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Among the instruments listed as required for IFR operations in the US FAR is are various "gyroscopic" instruments, including those for the turn coordinator, attitude and heading. In modern avionics, though, there are no gyros. All the inertial sensing is done through some sort of non-spinning resonator (see https://en.wikipedia.org/wiki/Vibrating_structure_gyroscope). How are these things "gyroscopes" within the language of the FAR? Am I misunderstanding what the term means? I thought the "gyro" root meant "to turn in a circle" (as in "gyrate" or the making of the Greek sandwich), which is how heading/attitude/rate-of-turn devices all used to work. Obviously this is a nitpick since the point is to sense rotation. But then why does the FAR specify "gyroscopic" as a modifier for the various instruments?

ETA: The origin of this question was a Sheppard Air test prep q referencing the FAR. Supposedly, all of these questions are on actual FAA exams. Here's another one, which in light of the comments I marked "B" for. But according to SA, the correct answer is "C". So (if SA is correct) the FAA seems to think here that a gyro is actually a spinning object.

Sheppard Air IR test prep question

(Also, thanks whoever downvoted this, while a People Magazine question about JFK Jr's IR training gets upvotes.)

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  • $\begingroup$ I can’t cite a FAR, but it appears the FAA is accepting AHRS systems as “gyroscopic” if they provide the relevant functionality, regardless of whether they actually contain gyroscopes. $\endgroup$ – StephenS Aug 19 at 14:43
  • $\begingroup$ Remember, the "F" in FAR and CFR stands for federal. The federal government doesn't move as quickly as industry... $\endgroup$ – Michael Hall Aug 19 at 15:32
  • $\begingroup$ Vibrating structure gyroscopes are (surprise) referred to as gyroscopes. So that would explain why they are called gyroscopic. $\endgroup$ – DJClayworth Aug 19 at 15:44
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    $\begingroup$ Related. This seems like semantics to me: laser ring gyros, MEMS etc. are still called "gyros" even though they have a totally different operating principle from traditional mechanical ones. It's likely that for engineers etc. "gyroscope" now simply means a device that provides certain functionality, regardless of how it operates. That might be more of an engineering or even English question than an aviation one, though! $\endgroup$ – Pondlife Aug 19 at 16:01
  • $\begingroup$ Yeah, this was more a terminology question than anything else. It's still the case that if you just look up "gyroscope" online, you'll get a description of the spinning thing. And the term "gyroscopic effect" refers to rotating body rigidity of orientation. I guess with that context I'm just puzzled by the explicit use of the term in the CFR. $\endgroup$ – Foster Boondoggle Aug 19 at 16:01
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In regulations, instruments are always defined functionally. They say what the instruments must measure, not how. Any actual implementation then has to be certified, but that certification concerns with accuracy of the instrument, its reliability and ability to detect its failure – because the pilot can only handle the failure if it is obvious to them – not it's principle. So anything that can locally (without external reference) measure attitude and rate of turn qualifies.

Note that the "gyro" root does mean “circle”, but don't forget the other half of the word:

[…] from Ancient Greek γῦρος (gûros, “circle”) and σκοπός (skopós, “watcher”).[1]:

It means, and always did, an instrument for observing rotation, though it was only used to describe one of the methods for doing so known to the person that coined the term (that person was Leon Foucault and the other method was obviously Foucault pendulum).

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  • $\begingroup$ Understood, and I agree. This is really about terminology. (Which is a really annoying thing with the FAA exams.) The thing is that the regs specifically mention “gyroscopic” instruments, then another question in the exam prep bank specifically identifies a gyro as a “spinning wheel or disk”. Just an old man grumbling over here... $\endgroup$ – Foster Boondoggle Aug 26 at 6:00

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