$ Commented Mar 19, 2014 at 10:30
  • 4
    $\begingroup$ 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. $\endgroup$ Commented Mar 19, 2014 at 10:44

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