AMM says:
The construction of window No. 1 consists of a glass pane laminated to each side of a polyvinyl butyral (vinyl) interlayer .
The inner glass pane is the thicker of the two and is the primary load carrying member. The vinyl interlayer acts as the "fail-safe" load carrying member. The outer pane has no structural significance, but provides rigidity and a hard, scratch resistant surface.
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