THE ACFT DEPARTED LAS VEGAS, NV AS THE 2ND OF 5 ACFT ON A SCHEDULED GRAND CANYON SIGHT-SEEING FLT. THE 1ST PORTION OF THE FLT WAS OVER THE LAS VEGAS STRIP AREA & LAKE MEAD & WAS UNEVENTFUL. HOWEVER, WX CONDITIONS PRECLUDED THE PLT FROM CONTINUING OVER THE SHIVWITS PLATEAU. RADIO CONVERSATIONS BETWEEN THE PLT & OTHER COMPANY AIRCREWS INDICATED THAT THE PLT OF N88LV HAD ENCOUNTERED DETERIORATING WX CONDITIONS IN THE CANYON & THAT HE HAD DECIDED TO ABORT THE FLT & CLIMB OUT. LATER ATTEMPTS TO CONTACT THE ACFT WERE UNSUCCESSFUL. THE WRECKAGE WAS FOUND ON 8/18/83 WHERE THE ACFT HAD COLLIDED ON THE WEST WALL OF A MESA INSIDE THE CANYON. A HELICOPTER PLT FLYING ABOUT 14 MI NORTH OF THE CRASH SITE REPORTED CLOUDS AT GROUND LEVEL (4500 FT MSL). THE ELEVATION OF THE ACCIDENT SITE WAS 6320 FT. SHORTLY BEFORE HIS LAST TRANSMITION, THE PLT STATED HE WAS 'JUST SHORT OF TWIN PEAKS.' THE PLT HAD TRANSMITTED HIS POSITION FROM PEACH VOR, INDICATING HE WAS IN THE AREA OF THE CRASH SITE. TWIN PEAKS WAS NORTHEAST & HAD SIMILAR TERRAIN.