
Why do airplane windows have tiny holes? - mml
http://www.slate.com/blogs/the_eye/2015/05/29/what_s_that_thing_why_are_there_holes_in_airplane_windows.html
======
JakaJancar
Am I blind or is the question not really answered?

So the hole is there so that air gets through and the pressure difference is
at the outermost pane, OK. What's wrong with it being on the middle pane? What
would happen if there was no hole?

~~~
weinzierl
The purpose of the smudge pane should be obvious. There are two panes and a
hole left to explain.

First for the panes:

The article says the two panes are for safety reasons and I buy that. Equally
important in my opinion is another reason. It's the same reason you probably
have double panes in your home: thermal and noise insulation. Air has terrible
thermal and noise conduction. So you need two panes and air between them.

Why the hole:

Remove moisture, I buy that. Allow pressure to equilibrate between the
passenger cabin and the air gap between the panes. I buy that too but the
article does not really explain why this is necessary.

If the two panes would be sealed absolutely tightly the pressure between them
would stay at normal level in any case. As an aircraft climbs, the air
pressure drops in both the cabin and the outside air — but it drops much more
outside, as the aircraft’s pressurization system keeps the cabin pressure at a
comfortable and safe level.

Without the hole and with perfect seal if the pressurization fails for some
reason you'd have maximum pressure gradient on _both_ panes. The redundancy of
the second pane would be lost.

~~~
mturmon
Just to put a number on it, the cabin pressure at high altitudes is given by
Wikipedia as around 0.75 atm. (It varies by aircraft model.)

If the inner cavity between the two windows did not have a hole, the full sea
level pressure would be pressing outward against the outer window. But with
the hole, only about 75% of that pressure is present.

Additionally, if there were no hole, the inner cavity between the windows
would be positively pressurized with respect to the cabin. This means the
force on the inner window would be backwards from what it would have to bear
if the outer window failed.

~~~
beachstartup
> But with the hole, only about 75% of that pressure is present.

wait, what? this isn't how pressure works... am i missing something here?

~~~
mbrameld
I think it's saying if the two panes were sealed without a hole at ambient
pressure when the plane is manufactured then the outer pane at elevation would
have ~1atm of pressure pushing out on it and the middle pane would have ~1atm
pushing in on it. With the hole the outer pane only has cabin pressure pushing
out on it, which is ~75% of 1atm.

~~~
mturmon
Yes. This whole exchange could have benefited from a simple diagram (numbers
approximately correct for 35K feet):

    
    
              |       |
      [cabin] | [gap] | [outside]
      0.75atm | X atm | 0.23 atm
              |       |
    

If "what to make X" is your design choice, you would not want to have it
always pegged at 1 atm, as it would be if it was/could be completely sealed at
sea level. It's simplest to have it equalize with the cabin pressure.

~~~
weinzierl
The ASCII diagram is very helpful. I wish I had thought of including something
like this in my original comment. Thanks a lot.

------
Neil44
Its also interesting that the corners of the windows are rounded - it's to
avoid points of concentration for metal fatigue. We learned that lesson the
hard way. When the De Haviland Comet was designed with square ones this was
still poorly understood and there were several severe failures. The quest to
find the cause of these failures at the time could be seen as the birth of
modern air crash investigations.

Design evolution at work.

~~~
iwwr
The windows weren't quite square panes, they were slightly rounded, but not
enough.

Another issue the Comet had were the engines buried in the wings, which makes
them less accessible for servicing and more dangerous to the aircraft in the
event of failure/explosion.

~~~
nabla9
Engines buried in the wings was first and foremost aerodynamic problem. Today
we know that intakes must be kept from messing with the airflow above the wing
where most of the lift is generated.

~~~
rubyrescue
most lift is not generated above the wing but due to angle of attack which is
the angle of the wing relative to the flight path.

~~~
CONTRARlAN
It's way more complicated than that:

[http://flighttraining.aopa.org/magazine/1998/November/199811...](http://flighttraining.aopa.org/magazine/1998/November/199811_Features_Bernoulli_or_Newton.html)

~~~
nzp
That's a weird text. At the beginning the author lists reasons why it's
clearly Newton's third law, and then just basically says: "But then an
'authority' on aerodynamics waved hands around telling me it's not. The end."
Maybe I misread something, but nowhere in the examples that are apparently
inconsistent with Bernoulli's law, and for which the professor claims they
actually support it, does he _actually_ provide an explanation.

------
hellyeasa
Here's another cool random plane tidbit.

Commercial jetliner fuselage wall thicknesses are typically around 1-2mm. They
don't call 'em flying tincans for nothin! Think about that next time you fly!

~~~
oh_sigh
To put it another way: Fuselages are 20x thicker than the walls of soda cans.

~~~
re
If you shrank a 777 to the size of a soda can, its skin would be about four
times thinner than the average can's.

~~~
achow
> _If you shrank a 777 to the size of a soda can, its skin would be about four
> times thinner than the average can 's._

My take.. The difference between inside and outside of soda can is approx 175
(kPa). The difference between inside and outside of aircraft cabin at the
cruising altitude is 56.6 (kPa). So Soda can bears differential of approx 3
times than the pressure differential that aircraft cabin structure supports,
with material 4 times thinner.

Aircraft is 12 times more safer than a soda can (!)

Interesting.. aircraft safety engineers at work.

Reference:

[http://en.wikipedia.org/wiki/Cabin_pressurization](http://en.wikipedia.org/wiki/Cabin_pressurization)

[http://www.engineeringtoolbox.com/air-altitude-pressure-
d_46...](http://www.engineeringtoolbox.com/air-altitude-pressure-d_462.html)

~~~
marcosdumay
Well, except that aircraft pressure is applied outside-in while soda can
pressure is inside-out. Put some outside-in pressure on a can, and see how
well it handles it.

Both are engineered in a way to make pressure not a problem. The can is
subject to stress if you refill it, the airplane is a bit overengineered so
it's not subject to stress. Increasing the width of any wouldn't lead to an
increase on their safety.

~~~
allannienhuis
Sorry I'm confused by your statement. The air pressure inside an aircraft
during flight is much higher than the air pressure outside. So the pressure is
also inside -> out, same as a soda can.

------
leoc
Bears a strong resemblance to this io9 article from the 11th:
[http://io9.com/why-is-there-a-hole-in-airplane-
windows-17036...](http://io9.com/why-is-there-a-hole-in-airplane-
windows-1703660371) .

------
vhost-
Holy crap. I actually put this in my notebook to look into. I was flying home
from Atlanta a few weeks back and I was staring at the hole, wondering why it
was there.

I didn't get around to researching it, but here it is. It fell into my lap.

------
frik
More info: [http://www.airliners.net/aviation-
forums/tech_ops/read.main/...](http://www.airliners.net/aviation-
forums/tech_ops/read.main/170548/#6)

------
darkmighty
So, why not set the intermediate pressure (fixed) at the average of the
external and internal (typical) pressures?

The load on each pane is lower so I would think they'd be less likely to fail.

A few hypothesis:

\- If the main failure mode is a defect that doesn't depend too much on
pressure, then we have increased likelihood of failure;

\- The manufacuring cost of pressurizing the midpane is high;

\- Having a varying load direction on the external pane (inward on ground and
outward in sky) is not good.

Any opinions?

~~~
alok-g
In fact, one could package the three panes in a normal (or humidity-free) air
without the hole. Assuming the cabin pressure is close to ground air pressure,
the pressure difference will still be on the outermost pane as in the OP, and
if the outermost pane fails, it will come automatically on the middle pane as
in the OP. In other words, this reason cited by the OP does not explain at all
why a hole is there. Am I missing something here? :-)

The other reason cited by OP, fog and frost, would still be valid if it is not
desirable to package the panes in a humidity-free environment.

~~~
lukeschlather
Without the hole, the space between the two panes will be pressurized. I'm not
sure what the exact figures are (I'm not a phycisist) but I would guess that
without the hole the outer pane will be subjected to some force F due to the
presure, and the inner pane will be subjected to some force F/2, or even
equal.

Without the hole the system would probably be better modeled as a single pane
with an airgap between its inner and outer edges. It's likely that both panes
would fail at the same time.

~~~
darkmighty
You don't need to be a physicist: if the panel is sealed there will be a
constant pressure inside. The pressure on each face is then P_ext-P_int. It's
just as likely that both panels will fail at the same time in that case as in
the case with a hole; but the bonus in this case is that you can set up the
internal pressure so both panels have lower nominal pressure (but the internal
at least should be able to still handle the full pressure difference).

I know it's routine to manufacture even household glass panels with different
gases sealed in their interior (I think they put Argon inside? some low
conductivity gas), but afaik it's at atmospheric pressure.

------
jessaustin
Many people assume that panes must be fixed solidly within a solid frame. ISTM
the frame itself could be somewhat flexible, to accommodate Boyle's Law and
the regular sea-level-to-8k-ft pressure cycle. There may well be other factors
involved, especially the beautiful simplicity of drilling a hole. However, if
for any particular use case the hole proves problematic, we might expect to
see the backup pane in such a flexible arrangement.

