NMAC BTWN 2 COMPANY BRITTEN-NORMAN ISLANDER BN2 DSNDING TO LAND AFTER A SIGHTSEEING FLT. ACFT #2 LOST SIGHT OF ACFT #1 WHICH RESULTED IN OVERTAKING ACFT #1 WHILE IN A DSCNT.

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BOTH ACFT INVOLVED WERE FLYING A TOUR FOR THE SAME OPERATOR WHICH HAS A VFR ONLY CERTIFICATE. WE WERE THE 2ND (ME) AND 3RD ACFT OF 3 FLYING THE SAME TOUR RTE TOGETHER. THE DAY HAD RAIN SHOWERS; SOME LOW CEILINGS; BUT ALL WX WAS RPTED (AND WAS) VFR. AT THE TIME OF THIS EVENT WE WERE ALL DSNDING TOWARD LIH FOR LNDG. THE ACFT IN FRONT AND BEHIND ME HAD BEEN AT 4500 FT AND I HAD STAYED AT 3000 FT TO HELP PROVIDE SEPARATION. WE ALL CONSTANTLY COMMUNICATE ON 122.75 TO BE ADVISED OF OUR RELATIVE POS AND OTHER PERTINENT FLT INFO. WE WERE ALL RECEIVING VFR ADVISORIES FROM CTR. THE ACFT BEHIND ME BEGAN HIS DSCNT WITHOUT ADVISING ME AND AT A TIME WHEN HE DID NOT HAVE ME IN SIGHT ALTHOUGH HE HAD PREVIOUSLY TOLD CTR THAT HE HAD ME IN SIGHT; AND I THOUGHT HE HAD ME IN SIGHT. WE WERE MANEUVERING AROUND SEVERAL LAYERS OF SCATTERED TO BROKEN CLOUDS. HOWEVER; THIS WAS TRULY VFR WITH ADEQUATE CLOUD CLRNC. WHEN I FIRST SAW THE OTHER ACFT (WHO I THOUGHT WAS A FEW MI BEHIND ME) HE APPEARED JUST OVER AND IN FRONT OF ME. THE TAIL NUMBERS (ACTUALLY ON THE AFT FUSELAGE) WERE CLRLY READABLE; AND HE WAS DSNDING THROUGH MY ALT WITH A VERY SLIGHT SPD OVERTAKE. I PULLED BACK PWR DRAMATICALLY; MADE A 90 DEG TURN; AND DSNDED FOR SOME IMMEDIATE SEPARATION. I WAS ABLE TO KEEP HIM IN SIGHT UNTIL LNDG. AFTER LNDG I ASKED IF HE HAD SEEN ME AND HE INDICATED HE HAD NOT AND THAT HE STILL THOUGHT I WAS IN FRONT OF HIM (UNTIL RADIO COM MADE IT CLR THIS WAS NOT THE CASE). OURS IS A GOOD VFR OP WITH PLTS WHO ARE ALL PROFESSIONALS; MOST WITH SIGNIFICANT HVY TURBINE EXPERIENCE. YET HERE THE NEED TO MANEUVER AROUND CLOUDS; AN ACFT WITH VERY LIMITED SIDE AND NO AFT VISIBILITY; NO TCASII; AND FAILURE OF 1 PLT TO FOLLOW ATC OR COMPANY PROC AND ADVISE HAVING LOST SIGHT OF THE ACFT HE WAS FOLLOWING VERY NEARLY LED TO A MIDAIR. WE DISCUSSED THIS INCIDENT WITH OUR CHIEF PLT AND BELIEVE THAT EVEN BETTER COM; AND WIDER SEPARATION AS WE TRAVEL WOULD HAVE HELPED AVOID THIS NEAR MISS. I BELIEVE THAT IN ADDITION VFR CERTIFICATES SHOULD NOT BE GRANTED AND THAT ACRS SHOULD ONLY BE CERTIFICATED WHEN THEIR OP PERMITS PLTS TO FILE AND FLY IFR FLT PLANS WHEN WX IS LESS THAN IDEAL. I BELIEVE THAT THE TRAVELING PUBLIC DOESN'T UNDERSTAND THE DIFFERENCES IN CERTIFICATES OR IN REQUIRED EQUIP (TCASII) SO THEY ARE NOT AWARE THAT CERTAIN TYPES OF OPS ARE INHERENTLY MORE DANGEROUS THAN OTHERS.