Windshield failure . . .
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In fact, cracked or shattered cockpit windshield occurrences, during commercial aircraft flights at normal cruise altitudes, ranging 20-38 thousand feet, happen more often than one might think. Every week or two there is a cracked or shattered cockpit windshield incident happening on one of the nearly 90 thousands flights airborne each day (or nearly 33 million flights annually) around the world.
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More info:
https://www.linkedin.com/pulse/20140507001219-90103575-cracked-aircraft-windshields-are-not-rare
I had the odd thought that it might have been a small but very fast-moving meteor.