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Years ago my father worked for Eastern Airlines out of Miami (that should give you a timeframe) and he was an electrical engineer. I recall once he told me that the pilots (one or more) used to intentionally damage the CVR microphone in the cockpit by twisting it until the contacts broke. Their reasons are not relevant for this question.

My question is, knowing what model planes they flew back then, (727, Tristars, A300s, 757, etc) how did the mechanics know that the mic was not working? Would the system show it as a fault or did the CVR tape review show that there was no data from that mic? The former I'd think would allow them to track down the pilot(s) that were responsible for this act.

Also, on planes with more modern electronics, would this fault be detectable immediately or at least before the next flight?

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Interesting question. On the Boeing 727 there is a panel named "Cockpit Voice Recorder Microphone Monitor". A headphone can be plugged into the labelled jack to test if the cockpit voice recorder microphone is working properly. I have not checked the other model aircraft you mentioned but I assume they have the same test capability.

Boeing 727 CVR Microphone Monitor Panel

These microphones are usually passive devices. I doubt there is a warning system even on modern airliners to tell you the CVR mike is not working. But I would also be willing to bet good money they are no longer so easy to disable.

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    $\begingroup$ great info, thanks. Seems like a lot of surface area to test such a minute (yet important) function. Do you know if its a scheduled maintenance procedure to test this, part of an A check perhaps? $\endgroup$ – Gary Bak Nov 19 '18 at 16:43
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    $\begingroup$ I know that the checks are recommended to be performed regularly, but my guess is that each airline has a different schedule to do so. $\endgroup$ – Juan Jimenez Nov 20 '18 at 11:45
  • $\begingroup$ @JuanJimenez: Just recommended? Not required? $\endgroup$ – Sean Nov 21 '18 at 22:43
  • $\begingroup$ The FARs do not require the microphone itself to be tested on a regular basis, not that i can see. $\endgroup$ – Juan Jimenez Nov 23 '18 at 8:22

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