SMA PLT RPT ON A POTENTIAL CONFLICT WITH A 'STEALTH' GLIDER THAT WAS NOT PICKED UP BY JOSHUA APCH RADAR.

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THIS WAS NOT A NEAR MISS AS I HAD THE SAILPLANE IN SIGHT FROM APPROX 2 NM AWAY. MY CONCERN IS THAT THE SAILPLANE WAS NOT VISIBLE ON RADAR. THE GLIDER HAD APPARENTLY CAUGHT A THERMAL COLUMN AND WAS ASCENDING AT APPROX 800 FPM TO MY ALT. MY 'EVASIVE' ACTION WAS SUBTLE AS I GRADUALLY TURNED EASTWARD TO AVOID BEING IN THE VERT PATH OF THE GLIDER. I SHOULD HAVE SENT THIS RPT IN EARLIER; BUT DIDN'T THINK OF IT AS A DANGER AT THE TIME. UPON REFLECTING; I DECIDED THAT NASA/ASRS SHOULD HAVE THIS INFO. FOR SOME REASON (CONSTRUCTION MATERIALS?; 'STEALTHY' SHAPE?) THE GLIDER APPARENTLY NEVER APPEARED ON THE RADAR OF JOSHUA APCH. AS THE SAILPLANE CLOSED TO ESTIMATE 1000 FT I ASKED JOSHUA APCH IF IT WAS ON THEIR RADAR AND RECEIVED A NEGATIVE. THE GLIDER WAS ALL WHITE AND APPEARED TO BE A 2-SEATER. I DO NOT KNOW IF THEY SAW ME. THEY DID NOT APPEAR TO ALTER THEIR SPIRAL CLB. IN REVIEWING THIS EVENT; I CAN THINK OF NOTHING TO RECOMMEND IN CHANGING PROCS. JOSHUA APCH WAS ACTIVELY CALLING MY TFC (OTHER THAN THE GLIDER) AND I WAS RECEIVING EXCELLENT; PROFESSIONAL; SVC FROM THEM. I AM EQUALLY SATISFIED WITH MY PILOTING DURING THIS EVENT. I HAD MY HEAD-UP AND HAD SPOTTED A POTENTIAL CONFLICT EARLY. MY REASON FOR FILING THIS RPT IS TO SUGGEST FURTHER INVESTIGATION OR RESEARCH ON WHY SOME GLIDERS; AT SOME TIMES; APPEAR INVISIBLE TO RADAR.