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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

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.)
achow 11y ago
Agree, the answer to the question was not articulated properly. Right at the beginning they should have said that the hole is for "releasing the moisture from the air gap and stop (most) fog or frost from forming on the window." The hole actually dilutes the safety wee bit: "In the extraordinarily unlikely event that the outer pane fails, the middle pane takes over. And yes, in that case, there would be a small leak of air through the breather hole—but nothing the aircraft’s pressurization system couldn’t easily cope with." TL;DR: Aircraft window hole compromises safety to improve the view :-)
- LukeShu 11y ago
  Um... that's not how I read it. It's about controlling the failure modes. The pane with the hole is the "backup"--without the hole, it would be stressed roughly the same as the outer pane during normal operation. This has a major consequence: when one breaks, the backup will probably be in the same sorry state as the primary before it broke, so it will break soon too! Not a very good backup! Sure, because they are sharing the normal load, it is less likely to break in the first place. But, you'd have essentially doubled the strength, but removed the backup. That's not a good trade-off.
  - achow 11y ago
    You could be right. But if that is the case then why doesn't the outside pane fog up?
    - LukeShu 11y ago
      Because the hole also releases moisture, as your post said. That's just a bonus.
- WalterBright 11y ago
  A pinhole like that is not going to affect pressurization in the least.
waxjar 11y ago
I'm just guessing, but I think the breaking of the middle pane would damage the outer pane anyway, since the pressure is coming from inside. That would leave you with two broken panes.
weinzierl 11y ago
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.
- copsarebastards 11y ago
  This explanation makes a lot of sense--I agree that not having the hole removed redundancy--but I'm not sure some of your details are exactly right. > If the two panes would be sealed absolutely tightly the pressure between them would stay at normal level in any case. I think higher pressure inside the plane would push out on the inner pane, pushing it closer to the outer window, so the pressure between would go up compared to the outside. That is to say, cabin pressure would be highest, pressure between the panes would be in the middle, and pressure outside the plane the lowest. But ALL THREE pressures would be lower than air pressure on the ground. > 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. The air in between the panes would provide some "padding" so the outer pane would receive less pressure than the inner pane, but there's not much air between the panes so the padding, and difference in pressure, would likely be small, so yes, redundancy would be lost.
  - moioci 11y ago
    "That is to say, cabin pressure would be highest, pressure between the panes would be in the middle, and pressure outside the plane the lowest. But ALL THREE pressures would be lower than air pressure on the ground." I think this is incorrect. Assuming the windows are sealed at ground level, the pressure between the panes would be slightly below ground level at altitude, due to flexion of both inner and outer panes; the pressure in the cabin is substantially lower than ground level; and outside pressure is the lowest of all. Thus the pressure between the two layers is the highest of the three compartments at altitude, subjecting both layers to considerable stresses with every pressurization/depressurization cycle. Now if you sealed a near-vacuum inbetween, that could help with thermal insulation and prevent condensation, but would cause more stress on the ground.

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.)

achow 11y ago

Agree, the answer to the question was not articulated properly. Right at the beginning they should have said that the hole is for "releasing the moisture from the air gap and stop (most) fog or frost from forming on the window." The hole actually dilutes the safety wee bit: "In the extraordinarily unlikely event that the outer pane fails, the middle pane takes over. And yes, in that case, there would be a small leak of air through the breather hole—but nothing the aircraft’s pressurization system couldn’t easily cope with." TL;DR: Aircraft window hole compromises safety to improve the view :-)

LukeShu 11y ago
Um... that's not how I read it. It's about controlling the failure modes. The pane with the hole is the "backup"--without the hole, it would be stressed roughly the same as the outer pane during normal operation. This has a major consequence: when one breaks, the backup will probably be in the same sorry state as the primary before it broke, so it will break soon too! Not a very good backup! Sure, because they are sharing the normal load, it is less likely to break in the first place. But, you'd have essentially doubled the strength, but removed the backup. That's not a good trade-off.
- achow 11y ago
  You could be right. But if that is the case then why doesn't the outside pane fog up?
  - LukeShu 11y ago
    Because the hole also releases moisture, as your post said. That's just a bonus.
WalterBright 11y ago
A pinhole like that is not going to affect pressurization in the least.

waxjar 11y ago

I'm just guessing, but I think the breaking of the middle pane would damage the outer pane anyway, since the pressure is coming from inside. That would leave you with two broken panes.

weinzierl 11y ago

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.

copsarebastards 11y ago
This explanation makes a lot of sense--I agree that not having the hole removed redundancy--but I'm not sure some of your details are exactly right. > If the two panes would be sealed absolutely tightly the pressure between them would stay at normal level in any case. I think higher pressure inside the plane would push out on the inner pane, pushing it closer to the outer window, so the pressure between would go up compared to the outside. That is to say, cabin pressure would be highest, pressure between the panes would be in the middle, and pressure outside the plane the lowest. But ALL THREE pressures would be lower than air pressure on the ground. > 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. The air in between the panes would provide some "padding" so the outer pane would receive less pressure than the inner pane, but there's not much air between the panes so the padding, and difference in pressure, would likely be small, so yes, redundancy would be lost.
- moioci 11y ago
  "That is to say, cabin pressure would be highest, pressure between the panes would be in the middle, and pressure outside the plane the lowest. But ALL THREE pressures would be lower than air pressure on the ground." I think this is incorrect. Assuming the windows are sealed at ground level, the pressure between the panes would be slightly below ground level at altitude, due to flexion of both inner and outer panes; the pressure in the cabin is substantially lower than ground level; and outside pressure is the lowest of all. Thus the pressure between the two layers is the highest of the three compartments at altitude, subjecting both layers to considerable stresses with every pressurization/depressurization cycle. Now if you sealed a near-vacuum inbetween, that could help with thermal insulation and prevent condensation, but would cause more stress on the ground.