FLC OF AN LTT SIGHTS A PARACHUTE JUMP ACFT CLBING TO ALT WHILE THEY ARE DSNDING INTO A NEARBY METRO ARPT BUT WERE OUTSIDE OF THE CLASS C AIRSPACE.

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ESTABLISHED AT 7000 FT ON THE CCR-353; TRAVIS APCH CLRED (ACFT) TO DSND TO AND MAINTAIN 4000 FT. IN THE VICINITY OF YOLO COUNTY ARPT; WHILE DSNDING; I SPOTTED A DHC6 TWIN OTTER (PARACHUTE JUMP) AT 12 O'CLOCK TO 1 O'CLOCK. THIS (ACFT) WAS ESTABLISHED IN A CLB ATTITUDE AND MOVING FROM R TO L ACROSS MY FLT PATH. OUR ALTS WERE MATCHED AT 5000 FT. EVASIVE ACTION; PER SE; WAS NOT INITIATED ON MY PART DUE ONLY TO THE DIFFERENCE IN OUR RESPECTIVE VERT DIRECTIONS -- THE DHC6 BEING ESTABLISHED IN A CLB; AND MY FLT IN A DSCNT. I ESTIMATE A DISTANCE OF 2700 FT OF INITIAL SEPARATION UPON FIRST SIGHT; THIS BASED UPON A GND SPD OF 270 KTS AND AN ESTIMATE OF 6 SECONDS TIME INTERVAL PRIOR TO (ACFT) PASSAGE ABOVE MINE. VERT SEPARATION ESTIMATED AT 200 FT AT THAT POINT. 1) TRAVIS APCH WAS URGENTLY CONTACTED AND I QUERIED WHETHER OR NOT THEY 'HAD OUR 12 O'CLOCK TFC.' INITIALLY; TRAVIS DID NOT ANSWER MY RADIO CALL. AFTER A FEW SECONDS DELAY; TRAVIS THEN RESPONDED THAT THE DHC6 WAS NOT VISIBLE ON RADAR DUE ITS BEING BEYOND TRAVIS'S COVERAGE! YET MY ACFT WAS STILL IN RADAR CONTACT. THEN TRAVIS QUICKLY ADDED THAT THE 'DHC6 TFC WAS NOT IN RADIO CONTACT' AND THEREFORE NON- PARTICIPATING. 2) YOLO COUNTY ARPT IS HOME TO SKYDIVING; AND PARACHUTE ACTIVITIES -- ESPECIALLY ON WEEKENDS -- ARE COMMON AND EXTENSIVE. MY CREWS ARE ALL WELL AWARE OF THE DROP ZONE'S PRESENCE AND ASSOCIATED JUMP (ACFT) OPS. HOWEVER; IT SEEMS THAT THE JUMP (ACFT) FLCS ARE SOMEHOW NOT AS AWARE OF THE FREQUENT AND DAILY COMMUTER TFC UTILIZING THE AIRSPACE IMMEDIATELY W OF YOLO COUNTY ARPT. ONE SIMPLE RADIO CALL FROM THE DHC6'S TO TRAVIS AFB ANNOUNCING THEIR PRESENCE AND INTENTIONS WOULD ALL BUT ELIMINATE THE POSSIBILITY OF A MID-AIR COLLISION BTWN THESE ACFT IN THE FUTURE.