18 ms·

Why do airplane windows have tiny holes?

246 points by mml 11y ago

vhost- 11y ago
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.
- deleted 11y ago
  [deleted]
hellyeasa 11y ago
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 11y ago
  To put it another way: Fuselages are 20x thicker than the walls of soda cans.
  - re 11y ago
    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.
    - jonsen 11y ago
      So it would be unwise to fill a 777 with soda.
      - ceejayoz 11y ago
        This is definitely the important lesson.
    - deleted 11y ago
      [deleted]
    - smcl 11y ago
      This is potentially a pretty stupid question - but are the differences in pressure roughly the same between a full, pressurised soda can and a cruising-altitude pressurised airline cabin?
      - semi-extrinsic 11y ago
        From a 2-minute google search, it looks like both are ~10 psi pressure difference (psig). The numbers I found were 7-10 psig for a plane and an estimate of 17 psig for the soda can. So they're quite similar and it's not a stupid question.
        e12e 11y ago
        One of us appear to be wrong :-) I was just coming back to edit my reply to say that the difference appear to be 0.5:2, or the soda can holds roughly 4x the difference. Ah, but I'm tired, maybe I read/thought wrong.
        semi-extrinsic 11y ago
        Well, the 17 psig number I found was an estimate, so if you found the soda can holds about 4*7 = 28 psig, I'd say we agree.
      - e12e 11y ago
        I'm not sure why you'd think it's a stupid question (what is a stupid question?) -- but according to the Internetz[1,2], I think the answer is no. The difference of pressure between an soda-can and air (at ground level) would appear to be between to and three atmospheres worth (or like sea-level and a depth of 20-30m/~60-90', if the rule-of-thumb I've learned is about correct; see also: Why is parachuting into water OK, while diving and then flying a bad idea?). It would appear the interior and exterior and an air-plane typically differ at about half an atmosphere (5m/15' water). [1] http://hypertextbook.com/facts/2000/SeemaMeraj.shtml http://hypertextbook.com/facts/2000/SeemaMeraj.shtml [2] http://en.wikipedia.org/wiki/Cabin_pressurization http://en.wikipedia.org/wiki/Cabin_pressurization
    - achow 11y ago
      > 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_462.html http://www.engineeringtoolbox.com/air-altitude-pressure-d_46...
      - marcosdumay 11y ago
        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 11y ago
        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.
      - Dylan16807 11y ago
        I think you got something backwards there. The plane is thinner when you make them the same size.
      - gusmd 11y ago
        You are neglecting the fact that a soda can is pressurized once, depressurized when opened for consumption, and then wasted. An aircraft fuselage needs to withstand THOUSANDS of cycles of pressurization / depressurization during its service life, which is usually much more critical then the static pressure load, due to metal fatigue. This is why aircraft service life is given in flights (which corresponds to one pressurization cycle) and not in flight hours or miles or whatever. This is also why aircraft used on longer routes tend to last for more years (longer flights = fewer pressurization cycles). edit: If anyone wants a reference, here goes one http://www.airspacemag.com/need-to-know/what-determines-an-airplanes-lifespan-29533465/?no-ist http://www.airspacemag.com/need-to-know/what-determines-an-a... edit2: Additionally, consider checking my response to another comment. The diameter of the fuselage is so much bigger than the soda can that the actual stress on the walls are roughly only 1.5x bigger on the soda cans.
    - robryk 11y ago
      Scaling objects doesn't preserve their ability to withstand forces: for example cross-section areas scale with square of scaling factor and masses/weights scale with the cube of scaling factor; so objects are close to collapsing due to their own weight if you scale them up.
- tim333 11y ago
  That also makes me think about Aloha Airlines Flight 243 http://www.vfrmagazine.net/wp-content/uploads/2012/09/fretting.jpg http://www.vfrmagazine.net/wp-content/uploads/2012/09/fretti... http://en.wikipedia.org/wiki/Aloha_Airlines_Flight_243 http://en.wikipedia.org/wiki/Aloha_Airlines_Flight_243
- userbinator 11y ago
  For comparison, passenger car body panels are somewhat thinner, at 0.6-1mm.
darkmighty 11y ago
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?
- oh_sigh 11y ago
  It seems like you are asking why a more complex solution with no benefits and some drawbacks isn't being used. Is that fair?
- alok-g 11y ago
  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 11y ago
    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 11y ago
frik 11y ago
More info: http://www.airliners.net/aviation-forums/tech_ops/read.main/170548/#6 http://www.airliners.net/aviation-forums/tech_ops/read.main/...
Neil44 11y ago
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 11y ago
  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 11y ago
    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 11y ago
      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.
      - jessaustin 11y ago
        Right you are! Normally I would hate to be that guy, but here's the obligatory XKCD: http://xkcd.com/803/ http://xkcd.com/803/
      - CONTRARlAN 11y ago
        It's way more complicated than that: http://flighttraining.aopa.org/magazine/1998/November/199811_Features_Bernoulli_or_Newton.html http://flighttraining.aopa.org/magazine/1998/November/199811...
JakaJancar 11y ago
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?
- Neil44 11y ago
  I guess it's good that the hole is inaccessible to passengers? Also there's good progression of failure this way.
- lucaspiller 11y ago
  > a separate but related function of the hole: to release moisture from the air gap and stop (most) fog or frost from forming on the window. You could get around that by having the panes airtight and produced in a moisture free environment, but that would be more complicated. Think about failure rates in double paned windows which don't have to undergo the stresses on an aircraft. I'd assume even the outer panes aren't completely airtight to keep things simple: http://aviation.stackexchange.com/a/953 http://aviation.stackexchange.com/a/953
- leoc 11y ago
  My main guess (I'm not an expert) is that it's to ensure that the double-window system is fully redundant. Without the hole, in the event of an outer-window failure the inner window would already have gone through a variable and probably large number of hours of stress from differing air pressure. With the hole, the middle window is (hopefully) effectively factory-fresh with zero service hours at the moment it takes over. That would be especially good if the failure of the outer window had anything to do with a quality defect in that window (or mishandling of it) instead of being completely adventitious: in that case, maybe it's not the only window pane in the aircraft with a dodgy service-life rating? If the outer windows have to be replaced every n flying hours, then not stressing the middle windows would also make sure that they don't have to be replaced as often. (This seems a bit clearer from reading the io9 version http://io9.com/why-is-there-a-hole-in-airplane-windows-1703660371 http://io9.com/why-is-there-a-hole-in-airplane-windows-17036... , though I don't think it's stated explicitly there either.)
- 11y ago
leoc 11y ago
Bears a strong resemblance to this io9 article from the 11th: http://io9.com/why-is-there-a-hole-in-airplane-windows-1703660371 http://io9.com/why-is-there-a-hole-in-airplane-windows-17036... .
deleted 11y ago
[deleted]
jessaustin 11y ago
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.

246 points by mml 11y ago

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.

deleted 11y ago
[deleted]

hellyeasa 11y ago

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 11y ago
To put it another way: Fuselages are 20x thicker than the walls of soda cans.
- re 11y ago
  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.
  - jonsen 11y ago
    So it would be unwise to fill a 777 with soda.
    - ceejayoz 11y ago
      This is definitely the important lesson.
  - deleted 11y ago
    [deleted]
  - smcl 11y ago
    This is potentially a pretty stupid question - but are the differences in pressure roughly the same between a full, pressurised soda can and a cruising-altitude pressurised airline cabin?
    - semi-extrinsic 11y ago
      From a 2-minute google search, it looks like both are ~10 psi pressure difference (psig). The numbers I found were 7-10 psig for a plane and an estimate of 17 psig for the soda can. So they're quite similar and it's not a stupid question.
      - e12e 11y ago
        One of us appear to be wrong :-) I was just coming back to edit my reply to say that the difference appear to be 0.5:2, or the soda can holds roughly 4x the difference. Ah, but I'm tired, maybe I read/thought wrong.
        semi-extrinsic 11y ago
        Well, the 17 psig number I found was an estimate, so if you found the soda can holds about 4*7 = 28 psig, I'd say we agree.
    - e12e 11y ago
      I'm not sure why you'd think it's a stupid question (what is a stupid question?) -- but according to the Internetz[1,2], I think the answer is no. The difference of pressure between an soda-can and air (at ground level) would appear to be between to and three atmospheres worth (or like sea-level and a depth of 20-30m/~60-90', if the rule-of-thumb I've learned is about correct; see also: Why is parachuting into water OK, while diving and then flying a bad idea?). It would appear the interior and exterior and an air-plane typically differ at about half an atmosphere (5m/15' water). [1] http://hypertextbook.com/facts/2000/SeemaMeraj.shtml http://hypertextbook.com/facts/2000/SeemaMeraj.shtml [2] http://en.wikipedia.org/wiki/Cabin_pressurization http://en.wikipedia.org/wiki/Cabin_pressurization
  - achow 11y ago
    > 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_462.html http://www.engineeringtoolbox.com/air-altitude-pressure-d_46...
    - marcosdumay 11y ago
      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 11y ago
        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.
    - Dylan16807 11y ago
      I think you got something backwards there. The plane is thinner when you make them the same size.
    - gusmd 11y ago
      You are neglecting the fact that a soda can is pressurized once, depressurized when opened for consumption, and then wasted. An aircraft fuselage needs to withstand THOUSANDS of cycles of pressurization / depressurization during its service life, which is usually much more critical then the static pressure load, due to metal fatigue. This is why aircraft service life is given in flights (which corresponds to one pressurization cycle) and not in flight hours or miles or whatever. This is also why aircraft used on longer routes tend to last for more years (longer flights = fewer pressurization cycles). edit: If anyone wants a reference, here goes one http://www.airspacemag.com/need-to-know/what-determines-an-airplanes-lifespan-29533465/?no-ist http://www.airspacemag.com/need-to-know/what-determines-an-a... edit2: Additionally, consider checking my response to another comment. The diameter of the fuselage is so much bigger than the soda can that the actual stress on the walls are roughly only 1.5x bigger on the soda cans.
  - robryk 11y ago
    Scaling objects doesn't preserve their ability to withstand forces: for example cross-section areas scale with square of scaling factor and masses/weights scale with the cube of scaling factor; so objects are close to collapsing due to their own weight if you scale them up.
tim333 11y ago
That also makes me think about Aloha Airlines Flight 243 http://www.vfrmagazine.net/wp-content/uploads/2012/09/fretting.jpg http://www.vfrmagazine.net/wp-content/uploads/2012/09/fretti... http://en.wikipedia.org/wiki/Aloha_Airlines_Flight_243 http://en.wikipedia.org/wiki/Aloha_Airlines_Flight_243
userbinator 11y ago
For comparison, passenger car body panels are somewhat thinner, at 0.6-1mm.

darkmighty 11y ago

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?

oh_sigh 11y ago
It seems like you are asking why a more complex solution with no benefits and some drawbacks isn't being used. Is that fair?
alok-g 11y ago
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 11y ago
  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 11y ago

frik 11y ago

More info: http://www.airliners.net/aviation-forums/tech_ops/read.main/170548/#6 http://www.airliners.net/aviation-forums/tech_ops/read.main/...

Neil44 11y ago

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 11y ago
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 11y ago
  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 11y ago
    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.
    - jessaustin 11y ago
      Right you are! Normally I would hate to be that guy, but here's the obligatory XKCD: http://xkcd.com/803/ http://xkcd.com/803/
    - CONTRARlAN 11y ago
      It's way more complicated than that: http://flighttraining.aopa.org/magazine/1998/November/199811_Features_Bernoulli_or_Newton.html http://flighttraining.aopa.org/magazine/1998/November/199811...

JakaJancar 11y ago

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?

Neil44 11y ago
I guess it's good that the hole is inaccessible to passengers? Also there's good progression of failure this way.
lucaspiller 11y ago
> a separate but related function of the hole: to release moisture from the air gap and stop (most) fog or frost from forming on the window. You could get around that by having the panes airtight and produced in a moisture free environment, but that would be more complicated. Think about failure rates in double paned windows which don't have to undergo the stresses on an aircraft. I'd assume even the outer panes aren't completely airtight to keep things simple: http://aviation.stackexchange.com/a/953 http://aviation.stackexchange.com/a/953
leoc 11y ago
My main guess (I'm not an expert) is that it's to ensure that the double-window system is fully redundant. Without the hole, in the event of an outer-window failure the inner window would already have gone through a variable and probably large number of hours of stress from differing air pressure. With the hole, the middle window is (hopefully) effectively factory-fresh with zero service hours at the moment it takes over. That would be especially good if the failure of the outer window had anything to do with a quality defect in that window (or mishandling of it) instead of being completely adventitious: in that case, maybe it's not the only window pane in the aircraft with a dodgy service-life rating? If the outer windows have to be replaced every n flying hours, then not stressing the middle windows would also make sure that they don't have to be replaced as often. (This seems a bit clearer from reading the io9 version http://io9.com/why-is-there-a-hole-in-airplane-windows-1703660371 http://io9.com/why-is-there-a-hole-in-airplane-windows-17036... , though I don't think it's stated explicitly there either.)
11y ago

leoc 11y ago

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

deleted 11y ago

[deleted]

jessaustin 11y ago

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.