Darryl,

Although the encoder/altimeter weren't formally checked during the
biannual transponder check, the technician did me a favor and quickly
checked the correspondance at 0, 5000, 10000 and 20000 feet - they
were OK. I also routinely check the transponder altitude reading
against my alitimeter during each flight. So I'm pretty sure that's
all OK. As for your suggestion about chatting with ATC, that didn't
happen.

-John

On Sep 9, 6:48 pm, Darryl Ramm wrote:
John

So my very next thought was has the encoder/altimeter system been tested
recently?

A standard biannual transponder check for a non-IFR aircraft does not
include a check of the altimeter/altitude encoder system, it just
includes RF tests--a fairly useless throwback to the days of much less
reliable traveling wave tube based transponders. The tests on first
install does include altimeter/encoder tests.

I've seen quite a few owners assume that the encoder/altimeter is being
tested, but unless you've made arrangement with the test provider this
just usually won't be done for these subsequent checks.

A simple test is to compare the altimeter (when set to 29.92"Hg) and
what the TT21 display says for the altitude. But that does not exclude
problems like the static line being disconnected behind the panel or
contaminated with water etc.

If this had happened to me and I'm sure my transponder was working I'd
probably contact the ATC/approach folks and have a chat - this just
should not happen with a TCAS II equipped 737. Unfortunately we don't
have Mode-S RA downlink really used in the USA. When used (like more
widely in Europe) that provides ATC controllers with direct visibility
to RA events in their airspace.

Darryl