One note of caution about relying too much on TCAS for collision avoidance.
TCAS was designed as the last line of defense against collisions when all
else fails. It is NOT designed for use as the primary way to avoid
collisions. Unfortunately, given current FAA ATC procedures, this is the
way it is currently being used.

One of the problems with TCAS and gliders is that the TCAS logic is designed
for typical aircraft. As a result, TCAS is assuming that targets are
traveling in a relatively straight trajectory. Gliders don't do this, so
the TCAS RA may generate advice that actually creates a collision.

If you are really serious about minimizing the threat of collisions, you
also need a device that will show you where the other traffic is. If you
are in an area where there are ADS-B ground stations, an ADS-B UAT
transceiver is definitely the way to go. If you look at the FAA web site,
you will see that there currently is ground station coverage on the entire
east coast, in southern Florida, and in other isolated other areas of the
US. By next summer, there should be a major expansion of the ground station
coverage, with most of the US covered by 2011.

If you are not in an area with ADS-B coverage, a PCAS type of device is an
alternative.

Mike Schumann

"Darryl Ramm" wrote in message
...
On Jun 21, 7:14 am, jcarlyle wrote:
I need to replace a Terra TRT250D (it's failed for the 5th time). The
only transponder that might fit into the Terra's rectangular cutout in
my crowded panel (if I do some horizontal filling) is the Trig TT21.
The Trig's specifications look good and it has an attractive price,
but it's brand new on the market.

Does anyone have any experience or thoughts about the Trig TT21
transponder that they'd care to share?

-John

I am not sure that the TT-21 has FAA TSO approval yet. I can see the
TT-31 in the TSO database (TSO-C112 2/29/2008) but not the TT-21. Is
anybody selling the TT-21 in the USA yet? I see claims on foreign
reseller websites for TSO approval for the T31 but I can't find that
claim on Trig's web site (they claim it will be approved) and I can't
find it in the FAA database, it would be great if it was approved.
Their (sole?) USA dealer is www.seaerospace.com and they only seem to
claim the TT31 is TSO approved.

I have no experience with any Trig transponder, but on paper at least
the Trig looks like a really nice device. I am also very curious to
hear from folks who have use it. It is compact, low power, should be
easy to mount with it's two box construction and with an built in
altitude encoder. It is Mode-S and supports ADS-B 1090ES out, which
gives you future compatibility to meet mandated ADS-B requirements.
What that means is that the Trig will support connecting a GPS data
source and broadcast the GPS location over ADS-B using the 1090ES
standard.

BTW some comments on ADS-B....

One thing to be aware of with that, and any 1090ES or UAT ADS-B device
in gliders, is that there is currently no low cost (IFR) certified GPS
unit that can be connected to provide the location data for ADS-B data
out. This problem is identical regardless of whether we are talking
about ADS-B over 1090ES or a UAT device. We've got 10 years or so to
solve that regulatory problem. Coverage is the second issue, there is
currently not good ADS-B coverage across the USA. The FAA has coverage
maps for for ADS-B roll out (both UAT and 1090ES).

That's the "1090ES" or "extended squitter" stuff or sometimes just
written as "ADS-B". The data transfer can be "in" or "out". A
transponder will support out, or both in and out. 1090ES out supports
sending the aircraft location and 1090ES in reads TIS-B traffic data
(or on a UAT also reads FIS-B weather and other data, a UAT can
support data services than 1090ES). And UATs based devices can support
data in or out or both. There are currently no UAT devices approved
for use that are suitable for gliders.


Not all Mode-S transponders have to provide ADS-B support. But I am
not aware of any curent Mode-S transponder sold in the USA that would
interest a glider pilot that does not at least have 1090ES out
capability. If there are any (compact, low power) glider compatible
Mode-S trasnponders that do *not* even do 1090ES out it would be great
to know.

The Trig transponders do not have 1090ES ADS-B out capability but not
1090ES in. Which means they cannot receive TIS-B traffic information
for display on a traffic display. Becker probably make the most
popular transponders in the USA for use in gliders. The current line
of Becker Mode-S claim to support ADS-B in and out but I am not aware
of anybody using this in a glider or any glider compatible traffic
display. It will be nice to see what happens if there is some pricing
competition for glider compatible Mode-S/1090ES transponders once the
Trig TT21 starts being sold in the USA.

The choice of a Mode-S 1090ES transponder gives you flexibility in
future for compatibility with ADS-B and is a much better choice for
traffic avoidance than UAT based ADS-B in traffic areas of mixed jets
and airliners since UAT devices provide *no* visibility to TCAS (and
there is no technical standard in which they can). TCAS is the last
critical part of the collision avoidance safety net and we really need
this when flying in regions of high density fast jet traffic. The FAA
and the industry really assumes that UATs are a supplement to existing
transponders, not a replacement. The nice thing about a Mode-S
transponder is it gives you options for the future, but with mandates
about a decade away a Mode-C transponder may still provide useful
economical life (and could be complemented with a UAT later, or be
replaced with a Mode-S 1090ES out transponder).

Hope that provides some useful information, even if it does not answer
the direct question.


Darryl