AN ACR B737-300 LNDG AT HPN USES UNICOM FOR TFC INFO AND ANOTHER ACFT USES CTAF. NEITHER ACFT IS COMMUNICATING WITH THE OTHER WHILE IN THE TFC PATTERN.

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I WAS IN COMMAND AND FLYING ACR FLT XYZ ON OCT/XX/98. AT 3000 FT MSL; XING THE SHORELINE OUTBOUND; NEW YORK APCH CLRED US FOR THE SHORE VISUAL APCH AND DIRECTED US TO CONTACT UNICOM 122.95 FOR ADVISORIES BECAUSE HPN TWR WAS CLOSED FOR THE NIGHT. SHORTLY AFTER XING SHORELINE INBOUND; I NOTICED A TCASII TARGET 500 FT BELOW US AND 1/2 WAY TO ARPT. I THEN VISUALLY ACQUIRED THE ACFT TURNING BASE TO FINAL. I ELECTED TO ABANDON MY STRAIGHT- IN APCH AND ENTER VISUAL L TFC VIA THE UPWIND LEG. AT NO TIME WAS SAFETY COMPROMISED. THE SEE AND AVOID PRINCIPLE WORKED AS ADVERTISED. IT TURNED OUT THAT OTHER ACFT WAS ON CTAF 119.7 BUT NOT UNICOM. I AM WELL AWARE OF CTAF PROCS; BUT AT HPN; UNICOM VERY MUCH EXPECTS TO RUN THE SHOW. THIS EXPECTATION IS REINFORCED BY NEW YORK APCH SENDING IFR TFC TO THEIR FREQ. IN ORDER TO HEAR EVERYONE; IT WOULD BE NECESSARY TO RECEIVE AND XMIT ON 2 PRIMARY FREQS SIMULTANEOUSLY -- A SIT WITH GRAVE POTENTIAL FOR DISASTER IN A 2 PLT JET. SUGGEST NEW YORK APCH AND HPN MGMNT COME TO A MEETING OF THE MINDS REGARDING WHICH SINGLE FREQ WILL BE WORKED IN THE NO-TWR HPN PATTERN. PROMINENTLY PUBLISH AND LIVE BY THAT DECISION. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR RESTATED THAT HE DID NOT SEE THE OTHER ACFT; AN AZTEC; IN THE TFC PATTERN UNTIL THE TCASII PAINTED HIM. HE NEVER DID HEAR THE AZTEC ON THE UNICOM FREQ THAT HE HAD BEEN INSTRUCTED TO MAKE CONTACT WITH HPN. WHILE ON THE GND AT HPN; HE OBSERVED AN ARPT VEHICLE DRIVE UP TO THE AZTEC AND SHOUT THROUGH THE WINDOW AT THE PLT; THAT HE WAS TO BE ON UNICOM FREQ AND NOT CTAF FREQ. RPTR HAS MADE MANY ENTRIES INTO HPN AND SAID THEY ARE TENACIOUS ABOUT DOING THINGS THEIR WAY. THAT PLTS SHOULD BE GETTING ALL THEIR TFC INFO FROM UNICOM. THE AZTEC DROPPED OFF A PAX AND THEN LEFT. RPTR BELIEVES THE AZTEC PLT WAS TRANSIENT AND UNAWARE OF THE IMPORTANCE THAT HPN PLACES ON BEING IN CONTACT WITH UNICOM. RPTR HAS STRONG FEELINGS ABOUT ALL ACFT SHOULD BE ON THE SAME FREQ WHEN IN THE SAME TFC PATTERN. HE FEELS IT IS A HAZARD NOT TO BE ON THE SAME FREQ. HE ALSO STATED THAT IF A PLT UNFAMILIAR WITH THE AREA OR ARPT WHEN THE TWR IS CLOSED; THAT THE LOGICAL FREQ FOR THE ACFT TO BE TUNED TO IS THE CTAF FREQ.