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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.

User 'diegus012' answers here, on 2015 Feb 21.

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  • $\begingroup$ In Copa 201, they were able to determine that it was faulty due to a loose wire. It's hard to imagine that an impact would loosen a wire in such a way that the gyro sent correct data but only intermittently. $\endgroup$ – raptortech97 Feb 25 '15 at 3:06
  • $\begingroup$ You also have recordings from the instrument before the crash happened. If it was giving faulty or implausible readings then, then it is more conclusive. $\endgroup$ – user7241 Feb 27 '15 at 20:14
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Investigators have experience in inspecting damaged equipment. Over the years they develop an understanding of what kind of damage comes from crashes. Based on this experience they can determine whether the damage is consistent with a crash or just typical operation.

As raptortech97 commented, the gyro from Copa 201 had a wire nearly severed. This is exactly the kind of normal damage that could happen to a gyro in operation, but not the kind of damage that would happen inside a gyro during a crash.

Of course there are many instances where investigators are unable to determine whether equipment was damaged before the crash. Items may be too badly damaged to determine this.

The comment about USAir 427 is also relevant. See also British Airways 38. Some failure modes leave no evidence behind, and thus are difficult to diagnose afterwards.

The investigation will take all of these factors into account along with all other evidence they have. They will not claim a definitive answer based on uncertain test results alone. In the case of Copa 201, the data from the FDR matched the behavior of a gyro with a loose connection, which matched the circumstances of the crash. In the case of the 737 PCU (USAir 427) and RR FOHE (British Airways 38), once the failure mode was tracked down, it was tested in the lab to confirm that such a failure was possible and could cause the expected effects, and to determine the best method to prevent it in the future.

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