I have seen this many times in typically older aircraft passenger windows and on a recent flight from Sydney to Melbourne I took the following pictures. It appears as if there are small cracks in the window or one of the laminate layers used to construct the windows. Could anyone shed some light on what is the cause and if there is any risk?
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2 Answers
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Airline pax windows are cast acrylic plastic (plexiglass) and when the panels age they can get little microcracks in the surface called crazing. This can be formed from long term tension stress on the crazed surface as well as from degradation from chemicals (ammonia is one of the worst - NEVER use Windex or equivalent on plexiglass) or UV (acrylic cast for aircraft has a UV block in it that makes it relatively resistant to UV).
On airline windows it's mostly due to long term surface stress because the pressurization system is trying to blow the window out all the time and the outer layer is in tension. Minor crazing is not a structural problem because it is in the outer couple thousandths of an inch of the plastic and doesn't reduce its strength significantly (1/4" thick acrylic sheet at 8ksi tensile strength, like you would typically find in a pressurized aircraft window, is roughly equivalent in tensile strength to .050" 2024 aluminum, so it's not that much different than the adjacent aluminum skin with minor abrasion surface scratches, structurally speaking).
Airlines restore windows with crazing and scratches by removing the outer crazed layer with abrasive products made for the purpose (like Micro Mesh https://micro-surface.com/). There is an allowable minimum thickness of the acrylic and they can go down to that, and if crazing is still apparent at that point the window is scrap.
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$\begingroup$ Thanks John, That makes sense and I appreciate the time and effort providing such a detailed answer. Cheers Ian $\endgroup$ Nov 22, 2018 at 21:15
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$\begingroup$ I was in China on business once and rode on a Shandong Airlines 737 with cabin windows that appeared to have been restored using Micro Mesh (a very special type of rubberized sandpaper) and they didn't do a particularly even job of it. My window was wavy like a window pane from a 19th century farmhouse lol. $\endgroup$– John KNov 23, 2018 at 2:20
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There is usually a plexiglass cover on the inside of an aircraft window. There tends to be a little hole in the plexiglass cover to make sure there is not much pressure difference on the cover. The cracks are probably only on the cover, and not in the window itself. If it really is the window, it was probably replaced already. The cover has no structural role, it is just for thermal isolation, and to keep passengers away from the real window, so it is nothing to be worried about.
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$\begingroup$ This seems unlikely as there is no stress on the inner window due to the vent hole you mentioned. John K's answer seems much more likely. $\endgroup$ Nov 22, 2018 at 19:12
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$\begingroup$ These cracks are much deeper than a few thousands of an inch, you need a magnifying glass or a microscope to see cracks that small. Constant stress is not needed to cause cracks, the inner panes are loaded during (de-)pressurisation. $\endgroup$– OrbitNov 23, 2018 at 9:59
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$\begingroup$ There is a bit of an optical illusion effect that makes crazing look much deeper than it really is. $\endgroup$– John KNov 24, 2018 at 17:58
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$\begingroup$ I sure hope so, because it looks like they are about 2 mm deep. Bending resistance decreases with the thickness to the power 3, and on top of that you get a stress concentration factor due to the crack. So cracks can be a pretty big threat to a window. $\endgroup$– OrbitNov 24, 2018 at 21:13