A COMMUTER FLC ALLOWS THEIR ACFT TO DSND BELOW THE GS WHEN THEY FAIL TO REALIZE THAT THE GS AUTO CAPTURE IS NOT FUNCTIONING.

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I WAS THE PF (THE FO) AND THE CAPT WAS THE PNF. AT ABOUT 10 DME ON THE RWY 28R LOC AT PDX WE WERE 'CLRED FOR THE APCH' AND TO MAINTAIN 2500 FT UNTIL ESTABLISHED ON THE LOC. I WAS ALREADY DSNDING AND IN THE DSCNT MODE OF THE AUTOPLT PRIOR TO THE CLRNC AND SELECTED THE 'APCH' MODE ON THE AUTOPLT ONCE CLRED FOR THE APCH. THE AUTOPLT WAS ALREADY CAPTURED ON THE LOC AND THE GS WAS ARMED. THE AUTOPLT CONTINUED TO DSND BELOW THE GS AND THEREFORE DID NOT CAPTURE IT. I DID NOT UNDERSTAND FULLY THE MODE OF THE AUTOPLT AND THAT IT WOULD NEVER CAPTURE ONCE SELECTING 'APCH.' I ASSUMED IT WAS CAPTURED WHEN I SAW THAT WE WERE A DOT BELOW AND CALLED FOR GEAR DOWN; BEFORE LNDG CHKLIST. THE CAPT PROCEEDED WITH THE CHKLIST AND WHEN HE RETURNED TO THE INSTS WE WERE 2 DOTS LOW AT 1900 FT. I PUT TOO MUCH EMPHASIS ON OTHER INSTS AND TOO MUCH TRUST OF THE AUTOPLT THAT IT HAD CAPTURED THE GS. THE CAPT CALLED FOR IMMEDIATE PWR AND TO CLB. ATC THEN ISSUED A LOW ALT RPT. WE RETURNED TO THE GS; BROKE OUT OF THE CLOUDS AND LANDED VISUALLY. I WAS LOW TIME IN THE ACFT AND GOT TOO BUSY MONITORING OTHER INSTS ASSUMING THE AUTOPLT WAS CAPTURED. WE THOROUGHLY DISCUSSED THOSE ISSUES AND THE MODES OF THE AUTOPLT WHEN WE RETURNED TO THE GATE. NO GPWS WARNINGS WERE RECEIVED AND ATC ACTED ACCORDINGLY IN THE INTEREST OF SAFETY AFTER WE REALIZED THE DEV. SUPPLEMENTAL INFO FROM ACN 402958: THE FO WAS LOW TIME IN THE ACFT AND TOTAL TIME. THOUGH A QUICK LEARNER; WITH AN EXCELLENT ATTITUDE; EXPERIENCE WAS A FACTOR WITH THE SIT. FAILURE TO OBSERVE AND CORRECT GS DEV OCCURRED WHILE ASSUMING THE AUTOPLT WAS COUPLED.