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?
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?
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.
> 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.
If sea level pressure was trapped between the two windows you'd have pressure in the opposite direction on the inside window whenever the cabin altitude increased, which would happen every flight and induce cyclic stress on the sill that it wasn't designed for.
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.
wait, what? this isn't how pressure works... am i missing something here?
| |
[cabin] | [gap] | [outside]
0.75atm | X atm | 0.23 atm
| |
I think it's the lack of air in double-paned windows that helps. Usually double-paned windows have a vacuum so that air molecules can't move from the outer pane to the inner pane. Surely air is a fine conductor of heat if we consider our weather system. And surely it's also a fine conductor of sound, if we consider our hearing system. In soundproofing applications, open air channels are the top priority, and the simplest way to dampen the sound is by adding mass. Sound doesn't travel at all through a perfect vacuum though (no sound in space), so typical double-paned glass should have an effect there.
I think the purpose of the hole is to ensure that stress does not build up on the redundant pane so that it is only used in the case where the main pane fails. When you build redundancy into a system, it's important to ensure that failure of both pieces doesn't happen at the same time. Either can fail on its own, but they can't both fail.
So you want the middle window to be as close to the ideal condition as possible when the outer window fails. By adding the hole, you accomplish this. Moreover, once the outer window fails, the hole ensures that the middle doesn't need to take the full pressure differential and allows the pressure to equalized in a somewhat controlled manner. Yes, the pressure in the cabin will drop, but there are oxygen masks that will drop if it gets too low and the pilots can always descend to a lower altitude. The people onboard will have the time necessary to take both necessary actions.
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:
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 :-)
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.