How do AHRS systems meet the "gyroscopic" requirement of 14 cfr 91.205.d? EDITED
Among the instruments listed as required for IFR operations in the US FAR is are various &quot;gyroscopic&quot; 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 &quot;gyroscopes&quot; within the language of the FAR? Am I misunderstanding what the term means? I thought the &quot;gyro&quot; root meant &quot;to turn in a circle&quot; (as in &quot;gyrate&quot; 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 &quot;gyroscopic&quot; 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 &quot;B&quot; for. But according to SA, the correct answer is &quot;C&quot;. So (if SA is correct) the FAA seems to think here that a gyro is actually a spinning object.

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