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Comment by tlrobinson | Hacker News Reader

I don’t understand how these two statements are both possible:

> "Several passengers recalled that after takeoff the aircraft cabin seemed noisier and colder than they were used to," investigators wrote

…

> the plane had remained "pressurized normally," investigators wrote.

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Aircraft windows are double paned. On the inner pane, there is actually a small hole that allows air to pass to the outer pane, and makes it so that the outer pane is holding in the pressure during normal operation.

If the outer pane is cracked (or lost entirely), then the inner pane will take on the pressure. The hole in the inner pane is small enough that the aircraft’s pressurization system can still maintain pressure even if that hole is leaking air.

However that hole likely makes a lot of noise if air is actively rushing through it, and also without the outer pane, there would be less insulation against the cold air and noise outside, and also the missing pane would create an aerodynamic change that I think would cause a lot more noise from the wind.

edit: actually per the AVHerald article, some of the windows were missing both panes (although from the pictures it looks like the inner pane is still intact?), so it must have just been that the pressurization system was just strong enough to keep up with the hole in the fuselage.

At up to 10,000 feet I imagine the pressurization system could keep pace with the differential from the missing windows. If they had gone much higher, it might not have been able to keep pace. If the cabin loses pressure at higher altitude, the procedure is to descend to 10,000 feet or less where pressurization is not needed for passenger survival. In this case, the problem was detected around the time they were crossing above 10,000 feet.

Shouldn’t that fire an alert of some sort that the system is having to work at higher capacity than normal to maintain pressure for the current altitude?

On a modern airliner, you'll get an alarm when the cabin pressure exceeds the pressure equivalent 10K feet. A few thousand feet of cabin altitude higher and the rubber jungle of masks appears. You won't get an alarm for just cabin pressure differential begin lower than usual.

So the air conditioning system essentially runs at a fixed capacity (although some very modern aircraft with electric compressors may vary). Pressure is regulated by controlling the amount of air exhausted by the cabin outflow valve near the back of the airplane.

Since they're are so many variables that can go into the inflow and outflow rates it's unlikely that anybody had characterized outflow valve position in all possible conditions. Nor is it's specific position of any concern, just that it be in the right position to maintain the selected cabin pressure.

So no, it's unlikely that they're would be an alert attached to its position, what is however likely is that they're is an alert to warn that cabin pressure has deviated from the commanded pressure. In this case though they never found themselves in a position where the aircraft was unable to regulate cabin pressure.

Planes that old are designed to actively pressurize to 6k/8k ft equivalent plus 13k feet is not that high, less high than mt Whitney.

Airplanes are not airtight, so the system is designed to continue to pressurise the cabin as the air escapes.

As they didn’t go above 14,000 feet, I’m guessing it could keep up with the pressure loss from the windows.

Maybe the windows are double-paned, and the loss of the outer pane resulted in poorer thermal and sound insulation but not a drop in pressure.

Isn't that just because the pressurization system had enough capacity that it could keep up with the extra loss of air?

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