SEA87MA034

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.