According to witnesses, the acft was seen flying along the Trans Alaska Pipeline at an estimated alt of 200 ft AGL. As the acft entered a pass, where the pipeline crossed rising terrain, a witness heard the acft's eng power increase, then the plane disappeared from his view in falling snow. Shortly thereafter, he could no longer hear the eng & assumed the acft had made it thru the pass. Later, the acft was found by a pipeline helicopter plt where it had crashed at the 4700 level of the pass. The pipeline plt reported the visibility was 1/2 mi in snow & the ceiling was 100 ft obscured at the highest point in the pass. Witnesses at the base of the pass estimated the ceiling was 100 to 200 ft, the visibility was 1/2 to 1 mi & light to moderate snow was falling. The crash site was located just beyond the point where the pipeline transitioned from an above ground installation to a below ground installation. No evidence of a preimpact malfunction was found.