Bdtyktl

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?

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.

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.