On Tuesday, September 26, 2017 at 11:32:33 AM UTC-7, LOV2AV8 wrote:
I'm curious how this happens. ATC sees gliders on their radar in Tucson. Wouldn't they SW flight get a traffic alert with "type and altitude unknown"? from ATC. ATC has the job of deconflicting and should worst case it as the unknown traffic is the same altitude as the 737. We have been contacted by Tucson TRACON and they would like to meet in order to share their concerns over transponder equipped gliders being at 10,000' over Pinal Airpark (KMZJ). We haven't met yet but take a look at the proximity to the Tucson Class C. A long ways away and they're complaining about us that are transponder equipped. Seems like a bad omen.

Randy "AV8"

There should be no surprise at all that this can happen. ATC requires transponders in aircraft to provide reliable seperation and gliders are very hard to visually see and can be at altitudes and in locations that have high traffic densities of fast jets and airliners.

Can primary radar see gliders? Sure. Sometimes.

Is that anything as effective (or frequent useful at all in practice) as having a transponder and seeing the glider via SSR? No.

And having a transponder also makes the glider visible to TCAS, the wonderful last resort tool to help avoid mid-air collisions. And having a transponder makes your aircraft visible via TIS-B to ADS-B In and Out equipped aircraft in busy airspace (i.e. will should have ADS-B ground station coverage).. Primary radar is not relayed to TIS-B or anything else.

ATC may or may not see gliders on primary radar. And if they do approach primary radar does not provide altitude data. Primary radar relies on doppler discrimination to see aircraft against ground clutter, what that looks like varies dramatically depending on the area/terrain. Gliders don't fly often fast towards or away from the radar antenna, and they stop and thermal slow where they may disappear off the display. Operators may have to turn up the doppler discriminator significantly to reduce target clutter caused by things like traffic on freeways or wind power generators. The later for example makes gliders flying to the East of Travis AFB totally invisible to primary radar, all in a very busy area full of traffic coming into and out of the San Francisco Bay Area. Travis RAPCON (who provide civil as well as military ATC in that area) love gliders with transponders....

When you talk to those ATC folks, which is certainly a good idea, ask for a tour of their facility and have them show you what they are dealing with and ask them about ground clutter, and have them show you the primary radar and doppler discriminator effects.

For transponder equipped gliders it may be that they want to understand your operations a bit more, might want folks to be in radio communications with ATC, etc. Talking to them is the only way of finding out. It can really helps to have somebody from the glider community who is an experienced power pilot, ideally with an instrument rating or airline experience etc. to be in those conversations.

As I'll point out in other posts, you should not be just looking at Class C or simmilar proximity, you should be looking at STAR and SID procedures as well when evaluating local traffic risks. Of course traffic can come from anywhere at any time, but STAR and SID routes give you a good idea where lots of traffic is expected to be.