Transponder requirement confusion

Quick (hopefully) question to a confusing part of the FARs. With my new (to
me) solid-state transponder, I'm now willing to power it up for even local VFR
flights. Before, I would only turn on my KT-76 if I needed to (VFR advisories, IFR,
Class B/C operation) since I knew the cavity tube had a finite number of hours before
it died. Now that I read through the transponder requirement carefully, it appears
that that's not allowed. Google groups appear to contain previous usenet arguments
regarding this, but the answer's still not quite clear. In particular:

FAR 91.215(5)(c)
(c) Transponder-on operation. While in the airspace as specified in paragraph (b) of
this section or in all controlled airspace, each person operating an aircraft equipped
with an operable ATC transponder maintained in accordance with ??91.413 of this part
shall operate the transponder, including Mode C equipment if installed, and shall
reply on the appropriate code or as assigned by ATC.

"Controlled" airspace also includes Class E... so if you have one (and it's
within the 2-year test), you *have* to turn it on? ... but you aren't *required* to
have one?

If so, that's a very unclear requirement from most colloquial references on
equipment requirements...

Curious...
-Cory

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* Cory Papenfuss, Ph.D., PPSEL-IA *
* Electrical Engineering *
* Virginia Polytechnic Institute and State University *
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