THE HELICOPTER DEPARTED PENDLETON ON AN EMERG MED SVC/MED-EVAC (EMS) FLT TO TRANSPORT A PATIENT TO PORTLAND. AFTER DELIVERING THE PATIENT, THE PLT TOOK OFF ON THE RTN FLT WITH 2 MED PSNL ABOARD. DRG ARRIVAL AT 1658 PST, HE REQUESTED A SPECIAL VFR (SVFR) CLEARANCE TO THE PENDLETON ARPT. THE PLT WAS ADVISED TO HOLD & EXPECT CLEARANCE AT ABOUT 1725. AFTER INBOUND TRAFFIC HAD LANDED, A SVFR CLEARANCE WAS ISSUED. ABOUT 5 MIN LATER, THE PLT TRANSMITTED A MAYDAY CALL THAT HE HAD 'INADVERTENT IFR' & WAS NOT CAPABLE OF IFR FLT. AT 1732, THE LAST TRANSMISSION FROM THE PLT STATED HE WAS AT2200 FT MSL 'APPROXIMATELY 5.1 NM FROM . . .' SUBSEQUENTLY, THE HELICOPTER IMPACTED THE GROUND ABOUT 5 MI WEST OF THE ARPT, WHILE DSCNDG IN A RT BANK. NO PREIMPACT PART FAILURE/MALFUNCTION WAS FND; RPRTDLY, THE HELICOPTER HAD DEPARTED PORTLAND WITH THE OIL TEMP GAGE INOP, TRANSPONDER REMOVED & A LEAK IN THE PITOT STATIC SYS. AT PENDLETON, THE 1650 PST WX WAS IN PART: 700 FT OVERCAST, VIS 3 MI WITH FOG. THE PLT WAS NOT CERTIFICATED FOR INSTRUMENT FLT IN HELICOPTERS.