On Thursday, May 5, 2016 at 11:17:11 AM UTC-7, wrote:
Here at Chicago Glider Club we have the dubious distinction of flying right under the approach path into Midway airport. I have looked at SouthWest planes coming through from all angles, including head-on. Flying with a C-mode transponder helps a lot, Approach will guide the big iron around us although I'm sometimes wondering about the spacing.
My PFlarm will indicate through the FlarmView display a blue dot around my position that is blinking ONLY when a SW or bigger commercial plane (equipped with TCAS?) is coming through. Could there be a connection such that the transponder when interrogated by a TCAS responds in a way that is picked up by PFlarm? What else might the blue blinking dot mean?
Please enlighten me.
Herb, J7

You've already got the answer, but a bit more detail...

TCAS is a transponder interrogator and receiver, it interrogates on 1030MHz similar to SSR radar and listens for transponder replies on 1090MHz. TCAS is all about that aircraft being able to see transponder equipped aircraft (with typical ranges of several tens of miles). That all those TCAS aircraft also have their own transponders lets them be seen by other aircraft with TCAS and your PowerFLARM's PCAS.

PowerFLARM only receives the 1090 MHz transponder replies, it cannot see the 1030 MHz interrogation signals (and they are not that interesting anyhow).. The combination of airborne TCAS interrogators and ground based SSR interrogators is what causes transponders to reply so a PCAS can see them at all. You won't see a lone TCAS equipped aircraft on your PowerFLARM via PCAS unless there is something else around to interrogate its transponder (and in your environment you have an SSR interrogator so you would see a lone TCAS equipped aircraft).

As airliners equip with ADS-B/1090ES Out heading into 2020 your PowerFLARM will see them directly via that and suppress the PCAS alert. And that happens without any interrogators being needed.

In this environment it is good to see you have a transponder in your glider, hopefully all the gliders in your club do. TCAS can't see your glider, or issue a TA or RA without that. ATC likely can't see gliders at all that are not equipped with transponders, not even as primary radar targets. Visiting FAA folks at the Chicago TRACON or the airport tower and talking with them may be a great thing to do, and maybe you folks already have done that.