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


On 9/9/11 2:48 PM, jcarlyle wrote:
I'm also puzzled as to why he didn't react to my presence. I would
think that his TCAS would have detected me. Nevertheless, there he was
- no course or alititude variation at all, he just flew right over me,
going _real_ fast. The other thing about it was that he wasn't on a
normal arrival path for PHL. I had my biannual VFR transponder check 3
weeks after this experience, and the Trig TT21 passed just fine. I
don't know what happened, but I hope not to be caught in a similar
situation again.

-John

On Sep 9, 5:14 pm, Darryl wrote:
Something is a little strange here as this seems well within the
altitude difference window that should have generated an RA. Which is a
big deal to the 737 crew both from the immediate required response to
the RA and subsequent reporting/paperwork.

Darryl