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From a materials engineering standpoint, cockpit windshields, typically weighing between 25-40 pounds, depending on the type of windshield build, are typically constructed of several layers:

(1) A glass face-plate, roughly 1/10" thick;

(2) A Polyvinyl Butryal (PVB) layer, about 1/8" thick;

(3) A stretched acrylic layer, approximately 1" thick;

(4) An additional PVB layer, nearly 1/10" thick;

(5) An additional stretched acrylic layer, approximately 1" thick. Source

But even then, it is not unbreakable. We have had several cases of broken windshields, as mentioned in above source. Also, the co-pilot in Varig Flight 254 crash was wounded from a tree, which struck his shoulder after crash landing.

So, why are aircraft windshields not made from unbreakable Glass Ceramics ?

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    $\begingroup$ Simple: glass ceramics are not unbreakable. Furthermore, being a ceramic, it will shatter instead of break the way safety glass does. $\endgroup$
    – Sanchises
    Commented Jun 20, 2015 at 17:51
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    $\begingroup$ There is no such thing as "unbreakable" so I don't understand what you're asking. $\endgroup$
    – David Richerby
    Commented Jun 20, 2015 at 20:31
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    $\begingroup$ and even if there were the cost and weight penalties would be too high. $\endgroup$
    – jwenting
    Commented Jun 21, 2015 at 5:55

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Windshields

In the US, aircraft windshields must conform to 14 CFR 25.775 - Windshields and windows

(b) Windshield panes directly in front of the pilots in the normal conduct of their duties, and the supporting structures for these panes, must withstand, without penetration, the impact of a four-pound bird when the velocity of the airplane (relative to the bird along the airplane's flight path) is equal to the value of V C, at sea level, selected under § 25.335(a).

So, windshields are not indestructible partly because they are not required to be.

I guess windshields are not required to be indestructible because bodies like the FAA probably take the view that when an aircraft impacts a massive object at high speed, say a large mountain at 500 MPH, it is of no consequence whether the windshield survives intact at the bottom of a large crater of tiny aircraft fragments. It serves little purpose to make the windshield substantially stronger than the surrounding airframe.

Glass-Ceramics

Glass Ceramics are not unbreakable. For example a glass-ceramic might have a bending strength of 340 MPa (Ref p9)

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  • $\begingroup$ Thanks! Just for fun, the only natural choice for windshields would be Aluminium oxynitride (ALON) better known as transparent aluminium from Star Trek IV (aye captain!). en.wikipedia.org/wiki/Aluminium_oxynitride $\endgroup$
    – Peter
    Commented Jun 21, 2015 at 10:37

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