Finbar wrote:

I think it's an oversight, rather than foolishness: airplanes ARE
required to have transponders, and this rule was written for them.

Gliders aren't required to have transponders, and when the rules were
written, they were extremely rare in gliders.

Hmmm...as far as I know airplanes do not have a general requirement for
transponders. 91.215 is poorly written, but neither it nor 91.205
seem to require a transponder be installed in all airplanes. Not
sure where you may have seen this...

My reading seems to indicate that if an operable transponder is installed,
it must be inspected (91.411 and/or 91.413 as applies) and
turned on. There are various exceptions for how to operate
it if it's out of inspection or otherwise inop.

So if one IS installed, there are things to do, but I've never read any
requirement that a transponder MUST be installed in all airplanes.

Now beyond that there are some airspaces that require transponders,
with some exceptions for gliders, aircraft without engine-driven
electrics, etc. All to allow the Baby Aces and Cubs and gliders
and the like to access airports under B and C rings without requiring
a transponder. Grandfathered in, it seems... And we rode the coattails
of the ol' farts that kept it this way...

Beware, however, a much greater evil. The San Jose FSDO
required a local pilot who wanted to certify his Experimental
Speed Canard to install a transponder for certification.
Not optional, but required by the FSDO for his experimental.
I've never heard of this as a requirement for good ol'
152's and 172's when flown outside of 91.215(b)(1-5), so
I don't know why he got hit with it for his experimental
(especially since his home airport was in "G" airspace
with only "E" above).

As far as I know, the Fresno FSDO 100+ NM south didn't
require a transponder for the recent experimental certification
of an RV-3. Do FSDO's have the discretion to
require transponders of experimentals on a case by case basis?
Perhaps so. A bit odd and not too consistent it might seem...

I've personally had a mechanic pull out a transponder with
a log entry, and then flown a plane for 6 months without it
below 10,000 ft in "E" and "G" airspace. Even had an FAA DPE
do a checkride in the plane. Nobody even blinked...straight
faces all around...

But maybe I'm missing something and all airplanes with
engine driven electrics ARE required to have transponders.
Maybe someone can point me to a reference...

As far as getting a transponder OR getting a Proxalert (etc),
I'd definitely go proxalert first...no-brainer there.
Lots of squawking targets out there, and me squawking too
does almost nothing to improve safety (since radar watching ATC
will rarely be in contact with them in my most common
scenarios). Me watching the other squawks, and then
looking outside in that direction: now that'll help...