I encountered the original question here; I tried to improve the prose by editing it:
Secondly, about all air disasters involving high-speed impact with terrain, how can investigators trust results from testing instrumentation that hit the ground? In Copa Air Flight 201 they tested the captain's gyro and found it faulty. It probably was, but isn't there the possibility that the gyro was damaged as a result of the impact? I'd really like to hear from an investigator on how it is they determine that test results are so reliable that they can come to a definitive conclusion as to why the accident occurred.
[User 'bonmec101' writes, week of 2015 Feb 3]
+[user] Marvin Kitfox "If you test an instrument from a crashed plane, and it works, you can be pretty dang sure it worked before the crash."
you haven't seen "Hidden Danger" then [which examines USAir Flight 427). One of my most favorite episodes.