SMA PLT HAD NMAC IN AK.

Narrative

DURING MOUNTAINOUS SIGHT-SEEING OP WHILE IN A CONSTANT DSCNT; I PASSED DIRECTLY BENEATH ANOTHER ACFT. HE APCHED FROM MY 2 O'CLOCK POS AND I NEVER SAW HIM UNTIL HE WAS DIRECTLY ABOVE ME. THERE WAS NO TIME FOR AVOIDANCE MANEUVERING AND THE ACFT WAS NOT SEEN BY MYSELF OR ANY OF THE PAX. BECAUSE OF THE HIGH VOLUME OF TFC IN THE AREA (PART 135 AND 91) WE HAVE ESTABLISHED OPERATING PROCS OF A COMMON FREQ AND AT LEAST THE AIR TAXI PLTS MAKE REGULAR POS RPTS; SHARE WX INFO; ETC. I HAD MADE MY POS RPTS APCHING THE AREA BUT HAD HEARD NO CALLS FROM THE OTHER ACFT. THIS CAN HAPPEN IF WE BOTH XMIT AT THE SAME TIME OR BECAUSE OF INTERVENING TERRAIN. AS FAR AS LOOKING FOR OTHER ACFT; MY PRIMARY ATTN WAS TO MY L FRONT BECAUSE OF THE PROX OF TERRAIN AND I WAS ABOUT TO MAKE A TURN IN THAT DIRECTION. CORRECTIVE MEASURES: 1) INCREASED VIGILANCE IN ALL DIRECTIONS; 2) REDOUBLE EFFORTS TO ASSURE ALL XMISSIONS ARE MADE IN THE CLR; AND 3) EMPHASIZE TO PAX THE IMPORTANCE OF MINIMAL TALK ON THE INTERCOM BUT TO POINT OUT ANY ACFT THEY SEE.