ACR FO INADVERTENTLY DSCNDED BELOW THE PRESCRIBED ALT DURING AN INST APCH DUE TO AN IMPROPER SETTING OF THE ACFT'S FLT DIRECTOR.

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THE PROBLEM AROSE WHEN I MIS-SELECTED THE HDG MODE ON THE HSI FLT DIRECTOR. I WAS DSNDING INTO A 2000 FT UNDERCAST IN A R HAND TURN TO 145 DEG TO FLY THE DCA RWY 18 LDA DME APCH. I SHOULD HAVE SELECTED THE GS MODE. I ALSO MISUNDERSTOOD THE CLRNC. THE CLRNC WAS TO DSND TO 2000 FT FROM 3000 ONCE ESTABLISHED ON THE APCH. I WAS DSNDING BEFORE THE LOC WAS DECLARED 'ALIVE' BY THE CAPT. BY THE TIME I CORRECTED; I WAS ON THE LOC AND BEGAN DSNDING REFING THE COMMAND BARS ON THE HSI. AS I LOOKED AT THE COMMAND BARS; IT SEEMED AS IF I HAD THE GS 'WIRED.' OF COURSE; THEY WERE ONLY DISPLAYING COURSE CORRECTION. THE FIRST TIME I REFED THE 'RAW DATA' GS DISPLAY; IT WAS PEGGED AT THE TOP OF THE CASE. I HAD FLOWN THROUGH THE GS AND WAS NOW MORE THAN 2 DOTS LOW. I COULD NOT RECONCILE THIS WITH WHAT SEEMED A 'RIGHT ON' GS DISPLAYED BY THE ERRONEOUSLY SELECTED HDG MODE ON THE FLT DIRECTOR. AT 6.5 MI OUT; WITH VISUAL REFS BECOMING MORE DISTINCT; I WAS AT 1280 FT AGL. AT THIS POINT I SHOULD HAVE BEEN AT 1770 FT AGL. AS THE CAPT BEGAN ASKING ME ABOUT THIS DISCREPANCY (HE ASSUMED I WAS PICKING UP VISUAL REF ON THE RIVER APCH); THE APCH CTLR ALERTLY DECLARED A LOW ALT WARNING; AND THE CAPT COMMANDED I LEVEL OFF IMMEDIATELY AND RE- ESTABLISH THE GS. A BIT SHAKEN; I FLEW THE LAST 5 MI OF THE APCH REFING RAW ILS DATA ONLY; AND MADE AN UNEVENTFUL TOUCH DOWN. THE CAPT DEBRIEFED ME; BUT I WAS UNABLE TO EXPLAIN MY CONFUSION. IT WAS NOT UNTIL THE DRIVE HOME I REALIZED MY ERROR. I HAD RECENTLY BEEN USING TECHNIQUE OF PLACING THE FLT DIRECTOR TO THE HDG MODE WHILE ON A VECTOR FROM ATC. THIS SERVED TO REMIND ME THAT THE ACFT WAS NO LONGER COUPLED TO THE ONS; BUT ON VECTORS. I SELECTED HDG INSTEAD OF GS ON THE APCH DUE TO THE RECENT REGULARITY I HAD BEEN TURNING THAT SELECTOR TO THE HDG POS. I DID NOT CROSS CHK RAW DATA QUICKLY ENOUGH AND SHOULD HAVE IMMEDIATELY SAID SOMETHING ABOUT A GS DISPLAY THAT INDICATED AT LEAST A 2 DOT LOW FLT PARAMETER. SUPPLEMENTAL INFO FROM ACN 204277: I'M PNF; FO IS PF - ONLY HAS 150 HRS AS A PLT IN THIS ACFT IN THE LAST 6 MOS; ONLY 150 TOTAL ACTUALLY. MIL FIGHTER BACKGND MIL FGTS; LIMITED USE OF FLT DIRECTORS.