NMAC BTWN AN ATX SMT TWIN SIGHT-SEEING FLT LEVEL AT 3600 FT AND AN UNKNOWN HELI LEVEL AND HEADING IN A SIMILAR FLT PATH. THE RPTING SMT DSNDED UPON SIGHTING THE HELI TO ASSURE SEPARATION. THE HELI PLT STATED THAT HE OBSERVED THE SMT GO BY UNDERNEATH HIM.

Narrative

DEC/XX/97 FROM HONOLULU TO KAHULUI VIA ACTIVE VOLCANO FOR ONE PAX. I WAS FLYING OVER THE BIG ISLAND OF HAWAII WHEN A SIT OCCURRED IN WHICH AN ACFT CAME IN CLOSE PROX TO THE ACFT I WAS FLYING. TIME: APPROX XX30 AM LCL TIME. POS: 15 DME ON THE HILO VOR 170 DEG RADIAL AT AN ALT OF 3600 FT. CLOUD COVER: 4500 FT SCATTERED. VISIBILITY: 30 NM PLUS. I OBTAINED THE CURRENT HILO ATIS AND CONTACTED HILO APCH AT 18 DME 170 DEG RADIAL (ITO) FOR RADAR SVC; FOR A TRANSITION TO THE N; NW. I WAS TRAVELING ON A HEADING OF N IN A VFR DSCNT. HILO APCH GAVE ME A SQUAWK AND TO IDENT. APPROX 1 MIN LATER I WAS RADAR IDENTED. MOMENTS AFTER; I WAS AWARE THAT ANOTHER ACFT; A HELI WAS OFF MY R SIDE FRONT SLIGHTLY HIGHER AND TRAVELING IN A SIMILAR DIRECTION. THE HELI WAS PARTIALLY BLOCKED BY MY MAGNETIC COMPASS JUST ABOVE MY INST PANEL. I MADE A SMOOTH NOSE DOWN ATTITUDE WITH THE CTL COLUMN AND THE HELI WAS GONE. I IMMEDIATELY ASKED APCH IF I HAD ANY TFC IN MY IMMEDIATE VICINITY. THEY REPLIED THAT I HAD TFC IN MY VICINITY BUT GAVE NO WARNING AT ALL EITHER BEFORE OR AFTER. I SPOKE WITH THE PLT OF THE OTHER ACFT ON VHF FREQ 122.9 WHILE MONITORING APCH CTL ON THE OTHER RADIO TO SEE IF HE WAS AWARE OF THE ENCOUNTER. HE REPLIED THAT HE WAS AND THAT HE HAD SEEN ME PASS UNDERNEATH HIS HELI. I BELIEVE THAT BOTH ACFT WERE TRAVELING ON A SIMILAR HEADING. THE HELI WAS MADE BLIND TO ME BY MY ACFT R SIDE NOSE AND SPLIT WINDSHIELD AND COMPASS. IT WAS ONLY WHEN OUR ALTS WERE SEPARATING AND I WAS LOWER THAN THE HELI THAT I WAS ABLE TO SEE IT. IT IS MY UNDERSTANDING THAT APCH CTL IN HILO SOMETIMES OPERATES THE TWR FREQ AS WELL AS APCH AT THE SAME TIME. WHEN ASKED TO CONTACT THE TWR ON CHANGING FREQ THE VOICE REMAINS THE SAME. I AM NOT SURE THAT THIS WAS THE CASE ON THIS OCCASION BECAUSE I DID NOT CONTACT THE TWR ON THIS PARTICULAR DAY. I AM STILL WONDERING WHY I WAS NOT INFORMED BY APCH OF THE CLOSE PROX OF THE OTHER ACFT. I BELIEVE THAT I HANDLED THE SIT APPROPRIATELY AS THE OTHER ACFT ONLY CAME INTO MY VIEW FOR A SHORT TIME. A REMEDY FOR THIS PARTICULAR OCCURRENCE MIGHT NOT BE FOUND; BUT I BELIEVE THAT COLLISION AVOIDANCE IN HIGH AREAS OF TFC SUCH AS THIS MUST BE CONSIDERED; POSSIBLY HAVING TCASII OR SOMETHING SIMILAR INSTALLED IN ALL COMMERCIAL ACFT OPERATING IN HIGH TFC AREAS.