AN MDT ACR CREW HAD A NMAC WITH AN SMA ON A RADAR CTLED DEP IN A TCA. THE ACR PLT SUSPECTS THAT THE SMA PLT MAY HAVE TURNED OFF HIS TRANSPONDER IN ORDER TO 'POP UP' THROUGH A BROKEN CLOUD CONDITION.

Narrative

TCASII ISSUED A DSCNT RA TO DSND. THE TARGET HAD APPEARED SO RAPIDLY AND IN CLOSE PROX THAT THE RELATIVE POS ON THE TCASII SCREEN COULD NOT BE DETERMINED. I BEGAN A PUSH- OVER MANEUVER SLOW AT FIRST; THEN MORE RAPIDLY SO AS NOT TO UPSET THE CABIN ATTENDANT. WE WERE CLBING THROUGH 7000 FT TO 9000 FT ON A 270 DEG ASSIGNED HDG. WE WERE JUST CLRING THE TOPS OF A SCATTERED TO BROKEN LAYER AND OBSERVED A BUILD-UP DIRECTLY IN FRONT OF US RISING TO ABOUT 8500 FT MSL. NO PREVIOUS TA HAD BEEN ISSUED BY THE TCASII OR BY ATC. 20 SECONDS HAD TRANSPIRED FROM THE 'DSND; DSND NOW' RA WHEN I SPOTTED THE TFC AT 10 O'CLOCK AND 1/4 MI ON A PATH THAT WOULD HAVE BROUGHT US TO WITHIN 50 FT WITH HIM PASSING JUST BEHIND. A FURTHER RA 'INCREASE DSCNT' WAS THEN ISSUED AT WHICH OUR ACFT WAS DSNDING THROUGH 7200 FT. ATC THEN GAVE US A TA OF 10 O'CLOCK AND 1 MI. I BEGAN TURNING TO THE R AND INCREASING DSCNT WHEN THE SMA; WHITE WITH RED AND BLACK TRIM AND WHEEL PANTS; MADE AN IMMEDIATE MANEUVER ALSO TO THE R. THE OPPOSING ACFT MOST LIKELY HAD TURNED OFF THEIR TRANSPONDER TO CLB THROUGH THE CLOUD CONDITIONS ON TOP. HIS PROX TO THE CLOUD WAS ABOUT 200 FT ABOVE THE CLOUD TOPS AND HAD JUST GONE RIGHT ALONGSIDE A CLOUD BUILD-UP. ATC WAS ADVISED OF THE ACFT PROX TO US AND THE CLOUDS TO WHICH THEY RESPONDED THAT HE HAD JUST POPPED UP. WE THEN ADVISED ATC OF HIS VIOLATION OF REQUIRED CLRNC FROM CLOUDS IN CTLED AIRSPACE. THE OTHER ACFT HAD PROBABLY JUST TURNED HIS TRANSPONDER BACK ON WHEN WE GOT THE RA. THE PUSH-OVER MANEUVER WAS ALSO EXECUTED SLOW AT FIRST BECAUSE THE OCCURRENCE OF THE RA WITH THE TARGET SO CLOSE WITH NO PREVIOUS WARNING; THAT A PHANTOM TARGET WAS SUSPECTED.