What's the little hole in the bottom of the window for?
 

For years I thought it was a silver screw. Now that my trips are longer I've realised that it's a hole. And every aeroplane window seems to have one.

I can only guess that it has something to do with pressure, but I'm clueless.

What is the hole for? When it was introduced? And what happens if it isn't drilled in?

An answer to any of these questions would be most appreciated.

Great question, I've often wondered the same thing. Anyone.....anyone......Bueller.......Bueller...... :p ;)

I am giddy with anticipation...thanks for asking.

It lets the condensation out and equalizes the pressure between the panes of whatever that is. Without it the windows would be constantly fogged.

Condensation, right?
 

I think it's to allow air to get between the two windows to equalize the temperature between the outside window and the inside of the inside window - if it didn't exist, you'd never be able to see outside the window because it'd constantly be fogged up...

Similar to opening your car window to get rid of the condensation on the other windows.

-Peter

FWIW what you guys are calling the "inside window" is officially known as a "dust cover" in the aviation trade.

Gee and I was told that if you press the little button above your seat that depicts an FA, you will be sucked right through the hole in the window. Go figure. :mad:

'silver screw'
 

Thanks so much for the info!
I'm still a bit confused though.
The air pressure thing I can get my head around. BOAC used to have square windows, but 90 degree angles in a highly stressed pressure chamber wasn't a good idea and it cost lives (amazing they could figure out how to get all those tons of metal of the ground and not realise that - but then most things are obvious retrospectively), so anything that will relieve pressure on a flat shatterable surface is obviously a good idea.

I think the main reason that I used to think that the hole was a metal screw was because little frost patterns usually congregate around it - so I guessed the metal screw acted like a metal spoon does when you pour hot tea into a glass, and was there to conduct temperature. Now I know it's a hole, that changes things. If there was a hole that small in a car, it wouldn't make a jot of difference to the condensation levels. So I guess some other forces are at work:

Does the difference in pressure help draw the water vapour out of such a small hole?
How does water get into the sealed gap in the first place? And what stops it accumulating more and more with every flight?

Sorry to be such an idiot, but this sort of thing becomes momentous on a 12 hour flight.

There are over a million parts in a modern aircraft, and I have to go and obsess about a non-part, or rather, the absence of one.

You're thinking of the Comet I, rather than BOAC generically. You have to remember that metallurgy was then not really the science that it is now. The behaviour of metals was much less well understood. The causes of "metal fatigue" (as the phenomenon became known) were obscure, and this type of use of metal in a pressure vessel with this type of cutout was novel. We live and learn.

That was precisely my point - but you've expressed it so much more eloquently.
Isn't it amazing that they could make these huge lumbering contraptions fly despite not fully understanding the nature of the materials they were using? My mind still reels over this kind of thing whilst I sit meditating over a little hole in the window.

What's the little hole in the bottom of the window for?
 

It's a decorative snowflake generator. During cruise flight it automatically produces a little snowflake that appears on the inside of the outside window. :D

^ :p

Great reply about the snowflake generator :-:

I asked a friend who works in a top Formula One team to explain how the hole might prevent condensation, here's what he came up with:

Of course they do equalize pressure, without them the inner window (i.e. the one closest to you) would probably pop out. Maybe the flow out (when ascending) and in (when descending) means that any damp air in there gets recycled.
The air inside the plane is of course very dry. I think the pressure in a cruising aircraft is about 80% of atmospheric, so you'd expect 20% of the
air in the window space to be removed and replaced on each flight.

I expect the space between the outer and middle windows is completely
sealed, double-glazing fashion.

At the National Center for Atmospheric Research in Boulder CO is an exhibit where a person is able to pump up the pressure in an enclosed space, and the release the pressure quickly. The result? Fog forms. So probably the pressurizing and depressurizing of the plane could result in fog (and thus moisture) forming in a sealed environment such as between two windows. Thus the hole helps to keep the pressure balanced (Yes, I stayed at a Holiday Inn Express last night. ;) :D )

Just like building a layer cake!
 

Thank you all for a great cumulative effort!
I’m entirely satisfied now (belch!).
I really did need every one of your answers to make a pragmatic (and poetic) sense of the little hole.
Personally, I find the explanation is much more interesting than the mystery – sorry if it’s been ruined for anybody else.