AN AEROSTAR 601 PLT INADVERTENTLY DSNDS 400 FT BELOW ASSIGNED ALT IN MOUNTAINOUS TERRAIN.

Narrative

AS I WAS BEING VECTORED ON DSCNT TO SLC ON AN IFR FLT PLAN FROM 7S0 (RONAN; MT); I WAS CLRED TO DSND TO 12000 FT MSL AND VECTORED ON A COURSE OF APPROX 150 DEGS TO INTERCEPT THE LOC TO RWY 17 AT SLC. I WAS BTWN 15-20 MI N OF THE ARPT; AND WHEN I SWITCHED MY NAV RADIO TO THE LOC FREQ; IT WAS FLAGGED; I INFORMED THE CTLR I WAS NOT RECEIVING THE SIGNAL AND TUNED MY OTHER NAV RECEIVER TO THE LOC. WHILE I WAS DISTR BY THE RADIO PROB; I DSNDED TO 11600 FT (APPROX); WHICH WAS 400 FT BELOW THE ASSIGNED ALT. THE CTLR THEN INFORMED ME OF THE ALT PROB; AND I IMMEDIATELY CLBED TO 12000 FT MSL. THE WX WAS CLR; WITH OGDEN; UT; AND SLC TO THE S; ALONG MY PATH OF FLT; AND THE WASATCH MOUNTAINS TO THE E AND N OF MY POS. AS I PULLED UP; I NOTICED I COULD SEE THE RIDGE OF THE RANGE OUT MY L WINDOW; AND I ESTIMATED I WAS 2000 FT ABOVE THE HIGHEST TERRAIN. THIS WAS; HOWEVER; BELOW IFR MINIMUM ALT FOR THIS POS. THE APCH CTLR ASKED ME TO CALL TRACON UPON LNDG. THE APCH CTL SUPVR INFORMED ME THAT MY DEV IN ALT PUT ME TOO CLOSE TO THE TERRAIN FOR IFR FLT. THE PWR OUTPUT OF THE LOC XMITTER MAY HAVE BEEN LOW; OR MY PRIMARY RECEIVER MAY NEED TO BE CHKED. JUST BEFORE THE CTLR INFORMED ME OF MY DEV; THE LOC SIGNAL WAS CORRECTLY RECEIVED; INDICATING I WAS JUST OUT OF RANGE OF THE SIGNAL WHEN THE EVENT OCCURRED. THE SUPVR INFORMED ME THAT SINCE I WAS CLOSE TO TERRAIN; THE INCIDENT WOULD BE RPTED FOR POSSIBLE FURTHER INVESTIGATION.