I've always wondered, what are those dual-layered (bonus: sometimes they have a little <2mm hole, why is it there?) windows commercial jets put next to their seats made of?
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3$\begingroup$ Related question: Why is there a hole in one of the layers of an airplane's window $\endgroup$– voretaq7Commented Mar 19, 2014 at 21:06
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4 Answers
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Generally aircraft windows are made of what we colloquially call "plexiglass" of some kind (Lexan polycarbonate is common in light General Aviation aircraft, acrylic plastics are also used). This material is light, relatively strong (not shatter-proof, but it'll take a moderate beating) and has decent optical properties.
Its major drawback is "crazing" -- hairline cracks on the surface of the material that form over time from dirt, stress, temperature, UV exposure, and the like.
Bonus answer: The little hole serves a couple of purposes, but the biggie is pressure relief/equalization. This gets discussed over on airliners.net a lot and they've covered it pretty thoroughly.
Basically the hole ensures that the cabin pressure is pushing against the outer (primary, usually thicker) sheet of plexiglass, which is plug-wedged into the fuselage structure and can't go anywhere.
Should the outer pane be damaged the hole will also theoretically bleed cabin pressure off at a controlled rate - though how successful that equalization is would depend on your altitude (and thus the pressure differential between the cabin and the outside air) and the degree of damage to the outer pane.
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$\begingroup$ As for shattering, it only depends on thickness - fighter aircraft feature moderately bullet-proof canopies (not even talking about birdstrike) and all the huge show aquaria sport PMMA panes. $\endgroup$ Commented Jan 7, 2014 at 18:40
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$\begingroup$ I wonder about the pressure relief thing, especially concerning pressure loss - not sure if this hits the spot (see also this). $\endgroup$ Commented Jan 7, 2014 at 18:45
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$\begingroup$ @yankeekilo Yup - thickness and design (you can get layered polycarbonate or acrylics which basically function like kevlar) - if you've been to a bank in the US recently you've probably seen the wall of bullet-resistant plastic between you and the tellers. Re: the hole, I'm not sure I completely buy the pressure transfer/relief thing, but I'm not sure I completely buy the condensation thing either (why not fill the middle chamber with dry nitrogen before sealing them together?) I suspect it's a little bit of both honestly, or possibly something else entirely. We need a jet A&P in here! :) $\endgroup$– voretaq7Commented Jan 8, 2014 at 2:54
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3$\begingroup$ The cabin-side screen is just a flimsy, easily replaceable thing that is used to protect the actual window and add a bit of thermal insulation. It does not form any kind of hermetic cavity with the window, too, but sits inside the pressurized cabin (otherwise you´d get one hell of a draft). I´ll try to remember to ask a colleague for confirmation $\endgroup$ Commented Jan 8, 2014 at 9:26
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1$\begingroup$ @Yankeekilo: I remember seeing some documentary where Israeli air force tested it and showed that bird strike at supersonic speed can break fighter canopy (Israel is bottleneck on migratory paths with sea at one side and desert on the other, so they get really huge flocks of birds in spring and autumn). $\endgroup$ Commented Mar 19, 2014 at 21:07
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Windows are either made of acrylics (PMMA, brand name e.g. "Plexiglas") or polycarbonate (PC, "Lexan" or "Makrolon") materials.
The small hole is there to provide ventilation and enable removal of moisture/condensation, but the hole is only in the thin protective screen on the inside. The much thicker actual window that holds the pressure is (hopefully) not punctured.
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Most windows (Boeing) are a triple layer of plexiglas - Cabin air circulates between panes for defogging - Except the windows found on cargo doors (electrical heat instead) - Elasticity is the main reason why they are strong -
Windshields are different, must resist to bird strikes and be heated - One inch or more in thickness and special optical corrections - A windshield pane for a 727 was $10,000 in the 1980s...!
Note that some airplanes have two different Vmo - Because of windshield bird strike resistance - As an example, some Learjets - Vmo (low altitude where birds are) is 305 KIAS - Vmo (high altitude, above FL 140) is 358 KIAS -
I did fly as Learjet instructor in the early 1970s - I was told the windshields of the 24/25 were "birdproof" - Could resist a 4 lbs bird hitting the windshield at 350 mph - I wondered if it could resist a 350 lbs turkey at 4 mph...? ROFL -
One note also about a fire axe - If you wish to make a hole to escape, do not try windows - Windows are extremely resistant - It is better to use a fire axe against metal (fuselage skin) -
Look at UK-CAA airplanes and "cut here" frames on side of fuselages - Airfield firemen are knowledgeable about not trying to "break" windows - And would open fuselages within the "cut here" frame indications -
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There is a term called Tg, glass transition temperature. In PC this is around 150C. Therefore the mobility in the PC molecular chains at RT are unmobile, which in most amorphous polymers would cause them to be brittle. However due to the chemical bonding of PC this does not occur.
However, upon an impact, the stress energy resulting from a certain point will be transferred across many of the Polycarbonate chains. Induced impacts can cause large amounts of stress, especially at the speeds we are talking about. Therefore the holes are not only there as people have already said, for the equalisation of pressure differences, but it also a stress relieving point for the material.
Sources: I am a MEng Materials Engineer with experience in this field
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2$\begingroup$ This seems to focus on the reason for the hole, and not the type of material, which is what the question is asking. $\endgroup$– foootCommented Oct 15, 2015 at 19:50
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1$\begingroup$ In PC the cooling rate makes all the difference. Cool it too quickly or very slowly, and PC will become very brittle. The pressure relief hole has nothing to do with it, Mr. Materials Engineer. $\endgroup$ Commented Oct 15, 2015 at 19:54
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$\begingroup$ @fooot The question explicitly asks what the hole is for, and the answer covers that. This is a problem with the question (and exactly why we don't like multi-part questions), not the answer. $\endgroup$ Commented Nov 11, 2017 at 11:19
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$\begingroup$ However, this answer is obviously wrong. The small hole is in the middle layer of a three-layer side window. Any impact is going to be on the outer layer and, in any case, the side windows don't receive significant impacts. A bird strike to the windscreen is dangerous because of the 500mph+ closing speed. A bird strike to a side window would be no more dangerous than a bird flying into your house windows: the closing speed is just the bird's airspeed and the window is trying to get out of the way at 500mph perpendicular. $\endgroup$ Commented Nov 11, 2017 at 11:22