FLC OF AN MLG DSNDED 1000 FT BELOW AN ALT FIX ON A BACK COURSE LOC APCH; AND THEN CORRECTED BACK AS SOON AS IT WAS NOTICED. ATC RECEIVED A LOW ALT ALERT IN THE CTL ROOM; BUT DID NOT PASS IT ON TO THE RPTR.

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PLEASE REFER TO BOISE; ID; COMMERCIAL CHART AUGUST 1996; LOC (BACK COURSE) RWY 28L. WE WERE VECTORED TO INTERCEPT THE BACK COURSE BTWN THE D9.1 AND D15.1 FIXES AT 6500 FT MSL AND CLRED FOR THE APCH. MY (CAPT/PF) EHSI WAS CONFIGURED TO RAW DATA ONCE WE WERE ESTABLISHED ON THE INTERCEPT COURSE. FO EHSI WAS ALSO CONFIGURED FOR RAW DATA; ALTHOUGH HIS INTENTION WAS TO HAVE BEEN IN A MAP CONFIGN; HE REMEMBERS BEING DISTRACTED AS HE REACHED FOR THE MODE SELECTOR. THE FO HAD CONFIGURED THE FMC TO DISPLAY FIX XING ALTS ON ANY EHSI IN MAP MODE AS A XCHK. I BELIEVE THAT I CALLED FOR THE MCP ALT WINDOW TO BE SET TO 5900 FT. WE AGREED AS A CREW THAT THIS WAS CORRECT; BUT 4900 FT WAS SET INSTEAD AND I DID NOT CHK IT PER SOP OR; I INCORRECTLY CALLED FOR 4900 FT AND THE FO SET THIS IN THE WINDOW WITHOUT CHKING ITS VALIDITY. I USED A VERT SPD MODE TO DSND (AUTOPLT IN COMMAND MODE AND AUTOTHROTTLES ENGAGED PER SOP) AS I INTENDED TO DSND ONLY 600 FT AND HAD PERHAPS 6 MI TO THE D9.1 STEP-DOWN FIX. APCH HANDED US OFF TO TWR AT ABOUT THIS POINT. GEAR DOWN WAS SELECTED DURING THIS DSCNT AND THE 'FINAL DSCNT CHKLIST' WAS ACCOMPLISHED BEFORE MAKING A VHF CALL TO BOISE TWR. AS I CONCENTRATED ON TRACKING THE BACK COURSE WITH THE HEADING SELECT MODE (PER SOP); I HAD THE SENSATION OF LEVEL FLT (THROTTLES MOVING FORWARD WITH GEAR DOWN). WE DISCOVERED THE DISCREPANCY AS WE WERE LEVELING AT 4900 FT AT APPROX D11.0. I INITIATED A CLB TO CORRECT AS OUR SIT BECAME CLR; BUT THE D9.1 FIX WAS SOON ACHIEVED AND 4900 FT WAS REACQUIRED. THE APCH WAS CONTINUED TO LNDG. AWARENESS OF PROGRESS ON THE APCH WAS NEVER LOST. INST SCAN BROKE DOWN HOWEVER; AND ALT AWARENESS WAS LOST. THE LOC WAS TRACKED CLOSELY; WITHIN 1/4 DOT. HIGHEST TERRAIN WAS CLRED BY 1000 FT; HIGHER TERRAIN IS CHARTED AT 5000 FT PERHAPS 2.5 MI N OF THIS APCH SEGMENT. NO GPWS ALERT WAS EXPERIENCED. NO WARNING WAS GIVEN BY APCH CTL OR TWR. I INITIATED A PHONE CALL TO THE APCH CTLR LATER AND LEARNED THAT A LOW ALT ALERT HAD IN FACT BEEN TRIGGERED; BUT NOT PASSED TO US.