PA34 LNDG SBP EXPERIENCED CONFLICT WITH OPPOSITE DIRECTION HELI IN TFC PATTERN.

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WAS ON WITH ZLA FLT FOLLOWING TO SBP. AN AIRLINER WAS SLOWLY OVERTAKING ME; ALSO ON WITH ZLA; HEADED TO SBP. CTR ASKED US TO ALLOW THE AIRLINER TO LAND BEFORE US; SINCE WE WOULD BOTH BE ARRIVING AT SBP AT THE SAME TIME. THE AIRLINER WAS MAKING A STRAIGHT-IN APCH AND CTR ASKED US IF WE WANTED TO MAKE A 360 DEG TURN TO MANEUVER BEHIND THE AIRLINER. DUE TO OUR LOW ALT AND THE TERRAIN; AND THE WX RPTING WIND CALM; WE OPTED TO INSTEAD ENTER R TFC FOR THE OPPOSITE RWY THAT THE AIRLINER WAS LNDG ON. WE ADVISED CTR WHAT WE WERE PLANNING ON DOING AND THEY THOUGHT IT WAS A FINE IDEA; AS DID THE AIRLINER. MEANWHILE; WE'RE MONITORING THE FREQ AT THE ARPT AND WERE HEARING NO TFC RPTING. DUE TO FREQ CONGESTION; CTR DIDN'T RELEASE US TO CHANGE FREQ UNTIL WE HAD ENTERED THE R DOWNWIND. AS WE WERE SWITCHING FREQS WE SPOTTED TFC FLYING OPPOSITE US; ON A L DOWNWIND TO THE RWY THE AIRLINER WAS LNDG ON. WE BOTH SPOTTED ONE ANOTHER AND ADJUSTED OUR COURSES TO THE R TO AVOID ONE ANOTHER. WE ENDED UP TURNING 120 DEGS TO FOLLOW THE TFC IN; WITHOUT INCIDENT. CONTRIBUTING FACTORS: ZLA NEVER SAW OR ADVISED OF THE TFC; AND I TALKED TO THE HELI PLTS AFTERWARD WHO CLAIMED ZLA COULDN'T EVER PICK THEM UP ON RADAR; HOWEVER; NO ONE ELSE HAS EVER EXPERIENCED THAT PROB. I BELIEVE INSTEAD THEIR XPONDER WAS NOT ON. ALSO; THEY WERE NOT MAKING STANDARD RADIO CALLS OR WE WOULD HAVE HEARD THEM AND KNOWN NOT TO CONTINUE WITH OUR PLAN. THEY DID NOT MAKE A CALL WHEN THE AIRLINER CHKED ON IN THE AREA; EITHER; WHICH ALSO WOULD HAVE ALERTED US. WE WOULDN'T HAVE HAD THE PROB EITHER IF WE HAD DONE A STANDARD TFC PATTERN ENTRY TO AN UNCTLED ARPT. FINALLY; IF WE HAD BROADCAST OUR INTENTIONS FURTHER OUT; THE HELI MAY HAVE ALSO BROADCAST; THEN WE WOULD NOT HAVE ENDED UP CLOSE TO ONE ANOTHER. ANOTHER FACTOR IS COMPLACENCY. I HAVE FLOWN THIS FLT INTO THE SAME ARPT AT THE SAME TIME OVER 100 TIMES AND THERE HAS NEVER BEEN TFC IN THE PATTERN BEFORE. NORMALLY; TRAINING FLTS DON'T OCCUR AT XA00 AT THIS ARPT.