On Jun 21, 10:23*am, "Mike Schumann" mike-nos...@traditions-
nospam.com wrote:
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

[snip]


If all you want is TIS-B traffic information then a Mode-S transponder
can get you traffic data, provide TCAS compatibiiey, and also
compatibility with PCAS systems used in some gliders and GA aircraft.
I know Mike knows all this, that comment is to make it clear to
others.

TCAS is the absolute last defence and for high-traffic jet/airline
areas I just hope people will not think a glider would be better off
with ADS-B TIS-B traffic data *instead* of a transponder that provides
visibility to TCAS equipped jets.

While there are arguments about TCAS RA predictions, the RA are based
on altitude and jet traffic can climb or descend rapidly when needed
and the TCAS will monitor what is happening as the threat target
converge. This is not a satisfactory answer but I just worry that many
readers might thing that TCAS is doing some sophisticated track
avoidance and therefore a glider maneuvering might cause far more
complexity than the altitude based avoidance that actually happens.
Being primarily altitude based makes the resolution determination a
lot simpler. Gliders are also relatively slow moving, so even if
maneuvering their relative location to a fast jet does not move
rapidly. Gliders might change climb rates at +/- thousands of feet per
minute but with altitude margins in TCAS and the continuous monitoring
of altitude as targets converge probably make this less of an issue
than it could be. I am not aware of any technical study that shows
serious problems with gliders (or other slow traffic) and TCAS. I'd
love to see any if they exist.

With TCAS on TCAS targets the Mode-S transponders are communicating RA
data to ensure that both aircraft are not instructed to climb etc.
Since a glider won't have TCAS that won't happen, but the real issue
with that is avoiding TCAS directing both the jet aircraft with high
climb/descent performance to do the same thing, and the ultimate fall
back here is the ability of a jet to normally out-climb or out-descent
a glider even if the glider pilot decides to change altitude abruptly
to avoid a collision.

So again, the problem if people just relied on ADS-B UAT devices and a
TIS-B traffic display is that the glider would be entirely invisible
to TCAS. I would much rather have the TCAS help the fast jet avoid me
than me avoid the fast jet. But ideally have both. And ADS-B TIS-B
(either over UAT or 1090ES) offers more range than PCAS - one of the
issues with PCAS (I fly with a Zaon MRX) is it may not have sufficient
range to be useful for fast jet traffic. And even if you have a fancy
TIS-B display you may still decide to make an avoidance manouvre that
conflicts with what TCAS is telling the other aircraft to do. Again
this would rely on the jet typically being able to out climb or out
descend a glider.

Had the ASG-29 flying near Reno a few years ago had a simple
transponder the Hawker jet very likely would not have collided with
with it. ATC hopefully would have given the Hawker a traffic advisory
(but as Mike points out there is no guarantee that currently they
would have since both aircraft collided outside controlled airspace),
failing that the TCAS should have done its job. Likewise with another
incident near Reno more recently a transponder would likely have
avoided the jet pilots and ATC being "surprised" by a glider while on
approach to Reno, and the subsequent hassles for all involved.

BTW people wanting to see what TIS-B traffic on a UAT device might
look like in a real product -- the NavWorx PADS600 receives TIS-B (and
FIS-B) data, it does not do ADS-B out. See http://www.navworx.com. The
receiver is currently ~$1,500 and can display traffic data on a Garmin
496 or several other types of popular GA displays. As Mike points out,
this technology is quite interesting for the future.

Darryl