Interesting thing with transponders

On Mar 9, 6:31*pm, "
wrote:
On Mar 6, 8:46*pm, Darryl Ramm wrote:



On Mar 6, 4:32*pm, JS wrote:

On Mar 6, 11:47 am, Darryl Ramm wrote:

If you are in close truly close formation (which might happen in some
buddy flying) ATC may prefer you to deactivate the transponder on all
but one aircraft in the formation.

* Correct, that is what was suggested by ATC... Especially if a glider
has been issued a discrete code. If the glider is on the discrete code
during tow, turn the transponder in the towplane to Standby until the
glider releases. All but one glider in the thermal or group flight
turns their transponder to SBY.
(Think of what these standby transponders do for PCAS use.)

Jim

Sorry but I'm not following the last part of the comment either about
"standby transponders" or at least I am not follwoing if you are
thinking this is a problem or not. Once off tow both transponders
would be on and the PCAS system should operate normally. While on tow
a PCAS unit like theZaonMRXin the glider squawking standby might
think the towplane's transponder is its own and supress the tow plane
as a threat. I'd argue that is not a *problem and actually what you
want--you want the PCAS to suppress the alert and while on tow you
know where the tow plane is (I hope!). And that allows any other
threats coming close to be warned about. Experience from people who
fly with a PCAS in their glider without a transponder (or transponder
in standby) with a tow plane with a transponder would be interesting.

I think it is absolutely the wrong advice to say that gliders should
be turning off and on their transponders without coordinating this
with ATC. That seriously cannot be what ATC wants? For example in
discussions of this with Reno approach, they want this only for close
formation flights (i.e not typical glider flights) and only when those
flights have contacted Reno approach and advised that it is a flight
of multiple aircraft with leader squawking.

The advice to turn off transponders when in thermals just seems
impractical and dangerous, let alone a violation of FARs. You want
people to do this if the other gliders are squawking 1200 or if they
are assigned a discrete code? How do you know they are on a discrete
code or not? Who decides who turns off a transponder and who turn one
on and when. Is this radio chatter going to happen over the approach
frequency?

If a tow plane contacts approach for flight following then they need
to let approach know it is a tow/formation flight with glider
squawking standby--so the controller is not surprised when the glider
pops up on his radar.

Pilots also should not be trying to docuble-guess the decorrelation
capability of TCAS interrogators. i.e. Even with just Mode-C equipped
threats a TCAS equipped aircraft can see multiple aircraft even if
their signals overlap a fair amount. You don't want to be thinking you
are turning off your tranpodner might be helping ATC and then make you
invisible to TCAS that could otherwise "see" you. Especially since
TCAS-II issues vertical resolution advisories there is a risk a TCAS-
II could issue an RA that would fly the aircraft straight into you as
it tries to avoid a gilder in the thermal that is above or below you
with its transponder on. (I'm not talking about the case of a thermal
stacked with many tens of gliders with Mode-C transponders, there have
been some studies of the problem of transponder synchronous garbling
in those cases). BTW if any of those gliders have a Mode-S transponder
like the Trig TT-21 then this avoids the Mode-C synchronous garbling
issues and TCAS and ATC are capable of unambiguously seeing a
relatively large number of individual threats in the same proximity.

Bottom line. I would hope people turn on the transponder and leave it
on unless they are using flight following for a formation/tow flight
and if so then talk to the local ATC facility about how they want to
handle the radio procedures. I know there are battery concerns, but
turning off for long periods to save battery power is different (but
also a violation of FARs), and I'd hope with modern transponders this
is really a not a requirement.

Darryl

I have been only loosly following this thread. *I'm pretty confused at
this point, but now have specific questions.

We have a ZAON PCAS in our club glider. * We are now installing a
transponder too. *I understand that the PCAS will know that the
glider's own transponder is not a threat so it will filter it out.

If we tow behind a towplane which also has a transponder, what will
happen? *Will the PCAS see the tow plane as a threat, and display
that? *But another airplane might be on a collision course... * but
the PCAS will not see that because it sees the towplane as the closest
threat?

OR..........will the PCAS think that the towplane transponder is
actually the glider's transponder since it stays at the same relative
altitiude and position? *Will it filter out both the glider's
transponder and the tow planes' transponder?

Should we put the glider's transponder to "stand by" during tow? *If
we do, will the PCAS then think the tow plane's transponder is really
the glider's transponder and filter it out, allowing the PCAS to see
other targets?

Should we just say "screw it" and ignore the PCAS entirely during tow?

Cookie

Probably yes to all the above, with a big does of "it depends" like it
depends on the exact pressure altitude the Zaon reads, the relative
power it sees from the different transponders as to which of the
transponders it locks onto (or if it sees synchronous garbling).

If the Zaon thinks the tow plane is a threat it will not display other
threats. Until they become more of a threat in which case you see the
"NEW" message. It is also possible it thinks your own transponder is a
threat at times. Remember the threat algorithm is biased towards
threats at a similar altitude.

What do people who tow behind a tow plane with a Zaon MRX see (with or
without their own local transponder?) I often self launch my ASH-26E
(with Zaon MRX and transponder) so am not the right person to answer
the question.

One thing I am pretty sure on is you should not be turning on and off
transponders unless in touch with ATC. And if ATC does prefer that
talk to them about the radio and other procedures they want followed.



Darryl